Earth is round, like an orange. Oranges have a skin and Earth has a skin, too. We call this skin Earth's crust. Under the crust there is very hot rock.
Earth's crust has different pieces. These pieces move very, very slowly. Millions of years ago, the pieces moved and made mountains. Under mountains the crust is trick, but under the ocean it's thinner. When two pieces of the crust move and meet, there can be earthquakes.
A volcano is a hole in Earth's crust. When a vocano erupts, hot rock flies out from under the ground. Volcanoes under the ocean sometimes make new island. In 1963, a volcano in the Atlantic Ocean made e new island called Surtsey.