Ch. 7 VolcanoesCh. 7 Volcanoes

Ch. 7.1 Volcanoes and Plate Ch. 7.1 Volcanoes and Plate TectonicsTectonics

• Most rock in the very hot mantle / asthenosphere remains solid due to very high pressure.

• Sometimes conditions (lower pressure or added water) allow part of the solid mantle to melt and become flowing liquid magma.

VolcanismVolcanism

• Volcanism is movement of magma toward or onto the surface of the earth.

• Magma forms pockets, and the less dense magma rises through the surrounding rock.

• Most magma forms at plate boundaries, especially at subduction zones, where water enters with the subducting plate.

Some definitions….Some definitions….

• Lava—magma that erupts onto the earth’s surface.

• Vent—the opening through which lava flows onto the surface.

• Volcano—The structure formed by the vent and the built-up volcanic material on the surface around the vent.

Major Volcanic ZonesMajor Volcanic Zones

• Mostly at divergent and convergent plate boundaries.

Subduction ZonesSubduction Zones

• With oceanic-continental plate convergence, an ocean trench forms, folded mountains form along the continental edge, and magma forms at the zone of subduction.

• Some magma rises to earth’s surface, forming volcanic mountains near the continent’s edge.

• With oceanic-oceanic plate convergence, a trench and magma result, and a string of volcanic islands (island arc) forms.

• As the islands grow larger, they may join and form larger landmasses such as Japan or the Philippines.

Mid-Ocean RidgesMid-Ocean Ridges

• Divergent boundaries, where the greatest amount of magma comes to the surface.

• New ocean floor created.• Mostly unnoticed, except in Iceland, where

the Mid-Atlantic ridge is above sea level.

Hot SpotsHot Spots

• Areas of magma in the mantle that remain stationary while the lithospheric plates move above them.

• Creates a chain of volcanic islands.