Geologists may need to find a new explanation for sources of volcanic activity in places like Iceland and Hawaii
January 6, 2022
The rock plumes feeding Earth’s volcanic hotspots are much colder than previously thought, suggesting geologists need to find a new explanation for the sources of volcanic activity in places like Iceland and Hawaii.
Volcanic hotspots are not connected to volcanic regions at tectonic plate boundaries. They are believed to be fed by hot plumes of rock from deep within the Earth’s mantle, which have expanded and risen due to high temperatures.
But Carolina Lithgow-Bertelloni of the University of California, Los Angeles and her colleagues found that a number of these hotspots are fueled by relatively cold material, suggesting that other dynamics may be at work. .
“We’re not saying these aren’t hotspots; we say yes, but there are different mechanisms that help them increase,” says Lithgow-Bertelloni. “You may still have some warm material coming up, but it doesn’t all come up from the depth and not all in the same way; it is aided by other processes in the mantle.
Calculating the temperature under volcanic hotspots is difficult. The upper mantle can be between 250 and 600 kilometers deep, which excludes any direct access. Lithgow-Bertelloni and his team measured the speed of seismic waves traveling beneath volcanic hotspots and inferred temperatures based on a model of rock composition.
The researchers then compared these temperatures with the relatively cold volcanic regions below the ridges, at the tectonic boundaries. According to classical theory, plumes must be between 100°C and 300°C hotter than the ridges in order to rise.
But more than half of the hotspots the researchers studied were less than 100°C warmer than the ridges. Nearly a sixth of the hotspots were mostly cold, meaning they were no hotter than the ridges over 36C.
“You wouldn’t expect very low temperature excesses, because that means you have very little driving force for the thing to inflate and rise in the first place,” says Oliver Shorttle from the University of Cambridge.
The study found that the ratio of helium isotopes differed between cold and hot hotspots, suggesting they may come from different parts of the mantle. Understanding the differences between different volcanic hotspots and what lies beneath them could reveal the history and development of Earth’s tectonic plates.
Journal reference: ScienceDOI: 10.1126/science.abj8944
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