The distribution area of karst landforms in China is more than 1 million square kilometers, the largest in Guangxi, Guizhou and eastern Yunnan, and also in Tibet and Northern China. What are the reasons for the formation of karst landform? Let's have a look.
Formation causes of karst landform
The formation of karst landform is the result of long-term dissolution of groundwater in limestone area. The main component of limestone is calcium carbonate (CaCO3). When there is water and carbon dioxide, a chemical reaction occurs to produce calcium bicarbonate [Ca (HCO3) 2], which is soluble in water, so the cavity is formed and gradually expanded. This phenomenon is most typical on the Karst Plateau along the Delia coast of southern Europe and Asia, so this terrain in limestone area is often generally called karst landform.
Characteristics and classification of karst landform
In addition to dissolution, it also includes mechanical erosion processes such as water erosion, potential erosion and collapse. Karst landforms are widely distributed in China, such as Guangxi, Yunnan and other places. Karst landforms are mainly characterized by karst caves, Tiankeng and other geographical phenomena.
According to the exposure conditions, karst landforms can be divided into three types: exposed karst, covered karst and buried karst.
According to the climate zone, it can be divided into five types: tropical karst, subtropical karst, temperate karst, cold karst and arid karst.
According to lithology, it can be divided into four types: limestone karst, dolomite karst, gypsum karst and salt karst.
Guilin Karst landform
The typical karst landforms in China are Guilin and Yangshuo in Guangxi. Guilin is located in a huge folded basin. The limestone in this area is thick, pure and abundant in precipitation. The Lijiang River passes through the basin. The dissolution of limestone by flowing water forms a rich and diverse karst landscape.
In the primary stage of dissolution, water only dissolves at the top of the rock to form shallow grooves, and the convex part is called stone bud. With the deepening of dissolution, the mountain peaks gradually appear. When the valley between the peaks is eroded to 1 / 3 ~ 1 / 2, and the peak is connected with the lower half of the peak, it is called peak cluster; When the valley is eroded to the bottom, it becomes a typical peak forest; As the water continues to dissolve, there is a distance between the peaks, which is the solitary peak, which belongs to the later stage of dissolution.
When the local crust rises and the groundwater level drops, water erosion will form broad karst caves, such as Reed Flute Cave in Guilin, Huanglong cave in SUOXIYU, Hunan and Swan cave in Ninghua, Fujian. There will also be various cave sedimentary forms - stalactites, stalagmites, stone pillars, etc.
In the karst area, the surface river flows through the area where the cave is developed, and the water flows into the underground through the vertical hole, forming a sinkhole, and the surface river may also be cut off.
Not all places with limestone can form the landform like Guilin mountains and rivers, but the limestone with large area, large thickness and pure texture, and the warm and humid climate conditions are required to develop into such a perfect landform and form such beautiful natural scenery.
Karst landforms in subtropical areas of China are also developed in Sichuan Xingwen Shihai, Chongqing Wulong, Qianjiang Xiaonanhai and other places. The Three Gorges area of the Yangtze River is also a limestone area, and the Three Gorges is also made of limestone. Above Qutang gorge, the Yangtze River passes through the soft sand shale. When it reaches Kuimen, it enters the hard limestone area. The flowing water cuts through the cracks of limestone, and the river channel suddenly narrows, forming a steep and deep canyon with steep rock walls, creating the magnificent Kuimen in the world.
In the Three Gorges area of the Yangtze River, not only the main stream runs through the limestone Canyon, but also many tributaries run through the limestone Canyon, such as Daning River, Xiangxi River, jiumianxi River, etc.
Karst not only has a unique mountain form, but also has different underground structures - Tiankeng, karst cave and underground river.
Limestone has many fissures, which can be permeable and soluble in water, but the water capacity is limited. When the water seepage of a-stone reaches saturation, the groundwater begins to flow horizontally, which is the groundwater level. The flowing water will dissolve along the horizontal bedding of J rock stratum and form caves. If the water level changes little and the caves gradually expand, underground rivers will be formed. Many famous underground caves and underground rivers in China belong to this, such as Xingyan (formerly known as Qixia cave) in Guilin, Guangxi, Qixingyan in Zhaoqing, Guangdong, Shanjuan cave in Yixing, Jiangsu, Longgong cave in Pengze, Jiangxi, etc.