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Earth: Plate Tectonic Theory
There are two types of motions, absolute and relative. Relative motion between the plates, however, is much more involved. The boundaries at which two plates move away, come together, or slide past one another can be classified as divergent, convergent, or transform, respectively. A less common boundary is between three, forming a triple junction. Examples include the Mendocino Triple Junction and the East African Rift Zone.
A divergent boundary occurs as two plates move away from each other. The phenomenon known as seafloor spreading occurs with oceanic crust. On the other hand, two continental plates moving away from each other create rift valleys, where broad areas of land are moved upward. The mid-ocean ridges today are 60,000 km long, making them the largest continuous mountain chain on the planet. An example of an oceanic divergent boundary is the Mid-Atlantic Ridge, which is an underwater mountain range in the Atlantic. Due to plate tectonics, the ridges are spreading out further at a rate of 2.5 cm per year.
A convergent boundary occurs as two plates come together. As this occurs, one plates goes beneath another at a subduction zone. Earthquakes are frequent here. Volcanic ridges and oceanic trenches are also formed. One example in the Pacific Ocean is the Mariana Trench, a chain of active volcanoes. Between an oceanic and continental plate, a continental margin arc and trench systems are made. Earthquake faults and volcanoes are common as well as thrust faults, which are breaks in the Earth's crust. Converging plates involving at least one continental plate also have the characteristic of thickening some of the land at the edges. This increase in thickness forms some mountains and plateaus. One example of a convergent boundary mountain range is the Andes Mountains in South America and the Tibetan Plateaus in Asia.
Plate tectonic theory has helped to explain many phenomena on Earth. The plates have shaped the planet's topology since its formation 4.65 billion years ago. Today, researchers are continuing studies in this area.