Earth’s volcanic hotspots are surprisingly cool

The hotspots that create volcanic islands like those of Hawai’i, Iceland or the Galápagos are surprisingly cool, according to a new study, and may not originate from active lava plumes in Earth’s deep mantle. The findings challenge the classical plume theory of the origin of hotspots. There are two types of volcanism observed on Earth’s surface. The dominant type occurs where tectonic plates meet and is driven by the large-scale circulation of the planet’s mantle. The other type occurs as isolated intraplate “hotspot” volcanoes, which are thought to be fed by hot, active upwelling plumes rising up from the deep mantle, with excess temperatures (T_ex) roughly 100-300 degrees Celsius higher than those located along mid-ocean ridges. However, T_ex estimates for volcanic hotspots are limited in geographical coverage and are often inconsistent for individual hotspots. To determine whether oceanic hotspots are indeed hotter than ridges, Xiyuan Bao and colleagues converted seismic velocity measurements from oceanic ridges and hotspots into temperature. Contrary to previous assumptions, Bao et al. found that some hotspots are surprisingly cool. According to the authors, while ~45% of plume-fed hotspots are hot (T_ex being 155 °C or higher), roughly 40% are not hot enough to actively upwell from the deep mantle. What’s more, 15% are cold (T_ex 36 °C or lower.) Bao et al. suggest that cooler hotspots may instead originate at upper mantle depths or are fed by deep plumes that are entrained and cooled by small-scale convection dynamics.