why volcano shapes differ

8/7/2019 why volcano shapes differ

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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.



The nature of the extruded material (and the volcanoitself) depends on the properties of the magma.

Volcanoes are classified according to their form.

The form of a volcanoes depends on the type of material

that it is made up of.

Magma: Molten rock within the Earth.

Magma is called lava when it reaches the surface.



SiO2 content controls the viscosity of a magma.

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).

Viscosity: a measure of how easily a fluid flows. Water has

a low viscosity, molasses has a much higher viscosity.



There are three basic types of magma:

The names are based on the rock type that forms when the

magma crystallizes.

Basaltic Magma

Andesitic Magma

Rhyolitic Magma

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.



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 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.



Classification of volcanoes

The processes and deposits dictate the morphology of

volcanoes.

Three types of volcano:

Volcanoes are classified according to their morphology.



Shield volcanoes: dominated by lava flows.

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

Muana Loa Volcano ± the world¶s largest volcano.



Cinder cones: dominated by pyroclastics.

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

Photograph by J.P. Lockwood on 1 December 1975



Stratovolcanoes: mixture of lavas and pyroclastics.

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

Mount Mageik volcano, Alaska

Photograph by R. McGimsey on 15 July 1990



Shield Volcanoes

Dominated by fluid, high temperature, low viscosity

basaltic magma.

Low, dome-shaped profile, like an inverted shield.

http://geoimages.berkeley.edu/GeoImages/Johnson/Landforms/Volcanism/ShieldVolcano.html



Typical slopes approximately 15 degrees.

Lava flows downslope, away from a central vent or a series

of vents.

Many shield volcanoes have a central caldera:

USGS

Calderas form after an

eruption when the surface

collapses.

Each caldera is located at

the site of a former

eruption.



Hawaiian Isands and Iceland are built from shield

volcanoes.

M auna 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.



Low viscosity lava forms fountains of lava flowing from vents near

the volcano summit.http://www.youtube.com/watch?v=

Hec9yK-QQ4o



The lava flows easily down the

gentle slopes«.reaching the ocean

during some eruptions.



Volcanoes that alternate

between periods of lava flows(constructive phase) and

periods of explosive eruptions

(destructive phase).

Commonly called ³composite

volcanoes´ because they are

made up of both lava and

pyroclastic deposits.

Steep slopes, at angle of

repose or greater.

Stratovolcanoes

©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 conduitbecomes 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.



http://www.youtub

e.com/watch?v=bgRnVhbfIKQ&featu

re=related



Describe and explain the differences between acomposite cone and a basic lava sheild volcano

HINT: Non-explosive volcanoes erupt more oftenbut the runny lava takes longer to solidify.Explosive volcanoes erupt less often but theviscous lava soldifies more quickly.

You must compare the height and width You must compare the type of material ejected

You should describe the location of the volcano onthe earth's surface for each volcano

You should draw a simple labelled sketch diagram You could explain the link between the type of

material and the effect on the height and width ofthe volcano

why volcano shapes differ

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