Volcanoes form when magma from under the Earth's surface escapes through openings called vents. There are over 600 active volcanoes worldwide, with the most active being Kilauea in Hawaii. Magma is less dense than the surrounding rock so it moves upward towards the surface, eventually erupting and causing effects like lava flows, ash in the air, and new land formations, as well as destruction. Volcanoes occur at divergent plate boundaries where plates move apart, convergent boundaries where they move together, and at hot spots where the Earth's crust is thinner.