A volcano is a rupture in the Earth’s crust that allows magma, ash, and gases to escape from a magma chamber below the surface. This molten rock is called magma underground and lava once extruded onto the surface. The distribution of these vents follows specific geological patterns, tied to the motion of tectonic plates. The conditions required to melt rock and force it to the surface only occur in three primary settings.
Volcanoes at Convergent Plate Boundaries
Volcanism occurs most commonly where two tectonic plates collide at subduction zones. Here, one denser oceanic plate slides beneath a less-dense plate. As the subducting plate descends into the hotter mantle, it releases water and volatile fluids locked within its minerals.
The introduction of this water into the hot mantle rock significantly lowers the rock’s melting temperature. This process, known as flux melting, generates buoyant magma that rises through the overlying crust. The resulting chain of volcanoes forms a volcanic arc parallel to the plate boundary. If the overriding plate is continental, a continental arc forms, such as the Andes Mountains.
If the collision involves two oceanic plates, the volcanoes form an island arc, like the Aleutian Islands or Japan. This volcanism accounts for the majority of the world’s active volcanoes, encircling the Pacific Ocean in the “Ring of Fire.” Volcanoes here are often explosive because the magma tends to be thicker, trapping gases and building up considerable pressure before eruption.
Volcanoes at Divergent Plate Boundaries
Volcanism also occurs where tectonic plates are pulling apart at a divergent plate boundary. As the plates separate, hot mantle rock rises into the space created by the thinning crust. This upward movement reduces the pressure on the hot rock, a second mechanism for generating magma.
This process is called decompression melting and produces large volumes of basaltic magma. The vast majority of this activity happens beneath the oceans along the Mid-Ocean Ridge system, the longest mountain range on Earth. Here, magma erupts on the seafloor to create new oceanic crust, known as seafloor spreading.
On continents, divergent boundaries form continental rift zones where the crust is being stretched and thinned. A prominent example is the East African Rift Valley, where volcanism is present as the landmass slowly splits apart. Unlike the explosive eruptions at subduction zones, volcanism at divergent boundaries is non-explosive, characterized by gentle outpourings of fluid lava through fissures.
Volcanic Activity Away from Plate Edges (Hotspots)
Not all volcanoes occur along plate boundaries; some are found in the middle of tectonic plates. These intraplate volcanoes are caused by stationary plumes of abnormally hot rock, called mantle plumes, rising from deep within the Earth’s mantle. These plumes create a localized source of melting known as a hotspot.
As the tectonic plate moves across this fixed hotspot, magma repeatedly punches through the crust to form a linear chain of volcanoes. The Hawaiian Islands and the Emperor Seamount chain provide the example. Active volcanoes are always located directly above the plume, while older, extinct volcanoes form a progressively older trail as the plate carries them away.
The Hawaiian-Emperor chain, over 6,200 kilometers long, demonstrates this movement. The Big Island of Hawai’i is the youngest and most active part. This mechanism allows geologists to track the past direction and speed of the tectonic plate’s movement. Decompression melting is the source of magma for these hotspots, as the buoyant plume rises and the pressure on the rock lessens.