Most volcanoes are cone shaped. Volcanoes are made of many layers of cooled and hardened magma or lava. Even when the lava is hardened, and you're able to walk safely on it, burning hot lava is flowing inside the volcano. Lava that flows out of the volcano and onto the ground, hardens and becomes volcanic rocks. Obsidian, or black glass, is a volcanic rock that is shiny and sharp, but dried up too fast for crystals to grow.

Volcanoes erupt because of the great pressure on the magma inside the earth's crust. The pressure comes from the heat and the earth's crust. when the pressure becomes too great, the magma flows upward looking for an opening to escape the pressure. when an opening is found, the magma flows out. This is a volcanic eruption.