Tectonic Plate Boundaries

Plate boundaries are the edges where two plates meet. Most geologic activities, including volcanoes, earthquakes, and mountain building, take place at plate boundaries. How can two plates move relative to each other?

  • Divergent plate boundaries: the two plates move away from each other.
  • Convergent plate boundaries: the two plates move toward each other.
  • Transform plate boundaries: the two plates slip past each other.


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.(In addition to the discussion below, click here for an explanation of plate boundaries and motions from the USGS.)

Divergent Plate Boundaries

 

Plates move apart at mid-ocean ridges where new seafloor forms. Between the two plates is a rift valley. Lava flows at the surface cool rapidly to become basalt, but deeper in the crust, magma cools more slowly to form gabbro. So the entire ridge system is made up of igneous rock that is either extrusive or intrusive. Earthquakes are common at mid-ocean ridges since the movement of magma and oceanic crust results in crustal shaking. The vast majority of mid-ocean ridges are located deep below the sea. Here is an animation of a divergent plate boundary at a mid-ocean ridge. As divergence occurs, shallow earthquakes can occur along with volcanoes along the rift areas. When the process begins, a valley will develop, such as the Great Rift Valley in Africa. Over time that valley can fill up with water creating linear lakes. If divergence continues, a sea can form, such as the Red Sea, and eventually an ocean, such as the Atlantic Ocean. Look at the eastern half of Africa and notice the lakes that are linear. Each of these lakes is located in a rift valley. Eastern Africa is tearing apart from these linear lakes, to the Great Rift Valley, and up to the Red Sea. The ultimate divergent boundary is the mid-Atlantic Ridge in the middle of the Atlantic Ocean.

Satellite view of northeast Africa (Egypt, Sudan, and Eritrea) and Saudi Arabia. The Red Sea is an elongated body of water between the two continents

Convergent Plate Boundaries

 

When two plates converge, the result depends on the type of lithosphere the plates are made of. No matter what, smashing two enormous slabs of lithosphere together results in the creation of magma and earthquakes. Ocean-to-continent convergence occurs when oceanic crust converges with continental crust, forcing the denser oceanic plate to plunge beneath the continental plate. This process, called subduction, occurs along oceanic trenches called subduction zones where lots of intense earthquakes and volcanic eruptions can occur. The denser, subducting plate begins to heat up under extreme pressure near the mantle and melts. The magma then rises to create volcanoes. These coastal volcanic mountains are found in a line above the subducting plate. The volcanoes are known as a continental arc. 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. This is called the Cascadia Subduction Zone. The movement of crust and magma often causes earthquakes. The Juan de Fuca plate is created by seafloor spreading just offshore at the Juan de Fuca ridge.

Edge of the pacific plate along Asia and North America. The edge of the plate is visible as a deep oceanic trench. The Mariana trench is found along this boundary. An oceanic-to-oceanic plate boundary occurs when two oceanic plates converge, causing the older, denser plate to subduct into the mantle. An ocean trench marks the location where the plate is pushed down into the mantle. The line of volcanoes that grows on the upper oceanic plate is an island arc. The Ring of Fire is a ring around the Pacific Ocean of subduction zones, most of which are oceanic-to-oceanic convergence. Along these subduction zones, volcanic islands (also called volcanic arcs) form. Examples of volcanic arcs include Japan, Indonesia, and the Aleutian Islands.

When two continental plates converge, instead of subduction, the two similar tectonic plates will buckle up to create large mountain ranges like a massive car pile-up. This is called continental-to-continental convergence, and geologically creates intense folding and faulting rather than volcanic activity. Examples of mountain ranges created by this process are the Himalayan mountains where India is colliding with Asia, the Alps in Europe, and the Appalachian Mountains in the United States. The Kashmir, India earthquake of 2005 that killed over 80,000 people occurred because of this process. More recently, the 2008 earthquake in China which killed nearly 85,000 people before the Summer Olympics was because of this tectonic force. The Appalachian Mountains are the remnants of a large mountain range that was created when North America rammed into Eurasia.

Transform Plate Boundaries

Transform plate boundaries occur when two tectonic plates slide (or grind) past parallel to each other. The most famous transform boundary is the San Andreas Fault where the Pacific plate that Los Angeles and Hawaii are on is grinding past the North American plate that San Francisco and the rest of the United States is on at the rate of 3 inches a year. Recently, geologists have stated that San Francisco should expect another disastrous earthquake in the next 30 years. Another important transform boundary is the North Anatolian Fault in Turkey. This powerful fault last ruptured in 1999 in Izmit, Turkey, killing 17,000 people in 48 seconds.

Comparison of the North Anatolian (Turkey) and San Andreas (California) faults. They share slip rates, ages, length, and straightness. The fault slip rate is 20 to 28 millimeters per year. The earthquake sequence in Turkey spans from 1939 to 1999. In California, there are two notable earthquakes: 1857 and 1906. The fault in Turkey runs east to west, while the fault in California runs north to south. To make the comparison more apparent, California has been rotated 90 degrees in the image.

Intraplate Activity

Hawaiian Islands A small amount of geologic activity, known as intraplate activity, does not take place at plate boundaries but within a plate instead. Mantle plumes are pipes of hot rock that rise through the mantle. The release of pressure causes melting near the surface to form a hotspot. Eruptions at the hotspot create a volcano. Hotspot volcanoes are found in a line. The youngest volcano sits above the hotspot and volcanoes become older with distance from the hotspot. A well-known example is the Hawaiian Islands. Geologists use some hotspot chains to tell the direction and the speed a plate is moving. Hotspot magmas rarely penetrate through thick continental crust. One exception is the Yellowstone hotspot.

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