Tectonic Plates
Tectonic plates are pieces of the lithosphere that slide around on top of the asthenosphere. Tectonic plates consists not only of the crust, but also part of the upper mantle. They can also contain continental crust, oceanic crust, or both. Some of the major tectonic plates are the Pacific, North American, Cocos, Nazca, South American, African, Eurasian, Indian, Australian, and Antarctic plates. A hypothesis, stated by Alfred Wegener, says that all the continents once formed a huge, single landmass, but they broke up and floated to their current positions. This hypothesis is Continental drift. This explains why the continents fit so well together and why different animal fossils were found on the different continents. The first huge continent was called Pangaea, which split into Laurasia and Gondwana. Then, it came into the current continents that we have today. This is all possible because of sea-floor spreading, which is the process in which magma rises to the surface and solidifies, then it forms the new oceanic lithosphere. Mid-ocean ridges are the places where sea-floor spreading is taking place. As the tectonic plates move away from each other, newer crust forms. Plate tectonics is the theory that the Earth's lithosphere is divided into different tectonic plates that float on top of the asthenosphere. Tectonic plates can slide past, collide, or separate form each other. Some scientists even use the GPS, or the Global Positioning System, to measure the rate of tectonic plates moving. A fault is an area of the Earth's surface where rocks break and slide past each other. There are three types of faults, which are normal faults, reverse faults, and strike-slip faults. Uplift is when regions of the Earth's crust rise to higher elevations, but subsidence is when regions of the Earth's crust sink into lower elevations. Tectonic plates move around on top of the Earth and can sometimes run into one another. (Holt Science & Technology, Earth Science, Pages 194-212)