Subduction zones

Along a convergent margin, two plates move towards each other. This is obviously going to create a problem with where to put the edges of the plates. Generally, the denser plate goes down into the mantle (subducts), where it is eventually absorbed. If one plate is of continental crust and the other is oceanic crust, the oceanic plate subducts, as it is the denser plate. If both plates are of oceanic crust, the older plate subducts, because as a plate gets older, it cools and contracts, becoming denser. If both plates are of continental crust, neither can subduct, as continental crust is of too low density to subduct. In this case, a collision zone forms, creating very large mountain ranges, such as the Alps and the Himalayas.

Trenches

Where the two plates meet, the subducting plate bends as it begins to go down. This creates a very deep trough on the sea floor, called a trench. All the deepest parts of the oceans occur in such trenches, such as the Marianas trench, the Peru-Chile trench, the Japan trench, etc.

As with any basin or trough, trenches tend to accumulate sediment. Trenches accumulate more than most because they are right next to the overriding plate, which has a substantial mountain belt along its leading edge. These mountains are eroding, which provides a major source of sediment. This accumulation of sediment in the trench gets disrupted by the bulldozer action of the overriding plate against the subducting plate. As a result, the chaotic mix is called melange, for its stirred aspect.

Earthquakes

As you might suspect, two plates grinding and scraping together as they converge generate a lot of earthquakes. Convergent margins can generate larger earthquakes than any other plate tectonic setting. This is true for two reasons: 1) rocks are stronger in compression than tension, and 2) the plates are thickened along the margin. Both factors contribute to larger earthquakes because the stronger the rock is, the more energy it takes to break it. The energy that goes into it is released when it breaks, generating the earthquake. So, as more energy goes in, the larger the resulting earthquake.

Earthquakes occur in rocks that are relatively cold, therefore brittle. The subducting slab is quite cold, having sat around as the sea floor for a couple hundred million years, and rocks conduct heat poorly, so they heat up slowly. Thus, the subducted plate stays cold enough to generate earthquakes until it is quite deep in the mantle. These earthquakes have been recorded as deep as 700 km. Hugo Benioff noticed that earthquake sources get deeper under the overriding plate proceeding away from the trench. He realized that this inclined array of earthquake sources indicate the position of the portion of the plate that has already been subducted. Thus, that pattern of earthquakes is known as a Benioff zone.

Mountain belts

The overriding plate buckles and crumples along its leading edge, forming large mountain belts of folded and faulted rocks. The Andes and the Canadian Rockies are examples of this type of mountain belt. Built on top of these already substantial mountains are a series of volcanoes, discussed next.

Volcanic activity

As the subducting plate goes down, it carries with it small amounts of water from the sea floor. This water acts exactly like antifreeze on the rocks it encounters in the subduction zone. It lowers the melting point of those rocks, partly melting them, which generates bodies of molten rock (magma). This magma is less dense than the surrounding solid rock, so it rises buoyantly through the crust (a slow, complicated process). Some of it rises as far as the surface, where it can erupt (molten rock on the surface is called lava). It cools and crystallizes, forming volcanoes made of a rock called andesite (the name given to this composition of rock). The magma that does not make it to the surface eventually cools and crystallizes as well, but does so very slowly, so the resulting rock looks quite different than the quickly cooled andesite. Because it looks so different, it gets its own name (diorite). More on that in the Igneous Rocks Lab.

Magma with an andesitic composition produces fairly violent eruptions. Some recent examples include Mt. St. Helens in the U.S. (1980), Mt. Pinatubo in the Philippines (1992), and Montserrat in the Caribbean (ongoing).