Researchers have discovered a 400-mile chain of deeply buried, extinct, fossilized volcanoes in southern China. The volcano was formed when two structural plates collided hundreds of millions of years ago during the split of the Rodynia supercontinent hundreds of millions of years ago, scientists reported in a new study. Ancient volcanoes may have extended the area of past volcanic activity in the region hundreds of miles, affecting the climate of the Earth.
About 800 million years ago, during the early Neoproterozoic period, South China sat on the northwestern margin of Rodynia. Shift plate tectonics allowed this area to penetrate the current Yangtze block plate and push it towards the China Ocean plate. When the two plates collided, the dense oceanic crust subsided beneath the more buoyant continental crust, sliding deep into the Earth. This is known as sinking.
When the oceanic crust subsides, the water is heated, the water is released, and magma is produced. The magma rises to the surface and creates a long, narrow chain of volcanoes following curved lines above the subduction zone. This is known as the volcanic arc.
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Volcanic activity and mountain buildings in the ARC system create new crusts and modify existing crusts. Therefore, researchers will study ancient volcanic arcs to understand how the crust was formed early.
Geologists previously discovered the remains of an extinct volcanic arc along the edge of Youngtzeblock, dating back to the early Neoproterozoic era. In a new study published in the Journal of Geophysical Research on June 30, Solid Earth, Zhidong Gu, senior engineer at Petrochina, Junyong Li, researcher at Nanjing University, and colleagues tested whether these ARC volcanoes had expanded further inland.
Fossil mountains can be difficult to find as they gradually wear out by wind and water and are buried beneath the layers of sediment. Today, several kilometers of sedimentary rocks cover the interior of the proton block, forming the Sichuan Basin.
Gu and Li’s teams use aerial magnetic sensors to “see” the crust beneath these sedimentary rocks. Different rock types contain different magnetic minerals, so geophysicists use magnetic signals to map underground rock formations.
They found iron-rich rock strips with a stronger than average magnetic field of about 4 miles (6 km) below the surface. It formed a belt of about 430 miles (700 km) long, stretching from the northeast to the southwest of the Yangtze River block, reaching 550 miles (900 km) inland. Iron-rich rocks like these are produced on the subduction of the marine skin.
The team also analyzed rocks from seven deep boreholes excavated into the top crust beneath the Sichuan Basin. They confirmed that these rocks came from magma and chemically resemble the new crust formed by the Ark Volcano. They dated magma rocks between 770 and 820 million years ago, confirming that the rocks formed during the early Neoproterozoic era.
Researchers concluded that during the Rodinia division, plate subduction formed a volcanic ring extending inside Youngtzeblock hundreds of miles.
The discovery is surprising, the team said. This is because most volcanic arcs form narrower belts along the continental margin. For example, cascades form a single chain of mountains above the Juan de Fuca plate as they sink under the North American coast.
Gu and Li attributed the wide Yangtze arc to a different style of tectonics called flat slab subduction. In flat slab subduction, the oceanic plate sinks to the earth at a shallow angle of several hundred miles, horizontally, shallowly under the continental plate. This process produces two different volcanic ridges. One is near the boundary where the oceanic plate first slides beneath the continent, and one is inland where it eventually sinks. Similar shallow subduction of the Nazca Plate, beneath the west coast of South America, forms the parallel mountain ranges of the Andes today.
Peter Cowd, a geoscientist at Monash University in Australia who was not involved in the study, agreed that this is one way in which an inland volcano could form. However, he suggested another explanation. “The two belts may represent two independent but time-equivalent systems that are not part of one broad arc system and flat slab, but rather two independent but time-equivalent systems that are stitched together,” he told Live Science.
Anyway, Cawood said the work presents “an exciting new dataset for areas that are difficult to study.” He added that “it shows that the amount of magmatic activity along this boundary may be significantly greater than previously realized,” adding that the impact on the Earth’s past climate needs to be assessed.
Scientists believe that the global carbon cycle has undergone major changes during this period, based on geochemical records from sedimentary rocks between 720 million and 1 billion years ago. Volcanoes release carbon dioxide into the atmosphere, but chemical weathering in the mountains consumes it. Both processes work to regulate the global carbon cycle and climate for millions of years. It remains unclear how the fire ring in southern China contributed to this perturbation and the resulting climate instability.
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