Intraplate volcanism refers to volcanic activity that occurs far from the edges of tectonic plates, deep within the interior of a plate. This is considered anomalous because most of the world’s volcanism is explained by the interactions between plates at their boundaries. The presence of volcanoes within a plate’s stable interior suggests a mechanism independent of the large-scale movement of the lithosphere. This type of volcanism hints at dynamic processes deeper within the Earth’s mantle that create localized thermal anomalies.
Volcanism at Plate Boundaries
Most of the Earth’s volcanic activity follows a predictable pattern, occurring where tectonic plates meet. At divergent plate boundaries, such as mid-ocean ridges, two plates move away from each other.
This separation causes the hot mantle rock beneath to rise and experience a sudden drop in pressure, leading to a process known as decompression melting. The reduction in pressure lowers the melting temperature of the rock, generating large volumes of magma that erupt to form new oceanic crust.
This mechanism accounts for the vast majority of volcanic output on Earth. In contrast, volcanism at convergent boundaries, where one plate slides beneath another in a subduction zone, is driven by a different process. As the subducting oceanic plate descends, it releases volatile compounds, primarily water, which rise into the overlying mantle rock. This causes flux melting, where the addition of water lowers the melting point of the rock, generating magma that forms volcanic arcs.
The Driving Force: Mantle Plumes
Intraplate volcanism requires a separate explanation detached from the mechanical stress of plate interactions. The prevailing theory involves the concept of a mantle plume: an upwelling column of unusually hot rock that originates from deep within the Earth, potentially near the core-mantle boundary. These plumes are distinct from the shallower convection currents that drive plate tectonics.
The rising column of hot material is imagined to have a bulbous head and a narrow tail, or conduit, feeding it from below. As the plume head nears the rigid lithosphere, the surrounding pressure decreases significantly.
This pressure release causes decompression melting to occur, similar to the process at divergent boundaries but driven by the plume’s ascent. This massive volume of newly generated magma then breaches the crust, creating a volcanic center on the surface. The mantle plume mechanism is a thermal anomaly, meaning the magma formation is due to the rock being hotter than its surroundings and rising.
Hotspots and Intraplate Volcanic Provinces
The surface expression of a persistent mantle plume is known as a “hotspot,” which acts as a stationary source of magma beneath the moving tectonic plates. The classic evidence for this model is the existence of linear chains of volcanoes that show a distinct age progression. As the overlying plate moves across the fixed hotspot, the magma continually punches through the plate. This results in a chain where the active volcano is located directly above the plume, and volcanoes become progressively older and more eroded farther away in the direction of plate movement.
The Hawaiian-Emperor seamount chain demonstrates this perfectly, with the youngest, active volcanoes located over the Hawaiian Hotspot and the seamounts becoming millions of years older to the northwest. Age dating of these volcanic rocks allows scientists to calculate the absolute velocity and direction of the plate’s motion.
Intraplate volcanism also manifests in continental settings, such as the Yellowstone Hotspot in North America. Here, the North American plate moves over the plume, creating a trail of progressively older, massive calderas stretching across Idaho and into Oregon. Particularly large mantle plumes can also produce Large Igneous Provinces (LIPs), which are enormous outpourings of flood basalt lava covering vast areas, like the Deccan Traps in India.