Hello Fifth-Grades: Here's some information on plate tectonics to help you better understand this phenomenon. Remember, the Earth is always changing, always moving. The Earth's rocky crust (both oceanic and continental) is comprised of a series of plates that float on the semi-solid or magma-like mantle. Convection currents cause the magma to rise and fall, therefore, moving the Earth's plates in different directions. This movement is responsible for mountain building, volcanoes, and earthquakes.
Convergent Plates: When two continental plates converge or collide, the crust is squished together, folding and buckling and forming mountains. This is just one way mountains are formed.
Subduction Plates: When one continental plate and one oceanic plate collide, the more dense oceanic plate is subducted or pushed under the continental plate. As it plunges deeper into the mantle, the rock of the oceanic crust melts, becoming part of the mantle. An ocean trench forms at the plate boundaries. Volcanoes and earthquakes may happen here. In the case of volcanoes, molten rock rises until it erupts at the surface.
Divergent Plates: When plates spread apart or diverge, magma rises up from the mantle and fills the gap, creating new crust. As this continues, volcanic islands may form at the surface.
Transform Plates: When plates grind against and past each other in opposite directions earthquakes may strike along these boundaries.
This map shows the continents and oceans and the plates on which they move.