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.