Historical evolution of the Loess Plateau

About 8 million years ago, the Loess Plateau was still a lake in Wang Yang. It starts from Sun Moon Mountain in Qinghai in the west, reaches Luoyang in Henan in the east, reaches Qinling in Shaanxi in the south, and reaches the Great Wall in northern Shaanxi in the north. The lake is very big, the water is ocean, and the area is equivalent to six Bohai Seas today.

It can be called the original lake of loess. On the west bank of this huge lake, there is a vast desert and a barren hill to the south and east. At that time, the weather on the earth was dry and cold, with strong winds blowing everywhere, and the wind blew up the dust on the ground.

Dust flying all over the sky was blown to the original lake of loess, which made the lake very turbid. Slowly, the dust sank to the bottom of the lake and accumulated bit by bit, layer by layer. With the passage of time, after tens of millions of years of accumulation, the dust at the bottom of the lake is becoming more and more abundant.

It has accumulated hundreds of meters to thousands of meters. Because this geological period is very cold, the dry and cold northwest wind is blowing on the mainland, so the dust powder mostly runs from northwest to southeast. In the process of dust powder operation, large particles always fall first.

Small and medium-sized ones fall into the vast loess lake in turn, thus forming a pattern of coarse to fine soil layer at the bottom of the lake from west to east. Near the northwest, the mud particles are coarse and close to the southeast.

Soil particles become finer in turn. With thousands of years of wind blowing, the soil at the bottom of the lake is getting thicker and thicker, and the sediment particles are arranged more and more closely under the induction force and gravity of water surface expansion, which lays the foundation for the hardness of loess in the future.

During the rainstorm season, strong mountain torrents will rush to the lake area with rocks, pebbles and coarse sand, and spread these stones on the clay layer accumulated by the lake. After the heavy rain, sandstorms and dust particles fall into the lake for a long time.

Cover those stones with thick Sha Meng, and the area near the lake is often repeated in this way, so that several layers of debris structures can be formed. This movement has been going on for tens of millions of years.

Slowly, the climate on the earth became warmer, the power of the storm gradually weakened, and less and less mud was blown into the lake. With the gradual warming of the weather, grass vegetation has gradually grown on the lake, and small creatures such as shellfish have gradually appeared in the lake.

About 20 million years ago, there were a lot of creatures here, as well as tropical animals such as elephants and rhinoceroses. Animals often play and eat by the lake, and occasionally some dead animals are buried in the mud by the lake and turned into fossils.

As time goes by, the earth is changing. About 6.5438+0.5 million years ago, great changes have taken place in the landform here. The earth plate pushed from the South Indian Ocean collided with the Eurasian plate, and the whole lake area was slowly pushed up.

The bottom of the lake rises, and the water pours down, rushing to the diwa area in the east and flowing to the East China Sea. Huge floods accumulated soil on the North China Plain, which laid the most important foundation for the formation of the North China Plain in the future.

With the passage of time, the area of Loess Lake has been raised higher and higher, while the lake has become shallower and shallower. Finally, in about 8 million years, the lake finally dried up, the loess was lifted out of the ground and gradually became a plateau. It has become the landform of today's loess high slope.

With the drying up of lakes and the formation of plateaus, the climate gradually became cold, and tropical animals such as elephants and rhinoceroses gradually became extinct, forming today's geographical environment and ecological environment. This is the formation process of the Loess Plateau.

Extended data:

The Loess Plateau is located between100 24 ′ ~14 and 34 ~ 40 20 ′ north latitude, which is the second step of the national landscape pattern. It spans China temperate zone and warm temperate zone from north to south, and includes semi-humid zone, semi-arid zone and arid zone from southeast to northwest.

Its eastern boundary is the west side of Taihang Mountain, its western boundary is Wushaoling, Riyue Mountain and Laji Mountain, its southern boundary is Qinling-Xionger Mountain and the northern foot of Songshan Mountain, and its northern boundary is the Great Wall. It spans seven provinces (regions) including Qinghai, Gansu, Ningxia, Mongolia, Shaanxi, Shanxi and Henan. The total area is 640,000 square kilometers.

The Loess Plateau is located in the middle and upper reaches of the Yellow River and Haihe River. The Yellow River Basin has been the birthplace of human civilization since ancient times. However, due to the inherent fragile natural environment and long-term unreasonable land use, vegetation has been destroyed.

Soil erosion is extremely serious, which has caused profound changes in the ecological environment of the Loess Plateau in the past two or three thousand years. The problem of soil erosion in the Loess Plateau has become an important restricting factor for its economic development.

The Loess Plateau is a unique geographical area in China, which has the following remarkable characteristics:

1, the ditch is steep and the terrain is undulating and broken.

The loess on the Loess Plateau is deep, porous and rich in calcium carbonate. Affected by long-term internal and external forces, the surface erosion and cutting are serious, fragmented and ravines are vertical and horizontal.

The climate of the Loess Plateau is dry, although there is little precipitation, but the precipitation is concentrated and mostly heavy rain, which makes the Loess Plateau develop into a landscape with many ditches, steep slopes and broken terrain.

2. The climate is completely different.

The geographical distribution of temperature and precipitation in the Loess Plateau decreases from southeast to northwest. As far as the temperature is concerned, the Loess Plateau has the continental climate characteristics of Leng Xia fever in winter, with drastic changes in temperature.

The Loess Plateau is deep inland, with a strong continental climate and a large daily temperature difference of 10℃ ~ 25℃. As far as precipitation is concerned, the annual precipitation in the Loess Plateau is basically between 400 and 600 mm. ..

Moreover, precipitation is concentrated in July, August and September, accounting for 60% of the annual precipitation, and often occurs in the form of heavy rain. In other seasons, there is less precipitation and strong evaporation, and drought is more common.

3. Water resources are scarce and the contradiction between supply and demand is prominent.

In the arid and semi-arid loess plateau, precipitation is scarce and evaporation is vigorous, and the evaporation on the water surface is more than twice that of precipitation, and precipitation is concentrated. The characteristics of the Loess Plateau determine that soil erosion is very serious, and the contradiction between supply and demand of water resources is increasingly prominent.

4. Mountains and hills cover a large area.

The loess plateau is dominated by hills and mountains, accounting for about 80% of the total area, while the valleys and plains only account for 20% of the total land area. This undulating landform structure is not suitable for farming.

In addition, poverty and backwardness, rapid population growth, single agricultural management, extensive planting and crop failure, extensive farming and so on. At the same time, the phenomenon of indiscriminate reclamation is very serious, forming a vicious circle of "more reclamation, poorer and poorer".

5. Low vegetation coverage.

On the loess plateau, about 30% ~ 40% of the places are barren hills and bare ridges, leaving a scene of devastation. There are few artificial grasslands, and what's more, in some places, due to over-exploitation of hillsides, the topsoil is seriously lost, exposing bedrock, and the further reduction of vegetation coverage has aggravated soil erosion in the Loess Plateau.

Baidu Encyclopedia-Loess Plateau