Existence of mantle plumes confirmed?
Geoscientists working with Bernard Bourdon at the Swiss Federal Institute of Technology in Zurich have found evidence that confirms the existence of so-called mantle plumes. In doing so, they consolidate the common hotspot theory, which has been the subject of frequent criticism lately.
Up to now, plate tectonics has not provided a plausible explanation for the formation of hotspot volcanoes, some of which are several thousand kilometers away from plate edges in the middle of the ocean. These volcanoes include Hawaii, the Galapagos Islands, the Azores and Iceland. According to the popular theory, these volcanoes are located above hot spots in the Earth's interior where hot material - the mantle plumes - rises in the Earth's mantle. This can then penetrate to the earth's surface as a concentrated columnar magma flow and form isolated volcanic islands there. Bernard Bourdon and his colleagues have now found new evidence that confirms the existence of these mantle plumes and thus supports the standard theory. They analyzed the proportions and ratios of various elements such as uranium, thorium and protactinium and their isotopes in bas alt rock from eight different island groups.
When mantle rock melts, the ratio of uranium isotopes to their decay products is massively disturbed. After cooling, the system returns to a steady, predictable state. Based on the change in conditions, the researchers were able to calculate how quickly and completely the material melted during the ascent and what the temperature difference was between the surrounding mantle and the ascending plumes: This difference determines the speed and magnitude of the buoyancy. The researchers estimate the temperature differences to be between 50 and 200 degrees Celsius. The temperature differences are larger for active volcanoes such as Hawaii and the Galapagos, and smaller for less active plumes such as Iceland and the Azores. For symmetrical plumes, Bourdon and his colleagues were also able to estimate mantle plume diameters - these estimates agree well with seismic measurements. The plume under Hawaii reaches an extent of at least 240 kilometers, that under Iceland at least 250 kilometers. © ETH Zurich
