Do you know how many meteorite craters have been discovered in China? Only 3, all in the three northeastern provinces!

Our Earth has been visited by "extraterrestrial visitors" since its birth. The threat of "asteroid impact" has never really disappeared.

Since 1988, there have been an average of 2-3 fireball events recorded on Earth every month. These meteoroids that enter the Earth's atmosphere are usually very small, and most of them will burn up in the atmosphere. The largest one (red dot) was the Chelyabinsk impact on February 15, 2013. The asteroid was estimated to be nearly 20 meters in diameter. It caused some damage, but no one died. | NASA/JPL-Caltech [1]

However, there are not many direct products of the impact event - " meteor craters " (or "impact craters") left on the earth. Only about 200 have been discovered so far.

Distribution of large meteorite craters currently discovered on Earth (incomplete statistics). Note that the size of the circle is not the actual size of the crater, but is proportional to the actual size of the crater | Wiki

If someone asks, how many meteorite craters have been discovered in China ?

Before this century, the answer was none. About 10 years ago, the answer was one. Two years ago, the answer was two. Now, that number has jumped to three .

In a paper published online on September 5, 2023, the team of researcher Chen Ming confirmed that a circular depression located in Baijifeng National Forest Park in Tonghua City, Jilin Province is actually a meteorite crater formed by the impact of a small celestial body [2] - this is the third meteorite crater discovered in China so far .

Baijifeng crater photographed by drone | Chen Ming research team

The other two are the Xiuyan crater in Anshan City, Liaoning Province, with a diameter of about 1.8 kilometers[3];

A bird's-eye view of the Xiuyan crater from 800 meters above the ground | People's Daily Online

The other is the Yilan crater located in Yilan County, Harbin City, Heilongjiang Province, with a diameter of approximately 1.85 kilometers.[4]

Panoramic view of the Yilan crater taken by a drone at an altitude of 500 meters | References [4]

All three meteorite craters were confirmed by Chen Ming's team. (Well, one for each of the three northeastern provinces, this is very fair)

The "Baiji Peak" that came out of the legend

Unlike the almost "dead" moon, there are still various active geological activities and animal and plant activities on Earth. Therefore , the craters formed on Earth are more likely to degenerate and be erased, making them more difficult to find and confirm .

In some ways, searching for ancient meteorite craters on Earth is a more challenging but also more interesting task - we can sometimes find clues in history books, classics and legends .

Baijifeng National Forest Park is named after the two peaks in the park: Front Baiji Peak and Back Baiji Peak .

Front Baiji Peak (left) and Back Baiji Peak (right) | Image source: Google CNES / Airbus Maxar Technologies Landsat / Copernicus Data SIO, NOAA, US Navy, NGA, GEBCO.

According to the local forestry bureau[5]:

According to legend, thousands of years ago, an alien meteorite fell on two peaks in the park . The stone is hard and silvery white like snow. Because the two mountains are shaped like chickens, they are named Baiji Peak.

Chen Ming's team's research confirmed that this legend is largely reliable: many years ago, a small celestial body hit this place at high speed, forming a meteorite crater with a diameter of about 1.4 kilometers . However, it is very likely that the "meteorite did not fall on the two peaks", but the shock wave caused by the impact changed the original terrain here, forming the two Baiji Peaks in front and behind.

The specific location of the newly discovered Baijifeng crater is 41°33'07" north latitude, 126°06'40" east longitude. Judging from the aerial images, the crater is not very round.

Google Earth image of Baijifeng crater

Certification criteria for Earth's impact craters

However, whether it is round or not is the least important thing when it comes to determining the Earth's meteorite craters. It's fine as long as it is almost round (many that are rounder than this are not round at all).

On the contrary, due to the constraints of the local geological structure before the impact, it is not uncommon for small earth impact craters to be non-circular. The most typical example is the Barringer crater with a diameter of 1.2 kilometers, which is almost square.

Barringer Crater, one of the earliest confirmed impact craters on Earth | Wikipedia

What is the important criterion? It is the traces of rocks being subjected to high temperature and high pressure in a short period of time !

Among these traces, the most common is the shatter cone , which is a typical product of a small celestial body impact or nuclear explosion. Rocks under pressure of 2-30 GPa may form this kind of cone-shaped divergent lines, which is a macroscopic impact landform that can be identified by the naked eye .

A typical shatter cone looks like this:

(Left) A fragmentation cone found in the Charlevoix crater in Quebec, Canada | Wiki by JM Gastonguay; (Right) A fragmentation cone found in the Ries crater in Germany | Wiki by Johannes Baier

Furthermore, there are some common rock impact metamorphic products that can be counted, such as coesite, stishovite, micro-diamond, etc., but these can only serve as auxiliary evidence that high-pressure impact has occurred here, rather than solid evidence.

The most critical proof is a feature called planar deformation features (PDFs), which is a microscopic feature that can only be seen under a microscope.

In the grains of silicate minerals (usually quartz or feldspar) that have experienced the high-pressure shock caused by meteorite impacts, one or more groups of linear cracks parallel to different directions may be produced, and the spacing between parallel cracks is on the order of tens to hundreds of microns.

Typical planar deformation structures in quartz grains, from: (left) Bosumtwi crater in Canada | Reference [6] (right) Keurusselka crater in Finland | Reference [7]. The pictures are all parallel to two different directions.

This is a product that can only appear at the site of high-pressure impact , and it is also the most important evidence on Earth for determining whether a pit is a meteorite crater .

Both the Xiuyan crater and the Yilan crater were truly recognized as "meteorite craters" only after the surface deformation structures were detected.

Planar deformation structures detected in the Xiuyan crater:

Quartz grains in the Xiuyan crater granite debris show planar deformation structural features under orthogonal polarized light, micrograph | Reference [8]

Planar deformation structures detected in Yilan crater:

Quartz grains in the granite breccia of Yilan Crater show planar deformation characteristics under orthogonal polarized light, micrograph | Reference [4]

This time is no exception. To say that it is a "confirmed discovery" is a no-brainer that Chen Ming's team did find planar deformation structures in the rocks of the Baijifeng crater through geological surveys and sampling.

Planar deformation structures (solid white lines) detected by optical microscopy in quartz grains collected from the Baijifeng crater, cross-polarized light | Reference [1]

When was the Baijifeng crater formed?

As for when the Baijifeng crater was formed, there is still a lack of more detailed radioactive dating conclusions. However, there are some geological evidences that can serve as constraints:

1) The granite basement of Baijifeng was formed between 172 million and 150 million years ago (Jurassic period), so the impact of Baijifeng should have occurred after the formation of these granites;

2) The rock debris layer covering the rim of the impact crater is well preserved, indicating that the impact structure is very young (in terms of geological scale, generally millions of years is very young) and has not been geologically eroded for too long;

The impact debris on the top of Qianbaiji Peak has sharp edges and is obviously not subject to long-term geological erosion. References [2]

3) Compared with the Yilan crater of similar diameter, both craters were formed in the temperate continental monsoon climate zone and are covered with vegetation, but the granite base of the Yilan crater was formed 49,000 years ago, and the sputtering debris near the crater edge has been severely eroded. In contrast, the Baijifeng crater, which is better preserved, was formed at least not much earlier than the Yilan crater .

In fact, the density of impact craters found per unit land area in China is far lower than the world average, which means that there are still more "hidden" impact craters that have not yet been discovered and confirmed . This is destined to be a very time-consuming and laborious task, but it often fails, because not all circular concave structures are meteorite craters. But each meteorite crater that has been certified after hardships is so fascinating, with so many stories connecting history and the present, legend and reality.

I hope that more and more remains of meteorite craters can be discovered in our country’s beautiful rivers and mountains, and I also hope that the local tourism bureau will explore and display these tourist resources. Anyway, Xiuyan, Yilan and Baiji Peak are already recorded in my little notebook.