More than 400 million years ago, an eruption of hot rock from the Earth’s mantle tore open the crust in Mongolia, creating an ocean that survived for 115 million years.
The geologic history of this ocean could help researchers understand Wilson cycles, or the process by which supercontinents break up and join together. These are slow, large-scale processes that advance less than an inch per year, the study’s co-author said Daniel Pastor-Galána geoscientist from the National Spanish Research Council in Madrid.
“It tells us about processes on Earth that are not easy to understand and also not easy to see,” Pastor-Galán told Live Science.
Geoscientists can fairly accurately reconstruct the breakup of the last supercontinent, Pangea, 250 million years ago. But before that, it is difficult to accurately model how the mantle and crust interact.
In a new study, scientists were interested in volcanic rocks in northwestern Mongolia from the Devonian period (419 million to 359 million years ago).
Related: Gravity anomalies reveal seamount three times the height of world’s tallest building
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 eventually became what is now Asia. These microcontinents gradually bumped into each other and joined in a process called accretion.
In 2019, researchers began fieldwork in northwestern Mongolia, where rocks from these continent-building collisions are exposed at the surface, studying the age and chemistry of the ancient rock layers. They found that about 410 million to 415 million years ago, an ocean called the Mongolian-Okhotsk Ocean opened up in the area. The chemistry of the volcanic rocks that accompanied this rift revealed the presence of a mantle plume—a stream 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 an ocean like the Atlantic Ocean,” lead author of the study. Mingshuai Zhuprofessor of geology and geophysics at the Chinese Academy of Sciences, told Live Science.
In many cases this will happen 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 was tearing apart crust that had previously been joined by accretion. The weak spots between the accreted microcontinents, combined with the plume, likely helped the ocean form, Zhu said. The researchers published their findings May 16 in the journal Geophysical Research Letters.
The ocean closed in the same place it opened, a common pattern in ocean life cycles, Pastor-Galán said, but the researchers in this study only looked at a snapshot of the ocean opening.
“The good thing is that the hotspot is relatively stable, so they stay in the same place for many millions of years,” Pastor-Galán said. As the continents in the crust move over the mantle hotspot, the hotspot leaves behind volcanic rocks and a distinctive chemistry; this helps researchers track plate movement over millennia, he said.
Asia is no longer adding new microcontinents, Pastor-Galán said, but the formation of the Mongolian-Okhotsk Ocean was probably similar to what is seen 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 an entirely new ocean in East Africa over tens of millions of years, although geologists don’t yet know whether other continental forces will prevent the ocean from opening up completely. Eos magazine.
Zhu and his colleagues now plan to use their data to create computer models to better describe the complicated tectonics of the ancient Devonian ocean.