Plate Boundaries, Earthquakes, and Volcanoes

The planet's rigid outer shell is cracked into plates that float on the slow-churning mantle below. Where plates meet, three things can happen. At a divergent boundary they pull apart, magma rises to fill the gap, and new crust forms, like the Mid-Atlantic Ridge. At a convergent boundary they crash together; if one plate is denser it sinks beneath the other in subduction. At a transform boundary they grind sideways past each other without creating or destroying crust, like the San Andreas Fault. Each type produces a different hazard signature: divergent zones give mild quakes and gentle lava, transform zones give sudden sharp quakes with little volcanism, and convergent zones give both big quakes and explosive volcanoes.

Subduction is the volcano factory. When a dense oceanic plate slides under a lighter continental plate, it drags water-soaked rock down into the hot mantle. That water lowers the rock's melting point, so the descending slab partially melts. The melted rock, now less dense than the solid rock around it, rises through the overriding plate and erupts, building a parallel chain of volcanoes inland from the trench, like the Andes or the Cascades. The same squeezing also locks and releases stress along the fault, producing some of the strongest earthquakes on Earth.

This explains the Ring of Fire: the Pacific plate's edges are almost entirely convergent, so subduction wraps nearly the whole ocean basin in volcanoes and quakes. It also sets up the next idea, what happens when neither plate can sink. Two continental plates are similarly light, so instead of one subducting, they crumple upward into mountain ranges like the Himalayas, with heavy earthquakes but little to no volcanism.