Death of a planet

Crust hole in front of the US West Coast provides insight into the end of an ocean plate

The Juan de Fuca plate gives important insights into how ocean plates "die". Already now a huge hole announces its coming end. © USGS / NASA
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How earth plates die: Geologists are currently witnessing the "death" of a tectonic plate off the US West Coast. The oceanic Juan de Fuca plate has already developed a huge hole - a preliminary stage of their final break, as the researchers report. Cause of the near end is the subduction of this long, but narrow plate under the American continent. In a few million years, this process could have swallowed the entire Juan de Fuca plate.

The earth's crust is a dynamic system: New crust is constantly being formed on the mid-ocean ridge, and this oceanic crust is pushed down and remelted at the subduction zones of the continental margins. At the same time, however, the drifting continent also leads to educational backings and subduction zones converging. The oceanic plate between them is getting smaller.

The educational backbones of the Juan de Fuca plate are already close to the subduction zone below the US west coast. © W. Jacquelyne Kious and Robert I. Tilling / USGS

Wan of an ocean plate

"In principle, the natural end of an oceanic plate is reached when the back forming the crustal plate reaches the subduction zone, " explain William Hawley and Richard Allen of the University of California at Berkeley. "But how this happens in practice and which geophysical phenomena are associated with it, has so far been poorly understood." This is also because there were hardly any examples of this "plate death".

Now, however, the geologists have made a record that seems to be nearing its end. It is the Juan de Fuca plate, a long but narrow ocean plate that stretches off the US west coast from the Canadian border to California. It is a relic of the once far Farallon ocean plate, which has already been subducted to a large extent under the North American continent plate. Also, the Juan de Fuca plate moves with 26 millimeters per year on the coast and is there pressed into the depths. Already, it is divided by dislocations into three parts.

Hole in the crust

Thus, the Juan de Fuca plate is the ideal candidate to watch the final phase of a tectonic plate live. "The fragments provide evidence of what the final stages of the Pacific North America Juan de Fuca system could look like, " the researchers say. For their study, they evaluated the data from two seismological measurement networks in this region. The data from 217 earthquakes allowed them for the first time to create a tomographic model of the "dying" Earth plate. display

Seismic fluoroscopy revealed that the Juan de Fuca plate formed a huge hole in the subduction zone. This is at least 100 kilometers below the surface and is the tip of a crack, which is at its peak about 200 kilometers wide. "This crack does not run laterally, but parallel to the subduction trench, " the researchers report. Magma has been spilling out of the mantle for millions of years through the extensive hole. This could be the cause of an ancient volcanic zone on the high plateau of Oregon.

Origin in submerged plate pieces

But what caused the crack in the plate? Further analysis showed that this hole was not created by tensions on the crustal surface, but has deep roots. Its origin is therefore in the piece of the ocean plate, which has already plunged deep into the mantle. There, the immense pressure has led to an old swell zone of the plate breaking up. Like an opening zipper, this rupture then spreads along this swelling zone towards the not yet subducted part of the slab, as the geologists explain.

"That could mean that the final stages in the life of an oceanic plate are bottom-up disintegrated, " Hawley and Allen state. Contrary to previous assumptions, a dying plate does not break from top to bottom, due to the contact of the old mid-ocean back with the subduction zone, but from bottom to top.

The end: first fragmentation, then merging

But what does this mean for the end of the Juan de Fuca plate? "This disruption could ultimately completely fragment the plate, " the researchers said. "What then remains on small pieces will attach to other nearby earth plates." The fragments could then either become part of the Pacific Plate or merge with the North American Continent Plate.

But it will take a few million years before it gets there. "But if our interpretation is correct, then this is an excellent example of the processes that control the end of a long-lived ocean plate, " Hawley and Allen state. (Geophysical Research Letters, 2019; doi: 10.1029 / 2019GL083437)

Source: Geophysical Research Letters

- Nadja Podbregar