Ch. 7 Volcanoes

1. Ch. 7 Volcanoes Ch. 7.1 Volcanoes and Plate Tectonics

2. 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.

3. 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.

4. 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.

5. Major Volcanic Zones • Mostly at divergent and convergent plate boundaries.

6. 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.

7. 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.

8. 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.

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