On the Earth’s surface, scientists have observed two types of volcanism. The dominant type, observed at tectonic plate boundaries, manifests the large-scale global circulation in the earth’s mantle. In contrast, isolated intraplate volcanoes, such as those of Hawaii, Iceland or Galapagos, do not fit with classical plate tectonics theory, and are thought to reflect dynamical processes rooted in the deep mantle. These hotspots usually appear as chains of intraplate volcanoes that may reflect relative movement between plates and the underlying mantle. The lavas found at these hotspot volcanoes provide a unique window into the thermo-chemical dynamics of Earth’s interior.

Hotspots are thought to be fed by hot, active upwellings from the deep mantle, with excess temperatures (Tex) ~100-300 °C higher than mid-ocean ridges. However, Tex estimates are limited in geographical coverage and often inconsistent for individual hotspots.

A research team led by Carolina Lithgow-Bertelloni, who holds UCLA’s Louis B and Martha B Slichter Chair in the Geosciences in the Department of Earth, Planetary, and Space Sciences, and EPSS graduate student Xiyuan Bao, have identified a new class of cold hotspots that do not fit classical plume theory by inferring mantle plume temperature from global seismic models. The researchers inferred the temperature of oceanic hotspots and ridges simultaneously by converting seismic velocity to temperature in new research published Jan. 6 in the journal Science.

They show that while approximately 45% of plume-fed hotspots are hot (Tex ≥ 155 °C), approximately 15% are cold (Tex ≤ 36 °C) and approximately 40% not hot enough to actively upwell (50 ≤ Tex ≤ 136 °C). Hot hotspots have extremely high 3He/4He ratios, which are a signature of the very deepest mantle, and buoyancy flux, but cold hotspots do not. Cold hotspots may instead originate at upper mantle depths. Alternatively, the deep plumes that feed them may be entrained and cooled by small-scale convection, the researchers report.

The presence of active, hot, upwelling plumes rising from the core-mantle boundary to the bottom of the lithosphere under hotspot volcanoes has been hotly debated since it was proposed 50 years ago. The idea of hot mantle plumes originating in the deep mantle that sample sources distinct from those that give rise to mid-ocean ridge volcanism reconciles many geophysical and geochemical observations. For example, seismic studies showing an expected thinner mantle transition zone under hotspots, and low-velocity columns in tomographic models that extend from the surface to the CMB beneath most hotspots, suggest that plumes can extend well into the deep mantle. That some of these may be cold, rather than hot, is a new finding.

The researchers compiled a dataset, extracted velocities for ridges and hotspots, velocity to temperature conversions, and inferred temperatures along with comparisons to other global seismic models.

Co-authors are Barbara Romanowicz of UC Berkeley’s Department of Earth and Planetary Science and Matthew Jackson of UC Santa Barbara’s Department of Earth Science.

Their research was funded by the National Science Foundation (grant EAR-1900633) and the Louis B. and Martha B. Slichter Endowment for Geosciences.

Much research has been devoted to determining whether ridges near hotspots are hotter than far-from-hotspot ridges.