area around the Pacific Ocean. These scientists were the first ones to use the term "plate tectonics" in print.

The theory of plate tectonics states that the surface of Earth is made up of about eight large and about a dozen small "chunks" called tectonic plates. These plates are between 30 and 90 miles (50 to 150 km) thick. Taken together, they make up the lithosphere, which is the outermost layer of the planet. The lithosphere is made up of rocks from both the crust and the upper mantle. The oceans and the continents we see at the surface are simply riding on the plates below. Some plates, like the one under the Pacific Ocean, just carry oceanic crust. Other plates, like the one under North America, carry both oceanic and continental crust. The plates of the lithosphere move by sliding on top of the semisolid rocks of the asthenosphere below.

Plate boundaries are areas with a large amount of earthquake and volcanic activity. The central areas of tectonic plates are fairly inactive and have very few active volcanoes or earthquakes. Three different types of boundaries separate tectonic plates. A divergent boundary is where two plates are being pushed apart. Divergent boundaries are commonly found at mid-ocean ridges where magma is flowing to the surface to form new oceanic crust. A convergent boundary, on the other hand, is where two plates are coming together. Convergent boundaries are where the process of subduction takes place. These can be found at deep ocean trenches or alongside continents. Most mountains form at either convergent or divergent boundaries.

The last type of boundary is a place where new crust does not form and old crust is not destroyed. At these so-called "neutral boundaries," plates just slide past each other. One of the most famous neutral boundaries is found at the San Andreas fault along the West Coast of the United States where the Pacific plate and the North American plate scrape past each other, triggering lots of earthquakes. Other neutral boundaries occur in the middle of the ocean and appear as transform faults, which cut across mid-ocean ridges and trenches.

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