Volcanoes are commonly found along tectonic plate boundaries like the Ring of Fire around the Pacific Ocean. Magma is melted rock below the Earth's surface that becomes lava when it reaches the surface. Different volcanic features like dikes, sills, and batholiths are formed by magma hardening in vertical cracks or between layers of rock. The three main types of volcanoes are shield, cinder-cone, and composite volcanoes which are distinguished by their shape and eruption patterns. Island chains like Hawaii are formed as tectonic plates move over stationary hotspots of magma or along island arcs where subduction causes magma to rise upward.