All About Convergent Plate Boundaries

When Tectonic Plates Collide

Two types of lithospheric plates, continental and oceanic, make up our Earth's surface. The crust that makes up continental plates is thicker, yet less dense, than oceanic crust because of the lighter rocks and minerals that compose it. Oceanic plates are made up of heavier basalt, the result of magmatic flows from mid-ocean ridges.  

When these plates come together, or converge, they do so in one of three settings: oceanic plates collide with each other (oceanic-oceanic), oceanic plates collide with continental plates (oceanic-continental) or continental plates collide with each other (continental-continental). 

In the first two cases, the more dense plate turns downward and sinks in a process known as subduction. When this occurs at an oceanic-continental plate boundary, the oceanic plate always subducts.

Sinking oceanic plates carry hydrated minerals and surface water with them. As the hydrated minerals are put under increasing pressure, their water content is released through a process known as metamorphic dewatering. This water enters the overlying mantle, lowering the melting point of the surrounding molten rock and forming magma. The magma erupts, and volcanoes form in long curving volcanic arcs.  

Earthquakes are common any time large slabs of Earth come into contact with each other, and convergent boundaries are no exception. In fact, most of the Earth's most powerful quakes have occurred at or near these boundaries. 

Oceanic-Oceanic Boundaries

Oceanic-oceanic convergent plate boundary.
An oceanic-oceanic convergent plate boundary. The defining features of these boundaries are volcanic island arcs and deep ocean trenches. Image by Wikimedia Commons user Domdomegg / licensed under CC-BY-4.0. Text labels added by Brooks Mitchell

When oceanic plates collide, the denser plate sinks below the less-dense plate and eventually, through the process of subduction, forms dark, heavy, basaltic volcanic islands.

The western half of the Pacific Ring of Fire is full of these volcanic island arcs, including the Aleutian, Japanese, Ryukyu, Philippine, Mariana, Solomon and Tonga-Kermadec. The Caribbean and South Sandwich island arcs are found in the Atlantic, while the Indonesian archipelago is a collection of volcanic arcs in the Indian Ocean. 

Ocean trenches occur wherever oceanic plates experience subduction. They form kilometers away from and parallel to volcanic arcs and extend deep beneath the surrounding terrain. The deepest of these, the Mariana Trench, is more than 35,000 feet beneath sea level. It is the result of the Pacific plate moving beneath the Mariana plate. 

Oceanic-Continental Boundaries

Oceanic-continental convergent plate boundary.
An oceanic-continental convergent plate boundary. The defining features of these boundaries are deep ocean trenches and volcanic arcs. Image by Wikimedia Commons user Domdomegg / licensed under CC-BY-4.0. Text labels added by Brooks Mitchell

As oceanic and continental plates collide, the oceanic plate undergoes subduction and volcanic arcs arise on land. These volcanoes have andesitic lavas that bear chemical traces of the continental crust they rise through. The Cascade Mountains of western North America and the Andes of western South America are prime examples with active volcanoes throughout. Italy, Greece, Kamchatka and New Guinea also fit this type.

The density, and thus higher subduction potential, of oceanic plates give them a shorter lifespan than continental plates. They are constantly being pulled into the mantle and recycled into new magma. The oldest oceanic plates are also the coldest, as they have moved away from heat sources like divergent boundaries and hot spots. This makes them more dense and more likely to subduct in an oceanic-oceanic boundary setting. Oceanic plate rocks are never more than 200 million years old, while continental crust rocks over 3 billion years are common. 

Continental-Continental Boundaries

Continental-continental convergent plate boundary.
A continental-continental convergent plate boundary. The defining features of these boundaries are large mountain chains and high plateaus. Image by Wikimedia Commons user Domdomegg / licensed under CC-BY-4.0. Text labels added by Brooks Mitchell

Continental-continental convergent boundaries pit large, buoyant slabs of crust against each other. This results in very little subduction, as the rock is too light to be carried very far into the dense mantle (about 150 km down at most). Instead, the continental crust gets folded, faulted and thickened, forming great mountain chains of uplifted rock. The continental crust may also be cracked in pieces and shoved aside. 

Magma cannot penetrate this thick crust; instead, it cools intrusively and forms granite. Highly metamorphosed rock, like gneiss, is also common. 

The Himalaya and Tibetan Plateau, the result of a 50 million-year-old collision between the Indian and Eurasian plates, are the most spectacular manifestation of this type of boundary. The jagged peaks of the Himalaya are the highest in the world, with Mount Everest reaching 29,029 feet and more than 35 other mountains higher than 25,000 feet. The Tibetan Plateau, which encompasses approximately 1,000 square miles north of the Himalaya, averages around 15,000 feet in elevation. 

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Mitchell, Brooks. "All About Convergent Plate Boundaries." ThoughtCo, Apr. 2, 2017, Mitchell, Brooks. (2017, April 2). All About Convergent Plate Boundaries. Retrieved from Mitchell, Brooks. "All About Convergent Plate Boundaries." ThoughtCo. (accessed June 18, 2018).