By Doug Bennett | Updated Mar 24, 2022
The vast majority of volcanic activity occurs where tectonic plates collide at convergent boundaries or spread apart at divergent boundaries. Yet a distinct class of volcanoes forms within the plates themselves, independent of plate edges. These inter‑plate, or hotspot, volcanoes arise from deep‑mantle heat sources known as thermal plumes.
Hotspot volcanoes are fueled by localized upwellings of exceptionally hot magma from the lower asthenosphere. Unlike the cooler lithospheric rock, this magma melts surrounding crust, creating magma chambers that, if fed to the surface, produce a sequence of volcanoes as the overlying plate drifts over the stationary plume. The age progression of the chain pinpoints the hotspot’s location and the plate’s motion.
Under oceanic plates, the resulting magma is basaltic, low‑viscosity and low in water content. Such magma produces fluid lava flows that build broad, gently sloping shield volcanoes. Because pressure does not accumulate, these volcanoes typically erupt as continuous, gentle lava effusions. Mauna Loa and Kilauea in the Hawaiian chain exemplify inter‑oceanic hotspot volcanoes.
When a thermal plume rises beneath continental crust, the melt is rich in silica, producing thick, viscous felsic magma. Pressure builds until the overlying crust fractures, releasing gas rapidly and causing explosive eruptions that empty the magma chamber and collapse the summit into a caldera. These powerful eruptions are classified as super‑volcanoes. Yellowstone is the most well‑known inter‑continental hotspot super‑volcano.
Super‑volcano eruptions eject vast volumes of pyroclastic material that can travel hundreds of miles. The 640,000‑year‑old Yellowstone eruption released approximately 250 cubic miles of ash—about 8,000 times the 1980 Mount St. Helens event—while the 2.1‑million‑year‑old event ejected 588 cubic miles, nearly 20,000 times that magnitude. Such eruptions inject enormous ash clouds into the atmosphere, causing global cooling. For context, Mount St. Helens’ crater covers 2 square miles, whereas Yellowstone’s caldera spans 1,500 square miles.
