10,000-year-old impact crater in China reveals meteorite strike equal to 40 atomic bombs

A quiet hillside in southern China holds the mark of something that arrived with enormous force from far beyond our planet. The Jinlin crater, tucked into the low mountains of Guangdong Province, offers a rare chance to see what happens when a fast-moving object from space slams into Earth.

The feature stretches almost a kilometer across and rests inside a thick layer of weathered granite that blankets the region. Its smooth, bowl-shaped form tilts gently to the southwest, mirroring the slope of the mountainside where it formed long ago.

Upon first glance, the crater looks like a natural depression created by rain and time. Upon further inspection, everything changes. The rim of the depression rises and falls steeply, with nearly 200 meters of elevation difference between the highest point on the uphill side and the lowest part on the downhill side.

The bottom of the depression is about 160 meters above sea level and drops roughly 90 meters from the top of the rim. The soil is loose and consists of broken pieces of granite, some of which are as large as a small truck. Despite the land being battered by thousands of years of storms and erosion, the overall structure is still intact and holds evidence of a violent origin.

The best evidence for this origin is something you cannot see with your naked eye. Scientists analyzed quartz grains picked from the debris field in the crater and discovered microscopic striations, or planar deformation features. Such stripes occur only as a result of quartz being subjected to shock waves that are strong enough to reposition the internal structure of the mineral.

These pressures can only occur as a result of an object that travels faster than a bullet impacting the ground. Over half of the planes in these quartz grains correspond to the same crystallographic patterns found at recognized impact sites globally. Nothing created through normal geological processes on Earth can form these features.

Scientists from Shanghai and Guangzhou described the site in the journal Matter and Radiation at Extremes. They concluded that it was likely a 30-meter-wide meteorite was responsible, traveling at speeds between 15-25 kilometers per second. The impact released energy equivalent to 600,000 tons of TNT – comparable to the destructive force of 40 Hiroshima-sized atomic bombs.

A comet of that size would have made a crater at least ten kilometers across, and so they ruled that out. The meteorite's composition is still unknown, although it was likely made of rock or iron.

There have only been about 200 confirmed impact craters on Earth. Most are erased over time because the surface of the planet is always in motion, altered by erosion, storms, shifting plate boundaries, and plant growth. In southern China, those erasing forces are at work even faster.

The region is located within the tropics of cancer, felt more severely during the summer monsoon. The combination of heat, humidity, and rainfall degrades granite rapidly, leaving behind a deep crust of loose soil. That weathering crust can be 80 meters thick and consists of a mixture of soil, rounded granite pieces, and deeper layers that crack and round into spherical forms.

The Jinlin crater is located within a granite body, called a batholith, from the Cretaceous period. The rim of the crater retains a sharp bowl shape despite an abundance of rain in the area. Further, the gullies cutting into the rim of the crater indicate that the crater is shaped and enlarged by continued storms. The sediment can drain out of the bowl shape of the crater because of a vertical groove on the southern side that creates a V-shaped drain. However, despite the ongoing process of change, the identity of the basin remains intact, and the overall shape is observable.

The evidence for preservation indicates a young age. In this section of China, granite weathers at a specific rate, and for example, a fragment of rock less than 30 centimeters would typically completely decompose into loose soil in less than 10,000 years. Nonetheless, there are many pieces remaining that have only partially weathered or decomposed.

This indicates to the researchers that the origin was likely during the early or middle Holocene, which commenced roughly 11,700 years ago. Most of the craters that have been discovered from the Holocene time frame are significantly smaller, generally no wider than 300 meters.

Therefore, the Jinlin crater diameter of roughly 900 meters is the most substantial known impact site from this time. Ming Chen, one of the lead researchers, stated that this will indicate that there are larger impacts during the Holocene than previously believed by researchers.

When the meteorite impacted the surface, it would have penetrated the surface between one to two times its own diameter to the point it reached the shallow or weathered soil, meeting solid granite. The explosion would have ejected debris radially in all directions.

The thick, weathered crust soil would have been ejected radially, with some of it contributing to the crater rim as it was expelled. It appears that the impact site on the upslope side likely had a thinner weathered layer, where excavated soil would be able to penetrate deeper into intact granite. Material from both weathered granite layers likely fell back into the crater or rested nearby, since weathered granite has a low density, which slows how ejected debris returns to the ground.

The crater is situated on a hillside that rises to approximately 630 meters. The slope may have influenced how debris spread during impact, and it appears to have tilted the final bowl to the southwest. Over thousands of years, water channeled through the southern gap in the crater, cutting the rim and carrying soil down slope. Despite this, the feature has survived as one of the most intact craters yet identified in such a wet and severely weathered setting.

Before this discovery, China only had four confirmed impact craters, all located in the northeastern quadrant of the country. None had been identified in the larger southern portion of the country. The discovery of the Jinlin crater opens the possibility of more impacted or undiscovered sites beyond the region.

It is also an additional opportunity to study the impact lifecycle in the context of how different soils, weather, and rock types influence the crater process. Improved understanding of how to acquire the geological history, including those events that have completely changed landscapes and modified local ecosystems.

The Jinlin crater is useful for better understanding how frequently Earth has had medium-sized impacts over the Holocene. Better frequency estimates and improved models are required to predict the frequency of similar impacts in the future and how close, or what impacts the probability of such impacts.

Improved estimations of impact rates can help in planning planetary defense, a new and growing field which is aimed at detecting and tracking all things that could impact Earth. In addition, the site represents a natural laboratory study of how craters survive and exist in a humid and highly dynamic setting.

Knowledge and insights gleaned can not only help to identify other hidden or eroded craters around the world but may also provide key details on gaps in the geological record and understanding of Earth's biological, physical, and environmental history over extended periods of time.

Research findings are available online in the journal Matter and Radiation at Extremes.

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