Explore the Deep Sea
Volcanoes & Vents
What happens when tectonic plates move towards each other: subduction, trenches and mountain building.
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.
Immense forces are involved when plates collide.
- As the lower plate melts is pushed down into the mantle and heats up, water and other volatile chemicals are released from it, causing parts of the overlying mantle to melt. Some of the resulting melt rises up through the crust, forming a chain of volcanoes (often these are referred to as island arcs).
- One plate or both can become crumpled on an immense scale (think of a rug crumpling when it is moved up against a wall), leading to the formation of mountain ranges. This has happened as the Indian Plate crashed into the Eurasian plate: the Himalaya mountain range is the result. Here, there is no trench; both plates were carrying continents in the zone of collision. Even today, the Greater Himalaya mountain range, including Mount Everest, is rising at about 5 mm per year as the Indian plate moves northward under Tibet at about 18 mm (three quarters of an inch) per year.
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.