One of the reasons we know that using seismic waves to map the interior of a planet is that we use similar techniques here on Earth. Earth may not have rings, but it does have plate tectonics that leads to earthquakes that create waves that move through the interior of the planet and provide a wealth of information. Again, this kind of analysis is pretty simple when you have enough data.

And now in a new paper published in the journal Science China Earth Sciences, a team of researchers has particularly focused on the effects of convergent plate margins, both active and fossil. Wait. Fossil plate boundaries? That’s a thing?

Apparently so.

According to this research, there are three stages in the formation and evolution of convergent plate boundaries. The first stage is characterized by what we call low-angle subduction, where the down-going plate is moving under the upper plate at an almost but not quite parallel angle. This type of convergent boundary gives rise to Alpine-type mountains but not magmatic volcanic arcs.

The next stage of subduction occurs with higher-angle subduction, where the down-going plate is sharply tilted with respect to the upper plate. And those situations are what create magmatic arcs as seen in around the Pacific Ring of Fire, with numerous volcanoes in Russia, Japan, Indonesia, Chile, Argentina, and even Alaska.

But then, there is a post-stage to both of those subduction types. And that is actually characterized by rifting as the boundary changes types. The subducted plate is still there, becoming thinner and more pliable from the heat of the mantle until it begins to founder. That means that the plate breaks away and leaves a gap where it once was being subducted. And instead, the hot, plastic mantle begins to well up through the gap and push the two plates apart.

But that leftover bit that is foundering and not attached? That’s a fossil plate. There is one under North America called the Farallon plate leftover from the formation of California and much of the west coast.

And this process repeats itself over and over in the history of our planet, leading to a mechanism that exchanges heat and energy between the interior and the surface.

Plate tectonics is fascinating. And we’re the only planet in our solar system that has it.

More Information

Tectonics of convergent plate margins: New insights into continental geology (EurekAlert)

“Tectonic evolution of convergent plate margins and its geological effects,” Yongfei Zheng, Yixiang Chen, Renxu Chen, and Liqun Dai, 2022 May 20, Science China Earth Sciences