Plate Boundaries - Back at the planet’s surface, the edges where two plates meet are known as plate boundaries. Most geologic activity, including volcanoes, earthquakes, and mountain building, takes place at plate boundaries where two enormous pieces of solid lithosphere interact. Think about two cars moving around a parking lot. In what three ways can those cars move relative to each other? They can move away from each other, they can move toward each other, or they can slide past each other.
These three types of relative motion also define the three types of plate boundaries:
- Divergent plate boundaries: the two plates move away from each other.
- Convergent plate boundaries: the two plates move towards each other.
- Transform plate boundaries: the two plates slip past each other.
What happens at plate boundaries depends on which direction the two plates are moving relative to each other. It also depends on whether the lithosphere on the two sides of the plate boundary is oceanic crust, continental crust, or one piece of each type. The type of plate boundary and the type of crust found on each side of the boundary determines what sort of geologic activity will be found there: earthquakes, volcanoes, or mountain building.
Divergent Plate Boundaries
Plates move apart, or diverge, at mid-ocean ridges where seafloor spreading forms new oceanic lithosphere. At these mid-ocean ridges, lava rises, erupts, and cools. Magma cools more slowly beneath the lava mostly forming the igneous intrusive rock gabbro. The entire ridge system, then, is igneous. Earthquakes are also common at mid-ocean ridges since the movement of magma and oceanic crust result in crustal shaking. Although the vast majority of mid-ocean ridges are located deep below the sea, we can see where the Mid-Atlantic Ridge surfaces at the volcanic island of Iceland.
Although it is uncommon, a divergent plate boundary can also occur within a continent. This is called continental rifting. Magma rises beneath the continent, causing it to thin, break, and ultimately split up. As the continental crust breaks apart, oceanic crust erupts in the void. This is how the Atlantic Ocean formed when Pangaea broke up. The East African Rift is currently splitting eastern Africa away from the African continent.
Convergent Plate Boundaries
What happens when two plates converge depends on the types of crust that are colliding. Convergence can take place between two slabs of continental lithosphere, two slabs of oceanic lithosphere, or between one continental and one oceanic slab. Most often, when two plates collide, one or both are destroyed. When oceanic crust converges with continental crust, the denser oceanic plate plunges beneath the continental plate. This process occurs at the oceanic trenches and is called subduction.
The entire region is known as a subduction zone. Subduction zones have a lot of intense earthquakes and volcanic eruptions. The subducting plate causes melting in the mantle. The magma rises and erupts, creating volcanoes. These volcanoes are found in a line above the subducting plate. The volcanoes are known as a continental arc. The movement of crust and magma causes earthquakes. The Andes Mountains, which line the western edge of South America, are a continental arc. The volcanoes are the result of the Nazca plate subducting beneath the South American plate.
The volcanoes of northeastern California Lassen Peak, Mount Shasta, and Medicine Lake volcano along with the rest of the Cascade Mountains of the Pacific Northwest, are the result of subduction of the Juan de Fuca plate beneath the North American plate. Mount St. Helens, which erupted explosively on May 18, 1980, is the most famous and currently the most active of the Cascades volcanoes.
Sometimes the magma does not rise all the way through the continental crust beneath a volcanic arc. This usually happens if the magma is rich in silica. These viscous magmas form large areas of intrusive igneous rock, called batholiths, which may someday be uplifted to form a mountain range. The Sierra Nevada batholith cooled beneath a volcanic arc roughly 200 million years ago. Similar batholiths are likely forming beneath the Andes and Cascades today.
When two oceanic plates converge, the older, denser plate will sink beneath the other plate and plunge into the mantle. As the plate is pushed deeper into the mantle, it melts, which forms magma. As the magma rises it forms volcanoes in a line known as an island arc, which is a line of volcanic islands. The Japanese, Indonesian, and Philippine islands are examples of island arc volcanoes. The volcanic islands are set off from the mainland in an arc shape as seen in this satellite image of Japan.
When two continental plates collide, they are too thick to subduct. Just like if you put your hands on two sides of a sheet of paper and bring your hands together, the material has nowhere to go but up. Some of the world’s largest mountains ranges are created at continent-continent convergent plate boundaries. In these locations, the crust is too thick for magma to penetrate so there are no volcanoes, but there may be magma.
Metamorphic rocks are common due to the stress the continental crust experiences. As you might think, with enormous slabs of crust smashing together, continent-continent collisions bring on numerous earthquakes. The world’s highest mountains, the Himalayas, are being created by a collision between the Indian and Eurasian plates. The Appalachian Mountains are the remnants of a large mountain range that was created when North America rammed into Eurasia about 250 million years ago.
Transform Plate Boundaries
Transform plate boundaries are seen as transform faults. At these earthquake faults, two plates move past each other in opposite directions. Where transform faults bisect continents, there are massive earthquakes. The world’s most notorious transform fault is the 1,300 kilometer (800 mile) long San Andreas Fault in California. This is where the Pacific and North American plates grind past each other, sometimes with disastrous consequences.
California is very geologically active. A transform plate boundary creates the San Andreas Fault. A convergent plate boundary between an oceanic plate and a continental plate creates the Cascades volcanoes. Just offshore, the Juan de Fuca ridge is subducting beneath the North American plate at a divergent plate boundary.
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