Loess Plateau
The Loess Plateau is a plateau in north-central China formed of loess, a clastic silt-like sediment formed by the accumulation of wind-blown dust. It is located southeast of the Gobi Desert and is surrounded by the Yellow River. It includes parts of the Chinese provinces of Qinghai, Gansu, Shaanxi and Shanxi. The depositional setting of the Chinese Loess Plateau was shaped by the tectonic movement in the Neogene period, after which strong southeast winds caused by the East Asian Monsoon transported sediment to the plateau during the Quaternary period. The three main morphological types in the Loess Plateau are loess platforms, ridges and hills, formed by the deposition and erosion of loess. Most of the loess comes from the Gobi Desert and other nearby deserts. The sediments were transported to the Loess Plateau during interglacial periods by southeasterly prevailing winds and winter monsoon winds. After the deposition of sediments on the plateau, they were gradually compacted to form loess under the arid climate.
The Loess Plateau is one of the largest and thickest loess plateaus in the world. Its area corresponds to around 6.6% of the land area in China. Around 108 million people inhabit the Loess Plateau.
Because of the strong winds, erosion is also powerful across the plateau. Therefore, erosional features, including wind escarpments, loess vertical joints and gullies are present. In the past few decades, the environment and climate has changed, including the rainfall pattern, vegetation cover, and the natural hazards. These changes may relate to human development in the plateau; Chinese environmental officials are trying to find sustainable ways to manage the region.
Geology
Geomorphology
There are three main types of morphology in the Loess Plateau. They are loess platform, loess ridges, and loess hills. Loess tableland is flat and with many loess strata. It is mostly located at south Loess Plateau. Loess ridges are formed by erosion and are located at the central Loess Plateau. Loess Hills are conical dunes and are located at the north Loess Plateau. The geomorphology of the Loess Plateau is formed by the erosion and deposition of loess.In the Loess Plateau, the geomorphology usually changes from rocky mountains to Alluvial plain at piedmont to river valley belt. This pattern keeps repeating from the Northwest to the Southeast of the Loess Plateau.
The height of the rocky mountains is much higher than the loess deposit. The height and morphology of the mountains are different in different locations.
One of the highest mountains in the Loess Plateau is called Mahan Mountain. The elevation of this mountain is around, which is higher than the loess line. It is a flat-topped mountain and has paleo-peneplain remnants on the mountain top.
Some of the mountain slopes, especially the windward slope, were forested in the past.
The alluvial plain at piedmont is composed of Alluvial fans which can be found in this area, and which are located at the foot of the rocky mountains.
The size of this belt depends on the amount of runoff and weathering materials from the rocky mountains.
Old alluvial fans are covered with eolian loess. Further from the rocky mountains, loess tableland and loess "Ping" can be found and even links with the next belt, which is the river valley belt.
The River valley belt includes flood plains, river terraces and river beds. The terraces with higher height are mostly covered with thick loess. It will change to another form of landscape, which is loess ridge, by strong erosion. If the erosion is weak, the higher terraces will change to loess tableland. These flat river basins, which include valley flat and lower terraces, are important for construction and agricultural activities.
Erosional features
Wind escarpment and bedrock ridges
is located in the northwest of the Loess Plateau. Wind escarpment in the Loess Plateau marks a boundary between Mu Us Desert and the Loess Plateau. It also represents a transition from loess accumulation to wind erosion in the Loess Plateau. Many linear bedrock ridges are formed behind the wind escarpment, which are parallel to the wind direction. In the northern Loess Plateau, the bedrock ridges are pointing towards the northeast. However, the direction of the ridges slowly rotates to the North in the central Loess Plateau. At North Loess Plateau, the ridges are oriented 118° ±14° while they are oriented 179° ± 11° at central Loess Plateau. This indicates the role of wind erosion.The monsoon wind direction in Quaternary is consistent with modern climatology. To observe near-surface wind vectors, they compared the wind in Quaternary and modern wind. The results show that the wind direction in winter and spring-storm events are the same as the orientation of the bedrock ridges. Therefore, modern windstorms also contribute to shaping the eolian geomorphology.
The Yellow River has provided sediments supply continuously which has been reworked by wind. Also, the wind erosion becomes stronger when it reaches the Loess Plateau wind escarpment. Because of the streamline compression of the wind escarpment, the wind speed is increased.
As a result, the Loess Plateau is not only a site of loess deposition but also a source of dust because of strong wind erosion. Wind erosion is very severe during the glacial period. During the glacial period, there is very little vegetation, so it favors wind erosion.
Vertical loess joints
Loess vertical joints distribution depends on the loess structure, water moisture, strata and microtopography. There are vertical development features and lateral development features.Vertical development features
Vertically, joints can be found in different loess strata, including late, middle and early Pleistocene loess layers. It is one of the most significant structures of the Loess Plateau. The development and size of the vertical loess joints depend on the vegetation coverage and slope. Steep slope and poor vegetation coverage favor the development of the joints. Many vertical loess joints can be easily found on the vertical cliffs of tableland.The joints and the loess-paleosol interface are orientated perpendicularly. Also, in a dry loess layer, vertical loess joints are the wet part of it. Therefore, it is very difficult to notice the loess vertical joints in deep strata.
The water from rainfall and irrigation will infiltrate into the loess strata through the vertical joint surface and pore concentration zone.
The joint systems in the loess strata are of different sizes, properties, periods, and origins. Loess Vertical Joints distribute all over the loess plateau. The joints in landslides can be categorized by their different features.
The original joints are formed on the major scarp, minor scarp, original vertical cliffs and flanks. They are no displacement and closed.
Unloading vertical joints and weathering vertical joints are at the top and edge of the slope or landslides and mostly in open shape and with little displacement.
Sliding joints are in the body of landslides. Usually they are step-shaped and with large displacement.
Collapsible joints are formed when there is asymmetrical settlement during rainfall or irrigation. They are located far from the edge of tableland and with apparent displacement.
Lateral development features
The lateral development of the vertical loess joints can be divided into four stages.In the development stage original vertical joints, unloading joints and weathering joints can be found. In this range, the joints are mainly weathering joints and unloading joints. Nothing fill in the joint surfaces.
In the micro-development stage, the distribution of the loess joints is sparser. The joints are filling with fine sand. This indicates the infiltration of water and accumulation of the sediments in the water.
In the underdevelopment stage a few or nearly zero joints are found.
And lastly in the undeveloped stage no vertical joint is found. The loess is very dry. The average moisture content is 16.22%.
Gullies
Gully erosion acts as an important source for sediments. If an area has gully erosion, it means that the area has serious land degradation. In the Loess Plateau, the contribution of gully erosion on total sediment production in the hilly areas is about 60% to 90%. It is serious in the Loess Plateau. To know the contribution of gully erosion, we can measure the gully volume changes.There are three types of gullies in the Loess Plateau, including floor gullies, hill slope gullies and valley bank gullies.
Geological development
In conclusion, the geomorphic outline of the Loess Plateau was shaped by the tectonic movement since Neogene. After that, because of the East Asia Monsoon in Quaternary, the loess and different erosional features started to form. However, because of human activities, many areas in the Loess Plateau turned into erosional environments.Loess deposits
Formation of loess
Loess does not necessarily mean the same as silt. Loess is yellow eolian sediments that were transported by wind from an arid or semi-arid region during the Quaternary period. Around 6% of the land in the world is covered with loess. Loess record the past climate and environment.The Chinese Loess Plateau is one of the largest sinks of loess in the world. When the sediments are transported to the Loess Plateau, they are silt materials. After they deposit in arid areas and under strong chemical weathering and the process of carbonation, loess is formed. Two types of loess are defined by their formation process.
Typical loess is loess that is deposited during late Pleistocene and Holocene. It is formed under arid or semi-arid conditions.
Secondary loess is loess that is compacted by upper loess and does not experience the weathering and carbonation process. Also, it is formed by the transformation of fluvial and lake loess in semi-arid areas.