New research shows hotspots are broad and deeply rooted

Sketch diagram of Hawaii and the hotspot responsible for it. "Hawaii hotspot cross-sectional diagram" by Joel E. Robinson, USGS. Copied from https://commons.wikimedia.org/wiki/File:Hawaii_hotspot_cross-sectional_diagram.jpg#/media/File:Hawaii_hotspot_cross-sectional_diagram.jpg

Several major questions remain about volcanic hotspots: how deeply are they rooted within the mantle – do they originate from partway through the mantle, or do they come from the core-mantle boundary? What shape are they in the mantle? Do they rise vertically or are they affected by mantle currents? These questions have been thoroughly debated, but studies of the subsurface lack the clarity to resolve this problem. New research appears to hold the answer to these questions: mantle plumes rise vertically from the core-mantle boundary, and are broader than previously thought.

Previous studies using seismic tomography weren’t very good at resolving the structure of plumes deep within the mantle. Seismic wave tomography combines signals from earthquakes across the globe, and shows where seismic waves travel more slowly, or more quickly, than normal. Waves travel more rapidly through solid material, and more slowly through more liquid material. Mantle plumes usually show up well because they are warm and fluid, so seismic waves travel more slowly through them than they do the surrounding mantle.

A new study by Drs. French and Romanowicz from UC Berkeley uses data not just from many earthquakes, but also from many different seismometers across the globe. This allowed the researchers to use many different types of seismic waves in their calculations, enabling them to have a significantly better resolution deep within the Earth, and see the deep parts of the mantle plumes with much more clarity.

This cross-section of the earth shows different seismic wave velocities, with blue being fast and red being slow. An area of slow seismic velocities occurs under Hawaii, and appears to connect stretch down to the core-mantle boundary. This suggests that the mantle plume responsible for the Hawaiian volcanic chain is broad and deeply rooted. (Adapted from figure 1 from French and Romanowicz, 2015)

This new technique shows that mantle plumes start at the core-mantle boundary, about 1798 miles beneath our feet. At the base of the plume, there is an area where seismic waves travel more slowly – this indicates that the wave is travelling through a warmer, more liquid area of the mantle. The added resolution of this data set shows that the mantle plume rises vertically from these low-velocity areas through the mantle. Once the plume reaches 621 miles below the Earth’s surface, it is frequently deflected by circulation patterns in the upper mantle. The crust above these plumes begins to melt, and creates massive volcanoes like those in Hawaii.

A three-dimensional model of the large mantle plume, also referred to as the Pacific Superswell, that lies beneath Hawaii and several other nearby islands (indicated by green dots). Separate panels in the figure show cutaways at different depths in the mantle. This shows the complex and broad nature of a mantle plume at different depths. (Figure 2 from French and Romanowicz, 2015)

Unfortunately, the mantle plume for some hotspots is more difficult to image, even using this new technique. Due to small velocity differences between the plume and the surrounding mantle, researchers were still unable to see the mantle plume in some locations – including under Yellowstone.

What does this mean for our knowledge of hotspots? This new research shows that mantle plumes are wider than previously thought. This is more realistic than previous models that predicted a skinny (<200 km) mantle plume; a broad mantle plume (800 – 1,000 km) is more consistent with temperature estimates in the mantle. Additionally, these results show that mantle plumes are sourced at the core-mantle boundary. This result will help us better understand hotspot dynamics, including how hotspots interact with the Earth’s crust and create volcanic systems.