Fold mountains

Extinct style of plate tectonics explains the flat mountains of early Earth

Source: Geophysical research letters

Geochemical and geological records provide key information about the tectonic history of the Earth, but in the case of mountains during the Proterozoic Eon, the records conflict with each other: geochemical evidence indicates that the crust was thin and warm, generally indicating that mountains could not have formed, but rocks left over from this period suggest mountains were present.

“How do you reconcile the geological evidence for mountain building with the geochemical evidence for thin crust?” “

“So how do you reconcile the geological evidence for mountain building with the geochemical evidence for thin crust? Asked Christopher Spencer, a geologist at Queen’s University in Ontario, Canada, and lead author of a new study published in Geophysical research letters.

Using a global database, Spencer et al. compared the rock records to the geochemical records from 1.8 billion to 850 million years ago, a period that begins about one and a half billion years after the first signs of life and ends 150 million years ago before complex life evolves. The geochemistry of continental rocks from this era suggests that the continental crust was thin (less than 40 kilometers) and hot – poor conditions for forming mountains.

Metamorphic rocks, like this 1.3 billion-year-old Australian sample, provided Spencer et al. with an overview of the formation of ancient mountains. Credit: Christopher Spencer, CC-BY-NC-4.0

“Proterozoic mountains were different from mountains at any other time in Earth’s history.”

Despite the hot crust, the mineral makeup of rocks around the world at this time suggests that there was significant crustal flow which resulted in lower mountain ranges. The process of building mountains under these unique conditions is something we have never seen on modern Earth or beyond, suggest the authors. “The Proterozoic mountains were different from mountains at any other time in Earth’s history,” Spencer said.

The authors argue that with a thin, warm crust, mountains could still form as a relatively weak crust slipped on itself in collisions that looked more like direct hits than head-on collisions. This behavior would have been due in part to the fact that the lower part of the hot crust “flows” like a very viscous fluid on geological time scales, even before the onset of modern plate tectonics. This explanation of an extinct style of plate tectonics connects geological and geochemical evidence, describing a relatively flat world that has persisted for a billion years.

“Continental motion is possible without global plate tectonics,” said Taras Gerya, a geophysicist at the Federal Institute of Technology Zurich (ETH Zurich) who was not involved in the study. According to Gerya, the authors “demonstrate that this period was indeed characterized by a rather particular tectonic style, different from the present. So something really changed when we went from the Proterozoic to the Phanerozoic about 541 million years ago. (Geophysical research letters, https://doi.org/10.1029/2021GL093312, 2021)

—Becca Dzombak (@bdzombak), science writer

Quote: Dzombak, B. (2021), Extinct Style of Plate Tectonics Explains Earth’s First Flat Mountains, Éos, 102 years old, https://doi.org/10.1029/2021EO210535. Posted on October 7, 2021.
Text © 2021. AGU. CC BY-NC-ND 3.0
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