A groundbreaking discovery in southern China has revealed a 400-mile-long chain of extinct volcanoes buried deep below the Sichuan Basin, a region historically part of the Yangtze Block. The findings, outlined in a recent study published in the Journal of Geophysical Research: Solid Earth, shed light on the ancient volcanic activity that took place during the early breakup of the supercontinent Rodinia about 800 million years ago. This discovery has important implications for understanding Earth’s tectonic processes and the role these ancient volcanoes played in the planet’s geological history. The study’s authors, including Zhidong Gu of PetroChina and Junyong Li of Nanjing University, used advanced geophysical techniques to uncover fossilized evidence of volcanic chains formed during a collision between tectonic plates.
The Tectonic Forces Behind the Volcanic Arc
The newly discovered volcanic chain formed as a result of plate tectonics, a dynamic process that has shaped the Earth’s surface for millions of years. Around 800 million years ago, during the early Neoproterozoic era, the Yangtze Block, part of Rodinia, drifted toward the China Ocean Plate. The collision of these two plates led to subduction, a process in which the denser oceanic crust was forced beneath the lighter continental crust. As the oceanic crust sank, it heated up and released water, which in turn generated magma. This magma then rose to the surface, forming a long, narrow chain of volcanoes along what is known as a volcanic arc.
The unique geological features of the newly discovered arc have sparked significant debate among researchers. The Yangtze volcanic arc is notably broader than other volcanic arcs, such as the Cascades along the Juan de Fuca Plate in North America. This anomaly has been attributed to a tectonic process known as flat-slab subduction, where the oceanic plate moves horizontally beneath the continental plate at a shallow angle before sinking into the Earth’s mantle. This style of subduction causes two distinct volcanic ridges to form: one near the subduction boundary and another further inland. The study’s findings, while groundbreaking, have prompted alternative theories, with some experts suggesting that the volcanic chains could have formed through different tectonic mechanisms.
The Role of Flat-Slab Subduction in the Formation of the Volcanoes
Flat-slab subduction plays a critical role in shaping the geophysical characteristics of a region. In this particular case, the subduction of the oceanic plate beneath the continental plate at a shallow angle created two distinct volcanic ridges. The first volcanic ridge is located near the boundary where the oceanic plate initially slips beneath the continent, while the second ridge is located further inland, where the oceanic plate finally sinks deeper into the Earth’s mantle. This two-ridge pattern is unique and presents a fascinating challenge for geologists trying to understand the tectonic processes at play.
The breadth of the Yangtze volcanic arc is a significant departure from the more typical narrow volcanic arcs observed along continental margins. Peter Cawood, an Earth scientist at Monash University in Australia, agrees that flat-slab subduction could explain the inland formation of these volcanoes. However, he proposes an alternative theory, suggesting that the two volcanic belts might not have been part of a single arc system but rather separate, time-equivalent systems that merged later in Earth’s history. According to Cawood, “It could be that the two belts are not part of one broad arc system and flat slab, but represent two independent but time-equivalent systems that were sutured together.”
Geophysical Techniques Used to Uncover the Volcanic Arc
The study of ancient volcanic arcs often faces significant challenges, especially when trying to identify geological remnants buried deep beneath layers of sediment. In this case, the research team employed cutting-edge geophysical techniques to detect the volcanic chain hidden beneath the Sichuan Basin. One of the key methods used was an airborne magnetic sensor, which allowed the researchers to “see” the underground rock formations beneath several kilometers of sedimentary rocks. This technique works by detecting the magnetic properties of different rock types. Iron-rich rocks, commonly found in subduction zones, produce a strong magnetic signal, which the researchers used to map the underground volcanic arc.
The airborne magnetic sensor revealed a continuous strip of iron-rich rock stretching 430 miles (700 km) from the northeast to the southwest of the Yangtze Block. The discovery also extended the volcanic arc further inland, up to 550 miles (900 km), offering a more comprehensive picture of the area’s ancient volcanic activity. Further confirmation of the volcanic origin of these rocks came from analyses of rocks obtained from deep boreholes drilled into the uppermost crust of the Sichuan Basin. These borehole samples showed chemical signatures consistent with those found in rocks formed by volcanic activity, dating them to between 770 million and 820 million years ago.
Implications for Earth’s Climate and Carbon Cycle
The volcanic activity that formed this chain of fossilized volcanoes likely had significant implications for the Earth’s carbon cycle and its climate during the Neoproterozoic era. Volcanoes release carbon dioxide (CO2) into the atmosphere, which can contribute to global warming. However, the process of mountain weathering can remove CO2 from the atmosphere by chemically breaking down the rocks on the Earth’s surface. These two processes—volcanic CO2 release and mountain weathering—work in tandem to regulate Earth’s climate over long periods.
Scientists believe that the Neoproterozoic era witnessed a significant shift in the global carbon cycle, which could have had widespread effects on Earth’s climate. The discovery of the Yangtze volcanic arc adds a new layer of complexity to our understanding of this climate transition. According to Cawood, “The work presents an ‘exciting new set of data in a region that has been difficult to study.’ It shows that the volume of magmatic activity along this boundary may be considerably greater than previously realized.” Cawood also emphasized the importance of evaluating the potential impact of this volcanic activity on Earth’s climate during the period, stating that “its impact on Earth’s past climate should be evaluated.”
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