Introduction to Convergent Plate Boundaries

A convergent plate boundary is a location where two tectonic plates are moving toward each other, often causing one plate to slide below the other (in a process known as subduction). The collision of tectonic plates can result in earthquakes, volcanoes, the formation of mountains, and other geological events.

Key Takeaways: Convergent Plate Boundaries

• When two tectonic plates move toward each other and collide, they form a convergent plate boundary.

• There are three types of convergent plate boundaries: oceanic-oceanic boundaries, oceanic-continental boundaries, and continental-continental boundaries. Each one is unique because of the density of the plates involved.

• Convergent plate boundaries are often the sites of earthquakes, volcanoes, and other significant geological activity.

Earth's surface is made up of two types of lithospheric plates: continental and oceanic. 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 magma flows from mid-ocean ridges.

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

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. 

How Convergent Boundaries Form

Model of the Earth showing the plates on the earth's surface, red dots showing eruptions of volcanoes

 

James Stevenson/Getty Images 

The surface of the Earth is made up of nine major tectonic plates, 10 minor plates, and a much larger number of microplates. These plates float on top of the viscous asthenosphere, the upper layer of Earth's mantle. Because of thermal changes in the mantle, tectonic plates are always moving—through the fastest-moving plate, the Nazca, only travels about 160 millimeters per year.

Where plates meet, they form a variety of different boundaries depending on the direction of their motion. Transform boundaries, for example, are formed where two plates grind against each other as they move in opposite directions. Divergent boundaries are formed where two plates pull apart from each other (the most famous example is the Mid-Atlantic Ridge, where the North American and Eurasian plates diverge). Convergent boundaries are formed wherever two plates move toward each other. In the collision, the denser plate is typically subducted, meaning it slides below the other.

Oceanic-Oceanic Boundaries

Oceanic-oceanic convergent plate boundary.

Domdomegg/Wikimedia Commons/CC BY 4.0

Text labels added by Brooks Mitchell

When two oceanic plates collide, the denser plate sinks below the lighter plate and eventually 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.

When oceanic plates are subducted, they often bend, resulting in the formation of oceanic trenches. These often run parallel to volcanic arcs and extend deep beneath the surrounding terrain. The deepest oceanic trench, the Mariana Trench, is more than 35,000 feet below sea level. It is the result of the Pacific Plate moving beneath the Mariana Plate.

Oceanic-Continental Boundaries

Oceanic-continental convergent plate boundary.

Domdomegg/Wikimedia Commons/CC BY 4.0

Text labels added by Brooks Mitchell

When oceanic and continental plates collide, the oceanic plate undergoes subduction and volcanic arcs arise on land. These volcanoes release lava with chemical traces of the continental crust they rise through. The Cascade Mountains of western North America and the Andes of western South America feature such active volcanoes. So do Italy, Greece, Kamchatka, and New Guinea.

Oceanic plates are denser than continental plates, which means they have a higher subduction potential. They are constantly being pulled into the mantle, where they are melted and recycled into new magma. The oldest oceanic plates are also the coldest, as they have moved away from heat sources such as divergent boundaries and hot spots. This makes them denser and more likely to subduct.

Continental-Continental Boundaries

Continental-continental convergent plate boundary.

Domdomegg/Wikimedia Commons/CC BY 4.0

Text labels added by Brooks Mitchell

Continental-continental convergent boundaries pit large slabs of crust against each other. This results in very little subduction, as most of the rock is too light to be carried very far down into the dense mantle. Instead, the continental crust at these convergent boundaries gets folded, faulted, and thickened, forming great mountain chains of uplifted rock.

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

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