Plate Tectonics, Earthquakes, Volcanoes and the Rock Cycle

Why the plates move

Heat from radioactive decay in the core and mantle drives convection currents in the asthenosphere. The rigid lithosphere above rides on those currents, broken into about a dozen plates. Every landform question in this unit begins with the same decision: which of the three boundary types is this?

• Divergent — plates move apart. Magma rises, cools and makes new crust: mid-ocean ridges, rift valleys, Iceland. Earthquakes are shallow and moderate; volcanism is basaltic and effusive.
• Convergent — plates collide. Oceanic crust is denser, so it subducts, melts, and feeds explosive composite volcanoes; earthquake foci get deeper away from the trench. Continent meets continent gives folded mountains such as the Himalayas and no volcanoes.
• Transform — plates slide past each other. Crust is neither created nor destroyed, there are no volcanoes, but friction stores strain and releases it as strong shallow earthquakes (San Andreas Fault).

The rock cycle

Rock is recycled, not permanent. Igneous rock forms when magma or lava cools. Weathering and erosion break it into sediment, which is buried, compacted and cemented into sedimentary rock. Heat and pressure without melting turn any rock into metamorphic rock; complete melting returns it to magma. Subduction zones are the main place where surface rock re-enters the mantle, which is why the rock cycle and plate tectonics are one story, not two.