Composition

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Composition
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Types of deposits
Types of volcanoes
Distribution
Prediction
Impact of eruptions
Supervolcanoes
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Volcano A mound of material that is extruded to
the Earths surface from a vent that is connected
to a magma chamber via a feeder conduit.
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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.
Magma is called lava when it reaches the surface.
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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.
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.
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Magma Type Chemical Composition Temperature (degrees C) Viscosity Gas Content
Basaltic 45-55 SiO2 High in Fe, Mg, Ca Low in K, Na. 1000 - 1200 Low Low
Andesitic 55-65 SiO2 Intermediate Fe, Mg, Ca, Na, K 800-1000 Intermediate Intermediate
Rhyolitic 65-75 SiO2 Low in Fe, Mg, Ca High in K, Na 650-800 High High
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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.
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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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.
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Lava can flow like viscous water, including
forming lava falls.
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Pahoehoe Lava with a ropelike surface texture
due to partial cooling as the lava flowed.
Relatively hot, low viscosity lava.
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Pahoehoe
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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.
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Lava tube A tube formed by cooling and
solidifying of the lava walls while fluid lava
continued to flow inside.
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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.
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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Ash tephra that is finer than 2 mm in diameter.
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Bombs soft, partially melted fragments greater
than 64 mm in diameter.
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Tuff A deposit made up of ash.
Welded tuff A deposit of pyroclastic material
that was laid down while still very hot and
particles become fused together.
Ash fall Fallout of very fine ash from the air.
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Ash flow Pyroclastic debris that flows downslope.
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Flow speeds can reach 160 km/hr and temperatures
can exceed 600 degrees C.
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Classification of volcanoes
Volcanoes are classified according to their
morphology.
The processes and deposits dictate the morphology
of volcanoes.
Three types of volcano
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Shield volcanoes dominated by lava flows.
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Cinder cones dominated by pyroclastics.
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Stratovolcanoes mixture of lavas and
pyroclastics. Syn. Composite volcanoes
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Shield Volcanoes
Dominated by fluid, high temperature, low
viscosity basaltic magma.
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Typical slopes approximately 15 degrees.
Lava flows downslope, away from a central vent or
a series of vents.
Calderas form after an eruption when the surface
collapses.
Each caldera is located at the site of a former
eruption.
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Low viscosity lava forms fountains of lava
flowing from vents near the volcano summit.
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The lava flows easily down the gentle
slopes.reaching the ocean during some eruptions.
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Where the lava is relatively cool eruptions form
small cinder cones on the volcanoes surface.
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Dominated by viscous, gaseous magmas
Relatively cool basaltic magmas or andesitic
magmas predominate.
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Internally constructed entirely of layers of
pyroclastic deposits (blocks, bombs, lapilli).
Slopes are steep, at angle of repose.
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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.
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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.
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May lay dormant for thousands of years.
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.
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.
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After an eruption a large caldera remains.
The eruption was 42 times more powerful than Mt.
St. Helens.
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The Distribution of volcanoes
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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
Examples Japan, most Pacific Islands, Caribbean
Islands, west coast of North and South America.
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2/3 of all volcanoes are along the Ring of Fire
that surrounds the Pacific Ocean.
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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).
Magma is very gaseous, particularly enriched with
water vapor.
Stratovoclanoes are constructed from feeder
conduits extending to the surface.
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Granitic (rhyolitic) intrusions are also formed,
becoming trapped within the volcanic pile
overlying the region of subduction.
Potential for very explosive eruptions.
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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
0.8 cubic km of ash, blocks, and bombs were
ejected during that eruption.
(Greater than Mt. St. Helens and there were no
fatalities).
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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.
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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.
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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.
Basaltic pillow lavas dominate the submerged
volcanoes.
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Unlike Hawaiian volcanoes, Icelandic shield
volcanoes deliver lava through fissures rather
than central vents.
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Volcanism associated with rifting
Volcanism Associated with subduction
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Volcanoes and Hot Spots
Hot Spot a point on the crust immediately above
a hot plume within the mantle.
Rising mantle material termed a mantle plume.
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Hot spots can occur beneath oceanic or
continental crust.
Mechanism first proposed by J. Tuzo Wilson (a
Canadian geophysicist) to illustrate that plates
actually move.
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The Hawaiian Islands consist of eastern active
volcanic islands and inactive volcanic islands to
the northwest.
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Further northwest of the islands are seamounts
(underwater mountains that are submerged
islands).
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http//www.biosbcc.net/ocean/marinesci/02ocean/hwg
eo.htm
Just southeast of Hawaii is an undersea volcano
known as Loihi.
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It began erupting in 1996 and the eruptions were
preceded by a cluster of small earthquakes
indicating the movement of magma.
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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.
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The Pacific plate is moving towards the
northwest.
The volcanic islands have been successively
pushed off the hot spot by plate movement.
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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.
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Recent studies suggest that the Hawaiian Hot Spot
has moved over time.
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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.
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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Zone 1 areas likely to be affected most
frequently. Most future flows from summit
eruptions probably would stay within this zone.
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Zone 1 areas likely to be affected 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.
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Short-term prediction
Based on the recognition of a pattern of events
prior to previous eruptions.
Gas emissions rates of emission and type of gas
changes in some volcanoes.
Important gases include sulfur dioxide (SO2) and
carbon dioxide (CO2)
Changes in concentration may reflect movement of
the magma up the vent.
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Surface tilting recognition of changes in the
land surface due to building pressure in the
conduit.
A surface bulge appeared on Mt. St. Helens prior
to its eruption.
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Earthquakes generated as the magma moves up the
feeder conduit to the 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.
The more earthquakes the further the magma has
moved.
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A combination of approaches is likely the key to
short-term prediction.
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The impact of volcanic eruptions
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Volcanic Hazards
Damage limited to the vicinity in the immediate
area of the volcano.
Fatalities rare due to slow speed of advancing
lava flow.
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Ash fall
Extensive property damage and fatalities can
result from heavy ash falls.
Significant ash in the upper atmosphere can
circle the globe in a matter of weeks.
More than 80 commercial jets have been damaged by
flying through volcanic ash clouds.
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An ashfall 10 million years ago killed these
rhinos that are preserved at Ashfall Fossil Beds
State Historic Park, Nebraska.
Death was not by burial but by lung failure due
to inhaling the ash.
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Pyroclastic flows
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.
Water and debris rushed down the slopes, picking
up more debris along the way.
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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.
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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People and animals died instantly from the
rushing cloud of hot gas and ash.
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Landslides
Landslides can be generated when a volcano
collapses during an eruption.
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.
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Volcanic Gases
In addition to making magma more explosive,
volcanic eruptions also include gases that can be
deadly to all life.
CO2, SO2 and CO are the most abundant of harmful
gases.
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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.
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)
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 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.
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Tsunamis
Caused by the displacement of seawater by
eruptions of volcanic islands and submarine
volcanoes.
Krakatoa (1883 eruption) killed 36,000 people by
the tsunami, alone (the most deadly outcome of
the eruption).
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Global Climate Change
Due to ash and gas that may spend years in the
upper atmosphere reduces incoming solar
radiation.
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 Earths 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.
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.
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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.
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.
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The atmospheric impact caused the year without a
summer along with 80,000 deaths due to famine
and disease.
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Approximately 260,000 people have been killed by
volcanoes in historic timesmost by a handful of
individual eruptions.
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Volcanic Explosivity Index
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http//pubs.usgs.gov/publications/msh/comparisons.
html
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Super Volcanoes
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.
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Toba the worlds largest Quaternary caldera.
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It is 100 km long and 30 km wide.
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840,000 years ago
500,000 years ago
74,000 years ago
Each producing a caldera.
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The eruption ejected 2,800 cubic km of material
and the pyroclastic flows covered an area of at
least 20,000 square km.
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.
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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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!).
This reduction had been estimated for
approximately the time of Tobas eruption on the
basis of genetic studies and is termed the human
population bottleneck.
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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 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.
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The magma chamber is only 5 to 13 km below the
land surface.
The caldera is 80 km long and 50 km wide.
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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 movement is due to the drift of the north
American continent over the hot spot.
Ancient, inactive calderas mark the path of the
hot spot.
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The current caldera was formed with an eruption
640,000 years ago (the Lava Creek Eruption).
This eruption ejected 1,000 km3 of pyroclastic
debris.
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 were overdue
for another one.
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Heightened monitoring of the Yellowstone Caldera
in recent years has led to media concern of an
impending eruption.
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.