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A new study finds that the volcanic islands created by the hotspots may be cool.
Scientists said that the findings suggest that such hot spots may not always come from giant hot rock welling up from Earth's core.
There are usually volcanoes found near the borders of tectonic plates, formed from clashing slabs of rock as they drift on top of the mantle layer between Earth's core and crust. The Ring of Fire on the Pacific Rim are examples of such volcanoes.
In the middle of plates, volcanoes erupt. The sources of these hot spots might be mantle plumes, mushroom-shaped pillars of hot rock ascending from the deep mantle to sear overlying material like a blowtorch. The geologists think volcanic isles can emerge as the plates wander.
Volcanic hot spots are hotter than mid-ocean ridges, where magma rises as tectonic plates spread apart underwater, according to previous research. The view was that the hot spots were heated by matter from near Earth's hot core and mid-ocean ridges.
Scientists are questioning the origins of many hotspots after finding them to be dramatically cooler than previously thought. A seismologist at the University of Maryland, College Park, who did not participate in the study, said that a lot of the hotspots did not fit the classical model.
The researchers used the velocities of the waves to estimate the temperatures at the sites. Seismic waves travel faster through cold rock.
More than half of the hot spots are hotter than mid-ocean ridges. Roughly 40% are hotter than mid-ocean ridges, but not particularly hot, and therefore not supportive of the upwelling of rock from the deep mantle. Only 36 C hotter or less than mid-ocean ridges are hotter than 15% of hotspots.
The scientists looked at the ratio of rarer helium to more common helium in their rock in order to learn more about the origins of the different types of hotspots. Each of the atomic cores of helium-4 has two neutrons.
Helium found in the Earth's crust is mostly helium-4, which is caused by the breakdown of radioactive isotopes over time, while helium found deep within the Earth is richer in helium 3, which is caused by the original ratio of these isotopes during Earth's first days. The researchers found hot hotspots had a higher ratio of the two gases than cold ones.
The classic model of hot spots may explain some of the famous ones, such as those underlying Hawaii and the Galapagos, but only a few truly behave like our classical model.
The term "hotspot" is misleading and should instead be referred to as "melting anomalies," said seismologist Ross Maguire from the University of New Mexico.
The upper mantle may be where the cooler hotspots originate, or they may be from slow- moving deep plumes that have more time to cool. "If this is true, it will be a challenge forgeosciences to explain such a finding," said the German Research Center for geosciences' Bernhard Steinberger, who was not a part of this work. "These results will prompt new research."
"The classical view of plumes is not so flawed as to be more complex than it was 30 to 50 years ago," she said.
This work points to a much greater variety among the mounds. Whenever you get a new close-up view of a planet or moon, it is like that. There are some totally unexpected features. But it is not over.
In the future, the scientists would like to analyze every hotspot in more detail to get a better sense of their temperatures. She said that they aim to conduct more computer simulations.
The findings were detailed in the journal Science.
The relative temperatures of Earth's volcanic hotspots and mid-ocean ridges are discussed in a paper. There is a science.abj8944
Science journal information.
Inside Science has re-posted this story. The original story can be read here. Permission was used. Inside Science is a news service of the American Institute of Physics.
The scientists found cool hot spots under the Earth's crust.
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