Trench

• When two plates of oceanic crust collide, one plate sinks beneath the other.

• One of the plates is pushed beneath the deep-sea trench down toward the mantle.

• A deep-ocean trench may form where oceanic crust sinks toward the mantle beneath continental crust. Some of the oceanic crust melts to form magma.

Volcanic Islands Beneath the trench, the plate melts

and becomes magma.

The magma erupts to the surface forming a row of volcanic islands.

Mid-Oceanic Ridge Sea-floor spreading occurs as oceanic

plates pull apart along the mid-oceanic ridge.

A mid-ocean ridge is a mountain range that winds through Earth’s oceans.

Lava erupts through narrow cracks along the ridge, adding new rock to the ocean floor.

Rift Valley

A rift valley forms when two pieces of continental crust pull apart.

Over millions of years, the rift valley could widen and sink below sea level, forming a new ocean.

Earthquakes

When two plates slide past each other, moving slowly in opposite directions, earthquakes can occur.

Mountains If two plates carrying continental crust collide, neither

plate sinks beneath the other.

Instead, the plates slowly fold and pile on top of each other, forming a mountain range.