Breakup of Supercontinent Cooled Mantle and Thinned Crust

The thinning is related to the cooling of Earth’s interior prompted by the splitting of the supercontinent Pangaea, which broke up into the continents that we have today, said Harm Van Avendonk, the lead author of the study and a senior research scientist at The University of Texas Institute for Geophysics. The findings, published in Nature Geosciences on Dec. 12, shed light on how mantle plumes and plate tectonics has influenced the cooling of the Earth’s mantle throughout geologic history.

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The mantle is the very hot, but mostly solid, layer of rock between the Earth’s crust and core. Magma from the mantle forms oceanic crust when it rises from the mantle to the surface at spreading centers and cools into the rock that forms the very bottom of the seafloor. The Earth’s mantle has been cooling almost from its creation.

“It’s important to note the Earth seems to be cooling a lot faster now than it has been over its lifetime,” Van Avendonk said. “The current rate of mantle plumes allows Earth to cool much more efficiently than it did in the past.”

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The research that led to the connection between the splitting of the supercontinent and crust thickness started when Van Avendock and Ph.D. student Jennifer Harding, a study co-author, noticed an unexpected trend when studying existing data from young and old seafloor. They analyzed 234 measurements of crustal thickness from around the world and found that, on a global scale, the oldest ocean crust examined – 170 million year old rock created in the Jurassic – is about one mile thicker than the crust that’s being produced today.

The link between crust thickness and age prompted two possible explanations – both related to the fact that hotter mantle tends to make more magma. Mantle plumes could have thickened the old crust by covering it in layers of lava at a later time. Or, the mantle was hotter in the Jurassic than it is now.

The finding that splitting up Pangea cooled the mantle is important because it gives a more nuanced view of the mantle temperature that influences tectonics on Earth.

BREAKING NEWS: Research Suggests Major Changes to Geology Textbooks Related to Mantle Plumes

Super-computer modeling of Earth’s crust and upper-mantle suggests that ancient geologic events may have left deep ‘scars’ that can come to life to play a role in earthquakes, mountain formation, and other ongoing processes on our planet.

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Mantle Plumes are columns of hot magma rising by convection in the mantle, believed to cause volcanic activity in hot spots, are often the source of mountain building such as the Hawaiian Islands, and are away from plate margins. This changes the widespread view that only interactions at the boundaries between continent-sized tectonic plates could be responsible for such events.

A team of researchers from the University of Toronto and the University of Aberdeen have created models indicating that former plate boundaries may stay hidden deep beneath the Earth’s surface. These multi-million-year-old structures, situated at sites away from existing plate boundaries, may trigger changes in the structure and properties at the surface in the interior regions of continents.

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“This is a potentially major revision to the fundamental idea of plate tectonics,” says lead author Philip Heron, a postdoctoral fellow in Russell Pysklywec’s research group in UT’s Department of Earth Sciences. Their paper, “Lasting Mantle Scars Lead to Perennial Plate Tectonics,” appears in the June 10, 2016 edition of Nature Communications.

Heron and Pysklywec, together with University of Aberdeen geologist Randell Stephenson have even proposed a ‘perennial plate tectonic map’ of the Earth to help illustrate how ancient processes may have present-day implications.

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“It’s based on the familiar global tectonic map that is taught starting in elementary school,” says Pysklywec, who is also chair of UT’s Department of Earth Sciences. “What our models redefine and show on the map are dormant, hidden, ancient plate boundaries that could also be enduring or “perennial” sites of past and active plate tectonic activity.”

To demonstrate the dominating effects that mantle plume anomalies below the Earth’s crust can have on shallow geological features, the researchers used UT’s SciNet – home to Canada’s most powerful computer and one of the most powerful in the world- to make numerical models of the crust and upper-mantle into which they could introduce these scar-like anomalies.

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The team essentially created an evolving “virtual Earth” to explore how such geodynamic models develop under different conditions.

“For these sorts of simulations, you need to go to a pretty high-resolution to understand what’s going on beneath the surface,” says Heron. “We modeled 1,500 kilometers across and 600 kilometers deep, but some parts of these structures could be just two or three kilometers wide. It is important to accurately resolve the smaller-scale stresses and strains.”

Using these models, the team found that different parts of the mantle below the Earth’s crust may control the folding, breaking, or flowing of the Earth’s crust within plates – in the form of mountain-building and seismic activity – when under compression. In this way, the mantle structures dominate over shallower structures in the crust that had previously been seen as the main cause of such deformation within plates.

“The mantle is like the thermal engine of the planet and the crust is an eggshell above,” says Pysklywec. “We’re looking at the enigmatic and largely unexplored realm in the Earth where these two regions meet.”

“Most of the really big plate tectonic activity happens on the plate boundaries, like when India rammed into Asia to create the Himalayas or how the Atlantic opened to split North America from Europe,” says Heron. “But there are lots of things we couldn’t explain, like seismic activity and mountain-building away from plate boundaries in continent interiors.”

The research team believes their simulations show that these mantle anomalies are generated through ancient plate tectonic processes, such as the closing of ancient oceans, and can remain hidden at sites away from normal plate boundaries until reactivation generates tectonic folding, breaking, or flowing in plate interiors.

“Future exploration of what lies in the mantle beneath the crust may lead to further such discoveries on how our planet works, generating a greater understanding of how the past may affect our geologic future,” says Heron.

The research carries on the legacy of J. Tuzo Wilson, also a U of T scientist, and a legendary figure in geosciences who pioneered the idea of plate tectonics in the 1960’s.

“Plate tectonics is really the cornerstone of all geoscience,” says Pysklywec. “Ultimately, this information could even lead to ways to help better predict how and when earthquakes happen. It’s a key building block.”

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