Vo l c a noes

Mt. St. Helen’s Eruption Sequence

Mt. St. Helen’sMay 18, 1980

Devastation

Post May 18, 1980 Crater and Resurgent Dome

From: USGS http://vulcan.wr.usgs.gov/

Crater lake - Mt. MazamaCrater lake, OR(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 covers about 80% of Iceland and its volume is approx. 12 cubic km.

Hekla erupted four times in the 20th century, the last time in 1991.

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).

http://www.biosbcc.net/ocean/marinesci/02ocean/hwgeo.htm

The modern active island (“Hawaii”) rests close to the hot spot and its volcanoes are fed from the magma that the hot spot generates.

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 subduction

Volcanism associated with rifting

Volcanism Associated with hot spots

To Summarize… (copy for notes)

Volcanoes can occur at:

• Convergent boundaries (most common)– Example(s):

• _________ boundaries– Example:

• _____ _______– Example: Hawaiian islands

Volcano: A cone of erupted material and a vent that is connected to a magma chamber.

•The properties of the magma determine the type of eruption and materials released.

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.

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)

and2. 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

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

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

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

Volcanoes are classified according to their forms.

Shield volcanoes: eruptions produce lava flows.

Photograph by J.D. Griggs on January 10, 1985http://hvo.wr.usgs.gov/maunaloa/

Muana Loa Volcano – the world’s largest volcano.

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.

http://volcanoes.usgs.gov/Products/Pglossary/CinderCone.html

Photograph by J.P. Lockwood on 1 December 1975

Dominated by relatively cool, viscous, gaseous magmas

Cinder Cones

Mount Edziza, British Columbia

Very steep slopes; the erupted ash and rock fragments fall close to the vent. Not very tall.

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.

http://volcanoes.usgs.gov/Products/Pglossary/stratovolcano.html

Mount Mageik volcano, Alaska

Photograph by R. McGimsey on 15 July 1990

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.

Composite volcanoes

© Noemi Emmelheinz 2001

The constructive phase often ends with a destructive phase – an explosive eruption.

May lay dormant for thousands of years.

Can grow to thousands of metres high during constructive lava flow phases.

On average, andesitic magmas with a high gas content.

Gases add great pressure when the feeder conduit becomes plugged, contributing to the explosive power.

Actually, a mix of basaltic and rhyolitic magmas in many cases.

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.

Photo by Peter Francis

Perched at 6,887 metres above sea level.

A composite volcano that has not erupted in historic time.

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