The largest known mass extinction may have been triggered by events deep inside Earth.
Hundreds of millions ofĀ yearsĀ ago, when the continents collided to form aĀ single supercontinent, hugeĀ amounts of material mayĀ haveĀ detached fromĀ their undersides, causing hot molten rock to rise up and trigger enormous volcanic eruptions.
There is strong evidence that massive volcanic eruptions were responsible for the Permian extinction 252Ā million years ago, which wiped out at least 80 per cent of species. These eruptions heated up the climate and caused the oceans to stagnate. But we donāt know what caused them.
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One possibility is that deep inside the planet, in the semi-molten mantle, a plume of unusually hot magma rose up and broke through the crust. Such plumes are thought to exist in the modern day: one under the Atlantic Ocean is believed to have created Iceland.
However, according to Chen Zhang at the China University ofĀ Petroleum in Beijing and his colleagues, it isnāt clear whether a plume could release enough carbon dioxide to cause the climate changes that would have caused a mass extinction. Instead, they have proposed another possibility.
Breakaway rocks
The team studied crystals called zircons from rocks taken from the Central Asian Orogenic BeltĀ ā a region that now stretches from the Ural mountains to the Pacific Ocean. The rocks are from volcanic eruptions millions of years ago. By studying their chemical make-up, the team could tell how hot the magma the rocks formed from was, which points to the source of the eruption.
There were two periods when volcanoes erupted unusually hot magma, the team found. One was about 252 million years ago, the time of the Permian extinction. The other was about 443 million years ago: when the Ordovician-Silurian mass extinction occurred.
Such hot magma is unlikely toĀ have been the result of a mantle plume, the team says, suggesting instead that the cause of the mass extinctions was rocks detaching from the underside of Earthās crust and slipping away into the mantle, aĀ process called delamination.
This can cause volcanic eruptions, because hot rocks find it easier to work their way up through the crust when its bottom layer is coming away. The hot magma is formed from a mixture of crust and mantle rocks, which would explain some of the teamās results.
To account for the volcanic eruptions at the end of the Permian period, delamination must have occurred on a hugeĀ scale. The team calls thisĀ āsuper-delaminationā, arguing that the cause was the formation of supercontinents.
Over hundreds of millions of years leading up to the super-delamination, several continents came together to form a supercontinent called Gondwana. Then the remaining land masses collided with Gondwana, creating an even larger supercontinent called Pangaea.
āThe formation of a supercontinent involves multiple collisions between continental fragments,ā the team writes. āThe resultant crustal thickening would inevitably trigger delamination, conceivably on a major scale.ā
It isnāt clear if this mechanism is plausible, says Katie Cooper at Washington State University. But āitās definitely an interesting idea and one that deserves more investigationā, she says.
Gondwana Research
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