What is the landform of the Loess Plateau? Where does the loess on the Loess Plateau come from?

The magnificent Loess Plateau stretches for thousands of miles, and countless literati have been h

What is the landform of the Loess Plateau? Where does the loess on the Loess Plateau come from?

The magnificent Loess Plateau stretches for thousands of miles, and countless literati have been here for thousands of years to recite poems and paint. While people are admiring, they can't help asking: Where did the loess on the Loess Plateau come from?

The Loess Plateau in northwest China extends to Taihang Mountain, borders Hebei Province and Shanxi Province in the east, borders Wushaoling in Gansu Province and Sun Moon Mountain in Qinghai Province in the west, reaches the vast area north of Guanzhong Plain in Weihe River Basin in the south, and reaches the Great Wall in the north, accounting for about 1/20 of China's land area.

The altitude of the Loess Plateau is about 65,438+0,000 ~ 65,438+0,500 meters, and the loess on the plateau is mainly loose silt without bedding. Thick loess completely covers the early terrain here. The soil layer is 30-50 meters thick, and the thickest part even exceeds 200 meters. Loess gradually becomes thinner from northwest to southeast, and the particles become thinner from coarse to fine.

This loess landform can be seen in many parts of the world, such as Europe, North and South America, but the area and thickness can't be compared with the loess plateau in northwest China. Loess is rich in calcareous nodules and soluble salts. Many fine-grained minerals such as timely, mica, feldspar, tourmaline, amphibole and epidote are the main components of loess, accounting for about 70%, and the rest are clay minerals. Where did such a large area of loess come from? How is it formed?

In order to explain these problems, geologists have studied and put forward more than 20 hypotheses of loess formation by using the knowledge of stratigraphy, paleontology, paleoclimate, material composition and chronology. At present, there are four theories that have great influence, namely, water formation theory, residual product theory, wind formation theory and multi-genetic theory. The main difference between these four theories lies in the source of loess material and the nature of loess itself.

Most scholars agree with the view of wind theory. It is particularly worth mentioning that Mr. Lu Xun also supports this view. Mr. Lu Xun wrote in a geological essay: "The Loess Plateau in China was formed by the wind blowing in the Central Asian desert in the early Quaternary, which turned the river yellow and became the Yellow River." Modern scholars have analyzed the basic characteristics of loess materials on the basis of a large number of facts, and think that a large area of desert in China may be the source of loess, and the main driving force for carrying loess materials is wind. The formation process of the Loess Plateau is a comprehensive geological process in geological history, which has three different stages: material source formation, transportation, sorting and accumulation into soil.

Geologists believe that in the late Tertiary or the second half of the early Quaternary, today's loess plateau has a humid and rainy climate, many rivers and lakes basins, and various flowing water geological processes prevail. Under the action of river water, a large number of flood, alluvial, lacustrine, slope and ice deposits have accumulated in the low-lying basins in bedrock mountainous areas, and the loose gravel and soil-like mixed deposits are getting thicker and thicker, and loess materials have a foundation for growth.

In the second half of Quaternary, about 654.38+0.2 billion years ago, the global climate changed and became cold sharply, from wet to cold and dry, and a new ice age came. Under the influence of Siberian-Mongolian high-pressure air flow, the cold air in northwest China drives straight in, and under the influence of Qilian Mountain, it is divided into two streams, one of which turns to the southeast, forming a northwest wind to enter Ordos area; The other branch forms the northeast wind direction and southwest wind direction, and enters Tarim Basin and Qaidam Basin.

At the same time, the west wind from Inner Mongolia Autonomous Region and the northwest wind from Siberia respectively entered the Junggar Basin in the northeast of Xinjiang Uygur Autonomous Region in China. Part of the deposits accumulated in the bedrock mountain area and the loose materials in the basin are lifted again by the strong wind, drifted, transported and sorted with the wind, and then deposited separately. Day after day, year after year, there are more and more kinds of deposits, which gradually form gravel desert, desert and extremely thick loess deposits in northwest China today.

The other three hypotheses about the formation of loess have little effect. According to the theory of water formation, loess moves and accumulates under the action of running water. Residual theory holds that the weathering of bedrock leads to the formation of loess in situ; The multi-genetic theory holds that loess is the result of the combined action of the above factors.

Up to now, although the four hypotheses all have some truth, Feng Cheng's theory still has an absolute advantage in academic circles. However, there is not enough evidence to deny the hypothesis of water formation and residual products. In recent years, the multi-cause theory has risen again, challenging the wind-induced theory, which seems to be more reasonable than other hypotheses. It's hard to tell right from wrong. When will the mystery of the Loess Plateau be solved? This can only be hoped for by scientists.