More than 400 million years ago, an upwelling of hot rock from the Earth’s mantle caused the crust in Mongolia to split apart, creating an ocean that lasted for 115 million years.
The geologic history of this ocean could help researchers understand Wilson cycles, or the process by which supercontinents break apart and reassemble. These are slow, large-scale processes that occur at less than an inch per year, said study co-author Daniel Pastor-Galánan earth scientist at the Spanish National Research Council in Madrid.
“It tells us about processes on Earth that are not so easy to understand and not so easy to see,” Pastor-Galán told Live Science.
Earth scientists can reconstruct the breakup of the last supercontinent with reasonable accuracy, Pangea250 million years ago. But before that it is difficult to model exactly how the mantle and crust interacted.
In a new study, researchers are fascinated 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 onto land. At this time, there were two large continents, Laurentia and Gondwana, and a long strip of microcontinents that would eventually become modern-day Asia. These microcontinents gradually collided with each other and merged in a process called accretion.
The researchers began fieldwork in 2019 in northwestern Mongolia, where rocks from these continental collisions are exposed at the surface. They studied the ages and chemistry of the ancient rock layers. They discovered that an ocean, called the Mongolian-Okhotsk Ocean, formed in the region between about 410 million and 415 million years ago. The chemistry of the volcanic rocks surrounding this rift revealed the presence of a mantle plume: a flow of particularly hot, buoyant mantle rock.
“Mantle plumes are usually involved in the first phase of the Wilson cycle: the breakup of continents and the opening of oceans, such as the Atlantic Ocean,” said the study’s lead author. Ming’shuai Zhuprofessor 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 piece of continent, tearing it apart. In this case, however, the geology is particularly complex, because the plume ripped apart crust that had previously been joined together by accretion. Weak spots between the accreted microcontinents, combined with the plume, likely helped form the ocean, Zhu said. The researchers published their findings May 16 in the journal Geophysical research letters.
According to Pastor-Galán, the ocean closed again at the same spot where it opened. That is a well-known pattern in ocean life cycles. In this study, however, the researchers only looked at a snapshot of the ocean opening.
“A good thing is that a hotspot is relatively stable, so they stay in the same place for millions of years,” Pastor-Galán said. As continents in the crust move over the mantle hotspot, the hotspot leaves behind volcanic rocks and treacherous chemistry; this helps researchers track the movement of plates over millennia, he said.
Asia is no longer accreting new microcontinents, Pastor-Galán said, but the formation of the Mongolian-Okhotsk Ocean was likely similar to what we see today in the Red Sea, where the crust is spreading about 0.4 inches (1 centimeter) per year. The Red Sea is part of a larger continental rift that could create a brand new ocean in East Africa in tens of millions of years, though geologists don’t yet know whether other continental forces will prevent that ocean from fully opening, he said. Eos magazine.
Zhu and his colleagues now plan to use their data to create computer models that will better describe the complex tectonics of the ancient Devonian ocean.