Human beings are very interested in anything from extraterrestrials. Even "stones" that cannot speak are also very attractive. People even compare the large craters formed after meteorites hit the earth to "kiss" "marks", thinking it is the best witness to wasted years .
The crater left by mysterious alien objects to mankind
What we are going to talk about today is that 49,000 years ago, a meteorite with a diameter of 100 meters hit our country, forming the huge crater. So, what's going on there now?
The huge crater under the satellite map
The young Yilan crater
First of all, let’s take a look at the classification of craters.
Since the number of craters on the earth is really limited, scientists also use the moon, Mars and other places as references when determining the crater classification standards, and classify craters into three types, namely simple craters, complex craters and polycyclic basin .
There are three types of craters
There are obvious differences in the size and appearance of these three .
The diameter of a simple crater is less than 4 kilometers, and the cross section looks similar to the shape of a bowl, which is more in line with everyone's idea of a crater. The ratio of complex pit depth to diameter is relatively small, and the diameter is several times larger than that of simple pits, but the overall appearance is flatter. As for the multi-ring basin, it is also called a giant crater, with a diameter of more than 100 kilometers, which is extremely rare on earth.
Multi-ring basins on the moon
So, what kind of crater does the crater we are talking about today, which was formed by the impact of a 100-meter meteorite, belong to?
This crater is located in Yilan County, Harbin City, Heilongjiang Province, Northeastern my country, on the edge of the southwest edge of the Lesser Khingan Mountains . Most of it is covered by birch forests.
Judging from the diameter of this crater, its diameter is only 1.85 kilometers, and its cross-section is obviously bowl-shaped , which is consistent with the characteristics of simple crater .
Meteorite impact relics of Yilan Crater
Data show that Yilan Cratermost of the crater rims are in good preservation condition, and the crater once developed into a lake. Based on the carbon 14 age analysis of soil and lake sediments at the bottom of the pit, scientists concluded that the lake disappeared between 11.14 and 8.34 thousand years ago.
In the eyes of many people, a simple crater is nothing to study, but this simple crater is the second crater discovered in our country and recognized by the international academic community , so it is of extraordinary significance to us.
And based on the field investigation and research of Chinese scientists, we also came to the conclusion that the Yilan Crater is not big, but is very "young" .
Contour map near Yilan Crater
Scientists believe that the impact that caused the formation of Yilan Crater occurred 249,000 years ago. The diameter of this extraterrestrial visitor is about 100 meters . Based on data such as the age of existing craters and size comparisons, the Yilan meteorite impact can indeed be regarded as the largest impact event that has occurred on Earth in the past 100,000 years.
At that time, human ancestors had entered the Stone Age and may have entered the Yilan area. Therefore, although there is no written record, these human ancestors are very likely to have witnessed the impact event, and were also affected by the impact event .
The "Crescent-shaped" Yilan Crater
However, when we pull back the time to modern times 49,000 years later, and then return to Yilan County to see this crater, it is actually difficult to see it directly. outlines and . After all, there are still small parts of the pit that have been damaged by time.
In addition, since the geotectonic structure of this area belongs to the East Asian Orogenic Belt, the structural and magma activities have been relatively intense since the late Neoproterozoic. Therefore, the Yilan Crater is located on the bedrock of Cretaceous granite. .
Schematic diagram of the geological structure of Yilan Crater
Speaking of this, we mentioned in the previous article that there is a large area of birch forest. In fact, in addition to the birch forest in the crater area, some areas have also been developed into cultivated land. Corn and soybeans are grown in it.
In this case, the presence of the "fresh kiss" left by the meteorite on the earth is not so obvious. At least in the eyes of the locals, before scientists announced the relevant findings, they did not regard it as a crater.
The lush green Yilan Crater
It is worth mentioning that scientists are trying to restore the crater formation process based on the granite impact breccia found at the bottom of the Yilan Crater. finally established the relevant numerical model and obtained the meteorite characteristics of the impact and the crater formation process .
Crater formation process
Scientists established a two-dimensional axisymmetric calculation model based on the vertical impact of a spherical asteroid on the ground. In order to record the formation process of impact melt in the crater, many tracking points were also subdivided. In this case, the formation conditions of the Yilan Crater were derived.
Conceptual diagram of a meteorite hitting the earth
First of all, the diameter of the meteorite hitting here is about 100 meters, and the speed is about 15 kilometers/second. In this case, the crater formation process has gone through three stages, respectively It is the contact and compression stage, the pit opening stage and the post-adjustment stage .
During the contact and compression stages, meteorites that entered the earth at super high speed began to invade downward after contacting the surface, and formed a high-temperature and high-pressure area in the area, making the radius and depth of the entire crater continue to increase. Therefore, it does not mean how big the meteorite is, the crater formed is exactly the same. The size of the crater is often 1 to 2 times more than the direct size of the meteorite .
Schematic diagram of the contact and compression stage
After this stage, it enters the pit opening stage. At this time, a large number of backsplash projectiles are formed, and their angles are also continuously reduced. The crater opening phase was not declared over until the bottom of the crater completely stopped growing. According to the simulation results, the depth of the Yilan Crater at this stage of may have reached 440 meters, which is much larger than the average depth of 150 meters measured today..
Schematic diagram of the pit opening stage
Finally entered the post-adjustment stage. At this time, the temperature value in the crater was still very high, and some of the materials in it slipped due to gravity, and finally gathered at the bottom of the crater . As a result, the bottom of the pit continues to rise, and gradually the depth of the pit becomes less deep than in the previous stage. Comparison of the
simulation results with the exploration results of the Yilan Crater shows that the Yilan Crater was most likely formed by a 120m-diameter globular granite asteroid impacting the earth's surface vertically at 12km/s, forming a crater with a final diameter of 1840m and a crater rim A simple crater with a depth of 263m.
Schematic diagram of material flow during crater formation
In addition, the simulation results also restored the dynamic cratering process of Yilan Crater. found that the mass of the melted layer produced by the impact could reach 5.64×10^10 kg , which is about about 224 times the mass of the 100-meter meteorite.
So, what can the study of craters bring us? Why is it that our country is so large, but the only two recognized meteorite craters so far are the Yilan Crater and the Xiuyan Crater that was previously discovered in Anshan City, Northeast China?
There is also a village inside the Xiuyan crater
It is very difficult to obtain an "ID card" in the crater
In fact, when it comes to the craters in our country, most of them People will subconsciously say that these two are not the only craters in our country, because in addition to those discovered by the private sector, there are many "candidate craters" proposed by the research community, such as Baisha Crater in Hainan, Duolun Ring in Inner Mongolia, and Taihu Lake. region and so on .
There are so many alternatives. In the end, and were recognized, but only these two craters in the Northeast were .
Aerial photos of the Xiuyan Crater
The reason why this happens is actually because to determine whether it is a crater, you need to examine various signs, such as landforms, mineral rocks, geochemistry and other aspects are within the discrimination mechanism, among which The mineral rock logo is the most important , and the remaining two are auxiliary logos.
Internationally, if a crater wants to be certified, it must have one of three characteristics: mineral shock planar leaf texture, mineral shock glass, and high-pressure polymorphism formed by impact.
Shock metamorphic samples of rocks in the Yilan Crater
And as we said in the previous article, there are very few craters on the earth compared to other planets. This is mainly due to active geological activities and the effects of weathering and erosion. , here In this case, a simple pit with a smaller diameter will lose its original appearance soon.
Therefore, in fact, not only in China, but also in many craters around the world, the road to obtaining identity is very bumpy. For example, the Barringer Crater in the United States and the Rees Crater in Germany have been regarded as caused by volcanic activity before they were recognized.
The 1,264-meter-wide Barringer Crater
However, even if it is difficult to obtain certification, we must continue to study, because the craters may contain rich mineral deposits, and as ancient "hickeys" on the earth, they are still It can reveal the evolution of geological history, geochemistry and other fields .
One of the witnesses of the Xiuyan meteorite crater in Liaoning Chen Ming said, "In addition to forming minerals and lakes, the craters are also museums and tourist attractions, and are of great scientific significance. The rich information about the universe they bring is also extremely precious. , In the United States, craters are an important training base for astronauts. "
Meteor craters are also one of the keys to mankind's understanding of the universe.
