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Composition
Volcano: A mound of material that is extruded to the
Earth’s surface from a vent that is connected to a magma
chamber via a feeder conduit.
Types of deposits
Types of volcanoes
Distribution
Prediction
Impact of eruptions
Supervolcanoes
Volcanoes are classified according to their form.
The form of a volcanoes depends on the type of material
that it is made up of.
The nature of the extruded material (and the volcano
itself) depends on the properties of the magma.
Magma: Molten rock within the Earth.
The composition of magma determines the type of rock
that forms when it cools and its behavior during an
eruption.
Main controls on behavior:
chemical composition (largely silica dioxide - SiO2 content)
and
gas content (largely water vapor and CO2).
SiO2 content controls the viscosity of a magma.
Magma is called lava when it reaches the surface.
Viscosity: a measure of how easily a fluid flows. Water has
a low viscosity, molasses has a much higher viscosity.
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Viscosity, in turn, controls the amount of gas that can be
trapped in the magma.
The greater the viscosity the more gas in the magma.
There are three basic types of magma:
Basaltic Magma
Andesitic Magma
Rhyolitic Magma
The names are based on the rock type that forms when the
magma crystallizes.
Overall, the behaviour of the magma determines the type
of volcano that develops:
Low SiO2 magmas, with little gas and low viscosity, flows
readily through their vents and across the land surface
when the lava escapes the vents.
Magma
Type
Basaltic
Chemical
Composition
Temperature Viscosity
(degrees C)
45-55% SiO2;
High in Fe,
Mg, Ca; Low in
K, Na.
Andesitic 55-65% SiO2;
Intermediate
Fe, Mg, Ca,
Na, K
Rhyolitic 65-75% SiO2;
Low in Fe, Mg,
Ca; High in K,
Na
1000 - 1200
Low
Gas
Content
Low
800-1000
Intermediate Intermediate
650-800
High
High
Types of volcanic deposits
(photos from USGS)
Volcanoes also vary in terms of the types of deposits that
they produce.
Lava: Hot (up to 1200 degrees C), fluid,molten rock
that flows along the land surface.
High SiO2 magmas, gaseous and with high viscosity, tend
to plug their vents until the force of escaping magma blows
the vent clear; such magmas cause explosive volcanoes.
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Lava can flow like viscous water, including forming lava
falls.
Pahoehoe
Pahoehoe: Lava with a ropelike surface texture due to
partial cooling as the lava flowed. Relatively hot, low
viscosity lava.
A thick deposit of pahoehoe lava
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Aa: Blocky, rough lava flow. Due to high viscosity lava
that flowed pushing chunks of solid and semi-solid
blocks.
Pillows: A form of closed lava tube (with a bulbous
end) that forms when a lava flows into water (e.g., a
lake or ocean) and cools very rapidly.
Lava tube: A tube
formed by cooling and
solidifying of the lava
walls while fluid lava
continued to flow inside.
Ash: tephra that is finer than 2 mm in diameter.
http://oceanexplorer.noaa.gov/explorations/04fire/background/volcanism/media/pillow_lava_video.html
Pyroclastic material: Debris formed by a volcanic
explosion. Results when magma is very viscous.
Tephra: The general term for all pyroclastic material
that is ejected from a volcano. Different terms apply
according to the size of the tephra. (syn. Ejecta)
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Lapilli: from 2 mm to 64 mm in diameter.
Bombs: soft, partially melted fragments greater than
64 mm in diameter.
Blocks: hard fragments greater
than 64 mm in diameter.
Tuff: A deposit made up of ash.
Ash flow: Pyroclastic debris that flows downslope.
Welded tuff: A deposit of pyroclastic material that was
laid down while still very hot and particles become
fused together.
Lahar: A water saturated slurry of ash and other
volcanic debris that flows downslope.
Ash fall: Fallout of very fine ash from the air.
Volcanic ash fall during
mid-day with the
eruption of Mount
Pinatubo in the
Philippines.
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Nuée Ardente (glowing cloud): A hot, gaseous cloud of
ash that flows down slope.
Classification of volcanoes
Volcanoes are classified according to their morphology.
Flow speeds can reach 160
km/hr and temperatures can
exceed 600 degrees C.
The processes and deposits dictate the morphology of
volcanoes.
Three types of volcano:
http://volcano.und.nodak.edu/vwdocs/volc_images/img_mt_pelee.html
Shield volcanoes: dominated by lava flows.
Cinder cones: dominated by pyroclastics.
Forms an isolated conical mound of tephra.
Muana Loa Volcano – the world’s largest volcano.
Photograph by J.P. Lockwood on 1 December 1975
http://hvo.wr.usgs.gov/maunaloa/
Photograph by J.D. Griggs on January 10, 1985
http://volcanoes.usgs.gov/Products/Pglossary/CinderCone.html
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Stratovolcanoes: mixture of lavas and pyroclastics.
Syn. Composite volcanoes
Mount Mageik volcano, Alaska
Shield Volcanoes
Dominated by fluid, high temperature, low viscosity
basaltic magma.
Low, dome-shaped profile, like an inverted shield.
Photograph by R. McGimsey on 15 July 1990
http://volcanoes.usgs.gov/Products/Pglossary/stratovolcano.html
Typical slopes approximately 15 degrees.
Lava flows downslope, away from a central vent or a series
of vents.
http://geoimages.berkeley.edu/GeoImages/Johnson/Landforms/Volcanism/ShieldVolcano.html
Low viscosity lava forms fountains of lava flowing from vents near
the volcano summit.
Many shield volcanoes have a central caldera:
Calderas form after an
eruption when the surface
collapses.
Each caldera is located at
the site of a former
eruption.
USGS
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Where the lava is relatively cool eruptions form small
cinder cones on the volcanoes surface.
The lava flows easily down the
gentle slopes….reaching the ocean
during some eruptions.
Cinder Cones
Internally constructed entirely of layers of pyroclastic
deposits (blocks, bombs, lapilli).
Dominated by viscous, gaseous magmas
Slopes are steep, at angle of repose.
Relatively cool basaltic magmas or andesitic magmas
predominate.
Angle of repose: the natural maximum angle that a
pile of loose, unconsolidated material will form.
Mount Edziza, British Columbia
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Stratovolcanoes
Typical angles: 30 to 40 degrees.
Range from several metres to over 300 m in height.
Commonly associated with old shield volcanoes with a
relatively cool, basaltic magma.
Volcanoes that alternate
between periods of lava flows
(constructive phase) and
periods of explosive eruptions
(destructive phase).
Commonly called “composite
volcanoes” because they are
made up of both lava and
pyroclastic deposits.
Steep slopes, at angle of
repose or greater.
May lay dormant for thousands of years.
© Noemi Emmelheinz 2001
Mt. St. Helens Before
On average, andesitic magmas with a high gas content.
Actually, a mix of basaltic and rhyolitic magmas in many
cases.
Gases add great pressure when the feeder conduit
becomes plugged, contributing to the explosive power.
Can grow to thousands of metres high during
constructive lava flow phases.
Mt. St. Helens After
The constructive phase often ends with a destructive phase
– an explosive eruption.
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Extensive ash falls and ash flows are commonly
produced during explosive phases.
After an eruption a large caldera remains.
Crater Lake is a caldera that remains following an
explosive eruption 7,700 years ago.
The eruption was 42 times more powerful than Mt. St.
Helens.
The Distribution of volcanoes
The vast majority of volcanoes are located:
Parallel to oceanic trenches.
Along the oceanic ridge.
Over hot spots originating from the mantle.
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Volcanoes along trenches
2/3 of all volcanoes are along the Ring of Fire that
surrounds the Pacific Ocean.
Examples: Japan, most Pacific Islands, Caribbean
Islands, west coast of North and South America.
Volcanoes result from magma rising off the melting
subducted plate.
The composition of the magma is andesitic (melted
basaltic crust plus sediment carried on the crust).
Granitic (rhyolitic) intrusions are also formed,
becoming trapped within the volcanic pile overlying
the region of subduction.
Potential for very explosive eruptions.
Magma is very gaseous,
particularly enriched with
water vapor.
Stratovoclanoes are
constructed from feeder
conduits extending to the
surface.
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Mt. Fuji, Japan
A stratovolcano that has erupted 16 times since
781 AD.
The most recent eruption was in 1707-1708
Similar situation on the west coast of North and South
America.
Volcanoes formed by intrusion into the mountain
chains that result from compressive forces between
oceanic and continental crust.
0.8 cubic km of ash, blocks, and bombs were ejected
during that eruption.
(Greater than Mt. St.
Helens and there were
no fatalities).
Volcanoes in Canada?
There are many inactive volcanoes in the Canadian Rocky Mountains.
None are erupting at the present time.
At least three have erupted over the past several hundred years.
For a catalogue of Canadian Volcanoes go to…..
http://gsc.nrcan.gc.ca/volcanoes/map/index_e.php
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Oceanic Ridge Volcanoes
Most volcanic activity is under water.
Intrusion of material from the magma chamber creates
new oceanic crust as the sea floor spreads.
Shield volcanoes occur where volcanic activity extends
to the surface (e.g., Iceland).
Iceland is growing by volcanic expansion of the ridge.
Unlike Hawaiian volcanoes, Icelandic shield volcanoes deliver lava
through fissures rather than central vents.
Basaltic pillow lavas
dominate the
submerged volcanoes.
Volcanoes and Hot Spots
Hot Spot: a point on the crust immediately above a hot
plume within the mantle.
Volcanism associated with rifting
Volcanism Associated with subduction
Heat from the mantle (and some magma) rises to the hot
spot.
Rising mantle
material
termed a
mantle plume.
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Hot spots can occur beneath oceanic or continental
crust.
The Hawaiian Islands
consist of eastern active
volcanic islands and
inactive volcanic
islands to the
northwest.
Mechanism first proposed by J. Tuzo Wilson (a
Canadian geophysicist) to illustrate that plates actually
move.
Further northwest of the islands are seamounts
(underwater mountains that are submerged islands).
http://www.biosbcc.net/ocean/marinesci/02ocean/hwgeo.htm
Just southeast of Hawaii is an undersea volcano known as
Loihi.
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The modern active island rests close to the hot spot
and its shield volcanoes are fed from the magma that
the hot spot generates.
Until 1996 Loihi was thought to be
an inactive seamount.
It began erupting in 1996 and the eruptions
were preceded by a cluster of small
earthquakes indicating the movement of
magma.
http://www.biosbcc.net/ocean/marinesci/02ocean/hwgeo.htm
The Pacific plate is moving
towards the northwest.
The volcanic islands have
been successively “pushed
off” the hot spot by plate
movement.
As the crust moves it
ages, becomes cooler and
more dense, causing it to
subside.
The seamounts are old
islands that have subsided
to below sea level.
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The seamounts represent even older islands that have
been pushed further from the hot spot.
Prediction of Volcanic Eruptions
Long Term Prediction
Identify volcanoes and the frequency and style of their
eruptions (a geological problem).
Establish probabilities of eruption, style and location for
individual volcanoes.
Recent studies suggest that the Hawaiian Hot Spot has moved over
time.
Hazard zones have been distinguished around Mt. Shasta
based on topography and past experience with eruptions.
Zone 1: areas likely to be affected
Zone 1 most frequently. Most future flows
from summit eruptions probably
would stay within this zone.
Establish the level of risk based on historic and geologic
record.
E.g., for individual volcanoes: determine most likely routes
for lahars, nuees ardentes, lava flows, etc., and avoid
construction in those areas.
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Hazard zones have been distinguished around Mt. Shasta
based on topography and past experience with eruptions.
Hazard zones have been distinguished around Mt. Shasta
based on topography and past experience with eruptions.
Zone 1: areas likely to be affected
Zone 2 most frequently. Most future flows
from summit eruptions probably
would stay within this zone.
Zone 1: areas likely to be affected
Zone 3 most frequently. Most future flows
from summit eruptions probably
would stay within this zone.
Zone 2: areas likely to be affected
by lava flows erupted from vents
on the flank of the volcano or that
move into zone 2 from zone 1.
Zone 2: areas likely to be affected
by lava flows erupted from vents
on the flank of the volcano or that
move into zone 2 from zone 1.
Zone 3: areas likely to be affected
infrequently and then only by long
lava flows that originate at vents in
zones 1 and 2
Short-term prediction
Surface tilting: recognition of changes in the land
surface due to building pressure in the conduit.
Based on the recognition of a pattern of events prior to
previous eruptions.
A surface bulge appeared on Mt. St. Helens prior to its
eruption.
Gas emissions: rates of emission and type of gas changes in
some volcanoes.
April 8, 1980
Important gases include sulfur dioxide (SO2) and carbon
dioxide (CO2)
Changes in concentration may reflect movement of the
magma up the vent.
April 26
May 2
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Earthquakes: generated as the magma moves up the
feeder conduit to the vent.
Mount Spurr, Alaska:
The 1992 Eruption of Crater Peak Vent
When viscous magma becomes stuck in the conduit strain
energy builds as more magma tries to push out.
Movement takes place in a series of “jerks” as the rock
material breaks. Each “jerk” produces an earthquake.
Magnitudes generally less than 5 M.
USGS
Black bars: earthquake
frequency.
Red lines: volcanic eruptions.
The more earthquakes the further the magma has moved.
A combination of approaches is likely the key to short-term prediction.
The impact of volcanic eruptions
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Volcanic Hazards
Ash fall
Lava flows
Extensive property damage and fatalities can result from
heavy ash falls.
Commonly destroy property in Hawaii and Iceland.
Significant ash in the upper atmosphere can circle the
globe in a matter of weeks.
Damage limited to the vicinity in the immediate area of the
volcano.
More than 80 commercial jets have been damaged by
flying through volcanic ash clouds.
Fatalities rare due to slow
speed of advancing lava
flow.
Mt. St. Helens’
ash cloud
An ashfall 10 million years ago killed these rhinos that are preserved at
Ashfall Fossil Beds State Historic Park, Nebraska.
Pyroclastic flows
Death was not by burial but by lung failure due to inhaling the ash.
Lahars are fast moving mudflows that can inundate
urban areas that are nearby the eruption.
Lahars can also dam rivers and which can lead to
extensive flooding.
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Lahars can be the most devastating outcome of many
volcanoes.
A 5 metre wall of
water and debris
slammed into the town
of Amero, 72 km from
the volcano.
A relatively small eruption of Nevada del Ruiz, Columbia,
in 1985, generated a lahar when the volcano melted a 2.5
km2 area of snow and ice.
Water and debris rushed down the slopes, picking up more
debris along the way.
The lahar killed
28,700 people and
destroyed over 5,000
structures in the city.
Nuée ardentes destroy life and property in their paths.
60 people, thousands of animals and fish, and
hundreds of acres of lumber were destroyed by ash
flows from Mt. St. Helens.
People and animals
died instantly from the
rushing cloud of hot
gas and ash.
A Nuée Ardent killed 20,000 people when Mt. Vesuvius
exploded and shed a pyroclastic flow across the village
of Pompeii in 79 AD.
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Landslides
Landslides can be generated when a volcano collapses
during an eruption.
Volcanic Gases
In addition to making magma more explosive, volcanic eruptions also
include gases that can be deadly to all life.
During the Mt. St. Helens eruption 2.3 km3 of debris slid
down the mountain at speeds up to 240 km/hr.
The slide traveled over 24 km and left a 45 m deep deposit.
350,000 years ago Mt. Shasta experienced a similar
eruption and landslide that was 20 times greater than that
of Mt. St. Helens.
CO2, SO2 and CO are the most abundant of harmful gases.
SO2 emissions can have direct effects on life in the vicinity
of a volcano.
An eruption in 1783 of Laki Crater (Iceland) produced a
sulfurous haze that lasted for 9 months and killed 75% of
all livestock and 24% of the Icelandic population.
Mammoth Mountain is
a relatively young
volcano that is emitting
large volumes of CO2.
Volcanoes release more than 130 to 230 million tonnes of
CO2 into the atmosphere every year
Humans add CO2 at the rate of approximately 22 billion
tonnes per year (150 times the rate of volcanic production)
Gas concentrations in the soil in
some areas near the mountain are
high enough to kill trees and small
animals.
Human CO2 production is equal to that if 17,000 volcanoes
like Kilauea were erupting every year.
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If the air that we breath has more than 10% CO2 it
becomes deadly because it displaces the Oxygen that we
need for respiration.
Lake Nyos 10 days after
the 1986 eruption
Lake Nios, Cameroon, is a very deep lake within a volcanic
crater.
The lake is so deep that hydrostatic pressure forces CO2
to remain at the lake bottom.
When the pressure of the CO2 exceeds a certain limit the
gas rapidly bubbles up out of the lake and flows as an
invisible gas cloud down the adjacent slopes.
On August 61, 1986 such a gas release flowed 19 km
suffocating 1,700 people along its route.
The fountain in the
background lifts CO2
up to the surface so
that it no longer
accumulates.
Tsunamis
Global Climate Change
Caused by the displacement of seawater by eruptions
of volcanic islands and submarine volcanoes.
Due to ash and gas that may spend years in the upper
atmosphere; reduces incoming solar radiation.
Krakatoa (1883 eruption) killed 36,000 people by the
tsunami, alone (the most deadly outcome of the
eruption).
This is the newly forming
summit of Krakatoa, growing
where the 1883 eruption blew
the top off of the original
volcano.
SO2 from an eruption forms tiny droplets of sulfuric acid
in the upper atmosphere.
The droplets significantly increase global albedo…..a
negative radiative forcing that leads to cooling.
Mt. Pinatubo (1991) released 22 million metric tons of SO2
and reduced the Earth’s average temperature by 0.5
degrees Celsius in the year following the eruption.
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A series of eruptions of Tambora (Indonesia) extruded up
to 150 km3 of magma (solid equivalent), much of it into the
atmosphere.
In June of 1816 there was widespread snowfall throughout
the eastern United States.
The normal growing season experienced repeated frosts
as cold air extended much more southerly than normal.
Food shortages and starvation are attributed to the deaths
of 80,000 people.
Tambora (1815 eruption) was followed in 1816 by the
“year without a summer”.
Average global temperature is estimated to have been
reduced by 3 degrees Celsius.
The global population was about 1 billion people in 1816.
Our current population is a little over 6 billion.
The 1816 fatality rate would have resulted in a death toll of
nearly 500,000 people due to starvation.
Approximately 260,000 people have been killed by volcanoes in
historic times…most by a handful of individual eruptions.
The concentration mercury
in ice cores from glaciers in
Wyoming record a peak in
atmospheric mercury that
corresponds to the Tambora
eruption.
The atmospheric impact
caused the “year without a
summer” along with 80,000
deaths due to famine and
disease.
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Volcanic Explosivity Index
http://pubs.usgs.gov/publications/msh/comparisons.html
Super Volcanoes
Toba: the world’s largest Quaternary caldera.
While not defined officially, lets say any eruption that ejects 1000
km3 or more of pyroclastic material (i.e., VEI 8 or more).
According to M.R. Rampino super eruptions take place, on average,
every 50,000 years. Three of the best known eruptions are
compared below.
The Australian Plate is subducting
beneath the Eurasian plate at a rate
of 6.7 cm/yr.
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Three eruptive events have been
recognized.
Today Toba is a caldera or
depression that is occupied
by Lake Toba.
840,000 years ago
500,000 years ago
It is 100 km long and 30 km
wide.
74,000 years ago
Each producing a caldera.
Toba last erupted about
75,000 years ago with the
largest eruption of the last 2
million years.
Samosir Island, rising 750 m above
the lake, is a dome built from lava
following the last eruption.
The eruption ejected 2,800 cubic km of material and the pyroclastic
flows covered an area of at least 20,000 square km.
It is estimated that the growing population of homo sapiens (i.e., us) was
reduced from 100,000 individuals to as few as 3,000 individuals (97% of
all humans were lost!).
In the immediate vicinity of the volcano ash deposits reach 600 metres in
thickness
Ash fall from the eruption covers an area of at least 4 million square km;
half the area of the continental United States.
This reduction had been estimated for approximately the time of Toba’s
eruption on the basis of genetic studies and is termed the “human
population bottleneck”.
Global cooling is estimated at between 3 and 5 degrees Celsius with
regional cooling of 15 degrees C.
Tropical plant life would have been all but eliminated
Temperate forests would loose 50% of all trees.
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Yellowstone Caldera
Known for its hot springs and geysers,
Yellowstone National Park, is likely
the most popular super volcano in the
world.
The magma chamber is only 5 to 13 km below the land surface.
The caldera is 80 km
long and 50 km wide.
The park sits on an active caldera that
rises and sinks in response to magma
movement and pressure fluctuations
within the Earth.
Over recent years the surface has risen
by as much as a metre and sunk back
by 1/3 of a metre.
Thousands of small earthquakes are
produced as earth surface moves.
The caldera and its magma chamber are due to a hot spot in the mantle
that has moved several hundred kilometres over the past 12.5 million
years.
The current caldera was formed with an eruption 640,000 years ago (the
Lava Creek Eruption).
The movement is due to the drift of the north American continent over
the hot spot.
This eruption ejected 1,000 km3 of pyroclastic debris.
Ancient, inactive
calderas mark the path
of the hot spot.
An earlier eruption (the Huckleberry Ridge Eruption, 2 million years
ago) ejected 2,500 km3
of pyroclastic debris.
A smaller eruption
happened 1.3 million
years ago, releasing
280 km3 of debris.
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Eruptions appear to have a 600,000 year period (that long between
eruptions) so we’re overdue for another one.
Heightened monitoring of the Yellowstone Caldera in recent years has
led to media concern of an impending eruption.
Previous eruptions spread ash over thousands of km2 across the US.
Government officials and geologists indicate that there have been no
clear indicators of high risk at this time.
If such an eruption were to take place, North America and the rest of the
world could experience another “Dark Ages”.
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