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Mt. St. Helen’s Eruption Sequence
Mt. St. Helen’s
May 18, 1980
Devastation
Post May 18, 1980 Crater
and Resurgent Dome
From: USGS http://vulcan.wr.usgs.gov/
Crater
Crater
lake,lake
OR - Mt. Mazama
(Eruption of Mt. Mazama)
From: http://www.igoweb.org/~wms/personal/photos/2000/craterLake/
Cascades
Volcanoes
The Distribution of volcanoes
2/3 of all volcanoes are along the Ring of Fire
that surrounds the Pacific Ocean.
What’s happening in
this location,
causing so many
volcanoes?
•Subduction!
•Convergent boundary:
•plates move toward each other, collide, and the
oceanic crust sinks under the continental crust.
•The oceanic crust melts, and the magma rises
upward.
Oceanic Ridge Volcanoes
•Most volcanic activity is under water.
•Divergent boundary:
•Rising magma
creates new oceanic
crust as the sea floor
spreads.
Iceland has many active volcanoes.
Hekla Volcano in the
distant background.
Hekla erupted four
times in the 20th
century, the last time in
1991.
Hekla covers about 80%
of Iceland and its volume
is approx. 12 cubic km.
In 1973 the Eldfell volcano
erupted on the island of
Heimaey
Photos from the USGS.
Volcanoes and Hot Spots
Hot Spot: a point on the crust immediately above a hot
plume within the mantle. (not on, or near, a plate boundary)
Heat from the mantle (and some magma) rises to the hot
spot.
Rising mantle
material
termed a
mantle plume.
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.
The Hawaiian Islands
consist of eastern active
volcanic islands and
inactive volcanic
islands to the
northwest.
Further northwest of the islands are seamounts
(underwater mountains that are submerged islands).
The modern active island (“Hawaii”) rests close to the
hot spot and its volcanoes are fed from the magma that
the hot spot generates.
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.
Volcanism associated with rifting
Volcanism Associated with subduction
Volcanism Associated with hot spots
To Summarize… (copy for notes)
Volcanoes can occur at:
• Convergent boundaries (most common)
–
•
_________ boundaries
–
•
Example(s):
Example:
_____ _______
–
Example: Hawaiian islands
Volcano: A cone of erupted material and a vent
that is connected to a magma chamber.
Volcanoes are classified according to their
form (shape, size, etc.)
•The shape and size of a volcano depends on the type of
eruption and the type material released.
•The properties of the magma determine the type of
eruption and materials released.
Magma: Molten rock within the Earth.
Magma is called lava when it reaches the surface.
The composition of magma determines the
type of eruption (mild, explosive, or
intermediate)
Two properties of magma influence an eruption:
1. chemical composition (largely silica dioxide SiO2 - content)
and
2. gas content (largely water vapor and CO2).
•SiO2 content controls the viscosity of magma.
•Viscosity controls the amount of gas that can be
trapped in the magma.
Viscosity: a measure of how easily a fluid flows.
•Water has a low viscosity, honey has a much greater
viscosity.
•Greater viscosity = more gas trapped = more
explosive eruption
•Think about shaking up a new soda vs. a flat soda.
Magma
Type
Basaltic
Chemical
Composition
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
Temperature Viscosity
(degrees C)
1000 - 1200
Low
Gas
Content
Low
800-1000
Intermediate Intermediate
650-800
High
High
Overall, the behavior 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.
Classification of volcanoes
Volcanoes are classified according to their forms.
The type of magma/eruption dictate the form of a volcano.
Three types of volcanoes:
1. Shield volcanoes
2. Cinder Cone volcanoes
3. Composite volcanoes
Shield volcanoes: eruptions produce lava flows.
Muana Loa Volcano – the world’s largest volcano.
http://hvo.wr.usgs.gov/maunaloa/
Photograph by J.D. Griggs on January 10, 1985
Shield Volcanoes
Dominated by fluid, high temperature, low viscosity
magma. Lava flows great distances.
Low, dome-shaped profile, like an inverted shield.
Gentle (non-steep) slopes. Uniform composition.
http://geoimages.berkeley.edu/GeoImages/Johnson/Landforms/Volcanism/ShieldVolcano.html
Low viscosity lava forms fountains of lava flowing from vents near
the volcano summit.
The lava flows easily down the
gentle slopes….reaching the ocean
during some eruptions.
Hawaiian Islands and Iceland are built from shield
volcanoes.
Mauna Loa is the largest volcano on Earth.
It makes up most of the
island of Hawaii.
The volcano rises 4,170 m above
sea level.
It covers an area of 5,271 km2.
Total volume of rock: 80,000 km3
Began to form 700,000 to 1,000,000 years ago when lava began to
flow to the sea floor.
Eruptions reached the surface 400,000 years ago.
Its great weight depresses the underlying crust by 8 km.
Cinder cones: eruptions produce ash and small rock
fragments.
Photograph by J.P. Lockwood on 1 December 1975
http://volcanoes.usgs.gov/Products/Pglossary/CinderCone.html
Cinder Cones
Dominated by relatively cool, viscous, gaseous magmas
Very steep slopes; the erupted ash and rock fragments fall
close to the vent. Not very tall.
Mount Edziza, British Columbia
Paricutin volcano began to erupt in
a corn field in Mexico in 1943 and
continued until 1952.
The farmer had noticed a fissure
(vent) had opened in the field one
morning and from it was pouring
black ash.
In the first year the volcano grew to
336 m (almost 1 metre per day).
Rate of growth decreased steadily;
by 1952 the volcano was 424 m in
height.
Composite Volcanoes (aka: Stratovolcanoes): eruptions
alternate between lava flows and eruptions of ash and
rock.
Mount Mageik volcano, Alaska
Photograph by R. McGimsey on 15 July 1990
http://volcanoes.usgs.gov/Products/Pglossary/stratovolcano.html
Composite volcanoes
Dominated by magmas of
intermediate gas content,
temperature, and viscosity.
Called “composite volcanoes”
because they are made up of
alternating layers of lava and
rock deposits.
Steep slopes and very tall.
© Noemi Emmelheinz 2001
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.
Mt. St. Helens Before
Mt. St. Helens After
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!
Mt. Fuji, Japan
A composite volcano 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).
Ojos del Salado, Chile – The world’s highest volcano.
Perched at 6,887 metres above sea level.
A composite volcano that has not erupted in historic time.
Photo by Peter Francis
Quick Review
• With your partner, discuss the following:
– What is the difference between a volcano on the
island of Hawaii and a volcano in the “Ring of
Fire”?
• Hint: think of origin, how it formed or is forming,
types of magma and eruptions, etc.
Basic Volcano Types
Volcanoes and the Rock Cycle
• Magma and/or
Lava cooling into
solid rock begins
the rock cycle.
– Following this,
many pathways are
possible.
http://www.rocksandminerals4u.com/images/rock-cycle-diagram-im.jpg