In many places around the world, the tectonic plates that make up the Earth’s crust, or outer layer, are slowly crunching together with enormous force.
The Atlantic is getting wider, pushing the Americas further west. Yet the Earth is not getting any bigger because as the American plates crash into the Pacific plates, the thinner, denser ocean plates are driven down into the Earth’s hot mantle and are destroyed.
The process of driving an ocean plate down into the Earth’s interior is called subduction.
Subduction creates deep ocean trenches typically 6-7 km deep at the point of collision. One of these, the Mariana Trench, could drown Mt Everest with 2 km to spare on top.
As an ocean plate bends down into the Earth’s mantle, it cracks. The movement of these cracks sets off earthquakes originating up to 700 km down. These earthquake zones are called Benioff–Wadati zones after Hugo Benioff, who discovered them in the 1950s.
As an ocean plate slides down, it melts and makes blobs of magma. This magma floats up towards the surface, punching its way through to create a line of volcanoes along the edge of the continental plate.
Subduction creates a ring of volcanoes around the Pacific Ocean called the ‘Ring of Fire’:
If volcanoes in subduction zones emerge in the sea, they form a curving line of volcanic islands called an island arc. Beyond this arc is the back-arc basin, an area of shallow sea that slowly fills up with sediments.
As a subducting plate sinks, the continental plate scrapes sediments off the ocean plate and piles them in a great wedge. Between this wedge and the island arc there may be a fore-arc basin, which is a shallow sea that slowly fills with sediment.
Where two continental plates collide, the plate splits into two layers: a lower layer of dense mantle rock and an upper layer of lighter crustal rock, which is too buoyant to be subducted. As the mantle rock goes down, the crustal rock peels off and crumples against the other to form fold mountains (see mountain ranges).
This is a cross-section through the top 1000 km or so of the Earth’s surface. It shows a subduction zone, where an ocean plate is bent down beneath a continental plate.