Convergent boundaries

On 9 June 1994 a magnitude 8.3 earthquake struck 636 km beneath Bolivia, inside a plate that had sunk into the mantle.

Where crust goes back down

The Earth has been about the same size for hundreds of millions of years, so crust made at ridges must be destroyed at the same rate somewhere. That somewhere is a convergent boundary, where plates move towards each other. What happens depends on what kind of lithosphere meets. Oceanic crust is thin and dense, with a basaltic composition. Continental crust is thicker and includes lighter rock like granite. When the two meet, the dense oceanic plate loses: it bends down and sinks, or subducts, into the mantle. The place where it sinks is called a subduction zone.

Where the plate bends down, the sea floor forms a trench. USGS says that if you could drain the Pacific you would see long, curving trenches thousands of kilometres long and 8 to 10 km deep. The deepest, the Challenger Deep at the southern end of the Mariana Trench, goes down nearly 11,000 m, farther below sea level than Mount Everest rises above it. Off South America, the oceanic Nazca Plate is sinking under the South American Plate along the Peru-Chile Trench, and the squeezed edge of the continent has been lifted into the Andes.

Earthquakes that get deeper inland

A sinking plate does not slide smoothly all the way. Its deep part breaks into pieces that lock and then suddenly slip, and the contact between the two plates sticks and lurches too. BGS notes that subduction zones produce most of the earthquakes larger than magnitude 6 and also the deepest ones. Because the earthquakes follow the slab, a map of their depths has a clear pattern: shallow near the trench, deeper and deeper inland, down to about 700 km. The 1994 Bolivian earthquake, 636 km down, was one of the deepest large earthquakes recorded in South America. It was felt as far away as Toronto but did little damage because it was so deep. In the model the dots build up the same tilted line.

Why there are volcanoes

A line of volcanoes stands on the continent, parallel to the trench and some distance inland. In the model the volcano sits above the point where the top of the slab is about 100 km deep. The sinking plate carries water locked in its minerals; as it heats up, the water is driven off and rises into the hot mantle above, which lowers the melting point of that rock so part of it melts. USGS notes that scientists still debate exactly which rock melts, the slab, the plate above or both, but the volcanoes clearly come from subduction. The Andes and the Cascade Range in the Pacific Northwest are both volcanic arcs of this kind.

Two other kinds of convergence complete the picture. When two oceanic plates meet, one sinks under the other and the volcanoes grow up from the sea floor into a curved chain of islands, an island arc, such as the Mariana and Aleutian Islands. When two continents meet, neither sinks, because continental rock is too light. Like two icebergs pushing into each other, the crust crumples and is pushed upward. India ran into Asia about 50 million years ago, and the slow squeeze since then has raised the Himalayas, the highest mountains on land, and the Tibetan Plateau, which averages about 4,600 m.