When tectonic plates collide and form a convergent boundary, their interaction can produce earthquakes, volcanic activity, underwater trenches and mountain formations. Any resulting geologic events on the Earth's surface are determined by the type of tectonic plates involved in the collision: oceanic crust, continental crust or a combination of both.
The Earth's outer shell, or lithosphere, is formed from several hard tectonic plates that drift over currents of molten rock. Plates boundaries are the contact points where two tectonic plates interact, such as crashing together or spreading part. Plates covered by continental crust, or landmasses, are buoyant, while plates covered with oceanic crust are extremely dense.
An oceanic-continental collision can produce trenches and volcanic mountains during a process known as subduction. Since oceanic crust is dense, the plate edge sinks beneath the continental crust and melts in the Earth's magma layer, which is known as the asthenosphere. An oceanic trench is formed at the deep impression where the two plates meet. In some cases, the submerged oceanic crust causes a buildup and release of high-pressure magma, which pushes above the surface to shape mountains.
The convergence of two continental plates typically causes the edge of one plate to be wedged under the other. The buoyancy of continental crust prevents either plate from sinking, so bulky sections of rock are shoved upward to form mountains. In a convergence of two oceanic crusts, the older plate has a higher density and sinks beneath the younger plate. This type of collision also produces subduction, causing the formation of volcanic islands.