Explore the Deep Sea

Volcanoes & Vents

Plates Colliding

What happens when tectonic plates move towards each other: subduction, trenches and mountain building.

Huge trenches

Diagram of tectonic plate subduction.

When tectonic plates with different densities collide, the denser plate sinks beneath the other: a process known as subduction. This happens, for example, when a plate carrying a continent (less dense) rides over an oceanic plate (more dense). The boundary between the two plates forms a trench that can be several miles deep and tens of miles wide. Because plates are so huge, a single trench can be thousands of miles long. All the trenches on Earth are found in oceans.

Mountain building

Diagram of tectonic plate subduction.

Immense forces are involved when plates collide.

Earthquake zones

The world's strongest earthquakes, as well as the deepest, have been recorded from zones of plate collision. Such earthquakes occur along the western coasts of the U.S. and Canada, where the North American plate is riding over several smaller plates. Earthquakes in the ocean floor can cause huge waves, or tsunamis, which can be devastating when they wash onto nearby land.

Next: Stretching: basins