Scientists Found an Ancient Tectonic Plate Buried Deep Inside the Earth’s Mantle

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Plate tectonics — the slow movement of the Earth’s continents across the globe, and their interactions with each other — is a relatively new discovery, having only been broadly accepted in the geologic community since the mid-1960s. In addition to studying the seven large plates that make up the major continents on Earth, some scientists study the remains of vanished plates that were subducted under the edge of a continent and driven down into the mantle. Now, one group studying northwest North America has discovered what it believes are the remnants of a now-vanished plate that slid underneath the Pacific Northwest millions of years ago.

Geologists agree that the West Coast of North America was formed by a complex interplay between multiple plates. Two of the now-subducted plates underneath this area of the United States and Canada are the Farallon and Kula plates. A third plate — dubbed Resurrection — has been controversially theorized to exist, with some geologists arguing that it explains certain features of the terrain (and terranes) in particular regions. Now that new evidence has come in, it looks as though the Resurrectionists have won.

Hrm. Probably shouldn’t use that name.

The “Pro-Resurrection” side of the debate seems to have won. After isolating masses that appear to belong to Kula and Farallon, the team reports there are reconstruction gaps in the record, which:

…correlate spatiotemporally to published NW Cordillera near-trench magmatism, even considering possible terrane translation. We attribute these gaps to thermal erosion related to ridge subduction and model mid-ocean ridges within these reconstruction gap mid-points. Our reconstructions show two coeval ridge-trench intersections that bound an additional “Resurrection”-like plate along the NW Cordillera prior to 40 Ma. In this model, the Yukon slab represents a thermally eroded remnant of the Resurrection plate.

Now, I’m sure nobody needs a translation of that, but just in case it’s helpful, here’s a YouTube video of how this process is believed to have unfolded.

“When ‘raised’ back to the Earth’s surface and reconstructed, the boundaries of this ancient Resurrection tectonic plate match well with the ancient volcanic belts in Washington State and Alaska, providing a much sought after link between the ancient Pacific Ocean and the North American geologic record,” says geologist Jonny Wu, from the University of Houston.

A few more interesting things about the relationships of really, really have things on Earth. Mountain ranges form at plate boundaries, but they don’t form at anything like a consistent speed. One explanation for this is that fast mountain formation is caused by pieces of subducting plates breaking off and plunging (very slowly) through the mantle, towards the core/mantle boundary. Imagine hurling a bowling ball into a bathtub, then freezing the water in the instant it hit maximum height, and you’ve got an idea for how these events played out over geologic time.

Plate subduction is not the only thing that reshapes the continents. After the last ice age, continents all over the world began to rise as the sheets of ice melted. This is believed to have had the side effect of making local volcanic eruptions more likely because there wasn’t nearly so much weight holding the continents down. Isostatic rebound is still occurring to this day, though it’s much slower than current rates of sea-level rise and is not beneficial to solving the problem due to the timescales involved.

Understanding where these vanished plates are underneath our feet can aid our understanding of geologic events on the surface, like volcanism. If volcanoes and earthquakes ever become predictable, it’ll likely be thanks to a greater understanding of the deeper layers of the Earth.

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