Over 400 million years ago, an upwelling of hot rock from the Earth’s mantle destroyed the crust in Mongolia, creating an ocean that survived for 115 million years.
The geologic history of this ocean can help researchers understand Wilson cycles, or the process by which supercontinents break up and merge. These are slow, large-scale processes that advance at less than an inch per year, the study’s co-author said. Daniel Pastor-Galána geoscientist at the Spanish National Research Council in Madrid.
“It’s telling us about processes on Earth that are not very easy to understand and that are also not very easy to see,” Pastor-Galán told Live Science.
Geoscientists can reconstruct with great precision the breakup of the last supercontinent, Pangea, 250 million years ago. But before that, it’s hard to model exactly how the mantle and crust interacted.
In a new study, researchers were intrigued by volcanic rocks in northwestern Mongolia from the Devonian period (419 million to 359 million years ago).
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The Devonian was the “Age of Fish,” when fish dominated the oceans and plants began to spread on land. At the time, there were two large continents, Laurentia and Gondwana, as well as a long stretch of microcontinents that would eventually become what is now Asia. These microcontinents gradually collided against each other and coalesced in a process called accretion.
Researchers began fieldwork in northwestern Mongolia, where rocks from these continent-building collisions are exposed at the surface, in 2019, studying the ages and chemistry of the ancient rock layers. They found that between 410 million and 415 million years ago, an ocean called the Mongol-Okhotsk Ocean opened up in the region. The chemistry of the volcanic rocks that accompanied this rift revealed the presence of a mantle plume—a flow of particularly hot and vibrant mantle rock.
“Mantle plumes are usually involved in the first phase of the Wilson cycle: the breakup of continents and the opening of an ocean, such as the Atlantic Ocean,” lead author of the study. Mingshuai Zhua professor of geology and geophysics at the Chinese Academy of Sciences, told Live Science.
In many cases, this happens right in the middle of a solid part of the continent, tearing it apart. In this case, however, the geology is particularly complex because the plume was ripping through crust that had previously been held together through accretion. The weak points between the accreted microcontinents, combined with the plume, probably helped form the ocean, Zhu said. The researchers published their findings May 16 in the journal Geophysical Research Letters.
The ocean closed in the same place it opened, which is a common pattern in ocean life cycles, Pastor-Galán said, but the researchers only looked at one view of the ocean opening in this study.
“One good thing is that a hotspot is relatively stable, so they persist, for many millions of years, in the same place,” Pastor-Galán said. As the continents in the crust move over the mantle hotspot, the hotspot leaves behind volcanic rocks and a telltale chemistry; this helps researchers track plate movement over millennia, he said.
Asia is no longer accreting new microcontinents, Pastor-Galán said, but the formation of the Mongol-Okhotsk Ocean was probably similar to what is seen today in the Red Sea, where the crust is spreading at about 0.4 inches (1 centimeter) per year. . The Red Sea is part of a larger continental rift that could create a whole new ocean in East Africa over tens of millions of years, although geologists don’t yet know whether other continental forces will prevent it from fully opening, according to Eos magazine.
Zhu and his colleagues now plan to use their data to make computer models to better describe the complex tectonics of the ancient Devonian ocean.