Volcanoes
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
Earth's volcanoes occur because its crust is broken into 17 major, rigid tectonic plates that float on a hotter, softer layer in its mantle.[1] Therefore, on Earth, volcanoes are generally found where tectonic plates are diverging or converging. For example, a mid-oceanic ridge, such as the Mid-Atlantic Ridge, has volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates coming together. Volcanoes can also form where there is stretching and thinning of the crust's interior plates
The Theory of Plate Tectonics
The basic theory of plate tectonics is that along seafloor spreading zones, the continents are separating from one another. As they spread apart, magma comes to the surface and becomes new continental crust. As the tectonic plates move away from spreading zones, they collide with one another. In some cases, the edges of two different plates will grind against each other in a horizontal fashion. These areas, called transform boundaries, experience many earthquakes. A well-known transform boundary is the San Andreas fault in California. In other cases, the plates directly collide, forcing one plate upward while the other plate is forced back into the mantle. These collision areas, called convergent boundaries, create mountain ranges. An active convergent boundary is under the Himalayan Mountains, which are being created by the subduction of the Indian crust sliding under the Tibetan crust.
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