Mantle plumes persevere

A recent census suggests that seamounts1 — typically extinct underwater volcanoes — are numerous. It has been estimated that about 125,000 seamounts with a height of more than one kilometre exist on our ocean floors. Most of these are postulated to form at volcanic hotspots that are the surface expressions of mantle plumes — hot material upwelling from Earth’s interior. Yet, many seamounts do not show the typical characteristics expected for volcanoes that have formed above a mantle plume. So, debate about the feasibility of the mantle plume hypothesis is ongoing. The most straightforward explanation is that not all hotspot volcanoes are alike, and that some groups of seamounts are better explained by mechanisms other than mantle plumes. Morgan’s mantle plumes Some hotspots form seamount trails along the surface of tectonic plates, far away from their volcanically active plate boundaries. Forty years ago, W. Jason Morgan introduced the concept of mantle plumes to explain this kind of hotspot volcanism2,3. According to his theory, plumes of hot material upwell from the deep mantle. During ascent and on impact with the overlying tectonic plates, these plumes drive melting and the production of magma, which erupts to form volcanic seamounts at the plate surface. Morgan proposed that the migration of tectonic plates over stationary and long-lived mantle plumes would generate chains of volcanoes on the ocean floor. However, because mantle plumes themselves cannot be directly sampled and the thin plume conduits are difficult to resolve using seismic data4,5, their existence has been difficult to confirm. Many question whether all hotspot volcanism is formed by mantle plumes6,7 and some doubt whether mantle plumes exist at all8,9. Perhaps the most captivating aspect of Morgan’s plume model is that he could explain the formation of the Hawaiian– Emperor and three other seamount trails along the Pacific Ocean floor. These three trails track each other in such a way that they can be explained by the rotation of a rigid Pacific plate that is drifting over four plumes fixed in the mantle (Fig. 1a). With this observation, Morgan supplied compelling support for the existence of plumes and also provided independent proof for the motion of tectonic plates relative to the underlying mantle. The mantle plume model also opened up potential new avenues of research into the Earth’s deepest regions10. Specifically, if the volcanic seamounts are the surface expression of a mantle plume, their erupted lavas could potentially preserve a record of long-lived variations in mantle composition and could provide insights into mantle convection.