A volcano generally begins with a crack in Earth’s crust.
Deep below the surface, a magma chamber, a place where magma has collected, contains the molten rock before the volcano erupts. The magma comes to the surface through a vent, which is a long pipe or tunnel in the rock that acts as a chimney.
In some volcanoes, the magma makes a depression, called a crater, when it reaches the surface. The crater fills with lava, which may harden in the crater or flow down the sides of the volcano.
Over time, as a volcano explodes again and again, the falling lava and debris may build up to form a cone.
Different types of volcanoes have different shapes. Not all volcanoes explode, so not all have cones. Some have only slopes of hardened lava leading up to the crater’s edge, because the lava flows out slowly.