Introduction
Recent research suggests that the plumes of rock feeding Earth’s volcanic hotspots are much cooler than previously believed. This finding challenges the existing understanding of the sources of volcanic activity in locations such as Iceland and Hawaii. Volcanic hotspots are not connected to volcanic regions at the boundaries of tectonic plates. Instead, they are believed to be sustained by hot plumes of rock from deep within Earth’s mantle. These plumes expand and rise due to high temperatures. However, new evidence suggests that a number of hotspots are being fed by relatively cold material, indicating the involvement of different mechanisms.
The Study
Carolina Lithgow-Bertelloni and her colleagues at the University of California, Los Angeles, investigated the temperatures beneath volcanic hotspots. Since direct access to the upper mantle, which can be anywhere from 250 to 600 kilometers deep, is not feasible, the team measured the speed of seismic waves traveling beneath these hotspots. They then inferred temperatures based on a model of the rock makeup in that area. The researchers compared these temperatures with the temperatures beneath ridges located at tectonic boundaries. According to traditional theories, the plumes need to be 100°C to 300°C hotter than the ridges in order to rise.
Surprising Findings
The study revealed that more than half of the hotspots examined were less than 100°C hotter than the ridges. Additionally, almost a sixth of the hotspots were essentially cold, with temperatures no more than 36°C higher than the ridges. These findings challenge the assumption that the plumes need a significant temperature difference to rise. Oliver Shorttle, from the University of Cambridge, highlights that such low temperature excesses indicate a weak driving force for the plumes to ascend in the first place.
Furthermore, the study observed that the ratio of helium isotopes differed between the cold and hot hotspots, indicating that they may originate from different parts of the mantle. Understanding the differences between various volcanic hotspots and the underlying mantle composition could provide valuable insights into the history and development of Earth’s tectonic plates.
