PTR - Plate Boundaries Lesson

 

Plate Boundaries

Interactions between tectonic plates have wide ranging effects on the surface of the Earth. Many of the Earth's major landforms and geologic hazards are due to the movement of these plates.

A convergent boundary occurs when two plates collide with each other. Densities of the plates involved play a key role in the resulting landforms that occur. For example, when an oceanic plate collides with a continental plate, the denser oceanic plate is forced under the less dense continental plate (a good thing for the continents when you think about it!). As the oceanic plate is forced downward into the asthenosphere (a process called subduction), it heats up and melts into molten rock called magma. This molten rock is less dense than the solid crust from which it formed, so it rises towards the surface, providing energy and pressure to power many of the world's active volcanoes.

The Pacific plate is converging on the North American, Filipino, and Australian plates, producing magma that powers a band of volcanoes that stretches along the entire boundary; the Nazca plate converges on the South American plate to complete the "Ring of Fire":

A convergent boundary between two continental plates produces a different result. Since the densities of the two plates are similar, neither plate subducts under the other. Instead, the collision plays out similar to that of a collision between two cars - a lot of crumpling that produces mountain ranges at the boundary of the plates. The collision between the Eurasian plate and the Indian plate produced the Himalayan mountain range, and this collision is actually still occurring; Mount Everest continues to grow several millimeters every year.

Other plates move away from each other. The Pacific and Nazca plates are one example of this type of boundary, as is the boundary between the South American and African plates. At divergent boundaries the crust is stretched apart and ruptures allow magma to rise to the surface. Once cool, this new crust is then stretched apart again, and the cycle continues. This area of constant stretching, pulling, and breaking at a divergent boundary is known as a rift zone.

Most divergent boundaries exist under the oceans, but one well-known divergence is actually found on land. Currents in the asthenosphere are moving the African plate in two different directions, and this process is slowly tearing the continent in two. The Great Rift Valley is the result of these forces; it is home to some of the deepest lakes on Earth, as well as a region of active volcanoes that runs the length of the weakened crust.

The third type of plate boundary occurs when two plates slide alongside of each other. This boundary, known as a transform boundary, does not result in subduction or thinning of the plates, so there is little volcanic activity as a result of this movement. However, the grinding of rock layers several miles thick can produce violent earthquakes, so transform boundaries are definitely a geographic consideration worth monitoring. One of the best known transform boundaries occurs where the Pacific plate interacts with the North American plate in California; coastal California is actually on the Pacific plate, and is sliding north towards Alaska at an average rate of 3-5 cm/yr.

 

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