Volcanoes are openings or vents in the Earth's crust through which lava, ash, and gases are ejected from below the surface. There are different types of volcanoes classified based on their shape and the material they erupt, such as shield volcanoes which are wide and gently sloping while stratovolcanoes are steep-sided cones built up by viscous lava flows alternating with explosive eruptions. Volcanic eruptions can have devastating impacts through lava flows, explosive eruptions, pyroclastic flows, lahars, and release of gases like sulfur dioxide.