Ch. 7 Volcanoes Ch. 7.1 Volcanoes and Plate Tectonics

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.

Volcanism 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…. 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 Zones Mostly at divergent and convergent plate boundaries.

Subduction 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 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 Spots Areas of magma in the mantle that remain stationary while the lithospheric plates move above them. Creates a chain of volcanic islands.