Volcanoes and volcanism
• Volcanoes represent
venting of the Earth’s
interior
• Molten magma rises
within the Earth and
is erupted either
quietly (lavas) or
violently (pyroclastics)
Terminology
Magma – molten rock sometimes containing
suspended minerals and dissolved gases. Magma
forms when temperatures rise sufficiently high for
melting to occur in the Earth’s crust or mantle.
Volcano – a vent at the surface in which magma,
solid rock, and gases erupt.
Lava – magma that reaches the surface and pours
out over the landscape.
A-Characteristics of Magma
Composition –
Controlled by the abundant elements in Earth (Si, Al, Fe, Ca, Mg, Na, K,
H, and O).
Most common types of magma are:
basaltic (~50% SiO2), andesitic (60% SiO2) and rhyolitic (70% SiO2)
Magma that solidify on the surface are called extrusive rocks and rocks
that solidify below the surface are called intrusive.
~70 - 75% of all magma erupted by volcanoes is basaltic the rest is split
between andesitic and rhyolitic.
Characteristics of Magma
Rock classification chart - USGS
Characteristics of Magma
Dissolved Gases
Comprise a small percentage of the magma (0.2 to 3 wt.%).
Although not present in abundance these gases strongly influence
the eruption style and explosiveness of the magma.
Dominantly H2O and CO2 with small amounts of nitrogen, chlorine,
sulfur and argon.
Temperature
Ranges from ~800 C to ~1200 C
Viscosity
a substances resistance to flow
**dependent on temperature and composition
The Viscosities of Foods as Analogs for Silicate Melts
From: Baker D. et al. (2004) J. Geosc. Education
Explosive Eruptions vs. Effusive Eruptions
• Three factors effect the explosivity of a volcano
– Temperature of magma
• High-temperature, less explosive
fluidity
– Composition of magma
• Less silica, less explosive
– Gas content of magma
• Less gas, less explosive
Volcano
types
Volcano types: cinder cones
• Cinder cones are
volcanoes which erupt
only during one episode
• They are explosive, but
small in size
• The cone is a pile of
pyroclastic debris which
piles up at the angle of
repose
Volcano types: cinder cones
• The cinders are generally of basaltic composition
• The eruptive activity typically lasts a few months or
years
Volcano types: shield
volcanoes
• Shield volcanoes are
broad, gently sloping
volcanoes
• They are composed
mainly of basaltic lava
flows
• This is a view of Mauna
Loa, Hawaii, from the
cinder cones of Mauna
Kea
Mauna Loa is the tallest
volcano on Earth, as
measured from the sea floor
Shield volcanoes on Mars
• Other planets also
have shield
volcanoes
• This is the largest
shield volcano in the
solar system,
Olympus Mons on
Mars
• Check out the scale !
Shield volcanoes: Earth vs. Mars
• Red = Hawaiian
chain, which is
superimposed on
Olympus Mons
• this says it pretty
well, I think !
Mauna Loa is about here
Volcano types: stratovolcanoes
• Stratovolcanoes consist
of alternating layers of
lava and pyroclastics
• They are dominantly
andesitic in composition
• These volcanoes are
typical of subduction
zones
Mt. St. Helens (pre-1980)
Volcanic landforms
Volcano types: calderas
A large depression generally caused by the removal of large
quantities of magma from beneath a volcano causing the
ground to collapse into an empty space.
Aniakchak Caldera, Alaska, formed during an enormous explosive eruption that
expelled more than 50 km3 of magma about 3,450 years ago. The caldera is 10 km in
diameter and 500-1,000 m deep. Subsequent eruptions formed domes, and explosion
pits on the caldera floor.
Now we need to answer………
A-How and where do magmas and volcanoes form ?
What tectonic environment do these volcanoes occur in
and why?
How and why do magmas and volcanoes form?
-Global distribution of volcanoes
1-Magma generation at hot spots
• Basaltic magmas at hot
spots are derived from
deep within the mantle
• the magmas are fed by
deep mantle plumes
which are stationary
relative to the drifting
tectonic plates
Intraplate Volcanism
USGS
Hawai’i
• Best example of intraplate volcanism
– More lava is extruded here constantly than
anywhere else on Earth!
A Bigger Picture
• Looking at Hawai’i, and volcanic seamounts nearby
(underwater volcanic islands)
Current Hawaiian
Islands
What is a hot spot?
• Some mantle anomaly allows the oceanic or
continental lithosphere to melt where it
would not normally melt
• The anomaly (usually) stays stationary
• The plate(s) moves over it
The Hawai’ian Hot Spot
University of North Dakota
Islands
and seamounts get older as you move away from the hotspot
Hawaiian island trail
Does the kink represent a change in plate direction?
2-Magma generation at mid-ocean
ridges
• In these zones, the mantle
rises and melts, producing
magma of silicate
composition
• the magma continues to rise,
and erupts mainly as basaltic
lava flows
**volcanism and earthquakes are separate issues
This rifting process is dramatic on
Iceland
• Iceland is literally being
torn apart by rifting of the
two plates…
• yet its center is
continually renewed by
new magma from the
mantle…
• the same thing is going on
under the ocean
One result of these processes
• Krafla volcano
erupts
frequently,
producing
spectacular
fountains of
fluid lava
3-Magma generation at subduction zones
• During subduction, the
subducted oceanic plate
is heated as it plunges
into the mantle
• At a depth of 80-120 km,
melting begins, and
volcanoes are produced
which parallel the
subduction zone
Andesitic to Dacitic magmas are
typical of these volcanoes
Volcanic landforms
Stratovolcanoes
-explosive
eruptions
-viscous lava
-built of
interlayered lava
and pyroclastic
material
-usually andesitic
in composition
Mt. Fuji, Japan
Volcanoes and Plate Tectonics
Stratovolcano
Stratovolcano eruptions
Pinchincha, Ecuador
Anak Krakatau, Indonesia
Indonesia
Krakatau
Volcanoes: 79 active 20% of the world total 600 eruptions since 1800
Population: 215 million
World’s fourth most
populous nation. 60%
on island of Java
Krakatau
Circa 1880
Height 2575 ft
(785 m)
Krakatau Eruption
August 27th, 1883
• Blast 10,000 greater than at Hiroshima
• >36,000 people killed
• 18 Cubic Km material
ejected
The explosion blew away the northern two-thirds of the
island and it was almost instantaneously followed by the
collapse of the unsupported volcanic chambers which
formed the huge underwater caldera
Other Features: calderas
NOW, WHAT ABOUT RHYOLITIC
VOLCANISM?
• SiO2 contents are even greater than is
the case in andesitic magmas,
therefore....... viscosities are even
greater.
• So, Incredible resistance to flow!
NOW, WHAT ABOUT RHYOLITIC
VOLCANISM?
• In addition, rhyolitic magmas tend to be richer in
H2O, because they form by partial melting of the
crust, and melting is only possibly there if H2O is
present.
• In other words, rhyolitic magmas exsolve more H2O
(more bubbles form as the magmas rise), yet the
bubbles cannot expand owing to the high viscosity of
the magma (they must expand as pressure decreases,
i.e., as the magmas rise through the crust)
• Recipe for a major disaster!!!
Yellowstone National Park, Wyoming,
offers an excellent example of rhyolitic volcanism
Yellowstone National Park, Wyoming,
• The volcanic eruptions, as well as the continuing geothermal
activity, are a result of a large chamber of magma located
below the caldera's surface.
• The magma in this chamber contains gases that are kept
dissolved only by the immense pressure that the magma is
under.
• If the pressure is released to a sufficient degree by some
geological shift, then some of the gases bubble out and cause
the magma to expand.
• This can cause a runaway reaction. If the expansion results in
further relief of pressure, for example, by blowing crust
material off the top of the chamber, the result is a very large
gas explosion.
Geysers and fumaroles!
The H2O is of near-surface origin, but the
heat is due to a batholith (still partially
molten) not far below the surface
Life Cycle of a Caldera
Uplift and ring
fracture formation
Initial eruptive stage
along ring fractures
Collapse alongside
eruption
Remaining lava
extruded
Smith and Bailey
Calderas can host lakes
-Active hydrothermal systems
Resurgent dome
forms
SUMMARY
• Calderas are primarily rhyolitic
– largest explosive eruptions are caldera-related
– lava is cool and viscous, rises slowly, allowing pressure to build up
– gas percolates slowly through the viscous magma, does not have an
easy way to vent
• Stratovolcanoes are dacitic-andesitic
–
–
–
–
have eruptions of intermediate explosivity
can undergo lava flows if lava is mafic enough and hot enough
lava domes if lava is more felsic and cooler
gas does vent, but slowly, through fissures
• Shield volcanoes are usually basaltic
– lava is very hot and fluid
– gases easily pass through magma to be released into the atmosphere
– experience gentle, effusive activity
• fountaining if pressure builds, usually at the start of an eruption
Volcanic activity
• In the following slides, I will give you some
examples of volcanic activity:
– lava flows, including flood basalts
– lava domes
– pyroclastic falls and pyroclastic flows
– lahars and debris avalanches
– volcanic gases
Volcanic activity: lava
flows
• This is a basalt lava flow in a
channel
• Due to its low silica content
and high temperature, it is
quite fluid (but stickier than
maple syrup)
• Yet lava usually flows fairly
slowly
Pahoehoe
lava
Do you want to walk
on pahoehoe ?
This is a Hawaiian
term for smooth,
ropy lava
It generally
exhibits fluid-like
textures
Aa lava
• This type of lava is quite
blocky on the surface,
and comparatively cool
• Yet below the surface,
the lava is fairly massive
and much hotter
• Do you want to walk on
aa ?
Fire fountaining
• Sometimes, basaltic
lava can contain lots of
gas
• Then, small explosive
eruptions form fire
fountains
• As partially liquid drops
fall back to the ground,
they may coalesce to
form a lava flow
Flood basalts
• The previous examples
represent small-scale activity
• But basaltic eruptions can be
huge, forming lava plateaus
• These huge outpourings may
occur quickly (1-3 Ma) and
may contribute to mass
extinctions
Global distribution of large igneous
provinces (LIPS)
Mainly flood
basalts
Lava domes
Mt. Unzen, Japan
Unzen began
growing a lava dome
in mid-1991. The
dome complex
continued to grow
until 1995
Lava domes at Unzen
• This dome was the first
to be erupted, in May
1991
• The lava is silica-rich and
thus highly viscous
(sticky) and cannot easily
flow
• Thus it tends to form
steep-sided domal
structures
Volcanic activity: lava domes
• By early 1995, the
dome complex had
grown substantially
and was highly
oversteepened
• As pieces of the dome
broke off, they would
fragment, creating
pyroclastic flows
Volcanic activity: pyroclastic
falls
• During explosive
volcanic eruptions, ash
falls downwind of the
volcano
• In the case of very large
eruptions, the ash may
be deposited over a
vast area
Ash fall-out from the Mt Pinatuba eruption
Volcanic activity: pyroclastic
flows
• Pyroclastic flows are
suspensions of hot
pyroclastic material, air, and
gas which descend under
the influence of gravity
• Their velocity is generally
very high (50-500 km/hr)
• This example is a flow from
Mt. St. Helens
Volcanic activity: pyroclastic
flows
• This is another example,
descending the slopes of
Unzen volcano after part
of the dome has
collapsed
• The flow has a dense
core which is hidden by
the billows of ash which
are rising
Unzen, 24 June 1993
Ash fall Hazards
Aviation safety
Death
Structural damage
Contaminated drinking water
Climate change
Volcanic activity: lahars
• Lahar is an Indonesian
word for volcanic
debris flow
• Lahars are flows of
water and loose
volcanic debris
• They are especially
prevalent at snow-clad
and ice-clad volcanoes
Lahars
• Lahars are volcanic mudflows
• They are triggered in one of three ways
– Volcanic activity melting snow and ice on the volcano
• Or a summit crater lake rupturing and draining due to volcanic activity
– Torrential rainfall providing the water source
• Does not require an active volcano
• Rainwater mixes with existing ash to create the lahar
– Rocks and ash from a landslide entering an existing drainage route
• Can destroy dams, unleashing more water
• As they progress, lahars can undergo a process called “bulking”
– Incorporate material from the area they flow over
• Erode sides of existing drainage channel
• As they progress, they are also diluted by existing river water
– A dilute lahar is called a hyperconcentrated flow
• Following a river valley, these fast-moving flows can transport ash and
sediments over 250 km away from the source
Impact of Lahars
• During bulking, large objects like rocks, cars,
houses can be picked up
– These often end up destroying bridges as the lahar
progresses
• People living in river valley drainage systems
of volcanoes are at risk
• Sediment load from the volcano can affect the
way a river flows, fish populations, etc.
Lahars
(Castatela 1980) – Mt. St. Helens
(Castatela 1980) – Mt. St. Helens
Volcanic activity: debris
avalanches
• Sometimes a volcanic
edifice is weakened
• Wholesale collapse of
part of the volcano may
ensue
• During collapse, a
debris avalanche
occurs, and a scalloped
scar remains
Unzen volcano, with the 1792
scar in the foreground
Volcanic activity: gases
• Volcanic gases are typically
highly acid
• Major constituents include
H2O, CO2, HCl, SO2, and HF
• This photo shows gas
emission from Masaya
volcano in Nicaragua
Volcanic activity: gases
• This is also Masaya
volcano…
• but this photo was
taken from the space
shuttle
• it shows the gas plume
being blown out over
the Pacific Ocean
Volcanic activity: gases
• About 15 km
downwind from
Masaya, the coffee
crop is adversely
affected by the
acid gases
Volcanic gases
• Don’t forget that powerful explosive eruptions
can inject large amounts of gas into the
stratosphere
• The impact is climate change...
Sizes of volcanic eruptions
• The Volcano Explosivity Index (VEI) is
similar to the Richter scale for quakes
• It is logarithmic
• It emphasizes the degree of explosivity of
eruptions
VEI – Volcanic Explosivity Index
• Eruptive styles
–
–
–
–
Hawaiian eruptions can produce lava, but not much ash
Strombolian eruptions produce bombs and lapilli and more ash
Vulcanian eruptions have an ash plume and only minor blocks
Plinian eruptions have lots of ash, minimal lapilli and blocks
Low sio2
High SiO2
Volcanic hazards of
Canada
Canada has “active”
volcanoes (black
triangles) which pose a
potential threat in B.C.
Another major hazard is
ashfall from explosive
eruptions of Cascade
volcanoes in
Washington state
Case history: Mount St. Helens, Washington
state, USA
Tectonic setting of Mt. St.
Helens
Previous eruptions of Mt. St.
Helens
• The volcano is the most
active of all the
Cascades volcanoes
• Based on previous
activity, there was a
fairly high probability
that the volcano would
again erupt before the
millenium
Stages at Mt. St. Helens
• Stage 1: precursory activity, 20 March - 18
May 1980
• Stage 2: the climactic eruption of 18 May
1980
• Stage 3: post-climactic activity, 1980-present
Stage 1: Precursory activity;
eruptions
• The first phreatic
eruption occurred on
27 March 1980
• then explosions
continued…the volcano
was preparing itself
• The eruptions of 13, 18
April consisted of
steam and ash
27 March 1980 eruption
Precursory activity:
seismicity
• A magnitude 4.1 earthquake was recorded on 20
March 1980 under the volcano
• By 1 April, harmonic tremor was observed
(continuous seismic signal of similar wavelength)
• this is a pretty good indication that magma is
involved
A seismogram from seismic station RAN on 2 April
showing the occurrence of harmonic tremor
Harmonic tremor
Precursory activity:
deformation
• A bulge was first detected on the NNE flank of the
volcano on 19 April 1980
• Deformation was as high as 5 feet/day !
• This was an indication that magma had moved into the
volcano itself
• At the same time, eruptive activity decreased during
14-23 April
Development of the bulge
28 September 1979
bulge
17 May 1980
Precursory activity: gas emissions
• Although some sulfur dioxide was detected, not
very much was actually being emitted
• This suggests that the volcano had somehow sealed
itself, and that pressure was building
• This interpretation is consistent with (a) the bulge
and (b) decreased eruptive activity
Stage 2: The climactic eruption,
18 May 1980
• At 0832 local time, a M 5.1 earthquake struck
the volcano
• A large portion of the volcano slid away
• This simultaneously developed a debris
avalanche and a lateral blast
Sliding...
Sliding and
depressurizing...
…andAnd
blasting
blasting
The blast
Note trees still standing
• The lateral blast was
comparatively cool at
100-300° C
• But its speed
approached 500 km/hr
• It was therefore
devastating to a very
large area (180°, up to
20 km distance)
Tree blowdown by the blast
Sector collapse and the debris
avalanche
• The sector collapse
reduced the height of
the volcano substantially
• A horseshoe-shaped
amphitheatre was
formed
• The avalanche deposit
was emplaced in the
Toutle River valley
Lahars
• The mixing of melted
snow and ice with
loose pyroclastic
material created the
perfect conditions for
lahars
• Also, the debris
avalanche deposit was
water-saturated,
producing lahars
Debris carried by lahars
Note the “high-water” mud
marks on the trees
Hazards associated with volcanoes
Monitoring and forecasting lahars
• To monitor lahars, a
series of acoustic flow
monitors (AFMs) have
been installed
• They measure the ground
vibrations from lahar
movement
• They are insensitive to
ground motion from
quakes, which occur at a
lower frequency
Final costs from the 18 May
devastation
• 35 people dead (it was a Sunday)
• 22 people never found
• $ 2.7 billion US in damage
• contrast this with the $ 10-20 billion in losses from
the 1989 Loma Prieta and 1994 Northridge
earthquakes
Would you call this
event a disaster?
Stage 3: Post-climactic activity
• Subsequent activity consisted of generally
diminishing explosive eruptions
• By mid-late 1980, lava domes began to grow
in the amphitheatre
• these were repeatedly destroyed by small
explosive eruptions
Explosive
eruptions
Explosive activity
diminished
gradually over the
next several years
Pyroclastic
flows
Spectacular
pyroclastic flows
were observed
forming from the
crater
These are some of
the best examples
you will ever see
Pyroclastic flows - note big rounded
pumices
Dome growth-destruction cycles
Dome growth-destruction
cycles
Early lava dome
Dome growth-destruction cycle
Late lava dome
Volcanic ash and aviation
safety
• Many of the world’s civil air routes cross active
volcanoes
• Ash erupted by a volcano frequently reaches
aircraft altitudes (9-12 km)
• Aircraft encounters with ash are potentially
fatal
• Ash can clog engines, causing them to fail
Global air
routes
North Pacific
air routes
Volcanic ash and aviation
safety
• All this shows that good
communication among
volcanologists, meteorologists, and
aviation people (especially pilots!) is
essential
Volcanoes - web
• Four good starting points for volcanoes:
• http://gsc.nrcan.gc.ca/volcanoes/index_e.php
• http://vulcan.wr.usgs.gov/home.html
• http://www.geo.mtu.edu/volcanoes/
• http://volcano.und.nodak.edu
Volcanoes - reading
• Francis, P., 1993. Volcanoes a planetary perspective. Oxford, Clarendon
Press.
• Sigurdsson, H., B.F. Houghton, S.R. McNutt, H. Rymer, and J. Stix, eds., 2000.
Encyclopedia of volcanoes. San Diego, Academic Press.
• Smith, W.S., 1983. High-altitude conk out. Natural History, v. 92, no. 11,
November 1983, pp. 26-34.
• Tilling, R.I., 1998. Volcanoes. http://pubs.usgs.gov/gip/volc/text.html