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Home > Earth > Plate Tectonics > Plate Boundaries

Plate Boundaries

At the same time, some of the oldest ocean crust occurs in deep sea trenches, which run parallel to continental mountain ranges. A lot of very large earthquakes have been plotted along deep ocean trenches, suggesting that these are seismically active areas (meaning the crust is moving). Scientists put two and two together, noting that the youngest oceanic crust was along the mid-ocean ridges and the oldest ocean crust was along the very bottoms of deep sea trenches. That neatly defined the edges of the tectonic plates and showed the direction of their movement. Where the deep sea trenches were found converging boundaries.

A Converging Boundary is the opposite of a spreading boundary. Typically you will see a converging boundary on a tectonic plate that is on the opposite side of a spreading boundary. As a plate moves in one direction it collides with the adjacent plate on its "front" end in a deep sea trench, while the trailing end of the plate is being pulled and stretched (spreading) from the plate on the other end at a mid-ocean ridge. For example, look at the Pacific plate (click to enlarge the plate tectonics map). The entire plate is moving north and westward as the top edge converges with the North American and European plates. You can see the left side of the Pacific plate is converging with the Indian plate. Then if you look at the bottom and right edges of the plate you can see it's spreading apart from the Antarctic and Nazca plates.

Sometimes you'll see volcanic activity at converging boundaries where plates are crashing into each other. When one plate (usually the lighter continental crust) rides up over the top of the other it's called a subduction zone - because one plate margin is being subducted under the other.

Plate tectonics mapA good example of this type of plate margin is where the Nazca and South American plates are crashing into each other. The lighter continental South American plate is riding up over the heavier oceanic Nazca plate. Deep down where the leading edge of the Nazca plate is diving down under the South American plate it's making contact with the molten magma of the earth's mantle. The long cordillera, or chord-like chain of volcanic mountains known as the Andes, are a result of the rumpling of the South American plate where the Nazca plate crashes into it, and the volcanoes that have formed from the increased seismic activity on the Nazca plate margin deep down.

In other converging boundaries, there is no volcanic activity because the tectonic plates are both continental plates, weighing the same. No subduction happens along these margins, just massive deformation of the edges of the plates. A good example of this is the Himalayan Mountains where the European and Indian plates meet. The two plates have continued ramming into each other, causing the crust to buckle, wrinkle, and uplift into the highest mountain range on earth.

The few transverse boundaries are places where the two plates are just sliding past each other. In many of these boundaries there is a lot of tension and strain where the two plates are sliding and scraping past each other. The resulting strain from the sliding action of the plates causes cracks in the crust called faults. As the larger plates move past each other some chunks of crust and overlying rock are broken into fault blocks. When there is a big enough movement along the cracks or faults in the earth's crust we feel it in the form of earthquakes.

One of the most famous faults is the San Andreas, which runs along the west coast of California. It's famous for generating many of the larger quakes in California, including the world-renowned San Francisco earthquake of 1906. Funny thing is, the 1906 earthquake itself didn't do nearly as much damage as the fires that burned the city afterwards - all the water mains had burst and broken during the 'quake so there was no water to put out the fires!

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