The Earth has two crusts called the continental and oceanic. On average, the continental crust is 25 miles thick and the oceanic crust is about four miles thick.
The crusts are broken up into 15 tectonic plates that cause earthquakes and volcanoes and create mountains as they move. The oceanic crust is made of more dense rocks than the continental crust. The different density between the two crusts causes the oceanic crust to slide underneath the continental crust, which forms volcanoes in a process called subduction. Over a period of millions of years, the oceanic crust is forced down far enough to enter the Earth's mantle, causing it to turn into magma and form a new crust.