New Delhi: The hotspots that created volcanic islands such as those of Hawaii, Iceland, and the Galapagos Islands may prove to be surprisingly cool, a new study has found. The findings were recently published in the journal Science.
Hotspots May Not Always Originate From Scorching Hot Rock
The findings suggest that such hotspots may not always originate from giant plumes of scorching hot rock welling up from near Earth’s core as previously thought, the authors noted in the study.
Volcanoes are mostly found near the borders of tectonic plates, and are born from clashes between giant 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 is a classic example of such a volcano.
Sometimes, volcanoes erupt in the middle of tectonic plates, and the sources of these hotspots could be mantle plumes, which are mushroom-shaped pillars of hot rock ascending from the deep mantle to burn overlying material like a blowtorch, the study said.
Geologists think chains of volcanic isles can emerge because tectonic plates wander over such plumes.
According to research, volcanic hotspots are roughly 100 to 300 degrees Celsius hotter than mid-ocean ridges, where magma rises as tectonic plates spread apart underwater. This suggested that hotspots were heated by matter from near Earth’s hot core and mid-ocean ridges by cooler mantle rock.
Hotspots Are Dramatically Cooler Than Previously Thought
However, scientists have now found that many hotspots are dramatically cooler than previously thought, which raises questions about their origins.
Researchers of the new study analysed the velocity of seismic waves rippling through the mantle underneath oceanic hotspots and ridges to estimate the temperatures at those sites.
The study found that roughly 45 per cent of hotspots are more than 155 degrees Celsius hotter than mid-ocean ridges. However, around 40 per cent of hotspots are only 50 to 136 degrees Celsius hotter than mid-ocean ridges. These are not particularly hot, and therefore, are not buoyant enough to support the active upwelling of rock from the deep mantle.
Surprisingly, the researchers found that around 15 per cent of hotspots are especially cold, and only 36 degrees hotter than mid-ocean ridges.
The scientists intended to gather more information about the origins of these different varieties of hotspots. They examined the ratio of the rarer helium-3 to the more common helium-4 in their rock.
The Helium found in Earth’s crust is mostly helium-4. It arises from the breakdown of uranium and other radioactive isotopes over time. Meanwhile, the Helium from within deep Earth is richer in helium-3, and likely originates from reservoirs of ancient material.
The researchers observed that hot hotspots possessed a much higher ratio of helium-3 to helium-4 than cold hotspots did, the study said.
Seismologist Ross Maguire from the University of New Mexico said that the term ‘hotspot’ is misleading, and that volcanoes which do not fit the tectonic paradigm should be referred to as ‘melting anomalies’, according to a statement issued by the American Institute of Astrophysics.
A substantial fraction of hotspots do not fit the classical plume model, according to Vedran Lekic, a seismologist from the University of Maryland, US.
The study found that cooler hotspots may instead originate in the upper mantle, from slow-moving deep plumes that have more time to cool, or from deep plumes that interact with and get cooled by swirling mantle rock.
Steinberger said that the work points to a much greater variety among plumes, and has some totally unexpected features.