What are the characteristics of the Loess Plateau?

(Loess Plateau)

In the north of China, Taihang Mountain in the east, Wushaoling in the west, Qinling Mountain in the south and the Great Wall in the north, mainly including Shanxi, Shaanxi, Gansu, Qinghai, Ningxia and Henan provinces, covering an area of 400,000 square kilometers, is the largest loess accumulation area in the world. Loess is 50- 180m thick, with dry climate, concentrated precipitation, sparse vegetation and serious soil erosion. The Loess Plateau is rich in mineral resources, with huge reserves of coal, oil and bauxite.

The north wind sends soil

For a long time, Chinese and foreign scholars have different arguments about the source of loess. Among them, the "wind theory" is more convincing. It is believed that loess comes from the vast arid and desert areas such as Gansu, Ningxia and Mongolian Plateau in the north and northwest, and even Central Asia. The rocks in these areas expand when heated during the day, contract when cooled at night, and gradually weather into stones, sand and clay of different sizes. At the same time, in these areas, whenever the northwest wind prevails in winter and spring, the wind suddenly rises, flying sand and stones, and dust covers the sky. Coarse stones remain in place and become "Gobi", while finer sand grains fall in nearby areas and gather into deserts, and tiny silt and clay fly to the southeast one after another. When the wind weakened or was blocked by the winding Qinling Mountains, they stopped accumulating, and after hundreds of thousands of years of accumulation, a vast loess plateau was formed. According to the different loess accumulation environment, the development of Huangshi in China can be divided into three periods: early Pleistocene, equivalent to the first ice age, the climate was drier and colder than Neogene, and loess accumulation occurred in Wucheng; In the Middle Pleistocene, the second glacial period appeared, and the climate dried up further, accumulating a wide range and thick soil layer of Lishi loess; In the third glacial period of late Pleistocene, the climate was drier and colder, and Malan loess was accumulated. Although the thickness is small, it is widely distributed, and it is called Xia Shu Loess in the south. The Holocene climate turned warm and humid, and the loose loess layer was eroded by running water, forming broken surfaces with gullies, beams and headlands alternately.

Science is constantly developing. In recent years, scientists have found that many phenomena cannot be explained by loess aeolian theory. For example, the coarse silt content in loess decreases from northwest to southeast, while the clay content increases from northwest to southeast. This regular arrangement from northwest to southeast is a step-by-step distribution transition, not a plane fuzzy transition. This step-by-step transition of tile distribution is more like a masterpiece of flood and so on.

In order to understand the "face-changing" process of the Loess Plateau, experts went to Jingning County, Qin 'an County and Dingxi County of Gansu Province in the west of the Loess Plateau to collect loess samples from six typical geological sections of the Loess Plateau, from which more than 700 sporopollen samples and 209 topsoil sporopollen samples were obtained. These nearly 1,000 sporopollen samples recorded the vegetation changes in the Loess Plateau from 46,000 BC to the present. According to the determination of carbon 14, there are 34 ages in 6 typical sections. After analysis by experts, it was found that the number of sporopollen particles isolated from 20g samples collected on the Loess Plateau reached at most about112, and at least less than 50, indicating that the environment and vegetation have changed greatly in the past 40,000 years.

Chunhai Li said that from the pollen analysis, dozens of plant pollen records such as pine, spruce, fir, hemlock, oak and compositae have been found. Experts believe that the Loess Plateau is not originally named "Yellow". In the 46,000-year history, more than half of the time, the Loess Plateau is the mutual growth and decline of forests and grasslands. During this period, the Loess Plateau experienced many rapid "face changes".

The formation of the Loess Plateau and the uplift of the Qinghai-Tibet Plateau accelerated the speed of erosion and weathering, and accumulated a large number of pebbles, sand and finer particles in the low-lying areas around the plateau. Whenever the wind suddenly rises, it will form a scene of flying sand and stones and flying dust in the western regions. The rolling dust settled in turn, and the fine dust finally landed on the Loess Plateau, forming a desolate land.

After the Indian plate moved northward and collided with the Eurasian plate, the Indian continental crust was inserted under the Asian continental crust to prop up the latter. As a result, the shallow sea of the Himalayas disappeared, the Himalayas began to form and gradually lifted, and the Qinghai-Tibet Plateau was also squeezed and lifted by the Indian plate.

However, the east-west Himalayan mountains stopped the warm and humid air mass in the Indian Ocean from moving northward. With the passage of time, the northwest of China has become more and more arid, gradually forming a large area of desert and Gobi. This is the birthplace of dust accumulated on the Loess Plateau. The huge Qinghai-Tibet Plateau just stands in the westerly belt of the northern hemisphere, and its height has been increasing for 2.4 million years. The width of the Qinghai-Tibet Plateau accounts for about one third of the westerly belt, which divides the surface of the westerly belt into two branches. The south branch flows eastward along the south side of the Himalayas, and the north branch flows eastward from the northeast edge of the Qinghai-Tibet Plateau. This kind of high-altitude airflow exists at an altitude of 3500-7000 meters all the year round and becomes the main driving force for carrying dust. At the same time, due to the uplift of the Qinghai-Tibet Plateau, the East Asian monsoon has also been strengthened. The winter wind blowing from northwest to southeast, together with westerly jet, formed the Loess Plateau in the north of China.

geomorphic type

Mountains, plains and Sichuan are the main landforms of the Loess Plateau. The mountains towering on the plateau are like islands in the ocean. Such as Wuqu Mountain, Huajialing Mountain and Maxian Mountain in the Longzhong Plateau west of Liupanshan Mountain, Ziwuling Mountain, Bai Yushan Mountain and Huanglong Mountain in the northern Shaanxi Plateau in eastern Gansu. The original (or tableland) refers to the flat loess plateau, and the famous ones are Longdong Dongdong Dongzhi tableland and Luochuan tableland in northern Shaanxi. The plateau is wide and suitable for mechanized farming, and it is an important agricultural area. However, tableland is easily eroded by flowing water, and valleys are developed, so it is divided into long tableland and becomes a mountain ridge, which is called "beam" land. If the beam land is cut by the valley, scattered and isolated, shaped like a steamed bread hill, it is called "Mao" locally. The loess hills composed of "beams" and "hairs" are mostly about 100 ~ 200 meters above the bottom of the nearby ditch, which has serious soil erosion and is the source of sediment in the Yellow River. Sichuan is a valley plain deep in the plateau. In Liangmao area, groundwater is exposed, and the sediment brought by rivers is deposited here, forming a small plain on both sides, which is called "Chuan". There are terraced fields on both sides of Sichuan, namely "palm" and "staff". Palm is a basin-like plain with the source of Sichuan land, which is different from the strip distribution of stick land.

The Yellow River flows through the world's largest loess plateau in the middle and upper reaches. The Loess Plateau has deep soil layer, loose soil, broken terrain, frequent rainstorms and extremely serious soil erosion, which is the main source of sediment in the Yellow River. Especially in the section from Hekou Town to Tongguan of the Yellow River, when the Yellow River crosses this section of the Loess Plateau, many tributaries flood in and "dye" the Yellow River. According to the measurement, the sediment entering the Yellow River in this reach accounts for 90% of the total sediment in the river.

Seen from the earth, loess is mainly distributed in Central Asia to northwest, north China and northeast China. The largest loess plateau in the world is located in the middle and upper reaches of the Yellow River. Its scope starts from Yinshan Mountain in the north, Qinling Mountain in the south, Sun Moon Mountain in the west and Taihang Mountain in the east, covering 6 provinces of Qinghai, Ningxia, Gansu, Shaanxi, Shanxi and Henan, covering an area of 640,000 square kilometers. The thickness of loess cover is generally below 100 m, and the loess layer in Longdong, northern Shaanxi and western Shanxi is the thickest. From the east of Liupan Mountain to the west of Luliang Mountain, the thickness of loess is between 100 meters and 200 meters, and the thickest is in Lanzhou, reaching more than 300 meters. The area and thickness of loess are the highest in the world.

Soil erosion

It is mainly caused by the scouring of loose loess by rainstorm gully. Loess has fine particles, loose structure, upright structure, calcium carbonate, easy to dissolve in water and easy to collapse. The steep slope on the ground, sparse vegetation and heavy rain in summer have caused micro-landforms such as strange peaks, steep walls, caves and natural bridges, which further contributed to the expansion of gullies and accelerated soil erosion. At the same time, it is also related to the rise of the modern crust, which makes the gully bed continuously undercut and eroded laterally, and the erosion of the gully source is intensified. Correspondingly, the valley slope is constantly expanding and the land between ditches is increasingly broken. In addition to the above natural factors, it is also closely related to human activities, especially social factors such as deforestation and destruction of natural vegetation. After the founding of New China, a series of comprehensive measures have been taken to control soil erosion in the Loess Plateau, such as planting trees and grass, returning farmland to forests and building water conservancy projects, and gratifying changes have taken place in the Loess Plateau.

Increase the construction of "Three North" shelterbelts and improve the vegetation coverage area and coverage rate. Especially for this loose loess plateau, the forest coverage rate must be higher than the national average of 22%. Only in this way can soil erosion be effectively prevented.

Geomorphological difference

According to the formation process and characteristics of landform, it can be divided into the following parts: ① Longzhong Plateau. It is called Longxi Plateau. Located in the west of Liupanshan, it is a Cenozoic depression basin, belonging to the basin plateau, with an altitude of 1500 ~ 2000 meters. Broken terrain, multi-beam, cape, valley and ridge topography. ② Longdong and Northern Shaanxi Plateau. Including the areas east of Liupanshan, west of Luliang Mountain, north of Beishan of Weihe River and south of the Great Wall. It is also a basin plateau with an altitude of 800 ~ 1200m. After intense erosion, except for a few remaining loess tablelands (Dong Zhiyuan and Luochuan tablelands), most areas have become broken Liangmaoshan. At the same time, only a few low bedrock hills protrude above the plateau like islands. ③ Shanxi Plateau. Including Wutai Mountain, south of Hengshan Mountain, north of Funiu Mountain, west of Taihang Mountain and east of Luliang Mountain. It consists of a series of folded fault-block mountains and subsidence basins. There are Lvliang, Hengshan, Wutai, Zhongtiaoshan and Taihang Mountain in the mountainous areas, and Datong, Xinxian, Taiyuan, Linfen and Yuncheng in the basin. Except for the valley plain, the elevation of most areas is1000 ~1500m. Rocky mountains constitute the main part of the plateau, and loess accumulation is limited to basins and intermountain valleys, accounting for about 40% of the whole area. ④ Weihe Plain is called Guanzhong Plain. Located between Beishan of Weihe River and Qinling Mountains, it starts from Baoji in the west.

Energy base

The loess plateau is rich in coal resources, and its reserves and output rank first in China. Coal resources are not only abundant and of high quality, but also have good mining conditions. Among them, the coal reserves available for open-pit mining reach 20 billion tons. About half of the super-large coalfields with proven reserves in China are located here. Shanxi Province is the largest coal base in China. Shenfu Coalfield and Changqing Oilfield in northern Shaanxi are also important components of the energy base in the Loess Plateau. The geographical position of the Loess Plateau is moderate. As a national energy base, it continuously provides coal and electricity for the whole country. People call it the "boiler room" of the whole country.

history

Since Qin and Han Dynasties, the Loess Plateau has experienced three climaxes of deforestation and land reclamation. The first time was a large-scale "reclamation" (organized reclamation by border guards) and "immigration" reclamation in the Qin and Han Dynasties. This large-scale "reclamation" has caused large-scale destruction of forests in northern Shanxi and northern Shaanxi. The second time was a large-scale "reclamation" in the Ming Dynasty, which caused unprecedented havoc to the ecological environment in the northern part of the Loess Plateau. According to textual research, in the early Ming Dynasty, the system of "ploughing fields" was vigorously promoted in northern Shaanxi (Yan 'an, Suide and Yulin areas) and northern Shanxi on the Loess Plateau, and the task of destroying forests and reclaiming land was imposed on every border guard. From here, we can easily see that the system of "reclaiming wasteland" implemented in the Ming Dynasty caused serious damage to the environment. The third reclamation was in the Qing dynasty, and the system of rewarding reclamation was implemented. The scope of reclamation moved from northern Shaanxi and northern Shanxi to southern Inner Mongolia, and millions of acres of grassland in the northern Loess Plateau and Ordos Plateau were reclaimed as farmland, which made the land desertification and soil erosion intensified.