1.
VOLCANOES
8.1. Volcanic Eruptions
8.1.1. Importance for Earth systems (How is volcanism important for
Earth’s systems?)
8.1.2. Viscosity of lava (What controls the viscosity of lava?)
8.1.3. Causes of volcanic eruptions (Why do volcanoes erupt?)
8.1.4. Feature: Santorini (What is the legacy of the eruption of the Greek
island of Santorini?)
8.2. Ejected Volcanic Materials
8.2.1. Lava (What are the ways lava flows?)
8.2.2. Gases and aerosols (What gases and aerosols do volcanoes eject?)
8.2.3. Pyroclastic materials (What forms do pyroclastic materials take?)
8.2.4. Volcanic resources (What material resources do we get from volcanic
eruptions?)
8.3. Structures of Volcanoes
8.3.1. Shield volcanoes (How do shield volcanoes like Hawaii form?)
8.3.2. Cinder cones and composite cones (How do cinder cones and
composite cones form?)
8.3.3. Magma chambers and calderas (What do magma chambers look like
and how are they responsible for the formation of calderas?)
8.3.4. Feature: Erta Ale and Lava Lakes (What goes on inside a lava lake such
as Erta Ale?)
8.4. Styles of Volcanic Eruptions
8.4.1. Magmatic and effusive eruptions (Why are some eruptions effusive?)
8.4.2. Explosive eruptions (Why are some eruptions explosive?)
8.4.3. Volcanic Explosivity Index (VEI) (How do we measure the size of
eruptions?)
8.4.4. Feature: Toba and Supervolcanoes (How dangerous are
supervolcanoes?)
8.5. Volcanic Hazards
8.5.1. Lava flows (What are the risks of flowing lava?)
8.5.2. Pyroclastic flows and ash falls (What are the risks of pyroclastic flows
and ash falls?)
8.5.3. Lahars and debris flows (What are the risks of volcanic debris flows?)
8.5.4. Volcanoes and history (How have volcanic eruptions changed the
course of human history?)
8.5.5. Feature (Human Impacts): Predicting Volcanic Eruptions (How do
volcanologists predict volcanic eruptions?)
8.6. Volcanism at Ridges and Rifts
8.6.1. Structure of ocean crust (What is the structure of the ocean crust?)
8.6.2. Pillow basalts (Why and where do pillow basalts form?)
8.6.3. Thermal vents (How to mid-ocean ridge thermal vents form?)
8.6.4. Continental Rift volcanism: African Rift Valley (What happens when a
continent like Africa rifts apart?)
8.7. Volcanism at Subduction Zones
8.7.1. Structure of Subduction Zones (What goes on inside of a subduction
zone?)
8.7.2. Eruption of volatiles (Where did the water come from for subduction
zone volcanoes?)
8.7.3. Pacific Ring of Fire (Why are there so many volcanoes around the rim
of the Pacific Ocean?)
8.7.4. Feature: Kawah Ijen acid lake (Why is Kawah Ijen the most acidic lake
in the world?)
8.8. Intraplate Volcanoes in the Ocean
8.8.1. Ocean floor seamounts (Why does the ocean seafloor have so many
seamounts?)
8.8.2. Hot spot volcano chains (Why are so many ocean volcanoes found as
part of linear chains?)
8.8.3. Feature: Hawaii (What makes the Hawaiian-Emperor seamounts the
model for a hot spot volcano chain?)
8.8.4. Hot spots at ridges: Iceland and Galapagos (Why are so many hot
spots at mid-ocean ridges?)
8.9. Intraplate Volcanoes on Continents
8.9.1. Large Igneous Provinces: Flood basalts (What happens when a hot
spot first erupts?)
8.9.2. Continental hot spot volcanoes: Yellowstone and the Columbia Flood
Basalts (How did the Yellowstone hot spot begin?)
8.9.3. Hot spots and opening supercontinents (Do hot spots play a role in
breaking apart supercontinents?)
8.9.4. Feature (How Geology is Done): EarthScope and Yellowstone (How
can we see deep beneath Yellowstone?)
8.10.
(Features: Nyiragongo and Erta Ale lava lakes, Kawah Ijen acid
lake, Toba caldera and supervolcanoes, Fuji, Columbia flood basalts,
Deccan traps, Great Meteor/New England hot spot chain, Tristan
hotspot and the opening of the southern Atlantic, Vesuvius/Santorini)
8.11.
(Human Impacts: Predicting Volcanic eruptions, EarthScope and
Yellowstone)
8 VOLCANOES
Volcanoes are the ultimate example of the wide range of processes that we call
Earth science. Volcanoes are the start of the rock cycles, erasing the history of
previous rocks and starting fresh with new igneous rock. They are part of a slow,
continual process of building new land at Earth’s surface – in places like Kilauea
the lava has been slowly bubbling out continuously since 1980. They are also the
epitome of catastrophic processes, with supervolcanoes like Yellowstone
occasionally blanketing the planet with thousands of cubic kilometers of ash.
Volcanoes are global, found in many places and in many geologic settings, but
each one is individual and unique in its own way. Volcanoes provide us with
many material resources such as rich soils, but also pose some of the deadliest of
natural hazards. Volcanoes seem to be what the philosopher Will Durant had in
mind when he said that “Civilization exists by geologic consent, subject to change
without notice.”
In this Chapter we will explore many different aspects of volcanoes. We will look
at the different forms that volcanoes can take and try to figure out the causes for
these differences. Likewise, we will look at the different styles of volcanic
eruptions and try to answer why it is that some volcanoes erupt so calmly that
you can stand safely next to rivers of lava, while others explode catastrophically.
We will travel (virtually) to volcanoes all around the world to understand the
plate tectonic forces that cause volcanoes to form when tectonic plates pull apart
and when they collide. We will investigate the enigma of hot spot volcanoes and
examine the continuing geologic controversies over what actually causes these
volcanoes that occur in the middle of plates. We will also look at the hazards
that volcanic eruptions pose to human societies and the attempts to predict
these extraordinary phenomena.
A: Photo ID: h57sxr.jpg:
8
VOLCANOES
8.1 Volcanic Eruptions
There are fewer aspects of geology more dramatic than the eruption of volcanoes. They
shape the land, build mountains, and create dangerous hazards, but do you know why they
occur? Volcanoes are just the top part of a complex system of Earth processes, but they give
us a glimpse into the workings of the interior of our planet. Not all eruptions are the same,
and an important control on this is how difficult it is for the lava to flow, or its viscosity.
When magma rises to the surface, its viscosity helps to determine the way it erupts at the
surface as lava. Volcanic eruptions have long been a source of fear and inspiration to
humans, and our earliest legends and myths are filled with our attempts to explain these
dramatic occurrences.
8.1.1
8.1.2
8.1.3
8.1.4
How is volcanism important for Earth’s systems?
What controls the viscosity of lava?
Why do volcanoes erupt?
What is the legacy of the eruption of the Greek island of Santorini?
8.1.1 How is volcanism important for Earth’s systems?
Volcanoes play an important role in almost all of Earth’s surface systems, and have done so
throughout Earth’s history. The connection to the rock cycle is obvious: magma that comes
to the surface cools and crystallizes to form new igneous rocks. The effects of volcanoes go
much farther than that:
1. Building land. A total of about 4 cubic kilometers [check] of lava erupts at Earth’s
surface each year. Most of this occurs at mid-ocean ridges, creating the ocean sea
floor. However, a significant amount of lava erupts on land. Because erosion
operates so quickly on Earth, there would be no land above water if volcanism did
not work together with orogeny (mountain-building through plate collisions) to
replenish the land.
2. Replenishing the ocean and atmosphere. Magma contains more than just molten
rock; it also contains large amounts of dissolved gases such as water vapor and
carbon dioxide. Volcanic eruptions replenish the gases of our atmosphere and water
of our ocean.
3. Affecting climate. The carbon dioxide emitted by volcanoes plays a vital role in
keeping Earth’s surface habitable through the greenhouse affect. During geologic
eras of significant volcanism, the greater volume of atmospheric carbon dioxide has
caused warmer global temperatures. In the short term, however, volcanoes also
emit ash and aerosols that reduce the amount of incoming sunlight that can cause
colder global temperatures over years to decades.
4. Creating fertile soils. Volcanic lava and ash contain minerals that create rich soils,
able to support abundant vegetation. Volcanic regions possess some of the world’s
most fertile soils.
5. The origin of life. The role of volcanism on life may go back to the very origin of life
on Earth. Volcanoes provide both the rich minerals and source of heat that may have
supported the world’s first microorganisms. Volcanic regions, such as mid-ocean
ridges, are thriving with complex ecosystems based on archaea and bacteria that
rely on the heat and minerals of the volcanoes to survive. The mid-ocean ridges may
well be the start of all of Earth’s life.
A: CGI video: CGI – 11
B: Photo - hflwlb.jpg:
C: photo of life at a mid-ocean ridge: expl2366.jpg
8.1.2 What controls the viscosity of lava?
When magma reaches the surface and erupts as lava, it can start flowing downhill. How far
the lava can flow will depend upon its viscosity. Viscosity is a measure of how resistant a
fluid is to flowing, and is measured in Pa-s (Pascal-seconds). The viscosities of everyday
objects vary by about 30 orders of magnitude. For example, some typical viscosities are: air
- 18x10-6 Pa-s; water - 10-3 Pa-s; peanut butter – 250 Pa-s; tar – 30,000 Pa; solid mantle
rocks – 1020-1024 Pa-s.
The major factors that control lava viscosity are temperature, composition, and volatile
content. Take two chocolate bars, put one in the freezer and one out on hot pavement and
you can see how important temperature is for viscosity. Lavas also vary greatly in
composition, usually a result of their geologic setting. For example, viscosities of basaltic
lavas like on Hawaii are typically in the range of 100-10,000 Pa-s. Andesitic lavas, such as at
Mt. St. Helens, might be 1-10 million Pa-s. Rhyolitic lavas can be even more viscous, at 0.1-1
trillion Pa-s. What controls this is the amount of silica. The covalent bonds between the
silica tetrahedra are very strong, even in lavas, and lavas that contain a lot of silica (such as
rhyolitic lavas) are very viscous. The presence of dissolved volatiles, such as water, also
affects lava viscosities. A dry rhyolite lava might have a viscosity of about a billion Pa-s, but
dissolve up to 10% water into it and the viscosity can drop to 1000 Pa-s.
Watch the three video clips and you can see that there is a wide in the viscosity for each.
These three cases all come from Hawaii and are all basalt, so their compositions are fairly
similar. In these cases, the differences in viscosity are a result of differences in temperature.
As the lava erupts and flows as a river, its temperature may be around 1200°C and its
viscosity can be as low as 50 Pa-s, like that of ketchup. As the lava cools, its viscosity drops.
The ropy, sluggish pahoehoe lava typically has temperatures of 1100-1200°C and viscosities
of 100-1000 Pa-s. The rough, blocky a’a lava typically has even cooler temperatures of
1000-1100°C and even higher viscosities of 1000-10,000 Pa-s.
No CGI.
Video Clips:
A: Film clip of ropy pahoehoe:
http://www.istockphoto.com/stock-video-11078518-lavaflow-at-volcanopacaya-in-guatemala.php?st=4b3a648
B: Film clip of aa:
http://www.istockphoto.com/stock-video-11078809-front-view-to-a-lavaflow.php?st=4b3a648
C: table of the viscosity of different fluids [We could either keep the examples of
viscosities in the text, or pull them out into a table][NEED]
Material
Typical Viscosity Value (Pa-s)
Air
0.000018
Water
Honey
Ketchup
Shortening
Basaltic lavas
Window putty
Andesitic lavas
Rhyolitic lavas
Mantle rocks
0.001
2-10
100
1000
100-10,000
100,000
1-10 million
0.1-1 trillion
1020 - 1022
Extra Notes:
Mt. St. Helens :
geology.rockbandit.net/2008/05/23/mount-st-helens-lava-dome-growth/
8.1.3 Why do volcanoes erupt?
Volcanoes are the surface eruptions of magma that forms underground from the melting of
rock. Remember from Chapter 7 that magma forms through several different mechanisms.
At mid-ocean ridges, mantle rock melts through the mechanism of pressure release. At
subduction zones, water leaves the top of the sinking slab and lowers the temperature of
the overlying mantle rock, causing it to melt. At hot spot volcanoes, the mechanism of
melting is variable, but often involves rising rock is anomalously hot and melts more easily
from pressure release. In all cases, only a very small percentage of the deep rock melts,
creating many small pockets of “partial melt” within the solid mantle
The process by which the partial melt rises to the surface to form a volcano is likely to be
very complex, and is a topic of active research. The driving force for it is gravity: the melt is
less dense than the solid rock, so it is pushed upward. The melt also has a much lower
viscosity than the solid rock, so it is mobile and able to crack its way upward through the
overlying rock. It is likely that the small channels of rising magma begin to merge, forming
channels that resemble the tributaries of a river. As the magma rises through the crust, it
often takes the form of a large conduit that may be meters to kilometers in diameter. In
many cases the conduit feeds into a magma chamber, and the infrequent eruptions at the
surface only occur when pressure within the magma chamber becomes great enough to
force the magma to the surface. It is important to remember that as the magma rises
upward it encounters many types of rock along the way. There are two important things
that result from this. Much of the magma cools and hardens along the way, forming new
igneous rock within the crust. However, some of the overlying rocks get melted and
becomes part of the magma, so the composition of the lava that erupts at the surface is a
combination of both the original magma and the various rocks it encounters on its way to
the surface.
A: CGI 43 – rising magma
B: Animation of magma channels (like from Katz and Weatherly, EPSL, 2012. I have
contacted them about using it.)
8.1.4 What is the legacy of the eruption of the Greek island of Santorini?
In 1628 BCE an enormous eruption occurred at Mt. Thera, at what is now the island of
Santorini, in the Greek islands north of Crete. The eruption likely played a prominent role in
affecting both the course of civilization in the Mediterranean region and the creation of
several mythological stories and legends. The island has an unusual shape; it is essentially a
giant caldera. All of the Cyclades Islands are volcanoes, a result of the northward subduction
of the Mediterranean Sea floor beneath Europe. Santorini is several million years old, and
goes through a cycle where small eruptions build up a large volcanic cone when then blows
away catastrophically. The last cycle began about 21,000 years ago, and ended with the
eruption 3600 years ago, which ejected about 60 cubic kilometers of rock and ash.
By 1628 BCE, the Minoan Civilization, centered on the island of Crete controlled the trade in
much of the eastern Mediterranean Sea. The largest Minoan city outside of Crete, Akrotiri,
was located on the south side of Santorini. It had 3-story buildings with indoor plumbing,
using hot and cold running water. That city now sits under 60 meters of ash. The eruption
also likely caused a giant tsunami that would have devastated the seafaring economy of
Crete. Within 100-150 years the Minoans were overrun by the Myceneans, who took the
Minoan language (known as “Linear B”) as their own, forming the basis of the Greek
language.
There are no written accounts of the eruption. The only Minoan language of that time,
Linear A, has yet to be deciphered. However, 1300 years later Plato wrote about the
destruction of the island of Atlantis in “one grievous day and night” when it was “swallowed
up by the sea and vanished.” Many anthropologists suggest that this referred to the eruption
of Thera. Santorini today is a beautiful island that is frequently visited by tourists, but it
stands as a significant tribute to the power of geological events. In the early days of
civilization, as languages were just beginning, one volcanic eruption might have shaped
history by allowing ending the dominance of the Minoans and allowing the emergence of
the Greek culture.
A: CGI – 48 – Santorini eruption, AND the next one on the Evolution of the
Mediterranean (that says “Not yet made into clip”)
B: Satellite Photo: h4vldm.jpg
C: One of my pictures from Santorini:
8
VOLCANOES
8.2 Ejected Volcanic Materials
When you think of what comes out of a volcano, what probably comes to mind is lava. Lava
is certainly a major component of what comes out of a volcano, and forms the extrusive
igneous rocks of basalt and rhyolite that we examined in the last chapter. However, the
materials that come out of a volcano take many other forms. Lava can be ejected into the air,
where it cools and hardens and falls as ash and other forms of ejecta. This ejecta can consist
of dust, ash, cinders, glass, bombs, and blocks, which vary greatly in size. However,
volcanoes also eject a lot of gases, mostly water and carbon dioxide, and also tiny liquid
droplets called aerosols. Nearby to a volcano, these materials can be tremendously
hazardous, but these materials also provide valuable resources such as fertile soils and
minerals such as sulfur.
8.2.1
8.2.2
8.2.3
8.2.4
What are the ways lava flows?
What gases and aerosols do volcanoes eject?
What forms do pyroclastic materials take?
What material resources do we get from volcanic eruptions?
8.2.1 What are the ways lava flows?
In section 8.1.2 we examined the viscosity of lava and saw that it can vary by many orders of
magnitude. Lava flowing quickly within a steep lava river on Hawaii is very liquid. However,
the lava that slowly squeezes up into the summit of Mt. St. Helens is very solid. The viscosity
of lava controls the way it flows, but there are other factors as well. For example, the
steepness of a slope affects the way lava flows. A thick, sluggish lava may suddenly behave
in a more fluid manner if it flows over a steep scarp.
Two of the most common forms of lava are pahoehoe and aa, both of which get their names
from types of lava found in Hawaii. Pahoehoe has a smooth, ropy texture that often has a
folded and convoluted surface. The surface cools quickly and has a shiny, glassy texture.
Pahoehoe flows smoothly because of the large amount of gases that are dissolved in the
lava. Evidence for this can be seen in the large number of air bubbles, called vesicles, that
are found within pahoehoe when it cools and hardens. Aa, on the other hand, has a very
rough, blocky appearance, with jagged, sharp edges. As aa flows, it has the appearance of a
heap of rock and cinders being bulldozed. Aa can have the same composition as pahoehoe,
but without the dissolved gases. In fact, pahoehoe can turn into aa as it flows if it loses too
much of its dissolved gases.
No CGI
A: Photo of lava river: h57ruw.jpg
B: Video Clip of lava flowing down a scarp and leveling off:
http://www.ngdigitalmotion.com/clips/4470258_088
C: Video Clip of pahoehoe:
http://www.volcanovideo.com/Movies/PahoehoeRopy2.mov
D: Video Clip of aa:
http://www.volcanovideo.com/Movies/LavaAaflow.mov
E: Photo of pahoehoe and aa: h2a2lj.jpg
8.2.2 What gases and aerosols do volcanoes eject?
In the last section we saw that gasses dissolved in lava can change the way it flows.
Sometimes these gases remain trapped in the lava when it cools, making large numbers of
vesicles in the rock (some volcanic rocks, like pumice, have so much air in them that they
are lighter than water and will float). Most of the gases, however, exsolve out of the lava and
enter the atmosphere. The most common gases emitted by volcanoes are water vapor
(H2O), carbon dioxide (CO2), and sulfur dioxide (SO2). It is estimated that XXX of water
erupts out of volcanoes each year. Volcanoes therefore a vital role in cycling gases like
water vapor through Earth’s systems. A great deal of water enters the mantle at subduction
zones, and most of it comes back out at volcanoes. As we will in section 8.4, these gases not
only affect the viscosity of the lava but also the way volcanoes erupt.
The carbon dioxide emitted by volcanoes each year ranges from about 60-300 million tons
[Moerner and Etiope, 2002; Kerrick, 2001]. That may seem like a lot of greenhouse gases,
but it is still less than 1% of the amount that humans release each year, which is about 35
billion tons. The sulfur dioxide also has a strong effect on climate, but in a different way. The
sulfur dioxide ejected into the atmosphere forms small droplets of liquid called aerosols.
These droplets can be in the size range of about 0.3 micrometers, which makes them
especially efficient at blocking incoming solar radiation. As a result, large volcanic eruptions
are often followed by several years of colder-than-average temperatures.
There are other gases emitted by volcanoes, including the gases hydrogen sulfide (H2S),
hydrochloric acid (HCl), and hydrogen fluoride (HF), which can all pose health hazards to
humans.
No CGI
A: Video clip of steam at a summit eruption:
http://www.istockphoto.com/stock-video-12690938-volcano-santiaguitoeruption.php?st=852f977
B: Video clip of Old Faithful erupting:
http://www.istockphoto.com/stock-video-3394647-old-faithful-yellowstonenational-park.php?st=0298ba8
8.2.3 What forms do pyroclastic materials take?
In the last two sections we talked about the emission of liquid rock (lava) and gases during a
volcanic eruption. When magma and gases are ejected simultaneously, a class of materials
called pyroclastic rocks, or pyroclastics, are erupted. As you might think from the name
(“pyro” = fire; “clastic” = broken), these are hot rocks that form as the ejected magma
comes in contact with the air and cools. The pyroclastics can range in size from tiny ash
particles to large blocks and bombs, depending upon the temperature and gas content of the
magma and the force of the eruption.
As the rising magma reaches the surface, the dissolved gases begin to form bubbles and
expand, often causing the erupted material to take a frothy, bubbly form. As the material
rises and falls, sometimes from tall lava fountains, the liquid solidifies into a variety of
volcanic rocks that are generically called tephra. Larger pyroclasts generally fall out quickly
and don’t travel far from the eruption. Fine ash, however, can be carried upward by hot
gases to altitudes of more than 80,000 ft, and can be carried far away from the volcano,
sometimes even circling the globe.
When the tephra lands, it often forms layers of new rocks like tuff (from ash) or pyroclastic
breccia (from larger blocks and bombs). Sometimes the ash is hot enough to weld together
to form a welded tuff (from falling ash) or ignimbrite (from a hot pyroclastic flow). These
rocks are therefore a combination of volcanic and sedimentary processes.
Size of Clast
> 64 mm
< 64 mm
< 2 mm
< 0.064 mm
Pyroclast
Agglomerate
Lapilli
Coarse ash
Fine ash
Pyroclastic Rock
Agglomerate, pyroclastic breccia
Lapilli tuff
Coarse ash tuff
Fine ash tuff
No CGI
A: Video clip of lava fountain, with helicopter for scale:
http://www.volcanovideo.com/Movies/HighFountains-chopper.mov
B: Video clip of lava fountain, showing red lava rising, dark pyroclasic debris falling:
http://www.volcanovideo.com/Movies/HighFountains2.mov
(just the third of the 4 segments, which is zoomed out)
C: Photo of plinian eruption: h6iuvy.jpg
D: Photo of debris layers: h32h96.jpg
E: Photo of Mt. St. Helens pumice: h32jxq.jpg
F (For assessment): Photo of large bomb: j28vz9.jpg
G: Photo of pumice field, with a person holding up a boulder: mmil0070.jpg
H: Video of bombs ejected and landing:
http://www.discoveryaccess.com/media/clip/80801908_132.do?assetId=clip
_23753211
8.2.4 What resources do we get from volcanoes?
Volcanic processes are vital to human existence; they provide many kinds of natural
resources. Most importantly, volcanic activity concentrates metallic minerals like copper,
gold, silver, lead, and zinc, allowing them to be easily mined. This process, explained further
in Chapter 23, explains why many metals are mined in areas of current or past subduction
zones. The history of human civilization is closely tied to discovery and exploitation of
regions of high mineral and metal concentrations, often the result of past volcanic activity.
Some early cultures, such as at places like Bandelier National Monument (New Mexico) and
Cappadocia (Turkey), even carved out their dwellings from soft layers of tuff.
At a more fundamental level, volcanism is responsible for much of the land. Erosion would
long since have washed land to the sea if it were not for new land being added by volcanoes,
both on the surface and along the coasts. Some of the world’s most fertile soils are also
formed from volcanic ash. Travelers to lush volcanic islands like the Hawaiian Islands, the
Philippines, Indonesia, etc., can easily attest to this. The volcanic ash is rich in minerals, and
the tephra can replenish surface soils that might otherwise lose their fertility over time.
Even volcanic rock such as pumice has many industrial applications.
Currently, there are many places in the world where geothermal power provided by heat
from shallow magmatic systems. For example, most of the homes in Iceland are heated
geothermally, and Iceland gets about 30% of its total electricity from geothermal power.
A: Video clip of lava pouring into the ocean:
http://footage.shutterstock.com/clip-607243-stock-footage-lava-flowinginto-the-ocean.html
and
http://footage.shutterstock.com/clip-1646347-stock-footage-lava-flowinginto-the-water.html
B: Photo of red volcanic soil, like on Hawaii [NEED]
C: Photo of industrial pumice [NEED]
D: Photo of a geothermal energy plant, like the Geysers [NEED]
E: Photo of ancient dwellings carved in volcanic tuff, like at Bandelier or Cappadocia
[NEED]
8
VOLCANOES
8.3 Structures of Volcanoes
As you flip through the pages of this chapter, one thing will become quickly obvious to you:
volcanoes occur in many different shapes and sizes. Within the ocean, enormous amounts of
basaltic lava can slowly and continuously bubble up for millions of years, creating enormous
broad shields of lava that make islands like Hawaii. On land, giant stratovolcanoes can build
from alternations of ash and lava. But lava also reaches the surface to form fissure
eruptions, cinder cones, lava domes, and even lava lakes.
8.3.1 How do shield volcanoes like Hawaii form?
8.3.2 How do cinder cones and composite cones form?
8.3.3 What do magma chambers look like and how are they responsible for the
formation of calderas?
8.3.4 Feature (Geologic Wonders): What goes on inside a lava lake such as Erta Ale?
8.3.1 How do shield volcanoes like Hawaii form?
If you visit certain parts of the world, such as Hawaii, Iceland, and the Galapagos Islands,
you see a certain kind of volcano called a shield volcano. The name (from the German
schildvulkan), refers to the shape of the defensive shields of ancient warriors. These
volcanoes are very wide and flat – they can be about 20 times wider than they are high.
What shield volcanoes have in common is their composition, which is low-silica basalt.
Remember that a high silica content causes magma to be viscous and “gummy.” The hot
low-silica basalt is able to flow easy across the surface, often traveling many tens of
kilometers away from an eruption, which allows the volcano to build out laterally. Mauna
Loa, on the Big Island of Hawaii, is the largest in the world – it is ~100 km in diameter and
rises ~9 km above the seafloor. However, because the weight of Hawaii has isostatically
depressed the seafloor, the actual thickness of the volcano is ~17 km.
Shield volcanoes are often associated with mantle hot spots, even in places like the African
Rift Valley, which is associated with the Afar hotspot. This also appears to be true
elsewhere in the solar system. Shield volcanoes are very common on places such as Venus,
Mars, and Jupiter’s moon Io, and even though these bodies do not have plate tectonics, they
still have mantle convection and hotspot activity.
One common feature of shield volcanoes is the occurrence of lava tubes. The tops of rivers
of lava solidify, insulating the flowing lava underneath, allowing it to flow horizontally even
greater distances. When the eruption stops, the empty subway-like tubes remain. The
longest, the Kazamura lava tube on Hawaii, extends more than 65 kilometers!
No CGI
A: Aerial video clip of Hawaiian lava
stream:
http://www.istockphoto.com/stockvideo-11680959-volcano-etna-lavaflow.php?st=4b3a648
B. [NEED] Diagram of shield volcano,
like:
C. Photos of Mauna Loa:
h6iold.jpg (from afar)
h0x7gb.jpg (aerial)
h4vmvl.jpg (from above, showing lava flows)
D: Photos of Lava tubes: h27tuw.jpg AND iejn30.jpg
8.3.2 How do cinder cones and composite cones form?
Volcanoes can take many different forms depending upon the relative amounts and
compositions of the erupted lava and pyroclastic materials. Nonetheless, there are some
common forms that appear in multiple places around the world.
When glassy pyroclastic materials called cinders or scoria are ejected from a volcano, a
cinder cone can result. These dark-colored cones tend to have steep slopes, as the
pyroclasts do not fall too far from the central vent. A symmetric bowl-shaped crater often
forms at the center. There can be lava flows at cinder cones, but the lava, which is much
denser that the cinders, usually flows out the base or flanks of the cone. Because of the loose
agglomeration of the cinders, cinder cones tend to erode relatively quickly, so old cinder
cones are rarely found.
The most common volcano found on continents is a composite cone, or stratovolcano, which
forms from different layers of both lava and ash. Most of the famous volcanoes – Mt. Fuji, Mt.
St. Helens, Krakatoa, Mt. Vesuvius, Kilimanjaro, and so on – are stratovolcanoes. The
composition of the lava is generally andesitic, which is more silicic than basalt, so the lava
doesn’t flow as far and the cone is steeper than a shield volcano. Lava flows also cover and
solidify layers of pyroclastic material, which also tends to fall closer to the volcanic vent.
Stratovolcanoes have the capacity to erupt explosively and catastrophically, discussed more
in later sections. When a Stratovolcano is rebuilding, eruptions of felsic silica-rich lava can
form another volcanic feature called a lava dome. This extrusion of very viscous lava can
currently be seen occurring within the summit of Mt. St. Helens in Washington.
A. [NEED] Video clip of stratovolcano eruption: [The MontserratTCScreener clips look
good. Please pull them so I can look at them]
B. Photo(s) of cinder cone: hhrmjp.jpg (San Francisco Mts, AZ)
C. Photo of composite cone: 5181460995_5bec8aa0b3_b.jpg AND h57qs7.jpg
D. [NEED] Diagram of
composite cone:
E. [NEED] Video clip – need a cinder cone (Pericutin? Surtsey? Etc.), [The ones of Etna
look good. Please let me see them.]
8.3.3 What do magma chambers look like and how are they responsible for the
formation of calderas?
An important part of a volcano that is never seen is the magma chamber. The formation of
magma can occur in small amounts more than 100 km beneath the surface, but it rises
toward the surface and accumulates in one or more magma chambers. Based on imaging
from seismic tomography, magma chambers tend to be small, perhaps only 100s of meters
across and only a few kilometers beneath the surface, though they can be also be several
tens of kilometers across.
Because magma can sit in a magma chamber for a long period of time, the composition can
change, causing different kinds of eruptions at different times. Low-density components can
rise to the top and erupt first, followed by the eruption of denser materials. For example, the
eruption of Mt. Vesuvius in 79 CE involved the initial eruption of white pumice followed by
a darker layer of gray pumice that came from deeper in the magma chamber. Different
minerals melt or crystallize at different temperatures, so as the magma chamber cools, the
composition of the erupted lava will change.
An interesting process can sometimes occur at the end of an eruption when the pressure
drops inside the magma chamber and a caldera forms at the surface. Following the eruption
of lava and gases or if magma drains back down, perhaps to a deeper magma chamber, the
pressure inside the magma chamber can be insufficient to support the ground above, which
collapses down to form a caldera. Famous calderas include the circular Greek island of
Santorini, the island of Fernandina in the Galapagos (which collapsed dramatically in 1968),
and Yellowstone National Park, which mostly rests inside a 70-km-wide caldera.
A: CGI – 43 would be good here
B: Aerial photo of Santorini: h4vldm.jpg
C: Aerial photo of Fernandina caldera (Galapagos): ISS005E06997.jpg
D Need an animation/infographic of the collapse of a caldera
8.3.4 Feature (Geologic Wonders): What goes on inside a lava lake such as
Halemaumau?
The volcanoes Halemaumau and Pu’u O’o (Hawaii), Erta Ale (Ethiopia), Nyiragongo (Congo),
Marum (Ambrym Island in Vanuatu), and Erebus (Antarctica) share a rare distinction: they
are the only 5 places in the world where you can go to see an active lava lake at the volcano
summit. In the case of Erta Ale, the summit crater has been a bubbling cauldron of lava for
over 100 years! Most volcanic eruptions are separated by long periods of dormancy, but
when the conditions are just right, lava can be continuously replenished by the magma
chamber underneath. The
The proximity of the two Hawaiian lava lakes to the Hawaii Volcano Observatory has
allowed them to be monitored and documented to an unprecedented degree. Halemaumau
was extremely active in the mid-1960s, and scientists could observed how the lava
repeatedly formed a thin solid crust at the surface that would then sink into the liquid lava,
to be replace by a new crust. Geologists later observed how the lava lake was like a minilaboratory that simulated the process of plate tectonics: the lava crust spread apart like
mid-ocean ridges, sank back down like subducting ocean lithosphere, and even displayed
transform faults. Geologists have also speculated that lava lakes might the closest thing
today to the way Earth’s surface appear very early in its history.
The activity within lava lakes can vary over time. Halemaumau was also very active in the
mid-1800s. Mark Twain visited it in 1866 and wrote that “all around the shores of the lake
were nearly white-hot chimneys or hollow drums of lava, four or five feet high, and up
through them were bursting gorgeous sprays of lava-gouts and gem spangles, some white,
some red and some golden--a ceaseless bombardment, and one that fascinated the eye with
its unapproachable splendor.” Halemaumau went into another active phase starting in
2008, when a large gas vent blew a big hole out of one side of the crater walls.
No CGI
A: Pu’u O’o lava lake video: 12-2011XDCAMStock (can I see this?)
8
VOLCANOES
8.4 Styles of Volcanic Eruptions
Watch movies of a few volcanic eruptions and you will realize that volcanoes can erupt in
many different styles. In some cases the lava bubbles up out of a vent or fissure so calmly
that you can stand right next to them. In other cases the eruptions are so catastrophic that
much of the volcano itself can be blasted away. Some of the factors that we have already
talked about – magma composition, temperature, volatile content, viscosity – play
important roles in determining the style of the eruption. The size of volcanic eruptions can
be determined by both the volume of material ejected and how explosive it is, quantified
using the Volcanic Explosivity Index (VEI). Fortunately, the most explosive eruptions,
referred to as “Supervolcanoes,” do not occur very often.
8.4.1
8.4.2
8.4.3
8.4.4
Why are some eruptions effusive?
Why are some eruptions explosive?
How do we measure the size of eruptions?
Feature (Human Impacts): How dangerous are supervolcanoes?
8.4.1 Why are some eruptions effusive?
We have already seen places like Hawaii where volcanic eruptions can occur relatively
peacefully and quietly. These are called effusive eruptions. Lava flowing out during effusive
eruptions typically has a relatively low viscosity, primarily because of having a basaltic
composition and low amounts of dissolved gases. These eruptions are therefore often found
occurring at ocean island shield volcanoes like Hawaii and Iceland that are associated with
hotspots. The low-viscosity lava can flow as sheet flows of pahoehoe or aa, or it can be
concentrated in rivers or lava tubes.
Effusive eruptions can also occur at subduction zone volcanoes, where the composition of
the lava is usually andesitic, provided that the eruption is not explosive. Here, the lava can
form thick stuffy flows, and can be extruded into steep-sided lava domes.
Some of the largest effusive eruptions are fissure eruptions. The largest historic effusive
eruption occurred in Iceland in 1783, from a 25-km-long fissure at Laki volcano. The
eruption lasted almost a year, with about 15 km3 of basaltic lava covering 600 km2. The
eruption released an estimated 120 million tons of sulfur dioxide, which created an aerosol
haze that drifted across Europe and around the world, greatly reducing global
temperatures. Severe famines were documented in around the world, and the resulting 6
million deaths made it the deadliest volcano in written history. The effects were worst in
Europe, and the famines in France played an important role in triggering the French
Revolution of 1789.
The largest effusive eruptions in geologic history are associated with the initiation of
hotspot volcanism, such as the basaltic Deccan Traps that covered a half-million km2 of
India 68-60 m.y. ago when the Reunion hotspot first erupted. In North America, the
initiation of the Yellowstone hotspot 17-14 m.y. ago coincided with the Columbia Plateau
flood basalts that covered the region of eastern Washington and Oregon with ~175 million
km3. These effusive flood basalts were released by a series of north-south trending fissures
that were hundreds of km long.
A: Video clip of lava bubbling out of the ground: 2007UpdateStock68mbps
B: [NEED] Photo of Laki fissure volcano, Iceland (if not, h27ui9.jpg of fissure eruption
on Hawaii will suffice). Can we get?:
8.4.2 Why are some eruptions explosive?
If magma contains a large amount of dissolved gases, primarily water, the rapid expansion
of the magma as it nears the surface can result in a very explosive eruption. The pressure
keeps the gases dissolved in the magma, but when the pressure is released at shallow
depths, bubbles begin to form, turning the magma into a rapidly expanding froth. The
process is similar to shaking up a bottle of soda and then watching the eruption when you
take the cap off. One cubic meter of magma can rapidly expand to form more than 500 km3
of lava and water vapor, creating an instantaneous runaway explosion that in cases like
Krakatoa and Mt. St. Helens can blow away large portions of the volcanic edifice.
Some eruptions, called phreatic eruptions, only involve expanding gas with no magma.
Explosive eruptions involving gas and magma can actually take many different forms that
often classified according to similarities to classic historic eruptions. We already saw
Hawaiian-style eruptions in the last section. Here are some more examples:
Strombolian eruptions (Stromboli Island, Sicily) are characterized by short volcanic blasts
that can send lava shooting hundreds of meters high, similar to some Hawaiian eruptions.
Strombolian eruptions, generally the lowest-energy of explosive eruptions, generally
involve relatively viscous basaltic lava that is ejected in the form of multiple bombs and
lapilli fragments. The result is a often a cinder cone, such as Paricutin (Mexico).
Vulcanian eruptions (Vulcano island, Sicily) are more explosive than Strombolian
eruptions, largely because of the more viscous andesitic or dacitic magma composition. This
means that greater pressures within the volcano are required for eruptions, and eruptive
columns of ash can reach 5-10 km in altitude. Though most of the pyroclastic material
ejected is ash, Vulcanian eruptions are also characterized by the ejection of large blocks and
bombs that wear away a pre-existing lava dome.
Pelean eruptions (Mount Pelee, Martinique) are deadly eruptions that involve the release of
fast-moving pyroclastic flows that move rapidly down the slopes of the volcano. During a
Pelean eruption, an existing lava dome of viscous rhyolite of andesite lava collapses in on
itself, causing the sudden release of dense pyroclastic flow. The name comes from the 1902
eruption of Mount Pelee, where 30,000 people of the coastal town of St. Pierre were nearly
instantaneously killed by the pyroclastic flow. Mayon volcano (Philippines) is one of the
most active volcanoes with Pelean-sytle eruptions in the world.
Plinian eruptions (from the documentation of the 79 CE eruption of Vesuvius by Pliny the
Younger) involve the ejection of pyroclastics as much as 40 km high into the atmosphere.
Rapid gas expansion, as the magma reaches the surface, is channeled by a narrow conduit,
which forces the gas and ash upward. Thermal convection carries the hot gas and ash to the
greatest heights. The most explosive and dramatic eruptions, which as Mt. St. Helens in
1980 and Mt. Pinatubo (Philippines) in 1991, are Plinean.
A: CGI – (the clips are all so short!) Perhaps we could link 44, 49, 50, and 51 into one
continuous CGI clip?
B: [NEED] Stromboli eruption
C: [NEED] Photo of Vulcan volcano eruption
D: [NEED] Photo of pyroclastic flow down slopes of Mt. Mayon, Philippines (Peleanstyle)
E: Photo of Mt. St. Helens erupting (Plinian-style): h6iuvy.jpg
8.4.3 How do we measure the size of eruptions?
It is often useful to have a simple scale of natural hazards. Hurricanes, tornadoes, and
earthquakes all have simple scales that allow for a quick assessment of their severity. For
volcanic eruptions, this scale, from 0 to 8, is called the Volcano Explosivity Index (VEI). Like
the moment magnitude (Mw) scale for earthquakes, the VEI is logarithmic – an increase of a
single unit is designed to represent a 10-fold increase in the size of the eruption
(determined by the volume of ejected material). However, unlike the earthquake Mw scale,
which directly corresponds to the amount of energy released, there is no exact quantitative
formula for computing a volcanic eruption’s VEI.
There are five general criteria for computing the VEI: 1) volume of ejecta, 2) height of the
erupted column of pyroclastics, 3) style of the eruption (effusive, explosive, cataclysmic,
etc.), 4) style of historic eruptions, and 5) height of the spreading of an erupted plume head
of ash. For ancient eruptions, all we generally have to go by is the volume of ejecta. The
more of these criteria that are observed, the more accurate the VEI is.
The VEI scale can be approximated in the following way:
VEI Plume
Height
Minimum
Erupted
Volume
Eruption Style
Approximate
Time Between
Occurrences
Examples
0
1
2
3
<100 m
<1000 m
1-5 km
5-15 km
1000 m3
10,000 m3
1,000,000 m3
0.01 km3
Hawaiian
Hawaiian/Strombolian
Strombolian/Vulcanian
Vulcanian
Continuous
Weeks
Months
Years
4
10-25 km
0.1 km3
Vulcanian/Pelean
Tens of Yrs
5
>25 km
1 km3
Plinian
100s of Yrs
6
7
8
>25 km
>25 km
>25 km
10 km3
100 km3
1000 km3
Plinian/Ultra-Plinian
Ultra-Plinian
Ultra-Plinian
1000s of Yrs
10,000s of Yrs
100,000s of Yrs
Kilauea
Stromboli
Galeras (1992)
Nevado del Ruiz
(1985)
Eyjafjallajökull
(2010)
Mt. St. Helens
(1980)
Krakatoa (1883)
Tambora (1815)
Yellowstone/Toba
This table displays an important characteristic of volcanic eruptions, which also holds true
for many other geologic phenomena such as earthquakes, meteoroid impacts, and river
floods. The largest volcanic eruptions don’t happen very often, but small eruptions can
happen almost continuously. Notice how each time the VEI increases by 1, the volume of
ejecta increases by a factor of 10, but the time between eruptions also increases by about
10. Geologists in previous generations often debated whether geologic processes occurred
continuously and gradually (called uniformitarianism) or occasionally and catastrophically
(called catastrophism). Both were right. Volcanoes continuously erupted around the world,
adding new land to Earth’s surface. However, even though a supervolcano such as Toba or
Yellowstone happens rarely, it can have enormous effects on some of Earth’s systems,
particular the biosphere.
No CGI; No Videos
A: [NEED] interactive graphic where the student could slide the bar and the volcano
would morph between historical eruptions of increasing VEI.
8.4.4 Feature (Human Impacts): How dangerous are supervolcanoes?
What happens when a VEI 8 eruption occurs? Only one has occurred since homo sapiens
first walked on Earth, but this was long before language was written, so we have no direct
records of it -- Toba volcano, in Indonesia, about 74,000 years ago. By the geologic and
biologic evidence that remains, however, it is likely that the effect of this eruption on
humans, and on other animals as well, was severe. It has been estimated that the amount of
ejecta erupted by Toba was 2800 km3, roughly 3000 times larger than the 1980 eruption of
Mt. St. Helens. Ice cores in Antarctica show that this period was followed some of the coldest
years of the most recent Ice Ages, as well as some of the driest and most dusty. The large
amount of aerosols ejected by the eruption could have significantly reduced the amount of
sunlight reaching Earth’s surface, causing a “volcanic winter” that could have lasted for
many years, followed by centuries of colder weather. Genetic evidence from humans and
other large mammals such as apes and tigers suggest that they suffered extreme population
losses and that surviving individuals today descended from a small number of individuals
that survived this period (called a population “bottleneck”).
There are other volcanoes that are capable of erupting as “supervolcanoes.” The bestknown of these is Yellowstone volcano, in western Wyoming, which had VEI 8 eruptions 6.6
m.y. ago (1500 km3 of ejecta), 4.5 m.y. ago (1800 km3), 2.1 m.y. ago (2500 km3), and
640,000 yrs ago (1000 km3). There were many other large Yellowstone eruptions during
this period, but these were the largest. The calderas from these eruptions have been as
great as 50 km across. The good news for human society is that these mega-eruptions do
not happen very often. However, they will undoubtedly occur again in the future, and the
effects will be extreme. Geologic evidence shows that ash from these past Yellowstone
supervolcano eruptions covered a large portion of North America.
No CGI
No video
A: Toba caldera: toba_AST_2006028_lrg.jpg
B: [NEED] Graph of temperature and dust levels from EPICA ice core, showing
anomaly at 74,000 years ago
8
VOLCANOES
8.5 Volcanic Hazards
In the last section we saw the effects of a supervolcano eruption like the 74,000 Ka eruption
of Toba, in Indonesia. Volcanic eruptions pose many different kinds of hazards however,
both local and global. At the site of the eruption, flowing lava can destroy towns and human
structures. Fast-flowing pyroclastic flows and mudflows can travel many miles from an
eruption, causing significant loss of life. Ejected ash can be blows tens of hundreds of miles
from an eruption, causing significant problems such as airplane cancellations over very
wide areas. However, the greatest impact of volcanoes on human history has been the affect
on climate from sulfur dioxide aerosols ejected into the stratosphere, which reduces
incoming sunlight and causes a decrease in global temperatures and affects weather
patterns.
8.5.1 What are the risks of flowing lava?
8.5.2 What are the risks of pyroclastic flows and ash falls?
8.5.3 What are the risks of lahars and volcanic debris flows?
8.5.4 Volcanoes and history (How have volcanic eruptions changed the course of
human history?)
8.5.5 Feature (Human Impacts): Predicting Volcanic Eruptions (How do
volcanologists predict volcanic eruptions?
8.5.1 What are the risks of flowing lava?
Imagine a river of dense, ultra-hot liquid rock flowing downhill into a town. The lava is
heavy, so it breaks through of flows over any barrier you might put before it. The lava is
more than 1000 °C and cools down very slowly, so it ignites houses and trees upon contact,
burning most everything in its path. In places like Hawaii, where the lava has a low
viscosity, it can also travel across the ground for tens of kilometers, doing a large amount of
damage. While it is rare that flowing lava is responsible for the loss of human life, the
financial cost of the destruction to buildings, roads, and other human structures can be very
large.
There is one famous instance where an erupting lava flow was halted by human activities.
In January, 1973, the EldFell volcano began erupting just behind a town on the Icelandic
island of Heimaey. The 5000 inhabitants were evacuated and most houses destroyed by the
initial ashfall, but when advancing lava threatened to cut off the harbor, a massive assault
was mounted to stop the lava. Seawater was sprayed onto the lava by ships in the harbor
and from hoses on land. People even drove upon the advancing lava from in tractors
carrying hoses. So much seawater was sprayed on the lava, in fact, that a quarter-million
tons of salt remains on land. In the end the advancing lava was halted the new lava that
flowed into the sea actually improved the harbor protection. Within a few years the town
was rebuilt, including a geothermal system that provided hot water to all homes.
No CGI
A: [NEED] Video clip of lava flow engulfing houses. Can I please see the three clips
from: 2007UpdateStock68mbps
B: [NEED] Hawaii lava: can we get this photo?
C: [NEED] Photos of Eldfell volcano eruption on Heimaey in 1973, like:
8.5.2 What are the risks of pyroclastic flows and ash falls?
The ejection of hot ash and other pyroclastic material poses an extremely high risk to
people living near active volcanoes. For Plinian-style eruptions the ash is ejected high into
the atmosphere and can fall over a broad area. The ash can be harmful to agriculture and
livestock, impairing human food supplies. The ash, which is often very fine, can also be
extremely damaging in urban areas, as the ash can penetrate and damage industrial
equipment, such as the air filters of cars.
When hot tephra and gases rush down the sides of volcanoes as pyroclastic flows, the
combination can be especially deadly. The descending wall of ash and gas can travel at
speeds of up to 700 km/hr and be more than 1000 °C, burning everything in its path.
Pyroclastic flows are also very unpredictable: a sudden change in direction of a pyroclastic
surge on Mt. Unzen (Japan) in 1991 killed 43 volcanologists and journalists who were
examining an eruption.
The combination of pyroclastic flows and falls can be seen nicely in comparing the
destruction of the towns of Pompeii and Herculaneum during the eruption of Mt. Vesuvius
in 79 CE. Archaeological excavations found both towns to have been covered with thick
layers of ash, but they were not deposited the same way. The eruption alternated between
being Plinean and Pelean many times: a large column of ash and pumice would rise up to 30
km high (Plinean), but would then collapse under its weight as it cooled, rushing down the
flanks of the volcano (Pelean). Pompeii was downwind of Vesuvius during the eruption, so
initial falls of pumice alerted the population, allowing large portions to escape. No such
warning was given to Herculaneum, upwind of the eruption, which was buried by more
than 20 meters of ash and other pyroclastic flow deposits. It is not likely that there were
many survivors there because of the sudden arrival of the fast-flowing pyroclastic surges.
A: [NEED] CGI – the Mt. Rainier sequence that hasn’t been made yet
B: Photo of ash falls: h32hoc.jpg
C: Photo of ash falls: h2aaij.jpg
8.5.3 What are the risks of lahars and volcanic debris flows?
When hot pyroclastic materials mix with water, the result can be a deadly rush of hot mud
called a volcanic debris flow, or lahar. This is more common than you might think. For
example, the peaks of tall mountains are often very cold and may have permanent packs of
snow and ice; even Mt. Kilimanjaro, near the equator in Tanzania, has glaciers at its summit
year-round. When an eruption occurs, this snow and ice are instantly melted by the hot ash,
and the result is a deadly lahar. Lahars can form any time ash and water combine, even long
after an eruption. A lahar can result from heavy rains falling on existing ash layers; heavy
rains in the Philippines in 1995 caused a lahar from the ash remaining from the 1991 Mt.
Pinatubo eruption, killing as many as 100 people.
Lahars usually have the consistency and viscosity of wet concrete as they flow, so can do
extreme damage. Like concrete, they solidify when they stop moving, permanently burying
whatever they surround. Because they are more fluid than lava, they can extend over huge
distances. Much of the city of Tacoma and many parts of Seattle are built upon lahar
deposits from past eruptions of Mt. Rainier, as recently as 500 years ago. One particular
lahar produced by an eruption of Mt. Rainier 5600 years ago covered an area of over 300
km2 from more than two km3 of mud.
The deadliest lahar in recent history occurred in 1985 in Columbia from an eruption of the
volcano Nevado del Ruiz. Pyroclastic flows melted glaciers along the slopes of the mountain.
Lahars rushed downhill at speeds up to 60 kph, overflowing the banks of rivers and into
villages along the rivers. The death toll totaled about 23,000 people.
A: CGI – 52 – Lahar;
B: The file also shows one as “A playground is drowned beneath a lahar,” but the
number given is also “52” which is not correct
C: Photo of lahar: h0x8pn.jpg
8.5.4 Volcanoes and history (How have volcanic eruptions changed the course of
human history?)
We have already seen several examples of volcanic eruptions that have had major impacts
on human society. The eruption of Toba volcano (Indonesia) 74,000 years ago may have
extinguished human existence from many parts of the world. The eruption of Mt. Thera
(Santorini) 3600 years ago may have triggered the collapse of the Minoan civilization. The
eruptions of Laki volcano (Iceland) that began in 1783 caused the famines that led to the
French Revolution. The list of significant impacts on humanity extends far beyond this,
particularly from the climatic effects of large eruptions. Volcanic aerosols, primarily sulfur
dioxide, can circulate throughout the atmosphere, globally reducing incoming sunlight. This
reduces agricultural yields and can change local weather patterns. The effects on
populations can be devastating, and can lead to collapse of governments and mass
migrations of people.
Accounts in ancient texts, such as from early Chinese dynasties or Byzantine eras, repeated
identify periods of time when observations of extended darkness or dimness of the sun
correlate with famines and extreme loss of life. For example, the Byzantine historian
Procopius wrote in 536 CE that “during this year…the sun gave forth its light without
brightness…and it seemed exceedingly like the sun in eclipse, for the beams it shed were not
clear,” and this observation correlates with a massive 536 CE eruption of the Tierra Blanca
Joven eruption in El Salvador.
A VEI 6 eruption of Huaynaputina volcano in Peru in 1600 CE caused famines around the
world, which in Russia led to the collapse of Boris Godunov’s reign. The VEI-7 eruption of
Mt. Tambora (Indonesia) in 1815 caused the “year without a summer” in 1816, causing the
worst famines of the 19th century including in New England, where it actually snowed in the
summer. This led to the large American push westward into the lands west of the
Mississippi, recently obtained from France in the Louisiana Purchase. The settling of the
American West was partly driven by a volcano in Indonesia.
No CGI
A: [NEED] Photo of Tambora caldera, Indonesia
8.5.5 Feature (Human Impacts): Predicting Volcanic Eruptions (How do
volcanologists predict volcanic eruptions?
There remain some natural hazards that remain unpredictable, such as earthquakes or
tornadoes. Volcanic eruptions, however, have become predictable to a large degree. Though
not entirely reliable, the set of geologic processes that lead up to an eruption provide for a
set of observations that can allow for its prediction. The majority of these observations
center on the rise of magma and build-up of pressure beneath a volcano.
As magma rises toward the surface it cracks its way through the rock, causing large
numbers of shallow small earthquakes. Vibrations within the magma plumbing system can
also be observed as a set of resonating vibration or harmonic tremors. Seismologists
monitor volcanic activity with networks of seismometers distributed around a volcano.
Becore the 2005 eruptions at Mt. St. Helens, seismologists located thousands of small
earthquakes beneath the cone with locations that slowly moved toward the surface as the
magma pushed its way upward.
As the magma nears the surface, gases begin to escape. The changing composition and
temperature of the gases can help to identify how close the magma is to the surface and the
likelihood of an eruption. Changing underground pressures can also be observed through
hydrologic measurements such as changes in water-well levels.
Another way to monitor underground activity is through observing the deformation of the
ground. One method involves the placement of tiltmeters on and around the volcano. As
magma moves up under the central vent, the flanks of the volcano bulge outward. This
ground deformation can also be observed by comparing multiple satellite radar images
taken over time.
In practice, many or all of these techniques are used simultaneously, and taken together, can
now provide a strong indicator of the time of a large eruption for many volcanoes.
A: Photo of sampling lava: h6iw7b.jpg
B: photo of sampling gases: hi4opc.jpg
8
VOLCANOES
8.6 Volcanism at Ridges and Rifts
Until now, we have largely concentrated on volcanoes at subduction zones and hot spots.
This is because they have the greatest impact on our lives. The reality is that most volcanoes
erupt in another way – when tectonic plates pull apart – but we rarely see this happening.
Ocean sea floor makes up over 60% of Earth’s surface, and it all forms from volcanic activity
at mid-ocean ridges. These ridges, such as the Mid-Atlantic Ridge, are divergent boundaries
where lava rises to the surface to fill rifts that open as plates move apart. Except for a few
places like Iceland, where the oceanic rift exists above land, we never see this unusual
volcanic process except with submersible vehicles that dive down to the sea floor. The other
form of rift-related volcanism occurs where continents are rifting apart, such as the rift
valleys of Africa.
8.6.1
8.6.2
8.6.3
8.6.4
What is the structure of the ocean crust?
Where and why do pillow basalts and sheeted dikes form?
How do mid-ocean ridge hydrothermal vents form?
What happens when a continent like Africa rifts apart?
8.6.1 What is the structure of the ocean crust?
In previous sections we have seen a wide variety of both volcanic forms (shield, cinder cone,
composite cone, etc.) as well as eruptive styles (Hawaiian, Plinian, Pelean, etc.). If you could
remove the ocean seawater and look at the volcanic activity along the 70,000 km of
connected mid-ocean ridges, you would see that that it was all very similar in appearance.
The result is that the structure of the ocean sea floor is everywhere quite similar. If you drill
into the ocean crust in nearly any two regions of the ocean, you will find that the vertical
structure is very similar in both places. In fact, there are even places where ancient ocean
crust has been thrust onto land during past tectonic plate collisions (called ophiolites), and
the structure of the ocean crust is still the same. Ocean crust has been made the same way
for billions of years.
The main reason for the similarity of ocean crust is the similarity of the lava, which is basalt.
The reason the basalt magma forms beneath mid-ocean ridges was discussed in section X.X.
The magma is stored in small magma chambers beneath the ridge segments, only hundreds
of meters to a couple kilometers across. The magma chambers are not uniform along the
ridge, and volcanic activity occurs in different places along the ridge at different times, but
the end result is a fairly uniform crust.
Looking at the figure, you see several layers to the ocean crust. The top is a layer of
sediment, which does not exist right at the ridge but gradually grows in thickness over time,
so gets thicker with distance away from the ridge. Layer 2 has two parts. The top consists of
hummocky basalt formations called pillow basalts, and the bottom consists of vertical dikes
of basalt. Layer 3 has a similar composition to Layer 2, though it often contains more
ultramafic rocks, but it cools more slowly from the magma and so takes the form of coursegrained gabbros.
A: CGI – 16 – Mid-Ocean Ridge volcanism
Animation from Richard Katz:
B:http://www.earth.ox.ac.uk/~richardk/res/magmaRidge/RidgeModelsKatz.mov
C: [NEED] Diagram of ocean crust structure
8.6.2 Where and why do pillow basalts and sheeted dikes form?
When ophiolites were first discovered in a few parts of the world, such as the Troodos
Mountains of Cyprus and Semail, Oman, geologists were puzzled by the textures of the
basaltic rocks and how they could have formed. Some of them looked like an enormous
number of basalt pillows piled on top of each other. Others were multiple parallel sheets of
basalt. When the ocean sea floor was finally drilled and cores removed, geologists realized
that they had been observing pieces of ancient sea floor.
There are places, such as off the coast of Hawaii, where you can observe basaltic lava
squirting up into the ocean and onto the sea floor. The lava cools instantly, forming a solid
shell, but fresh lava keeps breaking through the shell to form new “pillows.” The longer the
pillow sits on the sea floor, the colder and harder the shell gets, so new lava tends to get
pushed out the end of a long pillow. The appearance is similar to the lines of toothpaste that
would accumulate if you emptied out many tubes of toothpaste onto a table.
The vertical dikes of basalt that form in the underlying layer of ocean crust are a result of
magma that rises up to fill the rifts that continuously open as the two opposing plates move
apart. This occurs discontinuously, so that the magma rises into colder rock, so it cools
quickly to form the small crystals that distinguish basalt from gabbro. The entire Layer 2b
exists of a long sets of these parallel sheets of lava.
No CGI
A: Video clip of pillow basalt erupting on the seafloor: PillowLava2.mov
B: [NEED] Photo of ophiolite outcrop
8.6.3 How do mid-ocean ridge hydrothermal vents form?
The environment at a mid-ocean ridge system can be very strange. Whole colonies of
organisms, many never-before seen, have been observed living around tall chimneys
releasing hot water into the ocean. These chimneys are called hydrothermal vents, and the
water they release, heated by the underground magma, can be as hot as 450 °C. Liquid
water turns to water vapor at 100 °C at sea level, but can remain in liquid form to much high
temperatures at higher pressures. At pressures of the sea floor, this can be several hundreds
of degrees Celsius.
You might wonder where all this water comes from. A pattern of hydrothermal circulation
occurs, driven by the heating of the water beneath the ridge. As this water heats, expands,
and rises, it pulls in seawater from along the flanks of the ridge, which gradually is pulled
toward the ridge, heating all the time. As the water heats, its ability to dissolve minerals out
of the ridge increases. By the time the water gushes up out of the hydrothermal chimneys, it
is often full of metals and minerals. If these minerals are predominantly mafic, the water is
black and the vents are called “black smokers.” If the minerals are predominantly sialic, the
water has a grey, smoky appearance, and these are called “white smokers.”
The many minerals released out of these hydrothermal vents are important for two main
reasons. These minerals accumulate on the sea floor and are eventually carried into
subduction zones, forming the basis for a large portion of the world’s high-concentration
mineral and metal reserves. Future societies may eventually obtain minerals directly from
these seafloor thermal vents.
The other important implication for the thermal vents is that their ability to support whole
biological ecosystems far below the surface with no sunlight makes them strong candidates
for the initial locations for the origin of life on Earth.
No CGI
A: Photos of thermal vents: 5014975047_6c6e95726b_b.jpg (black smoker)
AND 5102285718_c73db1784f_b.jpg (white smoker)
B: [NEED] Video clip of thermal vents at mid-ocean ridges
8.6.4 What happens when a continent like Africa rifts apart?
We know from reconstructions of past tectonic plate motions that at various times in
Earth’s history continents can break apart through a process called continental rifting. This
can occur in the middle of continental areas or can re-rift along ancient boundaries between
continental fragments that came together in the assembly of supercontinents like Pangaea
(discussed more in section 8.9.3). As tectonic forces pull two continental pieces apart, a rift
forms, often with tall scarps that form a deep rift valley, and lava begins to erupt. This
volcanic activity can take the form of long fissure eruptions or single volcanic cones, and can
range in composition from pure basalt to incorporating continental material and having a
more andesitic composition.
There are several well-known continental rifts, such as the Baikal Rift (Russia), the Rio
Grande rift (New Mexico), and Rhine Graben (France/Germany), but by far the best example
of modern day continental rift is the set of African rift valleys in eastern Africa. The African
plate is in the process of breaking apart into three separate plates: the Nubian plate,
Somalian plate, and Arabian plate (which has already broken away). There are several
different kinds of volcanoes occurring here, which include basaltic shield volcanoes like Erta
Ale (Ethiopia) and stratovolcanoes like Kilimanjaro (Tanzania). The volncanism associated
with the continental breakup of Africa also demonstrates how continental rifting is the first
step in the process of forming an ocean. The magma begins with a more intermediate
composition, such as in the many volcanoes of the East African Rift, but transitions to the
pure basaltic composition of oceanic seafloor, such as is forming now in the Red Sea. Thirty
million years ago volcanism began along the rift of what is now the Red Sea, and it would
have then resembled the African Rift valleys of today.
A: CGI – 32 – start of continental rift
and
B: CGI – 26 (it says that the clip is not yet made: “Unzippering of a Continent,“ and
“Beneath the AFAR region lies a huge structure”, but it is!)
C: Map of Africa rift Valley system, like:
8
VOLCANOES
8.7 Volcanism at Subduction Zones
For most people, a discussion of volcanism brings to mind volcanoes at subduction zones.
They are not only large, majestic, and often visually stunning, but when they erupt, they
often do so spectacularly. The process of generating volcanoes at subduction zones is very
complicated, and depends strongly upon the water that is brought down deep into the
mantle, trapped within the oceanic lithosphere. The majority of the world’s stratovolcanoes
around found around the rim of the Pacific Ocean, known as the “Ring of Fire.” This is
because the Pacific Ocean is closing up, with subduction occurring around most of its
borders.
8.7.1 What goes on inside of a subduction zone?
8.7.2 What happens to the water within subduction zones?
8.7.3 Why are there so many volcanoes around the rim of the Pacific Ocean?
8.7.4 Feature (Geologic Wonders): Why is Kawah Ijen the most acidic lake in the
world?
8.7.1 What goes on inside of a subduction zone?
When you think about what is happening at a subduction zone, it might seem strange that
there are volcanoes forming. After all, cold ocean seafloor that has been sitting underneath
cold water (at about 0 °C) for often more than 100 million years gets plunged into the hot
mantle, and magma forms. Things don’t usually melt when you cool them off! The presence
of water, however, causes melting to occur, even though temperatures within the
subduction zone are cooled by the descending ocean lithosphere. In fact, even a small
amount of water can lower the melting point of the mantle rock (peridotite) by hundreds of
degrees.
The water within subduction zones exists within all layers of the ocean crust, and even
down into the mantle. Percolation of water into the crust and mantle began with the
hydrothermal circulation when the rock first formed at a mid-ocean ridge, and continued
throughout the aging of the ocean plate, especially during the significant faulting that
cracked into the lithosphere as it was entering the subduction zone and bending. Most of
the water is not in a liquid form, but actually trapped within minerals. For example, the
common mineral serpentinite can carry water as more than 15 of its mass.
When the ocean lithosphere descends to a depth of about 90-120 km, that water is released
by minerals back into a liquid form, where it rises up into the wedge of mantle rock of the
overlying plate. When the water comes in contact with the peridotite, magma begins to form
and travel upward. When you see a volcano like Mt. Rainier or Mt. Fuji, it is likely that the
top of the subducting ocean plate is about 100 km underneath it.
A: CGI – 17: subduction
B: Seismic tomography image of a subduction zone (Tonga or Japan or the Cascades).
[I will make this], like:
8.7.2 What happens to the water within subduction zones?
The water within subduction zone volcanoes plays many important roles concerning the
volcanic activity there. The wet magma is able to dissolve atoms of mineral and metals out
of the rock (atoms that do not fit well into the mineral structure of silicate rocks, like
copper, gold, silver, lead, etc.) and into the magma, concentrating it to high levels and
carrying it up toward the surface, where it cools and precipitates. There has been
productive gold mining in California and Alaska, but not in Connecticut or Alabama, because
the west coast states have had subduction occurring for hundreds of millions of years,
gradually concentrating metals to high-enough levels to mine.
The water is also critical in the style of the eruptions. As the water-rich magma approaches
the surface, the pressure drops until the water begins to form gas bubbles within the
magma and expands. This expansion, by a factor of hundreds, leads to the explosive
eruptions that blast away the sides of a volcano, such as at Krakatau and Mt. St. Helens, and
eject tephra high into the atmosphere.
However, not all water that enters the mantle at subduction zone coms right back up at
subduction zone volcanoes. Some stays within the core of the subducting lithosphere and
enters into the transition zone (410-660 km deep) and into the lower mantle (below 660
km). It is possible that there could be much more water trapped within the deep mantle,
more than 5 oceans full, than all of the water at or near Earth’s surface. This might be very
important for the way Earth operates. Water lowers not only the melting point of rocks but
also the viscosity, so water in the deep mantle might be allowing for much more rapid
convection and therefore a more rapid rate of planetary cooling. This might be what is
responsible for keeping Earth’s surface warm enough to support life.
No CGI, No Video
A: [NEED] Photo of Mt. St. Helens blast of the north face.
B: [NEED] Diagram of water circulation through a subduction zone, like:
8.7.3 Why are there so many volcanoes around the rim of the Pacific Ocean
More than a hundred years ago geologists noticed a strange pattern: the occurrence of large
stratovolcanoes and of large earthquakes that formed a ring around the border of the Pacific
Ocean. Before the theory of plate tectonics was constructed in the late 1960s, there was no
apparent reason for this. However, this pattern makes sense once you understand that
Pacific Ocean seafloor is sinking beneath the lands around much of the Pacific Ocean.
There are actually several different tectonic plates that make up the Pacific Ocean, and each
one of these is subducting into the mantle. The Pacific plate itself is subducting beneath
Alaska, eastern Asia, the Philippine plate, and the Australian plate; the Nazca plate is
subducting beneath South America; the Cocos plate is subducting beneath Central America;
the Juan de Fuca plate is subducting beneath the Pacific Northwest of North America; the
Philippine plate is subduction beneath Asia; and the Australian plate is subducting beneath
Asia as well as a part of the Pacific plate (note that the Pacific plate is also subducting
beneath a part of the Australian plate).
All of this subduction leads to huge numbers of stratovolcanoes. There are about 1500
active or potentially active above-ground volcanoes around the world, and about 500 of
these have erupted in historic times. Roughly 700 of these volcanoes are stratovolcanoes,
and most of these are found around the Ring of Fire.
No CGI; No Video
A: [NEED] Tectonic map of Pacific Ocean showing locations of volcanoes; It would be
great to have it interactive, so that as you dragged your cursor around, photos of a
selection of the volcanoes
8.7.4 Feature (Geologic Wonders): Why is Kawah Ijen the most acidic lake in the
world?
Imagine a place where hot steam has so much sulfuric acid in it that it would burn away
your lungs if you breathed it directly in. Imagine hot, flaming, liquid sulfur condensing from
this steam. Imagine a giant lake nearby filled with the equivalent of battery acid that would
dissolve you if you fell in it. Such a place exists, here on Earth! It is the crater lake of Kawah
Ijen volcano in Indonesia on the island of Java.
Ijen is the name given to both a group of stratovolcanoes on Java and a particular 20-km
wide caldera. The island of Java is essentially a continuous chain of subduction zone
volcanoes that have formed as the Australian plate subducts beneath Southeast Asia. We
have already seen stratovolcanoes like Mt. Fuji and Santorini that are beautiful touristfriendly attractions. The same is not true for Kawah Ijen. It has a crater at the summit that
contains a 200 meter-deep lake. Sulfur dioxide gas bubbling up through the lake causes the
water to have a pH of 0.5, and though the fumes from this are very dangerous, local people
work day and night collecting burning liquid sulfur that condenses from fumeroles. They
collect about 4 tons of pure sulfur each day.
No CGI
No Video
A: Kawah_Ijen_-East_Java_-Indonesia_-sulphur-31July2009-b.jpg and Kawah_Ijen_East_Java_-Indonesia-31July2009.jpg
B: [NEED] Is it possible to get any of the dramatic nighttime photos, which show the
burning liquid sulfur?
C: [NEED] This was also on the Wikipedia page with the others that were availabile. Is
this one available?
8
VOLCANOES
8.8 Intraplate Volcanoes in the Ocean
If you look at a map of the ocean with the water removed, you see that the ocean sea floor
contain large numbers of volcanoes. These do not seem to fit the traditional model of plate
tectonics, which predicts volcanism at mid-ocean ridges and subduction zone volcanoes.
When the tops of the volcanoes rise above sea level, the form volcanic islands. If their peaks
do not rise about sea level, we call these seamounts. It turns out that many of these islands
and seamounts form near the mid-ocean ridge system but along the flanks of it. They can
occur seemingly randomly along the ridge. Other ocean islands are the result of mantle hot
spots, the regions of continued volcanic activity at distinct locations discussed in Chapter 6.
These hot spots are usually distinguished by long linear collections of islands and
seamounts that result from ocean plates moving over the hot spot, such as the HawaiianEmperor chain in the Pacific.
8.8.1 Why does the ocean seafloor have so many seamounts?
8.8.2 Why are so many ocean volcanoes found as part of linear chains?
8.8.3 Feature: What makes the Hawaiian-Emperor seamounts the model for a hot
spot volcano chain?
8.8.4 Why are so many hot spots at mid-ocean ridges?
8.8.1 Why does the ocean seafloor have so many seamounts?
There are many more underwater seamounts than there are ocean islands. Sometimes
seamounts form because the volcano doesn’t rise high enough to break the surface. In other
cases, however, seamounts were once above sea level, but no longer. This involves a twostage process. Erosion rapidly, over a few million years, wears islands down to sea level.
However, because ocean lithosphere continues to sink long after it forms at the ridge, these
islands end up sinking beneath the sea surface. These can be distinguished by their flat tops
underwater.
The islands and seamounts form in two different ways. Asymmetric volcanic processes at
the ridge can cause volcanism to occur along one side of the ridge. As discussed in Chapter
5, the mid-ocean ridge system is very variable and changes over time, with rifts that can
propagate up or down the ridge. When this happens, a magma chamber can become isolated
on one side of the ridge, causing it to continue to release magma at one location on the sea
floor, forming a seamount. Because this process of ridge migration is more common when
plate spreading rates are high, like along the East Pacific Rise, there are more seamounts in
the Pacific Ocean than in the Atlantic Ocean.
The other way that seamounts form is from hot spot volcanic activity. The cause of the hot
spot activity is not always known, and is sometimes debated by geologists. In many cases,
the preferred hypothesis is that a plume of hot rock is rising up from the deep mantle at a
single location. However, there can be other causes of the volcanism, such as regions of high
water content in the mantle.
No CGI
No Videos
A: [NEED] Ocean sea floor map, showing multiple seamounts, like:
B: [NEED] Guyot bathymetry, like:
http://en.wikipedia.org/wiki/File:Bear_Seamount_guyot.jpg
8.8.2 Why are so many ocean volcanoes found as part of linear chains?
When hot spot volcanism persists for a long time, a linear chain of islands and seamounts
results. The volcanism stays in relatively the same location, but the plates move over them,
creating a chain of volcanoes. The farther from the hot spot you go, the older the volcano. In
actuality, the hot spots are not absolutely fixed relative to the mantle, but they move
relative to each other an order of magnitude more slowly (millimeters per year) than the
plates move (centimeters per year), so the result is fairly straight lines of volcanoes.
The Hawaiian-Emperor chain is the classic example of a hotspot chain, but there are many
others, and some have been releasing magma for more than 100 million years. The Great
Meteor hot spot track began beneath Hudson Bay (Canada) 215 Ma ago. The hotspot is now
located in the middle of the Atlantic Ocean, south of the Azores. The Chagos-Laccadive Ridge
in the Indian Ocean, which includes the Maldive Islands, is a result of the Indian plate
moving over the Reunion hot spot, which is now just east of Madagascar. The hot spot first
erupted to form the Deccan Traps in India around 65 Ma ago, when India was much further
south, adjacent to Madagascar. In the Pacific Ocean, the Louisville hot spot chain has more
than 70 seamounts that stretch in a line about 4300 km, the end of which is subducting into
the Tonga trench.
One interesting aspect is that the volcanoes do not exist as a single ridge of erupted basalt,
but rather as a set of discrete islands, sometimes separated by large distances. This has
been modeled as being a result of the magma conduits of the volcanoes carried away from
the hot spot for a finite amount of time, and then finding a new route to the surface.
A: CGI 45 – formation of a hot spot chain
B: [NEED] Ocean bathymetry map of Indian Ocean, showing 90-east ridge and ChagosLaccadive ridge, like
8.8.3 Feature: What makes the Hawaiian-Emperor seamounts the model for a hot
spot volcano chain?
By all standards, the Hawaiian-Emperor chain of ocean volcanoes is the largest in the world.
The chain stretches 5800 km from the big island of Hawaii to the subduction zone in
Kamchatka. The fact that the seamounts are being subducted there means that there were
once even more volcanoes in the chain. The Hawaiian hot spot has also produced more lava
than any other – more than a million cubic kilometers over the past 65 million years. The
eight main islands of the state of Hawaii (Hawaiʻi, Maui, Oʻahu, Kahoʻolawe, Lanaʻi,
Molokaʻi, Kauaʻi and Niʻihau), are part of about 130 islands and smaller islets that
stick up above the water, but extend westward in the form of about 80 independent
underwater seamounts. The reason that the western part of the chain is all
underwater is that the volcanoes are older; because ocean seafloor sinks over time,
these volcanoes, once above water, have all sunk below the surface.
Hawaii is a true mantle plume. Seismic tomography has identified the hot rock rising
up from the lower mantle. What is not known, however, is whether or not the
location of the plume in the lower mantle has changed over time. The HawaiianEmperor chain takes a significant bend at a location where lava was erupting 47 Ma
ago. It was long assumed that the Pacific plate suddenly changed direction at that
time. Before then, the plate was moving NNW; since then, the plate has been moving
WNW. It was proposed that a piece of the Pacific Ocean floor, a separate plate called
the Kula plate (which in the Pacific Northwest Tlingit language means “all gone”),
entirely disappeared beneath Alaska, and that this cause a shift in the motions of
other plates. A competing hypothesis, however, is that the Pacific plate motion does
not change, but rather that the base of the plume in the lower mantle was moving up
until 47 Ma, and then stabilized.
No CGI
A: [NEED] Interactive map of Hawaiian-Emperor chain, with ages, like this (though
bathymetry would be nice as well):
B: [NEED] Animation of change in Pacific plate direction with sinking of Kula plate,
like at:
http://emvc.geol.ucsb.edu/2_infopgs/IP3RegTect/dNoPacific.html
8.8.4 Why are so many hot spots at mid-ocean ridges?
When you look at the distribution of hot spot volcanoes within the ocean, what do you
notice about their connection to mid-ocean ridges? It turns out that a very large number of
active hot spot volcanoes exist at or near mid-ocean spreading centers: Iceland, the
Galapagos Islands, Easter Island, the Azores, Tristan, Jan Mayen Island, Bouvet Island, and
so on. The list is long. Given the strong correlation, it is unlikely that it is a coincidence, but
the reason for this is not clear.
One hypothesis is that ridges control the locations of hot spots. For a hot spot volcano that is
the result of a rising plume of hot rock from the deep mantle, the lower viscosity of rock
near the ridge may allow the plume bend and be channeled toward the ridge. For a hot spot
volcano that is the result of water or basaltic rock within the upper mantle, as it gets pulled
toward the spreading ridge, it would melt more easily. In either case, the location of the
volcanic island will be determined by the ridge location.
Another hypothesis, however, is just the opposite: hot spots control the locations of ridges.
A rising mantle plume could perforate the oceanic crust and cause the location of ridge to be
located on or near the site of hot spot volcanism.
No CGI; No Videos
A: [NEED] Interactive map (?) of all of the hot spots that are located near mid-ocean
ridges [a zoomed-in map and/or photo of each hot spot could pop-up with a cursor
drag]
B: [NEED] Animation or corn syrup tank image of plumes pulled toward mid-ocean
ridges [I will contact Mark Richards and/or Christopher Kincaid about this]
8
VOLCANOES
8.9 Hot Spot Volcanoes on Continents
Intraplate volcanoes occasionally erupt on continents. Sometimes these are associated with
continental rifting, such as the volcanoes that have erupted in New Mexico associated with
the Rio Grande rift. In other cases, they are the result of mantle hot spots located beneath
continents, such as the Yellowstone hot spot. Volcanoes on continents tend to have a much
more silica-rich composition, sometimes erupting rhyolite. When mantle hot spots first
erupt beneath continents, however, they can release enormous amounts of basaltic lava,
such as the Columbia River flood basalts that marked the beginning of the Yellowstone hot
spot. Continental hot spots may play an important role in the history of plate tectonics; they
may plan an important role in the break-up of supercontinents.
8.9.1
8.9.2
8.9.3
8.9.4
What happens when a hot spot first erupts?
What is the volcanic history of Yellowstone?
Do hot spots play a role in breaking apart supercontinents?
Feature (How Geology is Done): How can we see deep beneath Yellowstone?
8.9.1 What happens when a hot spot first erupts?
There have been times in Earth’s past when an enormous volume of basaltic lava has
erupted onto the surface of the land. These flood basalts can cover hundreds or even
thousands of square kilometers and can last for millions of years. Interestingly, volcanism
can last for tens or even hundreds of millions of years at these locations, but at a much
lower level. A common explanation for this behavior that the massive flood basalt
represents the birth of a rising mantle plume hot spot at the surface.
This pattern, of enormous flood basalts at the birth of hot spot volcanoes, has been repeated
multiple times. When the Iceland hot spot first began erupting, a huge flood basalt flowed
out over parts of Greenland and Scadinavia. When the Reunion hot spot first erupted 65 Ma
ago, it poured XXX km3 of basaltic lava onto what is now India to form the Deccan Traps. A
similar flood basalt in XXX created the Siberian traps in Russia.
One model for this phenomenon is that a rising plume of rock may have a head, perhaps
mushroom-like, that is larger than the stem below it. When that head of hot rock first
reaches the surface, it can trigger an eruption of many XX of cubic kilometers of lava that
can last for more than 10 million years. Subsequent eruptions are much weaker because a
smaller amount of magma rises through the remaining conduit.
No CGI
A: [NEED] Geodynamic animation of mantle plume rising, with a plume head reaching
the surface and widening (like by Farnetani – I will ask her). It looks like:
http://www.see.leeds.ac.uk/structure/dynamicearth/convection/plume/
8.9.2 What is the volcanic history of Yellowstone?
Yellowstone National Park has a long history. It was the first national park, of any kind, in
any country, signed into law in 1872 by president Ulysses S. Grant. However, Yellowstone’s
geologic history goes back a lot farther, more than 17 million years ago, when the hot spot
first erupted. At that time North America was farther to the east, so the hot spot lay under
western Nevada. As North America as moved WSW, at about an inch per year, the center of
volcanic activity has moved across the continent, across Idaho and into what is now
Wyoming. When you look at the map of Yellowstone-related volcanism, you can see how the
ages of the lava flows get progressively younger as you move to the northeast. This line of
ancient eruptions over the past 16 million years is parallel to the direction of the absolute
motion of North America over the mantle.
However, something very unusual occurred when volcanism first began, 17 Ma ago: a vast
outpouring of basaltic lava erupted out much farther to the north, onto what is now eastern
Oregon and Washington and parts of Montana. These eruptions occurred for more than 10
million years, creating layers of basalt that are now 1000s of meters in places. The lava
flowed down the Columbia River valley and actually flowed all the way to the Pacfic Ocean.
These flood basalts erupted out of a set of north-south trending fissures that extend for
hundreds of kilometers in length.
The Yellowstone hot spot is still very active, and another supervolcano eruption is very
possible for the not too distant future. Until then, Yellowstone remains the most
hydrothermally active place in the world. Over 10,000 geothermal features, including
geysers, fumeroles, mud pots, and hot springs, are found at Yellowstone – more than half of
all the world. The 300 geysers at Yellowstone are 2/3 of the world’s total.
No CGI
A: [NEED] Map of Columbia river basalts and historic Yellowstone Volcanism, like:
B: [NEED] Photo of grand prismatic springs (is there one more close-up than
hjz6ru.jpg?)
C: Photo of Mammoth springs: 09406.jpg
8.9.3 Do hot spots play a role in breaking apart supercontinents?
In Section 8.8.4 we examined hot spot volcanoes that are found at or near mid-ocean ridges.
In certain cases, there is a very unusual connection between the history of hot spot
volcanism and tectonic plate motions. A good example is the Tristan hot spot volcano, which
is currently found adjacent to the mid-Atlantic ridge in the South Atlantic Ocean. Lava has
been eruption at Tristan for over 135 million years. At that time, however, South American
and Africa were still connected: they hadn’t yet split apart as part of the break-up of the
supercontinent Pangaea. Tristan began with a massive flood basalt that is now on two
different continents: the Parana flood basalt in Brazil and Argentina (South America) and
the Etandeka flood basalt in Namibia and Angola (Africa). It is very possible that the
presence of the Tristan hotspot beneath the surface helped to determine the location of the
rifting between Africa and South America, analogous to weakening a piece of paper by
perforating it.
This phenomenon is actually very common. When North America began rifting away from
Europe, Greenland was initially part of Europe. However, when the Iceland hot spot began
to erupt, the ridge jumped to be on top of the hotspot, and Greenland began rifting away
from Europe. The Kerguelan hot spot first erupted as the Rajmahal flood basalt in India, and
may have played a role in the separation of India from Antarctica. Currently, the Afar hot
spot beneath Ethiopia seems to be playing a role in the separation of Africa into three
plates: Arabian, Somalian, and Nubian. Three-dimensional seismic imaging shows a region
of very hot rock in the mantle underneath the triple junction between the three plates,
which is supplying the magma for the volcanism as well as isostatically elevating the
surface.
No CGI
No Videos
A: [NEED] Map showing the locations of the hot spots mentioned here. It would be
good to have one that is interactive, showing the location of the hot spot and
volcanism as the plates open. Tristan is good for this, because it has put a chain of
volcanoes onto both plates.
B: [NEED] 3D seismic tomography of Afar hot spot beneath Africa (I will make this)
8.9.4 Feature (How Geology is Done): How can we see deep beneath Yellowstone?
In the last section we showed an image of the structure of the mantle beneath the African
Afar hot spot. How do we generate images like this? These images are made using a
technique called seismic tomography, whereby the waves from many different earthquakes
recorded at many different seismometers are analyzed to make an image of the rock that
they pass through. The process is analogous to medical tomography, only, instead of using
X-rays as the source of energy, geophysicists use seismic waves. These seismic waves travel
more slowly through hot rock, and this delay, sometimes only a fraction of a second, can be
used to identify a hot spot mantle plume.
In the case of Yellowstone, the seismometers were deployed as part of the USArray, a
transportable array of seismometers operated by the Incorporated Research Institutions for
Seismology (IRIS) as part of the National Science Foundation’s EarthScope project. An array
of 400 seismometers was slowly moved across the continental United States from California
to the East Coast over a period of ten years, and additional high-resolution arrays of
seismometers were deployed in places of particular interest, such as Yellowstone. The
result was a remarkable level of resolution of the three-dimension structure beneath North
America. As part of this, hot rock corresponding to the Yellowstone hot spot was mapped
out and shown to be similar to earlier geodynamic predictions of the structure of the
Yellowstone hot spot.
Tomographic images from USArray revealed many other structures. The plume of the
Yellowstone hotspot was shown to be cutting through the slab of oceanic lithosphere from
the Juan de Fuca plate that is subducting beneath the Pacific Northwest. It also identified a
place, beneath Nevada, where part of the North American lithosphere has separated from
the rest of the plate and is “dripping” down many hundreds of kilometers into the deeper
mantle.
No CGI; No Videos
A: [NEED] Map of EarthScope USArray station locations (I can provide this)
B: [NEED] Tomographic image of Yellowstone seismology compared to geodynamic
model (I can supply these)