800-Million-Year-Old Volcanic Arc Found Beneath South China

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A massive fossilised volcanic arc has been uncovered deep beneath South China, offering new insight into Earth’s ancient geology. Stretching nearly 400 miles, this chain of long-extinct volcanoes dates back roughly 800 million years to the early Neoproterozoic era and formed during the breakup of the supercontinent Rodinia. The discovery significantly reshapes scientists’ understanding of prehistoric tectonic and volcanic activity in the region.

Construction Of Arch

Around 800 million years ago, the Yangtze Block a major piece of what is now South China sat on the northwestern fringe of Rodinia. As tectonic forces caused it to drift toward the China Ocean plate, the denser oceanic crust slid beneath the lighter continental crust in a process known as subduction.

This movement generated magma as the subducting slab released water under extreme heat. The rising magma eventually created a vast volcanic chain along the subduction zone.

Making Arch Unique

Unlike most volcanic arcs, which are typically narrow, this one is unusually wide and extensive. It spans about 430 miles (700 km) in length, 30 miles (50 km) in width, and reaches 550 miles (900 km) inland.

Researchers attribute this size to flat-slab subduction, where the oceanic plate moved horizontally at a very shallow angle before finally descending deeper into the mantle. This unusual geometry produced two volcanic belts one near the plate boundary and another much further inland.

How Scientists Found It

Ancient volcanic arcs are often hidden under thick sediment layers, making them hard to detect. Researchers used airborne magnetic surveys to scan beneath the Sichuan Basin, identifying iron-rich rocks with strong magnetic signals about 4 miles (6 km) below the surface.

Deep borehole drilling confirmed volcanic arc chemistry in these rocks, while uranium-lead dating revealed their age to be between 770 and 820 million years old.

This Discovery Matters

Volcanic arcs are crucial to understanding crustal growth and mountain-building processes. This finding suggests that ancient volcanic arcs in South China were far more widespread than previously believed.

The immense volume of magma and its distribution highlight significant crustal expansion during the Neoproterozoic. Some geologists even suggest the two volcanic belts may have formed independently and later merged opening up new perspectives on early tectonic evolution.

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