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Juan de Fuca Plate: Remnant of a nearly extinct plate

Thu, 25 Feb 2021, 12:55
12:55 PM | BY: RL
Image showing the locations, relative to one another, of the Pacific Plate, North American Plate, and the Juan de Fuca Plate. (source: Google Earth)
Image showing the locations, relative to one another, of the Pacific Plate, North American Plate, and the Juan de Fuca Plate. (source: Google Earth)
A certain plate known as the Juan de Fuca Plate is subducting under the North American Plate faster than it is being created – and it has nearly disappeared!

An earthquake with a magnitude of 4.0 had been recorded by the Department of Natural Resources Canada on 17 February 2021. The epicenter was located 370 km off the coast of British Colombia, Canada, at about 10 km depth, very near to the mid-oceanic ridge underneath the Pacific Ocean. Its location suggests that the phenomenon known as a sea-floor spreading might have caused the quake.

Mid-oceanic ridges (MORs) or spreading centers are large mountain ranges rising from the ocean floor. To understand the function of these ridges, we should first be familiar with the concept of sea-floor spreading. This is simply the process describing how large slabs of the Earth's crust (oceanic crusts) are produced from continuous volcanic eruptions through the mid-oceanic ridges. These ridges are also called spreading centers because new volcanic rocks push the previously formed rocks outward. This results in the spreading or extending of oceanic crusts from either direction of the ridge.

At the other end of these oceanic crusts, however, there are subduction zones. These zones represent a type of convergent plate boundaries where one plate subducts or goes underneath another plate. This is where the density of crusts/plates actively plays a role in whether these crusts/plates will be subducted or not. The denser plate subducts under the less dense ones. In general, continental plates are less dense than oceanic plates. In general, we usually see oceanic plates subducting underneath continental plates – however there is always an exception (look up Australian plate-Pacific plate boundary).

Tens of millions of years ago, an oceanic crust called the Farallon Plate used to be found in between the Pacific Plate and the North American Plate. This Farallon Plate was actively subducting underneath the North American Plate and was also spreading from its mid-oceanic ridge boundary with the Pacific Plate. However, the spreading had been significantly slower than the rate in which it was subducting. This resulted in the Farallon Plate almost being entirely subducted under the North American Plate after tens of millions of years of subduction. Now, the northernmost part of the Farallon Plate – the Juan de Fuca Plate – is the only part that remains, and it is still being subducted!
It is still a mystery as to what will happen after an entire plate has been subducted. It takes a lot of time for this to happen, probably hundreds of thousands of years. Geologists can only speculate and develop geological models, and maybe just sit back and observe the slow geologic death of a tectonic plate.


References:
W. B. Hawley and R. M. Allen. July 2019. “The Fragmented Death of the Farallon Plate”. Geophysical Research Letters. Vol 46, Issue 13. pp 7386 – 7394.

Geologyin.com. web. 2020. “A Tectonic Plate is Dying Under Oregon”. Retrieved Feb. 17, 2021 via www.geologyin.com/2019/08/a-tectonic-plate-is-dying-under-oregon.html

Tarbuck, Lutgens, and Tasa. 2012. “Origin and Evolution of the Ocean Floor”. Essentials of Geology. 11th Ed. pp. 392-400.

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