The Earths crust is the outermost layer of the Earth. It is covered with continental crust and oceanic crust. Rising plumes of heat from the Earth's core cause the plates to move. The convection currents cause the plates to collide and move apart. Other plates move horizontally past another plate. All plate movement at these boundaries cause both large and small earthquakes.
Continental crust is much older than oceanic crust. The basement rocks of the continents are granitic rocks. Granitic rocks are lighter than oceanic crust rocks. The minerals that make up the crust are primarily quartz and feldspar.
When two crustal plates meet the continental crust is never destroyed. Instead, it overrides oceanic crust or smashes together with another continental crust to form great mountain chains. The Himalaya Mountains are growing as the Eurasian Plate collides with the Indian Plate and the Earths crust is not destroyed.
Oceanic crust is formed from periodite in the upper mantle. When this lava flows out on the ocean floors it forms pillow basalt. Basalt is an iron rich igneous rock made of magnetite, hornblende and other heavy minerals. The basalt flowing out of vents forms pillow basalt that covers all the ocean floors.
Sediment from the continent flows into the oceans and covers the ocean floor with silt and other rock debris. As the layers of sediment deepen they cover the pillow basalt with sedimentary rock.
The density of oceanic plates are approximately 3.3 grams per cubic centimeter. Continental crust is only 2.7 grams per cubic centimeter. When these two different types of plates meet the oceanic plate bends and begins to slip underneath the lighter continental plate forming a trench between the plates where they meet.
Density is why the continental crust always overrides the heavier oceanic crust. The oceanic crust melts as it forced downward beneath the continental crust. The rocks are recycled and form new igneous rocks when volcanoes erupt.