G. PLATE TECTONICS 87

"Volcanoes are among nature's most important crucibles of change - initially destructive, but in compensation, the source of enduring material and spiritual wealth"

Fischer, Heiken, and Hulen, 1997

1. Spreading centers - Because of plate motions and convection in the asthenosphere, portions of the mantle rise from depth under mid-ocean ridges, pressure is reduced, and the mantle melts to form basalt. The magmas intrude and extrude at the margins of the two diverging plates and solidify to form new oceanic lithosphere. These areas account for about 80% of the Earth’s volcanic activity. Ex: Mid-Atlantic Ridge, East African Rift Zone

2. Subduction zones - At places where oceanic lithosphere is being subducted, the heated lithosphere gives off volatiles like water, which cause the overlying mantle to melt.

a. where overriding plate is oceanic, basaltic and andesitic volcanism produces island arcs. Ex: Aleutian Islands

b. where overriding plate is continental, andesitic and rhyolitic volcanism produces volcanic mountain chains. Ex: Andes

The Pacific Ocean is rimmed by subduction zones > "Ring of Fire".

3. Intraplate - There are unusually hot areas of the Earth’s interior that cause mantle rocks to heat, rise, and melt to form a hot spot or mantle plume. These can produce volcanism in the middle of plates.

a. oceanic: basalt - Hawaii, Iceland

b. continental: basalt - Columbia River Plateau rhyolite - Yellowstone

4. Cascade Range - current volcanic activity (St. Helens, Shasta, Rainier, Crater Lake, etc.) Is a result of the subduction of the Juan de Fuca plate beneath the North American plate. 88

SUMMARY OF THE PLATE TECTONIC - VOLCANO CONNECTION

Tectonic Environment

Forms of Volcanoes

Composition

Topography

Example

Divergent Margins

Oceanic Crust

pillow lavas

basalt

mid-ocean ridge

Mid-Atlantic Ridge

Continental Crust

various type

basalt-rhyolite

rift valleys

East African Rifts

Convergent Margins

Oceanic-Oceanic

composite

basalt-andesite

trench & volc islands

Aleutian Islands

Oceanic-Continental

composite

andesite-rhyolite

trench & volc mtns

Andes Mtns

Continental-Continental

(none)

(none)

mountains

Himalaya Mtns

Transform Margins

Transform Faults

(none)

(none)

offsets

San Andreas fault

Intraplate Areas

Oceanic

shield

basalt

volcanic area

Hawaiian Islands

Continental

various types

basalt-rhyolite

volcanic area

Yellowstone

Main Page

Next Section