Geologists have found evidence of a 400-mile-long chain of extinct volcanoes hidden deep beneath the Sichuan Basin in the Yangtze Block of South China.
The team led by Nanjing University, China, revealed that the “fossilized volcanoes” are part of the Tonian magmatic arc belt that formed between 820 and 770 million years ago.
The findings could shed new light on the region’s ancient geological history and potentially its impact on Earth’s climate hundreds of millions of years ago.
“We present a data set of deep boreholes, geochronology, geochemistry, and geophysics to investigate the basement properties and evolution of the Sichuan Basin in the Yangtze Block, South China,” the researchers wrote in the study paper.
“These results provide evidence for a large Tonian magmatic arc belt hidden within the Yangtze Block’s interior,” it added.
Long volcanic arc
The study reports that these ancient volcanic behemoths formed during the tumultuous breakup of the supercontinent Rodinia, roughly 800 million years ago.
The plate tectonic shifts caused the South China region to separate from the supercontinent Rodinia, forming the Yangtze Block plate.
This plate then collided with the China Ocean plate, which triggered a process called subduction.
During subduction, the denser oceanic crust sank beneath the continental crust, generating intense heat and pressure that led to the formation of magma.
As this molten rock rose to the surface, it created a long, curved chain of volcanoes – a volcanic arc.
These geological powerhouses not only built new crust but also significantly altered the existing one.
But these ancient volcanic mountains haven’t been visible for eons, buried deep beneath kilometers of sedimentary rock in what is now the Sichuan Basin.
To investigate the hidden ancient volcanoes, scientists used “airborne magnetic sensors.” The sensors allowed them to detect a 430-mile-long, 30-mile-wide iron-rich strip of rock that exhibited a stronger magnetic field than its surroundings.
It was a clear indicator of past volcanic activity. Live Science reported that the belt is located about 4 miles beneath the surface in the lower crust.
The presence of these iron-rich rocks indicates a past subduction zone, as they are typically formed above subducting oceanic crust.
As per the study paper, further evidence came from analyzing rocks extracted from seven deep boreholes (∼3,600–6,500 m) drilled within the Sichuan Basin.
These samples were confirmed to be magmatic in origin, with a chemical composition consistent with rocks formed by arc volcanoes.
Radiometric dating placed the formation of these rocks between 770 and 820 million years ago – precisely during the period of Rodinia’s breakup.
The sheer extent of this newly discovered volcanic chain is surprising.
Most volcanic arcs tend to form narrower belts along continental margins, while the newly identified ancient volcanic arc stretches 500 miles inland.
Reportedly, the researchers suggest that a different style of tectonics, known as flat-slab subduction, may have been at play.
In this scenario, the oceanic plate slides horizontally for a long distance before finally sinking, potentially creating a much broader zone of volcanism.
Another possibility is that two separate volcanic systems merged over millions of years.
The discovery of such a massive volcanic system has implications for understanding Earth’s ancient climate.
The immense scale of this ancient volcanism in South China could have influenced the global carbon cycle and potentially contributed to climate instability during this critical period in Earth’s history.
The findings were published in the Journal of Geophysical Research: Solid Earth.