Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Cool from the Molten State
- Volcanic -- Erupted on Surface
- Plutonic -- Solidify Within Earth
Large Grain Size ----> Slow Cooling
- Volcanic Rocks -- Fine Grained
- Plutonic Rocks -- Coarse Grained
Large Crystals in Fine-grained Setting
- Slow Initial Cooling
- Rapid Final Cooling
Magma – molten rock beneath the surface Lava – molten rock on the surface
Where Does Magma Come From? Earth’s interior is hot (25 C/km near surface
= 1000 C at 40 km) Pressure inhibits melting Mantle is solid Never far below
melting point Volcanoes fed by small pockets 0-100 km deep Rising hot
material may melt Water can lower melting point
Why Igneous Rock Classification Matters
Silica Content governs viscosity or resistance to flow, because silica
tetrahedra link to form chains (polymerize), and the chains tangle around each
other to impede flow. Fluid lavas allow gases to escape easily so silica content
governs violence of eruptions. The principal volcanic rocks are
- Silica Poor (Basalt): Fluid lavas, generally little explosive activity
- Intermediate Lavas (Andesite): Pasty lavas, explosive eruptions common
- Silica-Rich Lavas (Rhyolite): Extremely viscous lava and explosive
Basalt (45-52% SiO2)
Slightly modified planetary raw material and therefore likely to be common on
all planets. We know it occurs on the moon, Mars, Venus, and some asteroids. On
earth basalt is derived directly from mantle and occurs in settings where magma
comes straight from the mantle to the surface:
- Oceanic crust
- Hot Spots and Flood Basalts
- Oceanic volcanic arcs
- Early stage of continental volcanic arcs
- Rift zones with rapid spreading
Fluid lava with little explosive activity, builds shield volcanoes and cinder
Andesite (52-66% SiO2)
Mixture of mantle material and continental crust and mostly restricted
tocontinental volcanic chains. Pasty lava with significant explosive activity,
Rhyolite (>66% SiO2)
Mostly remelted continental crust, occurs in settings where magma has a long
time to react with continental crust:
- Late stage of continental volcanic arcs
- Slow-spreading Continental Rifts
- Long-lasting continental Hot Spots (Yellowstone)
- Catastrophic explosive activity common.
In addition to stratovolcanoes, rhyolite sometimes builds small obsidian
domes. Occasionally there will be no volcano at all; instead the roof of a magma
chamber simply collapses and huge volumes of ash, pumice and gas are vented
suddenly. These are the most catastrophic of all eruptions. Some major examples:
- Yellowstone at least three times in the last 2 million years
- Long Valley Caldera, California 700,000 years ago
- Toba, Sumatra, Indonesia 70,000 years ago.
Some Igneous Rocks Are Named on Textural Criteria:
- Obsidian - Glass
- Tuff - Cemented Ash
- Breccia - Cemented Fragments
Types of Volcanoes
Classes of Eruption
Eruptions can be divided into two broad families
- Effusive eruptions erupt mostly lava (generally basalt) and have little
- Explosive eruptions are, well, explosive.
Proposed by Chris Newhall of the U.S. Geological Survey in 1982 as a means of
quantifying the violence of eruptions. Volume refers only to material ejected
explosively; Hawaiian and Icelandic eruptions may erupt vast volumes of lava but
eject very little material explosively. Also, the index isn't really concerned
with material that falls immediately around the vent. Hawaii is noted for
spectacular fire fountains and the total material ejected may be quite large,
pushing the upper end of explosivity 1, but the material mostly falls back
around the vent.
Like the Richter Scale for earthquakes,
the Volcanic Explosivity Index is logarithmic. Each step is about 10 times larger than the previous step.
||Volume of Ejecta
||< 100 m
||Nevado Ruiz, 1985
||St. Helens, 1980
||Yellowstone, 2 Ma
- Lava simply issues from fissures without building a volcano, though
repeated activity may build shields. The greatest historic example was the
Laki fissure flow of 1783, which killed about a fifth of the population of
Iceland, mostly through crop and livestock destruction. Explosivity index 0.
Note that merely being in Iceland doesn't make an Icelandic eruption; the
Icelandic volcano Hekla has has some of the largest explosive eruptions in
- Basalt issues from long-lived central vents and builds shield volcanoes.
Explosivity index 0-1.
- Steam explosions caused when lava or magma comes in contact with water.
Large events may blast out craters called maars. The few explosive
eruptions on Hawaii have been phreatic. One in 1790 killed several members
of a Hawaiian war party passing close to Kilauea. Another series occurred at
Kilauea in 1924 and one person was killed by flying ejecta. Explosivity index 0-1 in
most cases but maar eruptions may go as high as 3 or so.
- Named for Stromboli, Italy, which has been popping mildly since Roman
times and is nicknamed "lighthouse of the Mediterranean." Mild, long-lasting
explosive activity confined to the immediate vent area. Typically associated
with small stratovolcanoes (they don't erupt much material) and basalt or
andesite lava. Explosivity index 1-2
- Named for Vulcano, Italy, from which we also get the term volcano.
Typical explosive eruption, with a large eruption cloud but not much
pyroclastic flows. Generally associated with andesite stratovolcanoes.
Explosivity index 2-4
- Named for Pliny the Younger, who left a description of the eruption of
Vesuvius in 79 A.D., and his uncle Pliny the Elder, who died in the
eruption. Large eruption cloud, pyroclastic flows, and may collapse to
create a caldera. Andesite or rhyolite stratovolcanoes. Mount Pelee, 1902
and Mount St. Helens, 1980 are examples. Explosivity index 4-6
- Caldera-Forming (Ultra-Plinian)
- Catastrophic eruption usually associated with rhyolite stratovolcanoes
or magma chamber collapse. Extremely large volume of pyroclastic flows that
may travel for long distances. Tambora, 1815, Krakatoa, 1883, Katmai, 1912
and Mount Pinatubo, 1991 were historic examples. Mount Mazama 7000 years ago
and Thera (Santorini) about 1500 B.C. are other famous cases. Explosivity
Products of Eruptions
- Lava Flows
- Pyroclastic Debris
- Carbon Dioxide
Environmental Hazards of Volcanoes
Blast (Mt. St. Helens, 1980)
Pyroclastic Flow or Nuee Ardente
- Ash Falls
- Building Collapse
- Crop Destruction
- Mudflows (Lahars)
- Direct Damage
- Nevado Ruiz in Colombia, erupted in 1985. Melting of its snow
cap unleashed a mudflow that buried the town of Armero, killing over
20,000 in the worst disaster of its type in history.
- The Tangiwai disaster in New Zealand in 1953 has all the
ingredients of a movie script. It happened on Christmas Eve when a
volcanic crater lake drained suddenly, taking out a railroad bridge.
A few minutes later, a passenger train plunged over the collapsed
bridge. A railroad employee ran up the tracks with a lantern to
signal the train, and may have provided enough warning to allow the
train to begin braking, so that only the engine and first six cars
of the train fell into the flood. Of the 285 people on the train,
151 were killed. It is New Zealand's worst rail disaster.
- Mudflows can dam rivers, impounding a lake. When the lake
reaches the top of the poorly-consolidated dam, it can cut through
the dam quickly and drain catastrophically.
- Mudflows can flow into a lake and displace all the water. When
Mount St. Helens began erupting in 1980, reservoirs near the volcano
were drawn down to avert this hazard.
- Mudflows can fill river valleys, leaving no place for normal
river flow. The Army Corps of Engineers dredged valleys filled by
Mount St. Helens in 1980 to deal with this problem. For several
months, nearly all the dredges on the West Coast were cleaning up
after Mount St. Helens.
- Lava Flows
- Loom large in popular thinking but are minor hazards. They generally move
slowly and are predictable. In recent years several tourists have been
killed in Hawaii venturing too close to lava flows, some by falls, some by
inhalation of superheated steam. One tourist was burned badly by falling
onto still-hot lava.
The only major fatalities directly due to lava flows in
recent years are due to the volcano Nyiragongo in the Republic of the Congo
(former Zaire). Nyiragongo presents the paradox of a steep-sided volcano
that erupts very fluid lava, so that the lava flows can move at high speeds.
Much of the time its crater is filled with a lava lake. On January 10, 1977
the lava lake drained catastrophically through fissures in the crater wall.
The eruption happened in the middle of the night and overwhelmed several
villages by surprise, killing 50-100 people. Some sources claim over 1000
fatalities but offer no evidence.
In January 2002, as if the Congo didn't have enough problems
with a bloody civil war, Nyiragongo erupted and sent lava flows into the
city of Goma (population 400,000) destroying half the city. This is easily
the greatest damage to a city ever inflicted by lava flows. About 50-100
people were killed but there is no information on how many were killed
directly by lava. For example, several dozen people were killed while
looting an abandoned gas station when gasoline came into contact with lava
and blew the station up. Others were killed by fires or building collapse
rather than by the lava itself.
- The celebrated story of St. Pierre, Martinique in 1902 is a classic tale
of an unknown type of disaster abetted by political ambition. Despite
several months of volcanic activity at Mount Pelee, the governor refused to
evacuate the city of St. Pierre. A pyroclastic flow swept over the town,
killing 28,000 people. One survivor was a prisoner in the city jail, and
thanks to the efforts of P.T. Barnum he was considered to be the only
survivor for a long time. The story of a second survivor later came to
light, and most texts say there were only two survivors. In reality there
were other people found alive who later died of their injuries, and others
outside the town who were caught in the flow and survived. The exact number
of survivors is unlikely ever to be known but is almost certainly less than
- Mount Lamington, New Guinea wasn't even recognized as a volcano when it
began erupting in 1951. Three days later it unleashed a pyroclastic flow
that killed 4,000 people.
- For a long time it was believed that the 79 A.D. eruption of Mount
Vesuvius was fairly tame and the celebrated body casts found at Pompeii were
of people who miscalculated how soon to evacuate. Studies since the 1980's
have indicated that about 24 hours after the eruption began, the gas
pressure in the volcano began to falter, allowing the eruption cloud to fall
and roll down the mountain as pyroclastic flows. This phenomenon is called a
collapsing Plinian cloud. The seaport of Herculaneum was hit first,
Pompeii somewhat later. The eruption was much more violent than previously
suspected. The ring of hills around Vesuvius, a caldera rim called Monte
Somma, was formerly considered prehistoric, but many vulcanologists now
regard it as a result of the 79 A.D. eruption. Roman wall paintings of
Vesuvius show only a single cone. Vesuvius now rises well above Monte Somma
but paintings from the 1500's and 1600's show the summit below Monte
- Carbon dioxide is a common but little-noted hazard of eruptions. It can
be a problem in confined spaces and low-lying areas. The 1973 eruption at
Heimaey, Iceland resulted in toxic levels in many buildings. The only
fatality during the eruption was a looter who was overcome by carbon dioxide
while burglarizing a drug store for drugs.
- Lake Nyos is a crater lake in Cameroon, Africa. Gas is still seeping
into the lake from the magma below and the lake is supercharged with carbon
dioxide. Samples of water brought to the surface literally fizz like club
soda. In 1986 the lake vented several cubic kilometers of carbon dioxide,
which, being heavier than air, flowed down adjacent valleys and suffocated
about 1800 people. The carbon dioxide displaced all the oxygen, plus pure
carbon dioxide is highly toxic in its own right.
- Lake Kivu in the Congo and Rwanda is the largest gas-charged lake in the
world, holding huge amounts of methane and carbon dioxide. Disaster planners
are concerned that a volcanic eruption from nearby Nyiragongo, or under the
lake itself, might trigger a large release of gas.
Greatest Earthquakes and
Pyroclastic Flow or Nuee Ardente (French: Fiery Cloud)
- Gas Expands as Lava Rises
- Lava Breaks up into Fragments Supported by Escaping Gas
- Cloud Flows Downhill at 60-100 M.p.h. Temperature about
1000 C. You will not outrun this.
Nuee Ardente is a more "classical" term, but seems to be in the process of
being replaced by "pyroclastic flow." Pyroclastic flow deposits are typically
called just that, or also "ash flows." Really large and thick deposits,
especially those with evidence of violent eruption, are sometimes called
ignimbrites from the Latin ignis, fire and imber, storm - a
wonderfully descriptive term. Some pyroclastic flow deposits are so thick that
the hot interior fuses together into a glassy mass. Such deposits are called
Volcanoes with pyroclastic flow deposits in the vicinity should always be
regarded as dangerous when they erupt. Once pyroclastic flows do begin erupting,
the safest procedure is to estimate the greatest likely range of the flows, add
a safety margin, and evacuate. The only way to survive an oncoming flow is to
outrun it or evade it. Evasion might be possible if there's a quick route off to
the side or uphill. Outrunning it is only possible if there's a good straight
road, and then do not stop for anything - stop signs, police cars, red
lights, animals, anything. Anything that does not outrun the flow will die.
How Calderas Form
Calderas form when volcanoes collapse. In some cases, violent
explosive eruptions (left) can empty a magma chamber enough that
the summit collapses. In other cases, magma may erupt on the
flanks of a volcano or drain back to deeper levels, permitting
the summit to subside (right). These caldera collapses are
generally not violent.
In some cases, magma may be just a few kilometers below the surface. The roof
of the magma chamber may cave in, uncorking the gas pressure and resulting in
colossal pyroclastic flows and Ultra-Plinian eruptions with Explosivity Index 8.
Yellowstone, Long Valley Caldera and Toba are important examples in the recent
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Created 3 Feb 2006, Last Update
14 December 2009
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