On February 7, 1966, the people's Daily called for learning from Comrade Jiao Yulu. One of the important work that Comrade Jiao Yulu did during his term of office was to control the desertification of Lankao in Henan Province. Some people may wonder, Henan is far away from the northwest desert. Where does the sand come from? Listen to our experts to explain it to you
Desert refers to sandy desert, covering about 1 / 3 of the earth's land surface. Desert area has few vegetation and arid climate, which can be called "forbidden zone of life". The desert is a continuous sand dune in the natural landscape, and the big desert is also called "sand sea", which vividly expresses that the desert is composed of vast sand. Where does so much sand come from in the desert?
There are at least four explanations for this issue. First, many of the earth's deserts lie behind tall mountains or in basins surrounded by mountains. The former is like the Nevada desert behind the Rockies in the United States, the latter is like the Taklimakan Desert in China and the Gurbantunggut Desert in Junggar basin. The source of these desert sands has a common feature - it is closely related to nearby or surrounding mountains. The altitude of these high mountains is more than 3000 meters, and the temperature difference between day and night is large. Influenced by 'thermal expansion and cold contraction', the rocks on the mountains are easily broken, producing a large amount of rock debris. If the altitude is higher, that is to say, the rock mass enters into the cryosphere, more clastic material will be produced after the weathering of the rock due to the addition of glacial abrasion.
These clastic materials will eventually be brought out of the mountains by glacial melt water or flood and rivers, thus forming large, fan-shaped rock clasts in front of the mountains. If the piedmont area of the mountain range happens to be a dry and windy natural environment, these coarse and fine debris will be sorted under the long-term wind erosion. The coarse gravel is not easy to be blown by the wind, and it stays on the surface to form a Gobi, while the sand that can be rolled up by the wind gathers into sand dunes, and more and more sand dunes gather into deserts. Therefore, for the above-mentioned deserts, the weathering and fragmentation of rocks from nearby mountains is the main reason for the production of massive sand. The most typical representative is the Taklimakan Desert known as the sea of death.
There are many kinds of sand dunes in the desert, the most common one is crescent sand dune, which presents the shape of gentle windward slope and steep leeward slope. Others are pyramidal dunes, linear dunes, parabolic dunes and more complex compound dune chains.
Second, some deserts are not surrounded by mountains, but they can form deserts, such as the Horqin Sandy Land in Inner Mongolia, China. The source of sand in these areas is closely related to the fluvial process in the geological period. If an area is located on a large alluvial plain formed in geological period, and the climate is just in the arid area, then under the long-term wind force, the materials that make up the alluvial plain will also be sorted, so that more and more sand will gather on the surface and form a desert.
Third, some deserts are neither behind the high mountains nor on the large-scale alluvial plain in the geological period, so where does the sand of the above deserts come from? We know that there are some rocks, such as sandstone, mainly composed of sand in the geological period itself. After physical weathering of sandstone, it is easy to produce fine-grained sand. In Ordos Plateau of China, there are a large range of Cretaceous sandstone. There is a view that the sand in the Mu Us Desert of the Ordos Plateau is weathered from the Earlier Cretaceous sand mounds. Secondly, granite on the earth's surface is easy to be weathered to produce sand. If the surface of an area is mainly composed of sandstones, and the surface is in arid environment, then under the long-term physical weathering of rocks, the above-mentioned rocks gradually break up, producing a lot of sand, which can also form deserts.
The formation of desert oases cannot be separated from rivers. There are generally tall mountains around many deserts. Rivers from mountains will travel thousands of miles, and eventually reach into the desert, so life exists.
Fourthly, there are deserts on the west coast of the continent near the Tropic of cancer and the Tropic of cancer, forming coastal deserts, such as the Atacama Desert in South America and the Namib Desert in Africa. The formation of deserts in these areas is influenced by land, ocean current and weather system. The local climate is extremely dry. The sand source of the coastal desert is related to the long-term erosion of the Bedrock Coast by the waves and the physical weathering of the bedrock.
In a word, no matter where and when the desert is formed, in the final analysis, the ultimate source of sand is related to the clastic material produced by the breaking of rocks on the earth.