The gigantic accumulations of molten rock that feed isolated volcanoes such as Hawaii, Iceland or the Yellowstone supervolcano are often surprisingly cool. This calls into question the 50-year-old hypothesis about its origin. This is the result of a working group led by Xiyuan Bao from the University of California in Los Angeles after seismic measurements that provide information about the temperature in the subsurface. As the team reports in »Science«, about 40 percent of the hotspots are too cool to rise from the deep mantle due to their buoyancy. Thus, it is at least unclear whether they emanate from the boundary between the Earth's core and mantle, as the classical theory requires.
A hotspot consists of a canal through which material rises from a large depth and forms a mushroom -like bladder under the earth's crust, from which the volcanoes dine over it. The working group around Bao measured the speed of seismic waves in the coat below the hotspot material and calculated its temperature from it. This is heated up by the hotspot, but is more uniform, so that influences such as the composition do not disturb the temperature measurement. From the temperature calculated in this way, the team closed how hot the hotspot material is itself. In order to rise quickly enough from large depths, the material that has been pushed from below must be at least about 100 to 150 degrees more hotter than the melt used as a comparison of the Central Ocean back.
However, this is only the case with around 45 percent of hotspots. The experts point out that there are already assumptions behind this that favor a high temperature difference, so that even more hotspots could actually be cooler than expected. From these results, they conclude that the current theory about the origin of hotspots is probably not sufficient to explain volcanoes beyond the plate boundaries. It is possible that some of the hotspots come from completely different sources than expected.
This assumption also supports findings on the composition of the helium in the hotspot volcanoes. Accordingly, hotter hotspots have a higher proportion of helium-3 on average, which has spoken for deep material since the origin of the earth. The helium composition of cooler hotspots deviates from this and is closer to that of the Central Ocean back. A possible explanation for the differences in temperature and helium content is therefore that the classic theory only applies to some of the hotspots. Then only the hottest structures originated, as previously assumed in all hotspots, at the core mantle limit.
This is also supported by the fact that the hottest hotspots are located above anomalies at the core-mantle boundary from which they may be fed. The others could, the experts suggest, arise at shallow depths due to local convection. However, there are also explanations according to which these also have a deep origin. For example, classic hotspots could "get stuck" and cool down on the way up, or supposedly cool hotspots could actually be small and their temperature therefore underestimated. It is unclear why cold and warm seem to be unevenly distributed over the earth. Almost all hot hotspots are concentrated in the Pacific region, while the cooler ones are concentrated around Africa.
