Volcanoes are ruptures in the Earth[SQ]s crust that allow hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. They[SQ]re essentially vents or openings that connect the Earth[SQ]s interior to the surface. The eruptions can be explosive or effusive, depending on the magma[SQ]s viscosity (thickness) and gas content. Volcanoes are often found along tectonic plate boundaries, where plates collide or separate, but they can also form at hotspots, where magma plumes rise from deep within the Earth[SQ]s mantle. The landforms they create range from gently sloping shield volcanoes to steep, cone-shaped stratovolcanoes.