By any measure, the Andes Mountains are very, very large. They reach by running about 8,900 kilometers (5,530 mi) from South America up to 7 kilometers (4.3 mi) in height and sStretch up to 700 kilometers (435 mi) in width.
But how did the series grow to this massive scale? Plate tectonics—the movement of great slabs of Earth’s crust across the planet—can create mountain ridges as slower parts are pushed up by faster-moving regions.
Although the concept is simple in theory, it is difficult for geologists to track the pace of tectonic movement over periods shorter than 10 to 15 million years.
Researchers at the University of Copenhagen used a recently developed method to get a more detailed look at the movement of the South American plate that forms the Andes. They identified a 13 percent subsidence of plate segments around 10 to 14 million years ago and a 20 percent subsidence between 5 and 9 million years ago—enough to explain some of the features we see today.
“In the period leading up to the two depressions, the plate immediately to the west, the Nazca plate, was plowing into the mountains and compressing them, causing them to rise,” says geologist Valentina Espinoza of the University of Copenhagen in Denmark. .
“This result may indicate that part of the pre-existing range acted as a brake on both the Nazca and South American plates. As the plates slowed down, the mountains instead widened.”
The technique used in the study starts with Absolute Plate Movement (APM), which is the movement of the plates in terms of fixed points on Earth. APM is mostly determined by studying volcanic activity in the crust, where the trail of magma tells geologists how the plates have shifted.
Then there is the relative plate movement (RPM), the movement of the plates in relation to each other. These are calculated using a wide range of signals, including magnetization data embedded in the sea floor that show rock movement and provide higher resolution (smaller timescale) data than APM.
To determine the rate of movement in the South American plate, geologists used the high-resolution RPM data to estimate the APM through some detailed mathematics. We are confident that by validating inferred data with geological data, the method enables experts to learn more about the interactions between tectonic plates.
“This method can be used for all plates as long as high-resolution data is available,” says Giampiro Ifaldano, a geologist at the University of Copenhagen.
“My hope is that such methods will be used to refine historical models of tectonic plates and thereby improve our chances of reconstructing geological events that remain obscure.”
The team also considered the question of why these two significant recessions occurred in the first place. While several million years is a long time to us, it is like the blink of an eye on the geological time scale.
One possibility is that convection currents in the mantle have changed, moving different densities of material around. It is also possible that a phenomenon called delamination was responsible, where significant portions of the plates sink down into the mantle. Both events would have had knock-on effects that would have influenced the rate of plate movement.
Knowing for sure will require more research and more data, and a new method of analysis will help with that. Even with one question (probably) answered, there is much more to work on.
“If this explanation is correct, it tells us a lot about how these huge mountain ranges came to be,” says Espinoza.
“But there’s still a lot we don’t know. Why did it get so big? At what speed did it happen? How does the ridge sustain itself? And will it eventually collapse?”
This is published in research Earth and Planetary Science Papers.
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