Fixed Flames Beneath Moving Plates
In 1963, J. Tuzo Wilson proposed that the Hawaiian island chain formed as the Pacific plate drifted over a stationary source of magma — a 'hotspot' fixed deep in the mantle. This elegant idea explained why the islands get progressively older to the northwest and why only the Big Island of Hawaii (over the plume) is volcanically active today. The concept was extended by Jason Morgan in 1971, who proposed that hotspots are the surface expressions of deep mantle plumes.
Anatomy of a Mantle Plume
A mantle plume is modeled as a narrow conduit (~100–200 km diameter) of anomalously hot material rising from the core-mantle boundary, 2,900 km deep. The plume head, which can be 500–1000 km across, produces massive volcanism when it first reaches the lithosphere — potentially triggering continental flood basalts like the Deccan Traps. The narrower plume tail maintains volcanism for tens of millions of years, creating the linear island chain.
The Hawaiian-Emperor Chain
The Hawaiian-Emperor seamount chain stretches over 6,000 km across the Pacific, from the active Kilauea volcano to the 80-million-year-old Meiji seamount near the Aleutian Trench. The chain records the Pacific plate's motion: northwest at ~7 cm/yr for the last 47 Myr (Hawaiian segment), and more northward before that (Emperor segment). The famous 60° bend at 47 Ma marks a major reorganization of plate motion, possibly triggered by the India-Asia collision.
Subsidence and Coral Atolls
Once a volcano moves off the hotspot, it ceases erupting and begins to subside as the underlying lithosphere cools. The volcano erodes, creating a flat-topped seamount (guyot). In tropical waters, coral growth can keep pace with subsidence, building a fringing reef that evolves into a barrier reef and finally an atoll — a ring of coral surrounding a lagoon where the volcanic island once stood. Darwin first proposed this sequence in 1842, and it was dramatically confirmed by drilling at Enewetak Atoll in 1952.