volcanic phenomena.
aspects of volcanism, generation of magma, its transport and migration, eruption, and
formation of volcanic deposits. It also addresses volcanic hazards, their mitigation, the
monitoring of volcanic activity, and economic aspects and,
specific cultural aspects of volcanic activity, including the impact of volcanic activity on
archaeology, literature, art, and film.
Lecture Notes Study Guide: Volcanoes and Volcanic Activity
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Objectives
All of the Hawaiian Isles are volcanic in origin. There are about 1,500 active
volcanoes on Earth, almost 400 of which have erupted in the twentieth century.
Volcanoes occur on all seven continents, as well as in the middle of the ocean.
When human beings live in the path of an active volcano, the results can be
devastating. The exercises in this chapter will help reinforce the learning
objectives from the text:
Know the major types of volcanoes of Hawaiian Isles, the rocks they produce, and
their plate tectonic setting
Understand the main types and effects of volcanic activity, including lava flows,
pyroclastic activity, debris flows, and mudflows
Understand the methods of studying volcanic activity, including seismic activity,
topographic change, emission of gases, and geologic history, in order to better
predict volcanic eruptions and minimize the hazard
Please check out the websites: http://www2.hawaii.edu/~nasir/
A Teacher's Guide to the Geology of Hawaii Volcanoes:
http://www.volcanoworld.org/vwdocs/vwlessons/atg.html
A Teacher's Guide to the Geology of Hawaii Volcanoes National Park. The
following material is a web version of the book.
1. Plate Tectonics
2. Hot Spots and Mantle Plumes
3. Evolution of the Hawaiian Volcanoes
4.
5.
6.
7.
8.
The Volcanoes of the Island of Hawaii
Volcanic Landforms
Volcanic Landforms of Hawaii Volcanoes National Park
Lava
Tephra
o Background
o Teaching Suggestions and Activities
9. The Current Eruption of Kilauea Volcano
o Background
o Teaching Suggestions and Activities
10. Minerals, Magmas, and Volcanic Rocks
o Background
o Teaching Suggestions and Activities
11. Volcano Monitoring Techniques
o Background
o Teaching Suggestions and Activities
12. Kinds of Volcanic Eruptions
o Background
o Teaching Suggestions and Activities
13. Magma Pathways, Calderas and Pit Craters
14. Living With Hawaiian Volcanoes
Keep an eye on this page. We'll be adding more lessons soon.
Volcanoes – Hawaiian Isles
Landforms :surface expressions of subsurface igneous activity are named after the
Roman god of fire, Vulcan. Volcanoes are typically described in terms of their eruptive
history as:
1. Active - have erupted in historical time.
2. Dormant - have not erupted in historical time, but are capable of renewed activity.
3. Extinct - have not erupted historically, show major erosion, and have no signs of
activity.
There are about 650 known active volcanoes on the continents and islands; several may
be erupting at any given time. There are thousands of volcanic vents on the mid oceanic
ridges.
Products of Volcanism
Magmas can vary in composition from basic to silicic and carry some percentage of
gaseous materials. The magmatic products of volcanism can include:
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Lava is molten rock material that has reached the surface. Basaltic lavas can be
separated into three types:
1. Pahoehoe - Fluid, gas-rich lava having a ropy surface.
2. Aa - Viscous, gas-poor lava having a broken surface.
3. Pillow Lava — Lava, typical of submarine mid-ocean ridge basalts
(MORBs), extruded into water and quickly chilled.
Some of the surface and internal features shown by these lavas include:
4. Pressure ridges are buckled areas on the surface of lava flows.
5. Spatter cones are steep-sided cones built around the vent where escaping
gases throw globs of lava into the air.
6. Columnar joints are parallel fractures that form normal to the lava surface,
as the lava flow cools and contracts. They form polygonal (4-6 sided)
cracks on the surface of the flow and columns perpendicular to the cooling
surface.
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Volatiles are the dissolved gaseous materials in magma which can produce
explosive volcanic activity when magma nears the Earth's surface and pressure is
released. Typical volcanic gases include:
1.
2.
3.
4.
5.
6.
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water
carbon dioxide
nitrogen and sulfur oxides
carbon monoxide
hydrogen chloride
halogen gases (fluorine, chlorine)
Pyroclastic debris represents hot volcanic fragments blown out of a vent with
explosive force. Pyroclastic materials are subdivided on the basis of fragment size:
1. Ash - Fine (<2 mm) particles that may settle at considerable distance
around the vent. Volcanic dust can remain airborne for long periods.
2. Lapilli - 2-64 mm pieces of lava; basaltic fragments are commonly called
cinder.
3. Blocks - >64 mm angular pieces that were solid when ejected.
4. Bombs - >64 mm rounded masses that were molten when ejected; the term
pumice describes frothy pieces of rhyolite glass that can float on water.
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Accumulations of these fragmental materials form several types of deposits
that include:
1. Tephra - a general term for pyroclastic debris that accumulates through
vertical airfall.
2. Pyroclastic flows - avalanches of incandescent ash and gas (nuees
ardentes).
3. Lahars - hot mudflows formed when water mixes with hot pyroclastic
debris.
Large-scale Positive Volcanic Landforms
A number of different large-scale structures can be associated with eruption of magma
onto the Earth’s surface:
 Shield Volcano are most often found in oceanic areas and characterized by features
that include:
1. many thin basaltic flows with little to no pyroclastic material.
2. fluid lavas of high temperature (900-1200ûC), low silica (basaltic) magma
that undergo relatively quiet eruptions.
3. broad and low profiles (slopes < 10 degrees).
4. flank eruptions.
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Composite Volcano (Stratovolcano) are typically found on continents and
volcanic arcs and characteristized by:
1. alternating layers of lava flows, pyroclastic material, and lahars.
2. lower temperature (2950ûC) viscous lava of andesitic to granitic
composition that typically experience fairly violent eruptions.
3. high and steep cones (up to 30 degree slopes).
Cinder Cones may exist separately or on the flanks of larger volcanoes. These
pyroclastic mounds have characteristics that include:
1. steep slopes (up to 45 degrees) and low heights (usually < 400 m).
2. often symmetric shape around the vent.
Lava Domes have a bulbous surface structure formed when a viscous mass of
magma is extruded. Domes, which grow slowly in size, are:
1. steep-sided.
2. generally felsic in composition, but also may be intermediate.
3. often associated with extremely explosive eruptions.
Fissures represent fractures or cracks through which magma erupts in areas where
tectonic plates pull apart (rift). Typical fissure eruptions occur on the flanks of
shield volcanoes, they are characteristized by:
1. large volumes of lava flows.
2. fluid lava covers large areas and may form basaltic plateaus.
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Pyroclastic Sheet Deposits result from massive pyroclastic flows erupting from
fissures, rather than a central vent. They cover vast areas, are associated with
caldera formation, and have characteristics that include:
1. felsic composition.
2. thicknesses ranging from a few meters to 100s of meters.
Volcanic Structures
Volcanic landforms may be characterized by several structures that include:
1. Vent - an opening through which volcanic material passes.
2. Crater - a steep-walled circular depression (generally <1 km in diameter)
at the vent area.
Diamond Head Crater: Leahi is ~2 km south of the summit of Kaimuki Lava
Dome, and is probably Honolulu’s most famous landmark, named Diamond
Head Crater.
This extinct volcanic is a tuff cone, produced by hydromagmatic
eruptions. Growth position tree molds have been found in the lowest
layers of Leahi ash indicating that at least part of the eruption
occurred on land. The area of crater is about 225 hectares (475 acres).
The “diamonds” for which Diamond Head is named, are actually calcite
crystals, composed of calcium carbonate. The calcite was precipitated
in fractures and between ash layers having originally dissolved out of
coral xenoliths included in the ash. Some of the calcite may also
derive from weathering of the ash.
3. Caldera - a large summit depression (>1 km in diameter) caused by
subsidence or explosion.
4. Fumarole - a vents that expels only gas.
Distribution of Volcanoes
Most volcanoes occur in well-defined belts, but some isolated volcanoes also exist. The
major concentrations of volcanoes occur within:
1. Circum-Pacific Belt - More than 60% of all active volcanoes are in the circumPacific belt that encircles the margins of the Pacific Ocean basin. These are
mostly composite volcanoes, and consist largely of intermediate to felsic lava
flows and pyroclastic layers.
2. Mediterranean Belt - About 20% of all active volcanoes are in the Mediterranean
belt. Most of these are composite volcanoes of intermediate to felsic composition.
3. Mid-Oceanic Ridges - Most of the remaining active volcanoes on Earth are
located at mid-oceanic ridges, such as the Mid-Atlantic Ridge and East Pacific
Rise. Volcanism consists of mostly basaltic fissure eruptions.
Isolated volcanoes can consist of basaltic shield volcanoes like Hawaii or silicic volcanic
suites like the Valles, Yellowstone, and Long Valley calderas.
Plate Tectonics and Igneous Activity
Plate tectonics explains the existence of volcanic belts and the origin of different magma
compositions:
1. Igneous Activity at Spreading Ridges - New lithosphere is produced by igneous
activity as plates move away from each other at either Mid-Oceanic Ridges or
Continental Rift Valleys. Magmas originate as basaltic magma is produced by
partial melting of the underlying mantle peridotite. Temperature increases with
depth (geothermal gradient averages 25°C/km), but increasing pressure tends to
keep rocks from melting. Melting is produced at spreading centers by the release
of confining pressure (rifting) or by the presence of hot spots/mantle plumes
(possibly produced by concentrations of radioactive elements which release heat
as they decay). Most of the magmas form gabbroic plutons, but some reach the
surface to erupt as basaltic lava. In continental regions, more silicic minerals of
Bowen's Reaction Series (quartz and alkali-feldpars), that melt at low
temperatures, occur within the crust and melt producing magmas that are more
silica-rich than the rocks from which they are derived.
2. Igneous Activity at Subduction Zones - Melting at subduction zones beneath the
leading margin of the overriding plate produces island arcs along oceanic-oceanic
convergent boundaries or continental arcs along oceanic-continental convergent
boundaries. Intermediate to felsic magma are produced by partial melting of the
subducted oceanic plate and silica-rich continental shelf sediments or the mantle
overlying the subduction zone where release of water from descending wet
oceanic crust enhances melting (wet rock melts at a lower temperature than dry
rock). As magmas rise, they may be affected by assimilation of continental crust.
Most magma crytallizes as plutons, but some volcanism also occurs.
3. Intraplate Volcanism - Igneous activity typically results from tectonic plates
moving slowly over hot spots (mantle plumes).
Indirect Effects of Volcanism
Besides eruptions of volcanic material, other profound effects that volcanism can have on
society and on the environment include:
1. Earthquakes - movement of magma underground causes earthquakes as it forces
its way upward. Depth of earthquake activity varies but may gives clues to the
depth of the magma chamber feeding the volcano.
2. Lahars (Mudflows) - caused by volcanic debris mixed with rainwater, stream
water, or melted snow and ice.
3. Climate Effects - dust in upper atmosphere may block solar radiation.
4. Tsunamis - tidal waves generated by underwater earthquakes and volcanic
explosions.
5. Gas Clouds - can cause suffocation.
6. Lava baloons
Columns of white vapor streamed from the Atlantic this winter. About 8km west of an island called
Terceira in the Azores, a submarine eruption was under way. Hot lava squeezed up through cracks in the
ocean floor at about 500 meters below the surface of the ocean. The lava solidified into lava balloons.
These gas-rich lava balloons interacted with cold seawater as they rose. This process generated steam,
which emerged from the Atlantic like smoke from dozens of chimneys. The steam rose about 10 meters
high. As the lava balloons reached the surface, the gas that made them buoyant escaped through cracks, and
the balloons filled with water and sank.
Why? Because a hotspot is stationary, but the ocean plate and everything on it moves. After sea-floor
spreading drags a volcano off a hotspot, the hotspot generates a new volcano. But the Azores volcanoes
don't increase in age farther away from the ridge. Older volcanoes are mixed in with younger ones in no
particular order. This seems to contradict the idea that a hotspot created this chain of islands. One more
clue helps us understand how the Azores were created.