The teacher once said in the nature class that he lost his way in the wild and could find the direction by distinguishing the position of the sun and the North Star. In the forest, canopies or annual rings can also help. But what to do on the overcast Gobi Desert?
A scientist in the United States who studies the weathering of rocks in desert areas says that cracks in certain rocks may serve as a natural compass to point people in the direction.
Scientists have discovered that whether in Iraq or in the dry desert areas of the southwestern United States, a large amount of dark and fragmented gravel often appears and spreads into the distance like a naturally formed highway. The gravel was created by the collapse of huge rocks that dot the wilderness. In wet areas, stones often crack due to the power of water. Because water can penetrate the inside of the stone, once it freezes, the expansion can crack the hard rock. But in these areas, the temperature is not enough to freeze the water, and the cause of weathering and cracking of the rocks seems somewhat mysterious.
Geologists originally speculated that this weathering was caused by salt erosion. These crystalline salts are caused by the water that had penetrated into the stones earlier. But this type of weathering generally occurs near the sea, and it seems to be insufficient to explain the weathering phenomenon in the desert.
Finally, the scientists focused their eyes on the scorching sun above their heads. The scorching is likely to be the culprit of these rock breaks. If you burn a stone on fire, the silicate-based rock has poor thermal conductivity, the outer layer becomes very hot, and the inner part is quite cold. This temperature difference can cause the stone to collapse. Although fire rarely occurs in the desert, the heat from the sun is already considerable. Some dark rocks can reach 80 ° C during the day, which is enough to cause weathering of desert rocks.
However, the temperature difference between the stones exposed to sunlight is uneven on both sides, especially in the early morning. The shady side of the rock is still cold, and the rising side has become very hot. Therefore, the crack in the rock must extend along the dividing line of the rock towards the sun in the early morning, and if its shape is regular enough, the crack must indicate the north-south direction.
The scientists believe that this theory can also explain weathering in other regions. Buildings and other man-made objects also form cracks that extend along the light-shadow boundary. Even the weathering we have observed on Mars can be explained this way.