The Solar System

472 PART 4^ |^ THE SOLAR SYSTEM

Chains of composite
volcanoes are not found on Venus or Mars,
which is evidence that subduction and plate motion does
not occur on those worlds.

Mount St. Helens exploded northward on May 18, 1980,

killing 63 people and destroying 600 km^2 (230 mi^2 ) of

forest with a blast of winds and suspended rock

fragments that moved as fast as 480 km/hr (300 mph)

and had temperatures as hot as 350°C (660°F). Note

the steep slope of this composite volcano.

The Cascade Range

composite volcanoes

are produced by an

oceanic plate being

subducted below

North America and

partially melting.

Molten rock (magma) is less dense than the

surrounding rock and tends to rise. Where it

bursts through Earth’s crust, you see volcanism. The

two main types of volcanoes on Earth provide good

examples for comparison with those on Venus and

Mars.

On Earth, composite volcanoes form above

subduction zones where the descending crust melts

and the magma rises to the surface. This forms

chains of volcanoes along the subduction zone, such

as the Andes along the west coast of South America.

Magma rising above subduction zones is not very

fluid, and it produces explosive volcanoes with sides

as steep as 30°.

Magma collects in a

chamber in the crust

and finds its way to

the surface through

cracks.

Oceanic crust

Composite

volcanoes

Upper

mantle

Upper

mantle

Based on Physical Geology, 4th edition, James S. Monroe and Reed

Wicander, Wadsworth Publishing Company. Used with permission.

USGS

Oceanic plate

Subduction

zone

Continental

crust

Magma chamber

Magma forces its

way upward

through cracks in

the upper mantle

and causes small,

deep earthquakes.

A hot spot is formed

by a rising

convection current

of magma moving

upward through the

hot, deformable

(plastic) rock of the

mantle.

Ashield

volcano is

formed by highly

fluid lava (basalt)

that flows easily

and creates

low-profile

volcanic peaks

with slopes of 3°

to 10°. The

volcanoes of

Hawaii are shield

volcanoes that

occur over a hot

spot in the middle

of the Pacific

plate.

Ashield

volcanois

formed by highly

fluid lava (basalt)

that flows easily

and creates

low-profile

volcanic peaks

with slopes of 3°

to 10°. The

volcanoes of

Hawaii are shield

volcanoes that

occur over a hot

spot in the middle

of the Pacific

plate.

Shield volcano Lava flow

Washington

Seattle

Portland

St Helens Rainier

Oregon

Shasta

Lassen

California

Nevada

Washington

Seattle

Portland

Pacific Ocean

St Helens Rainier

Oregon

Shasta

Lassen

California

Nevada

HoodHood

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