Where do these "broken ice", "circles" and "gravel" growing on the ground come from?

Permafrost, a special type of soil with a temperature below zero degrees Celsius and containing ice, is distributed in high-latitude and high-altitude areas around the world, such as my country's Greater and Lesser Khingan Mountains and the Qinghai-Tibet Plateau.

Permafrost has extremely special properties, and therefore can form a variety of peculiar permafrost landforms. The "black magic" mentioned in this article is caused by permafrost.

01

The groundwater is coming out.

An important condition for the formation of permafrost is the presence of water in the soil. When this water is constantly freezing and melting, many interesting landforms will be formed.

Ground ice , as the name suggests, is ice formed in permafrost, including condensed ice, buried ice, vein ice, etc. Ground ice is an extremely important water resource in permafrost areas, and the existence of ground ice has a great impact on permafrost engineering and climate change.

Underground ice. Image source: Wikipedia

Frost heave mounds refer to landforms where the soil expands upward due to the increase of ice under freezing conditions. Specifically, in winter, the melting layer freezes from top to bottom and from bottom to top, the water-passing section is reduced, and the water on the frozen layer is under pressure. At the same time, the water migrates during the freezing process, which will produce an ice layer. As the frozen surface develops downward, when the pressure of the water on the frozen layer is greater than the strength of the overlying soil layer, the surface rises, forming a frost heave mound.

Frost heave mounds are a type of frozen landform that can be often seen in permafrost areas in China. The diameter of the bottom of a frost heave mound ranges from a few meters to tens of meters, and the height is 1 to 2 meters, and some can reach 3 to 5 meters. There are often crisscrossing cracks on the surface of a frost heave mound. The largest known frost heave mound in China is on the 62nd road section of the Qinghai-Tibet Highway. It has a bottom diameter of 40 to 50 meters and a height of 20 meters .

Frost heave hill cross section. Image source: Wikipedia

In permafrost areas, you can sometimes see silvery ice from a distance. This is an icicle. When the melted layer freezes back in winter, the groundwater pressure increases, breaking through the overlying soil layer and overflowing to the surface. The ice at the overflow outlet gradually increases in size and rises, and takes on a cone shape . The overflow freezes as it flows, and extends along the original groundwater flow path, thus forming an icicle.

The shapes and sizes of ice cones vary greatly, with some having a diameter of 2 to 3 meters, and some extending for tens or even hundreds of meters in the form of ice slopes, sometimes with several overflow ports. Ice cones are very common in frozen areas.

Melting ice cone. Image source: China Science Expo

02

Frozen soil also likes to circle

Have you ever seen circles of gravel? The unique freeze-thaw sorting effect of permafrost will separate different materials, so this "circle" phenomenon is more likely to appear in permafrost areas. Stone rings and frost heave grass rings are more typical "circle" landforms .

In the flat and mixed surface layer, after freeze-thaw sorting , soil and small rock debris are concentrated in the middle, and the rock blocks are squeezed to the periphery, forming polygonal or nearly circular shapes, as if someone consciously put the stones in a circle. This kind of frozen landform is called a stone ring.

Stone ring. Image source: Wikipedia

The area where the stone ring is formed must be a mixture of rocks and soil of varying sizes, and there must also be sufficient moisture, and the temperature must fluctuate around zero degrees Celsius for a longer period of time . Only in this way can the stones mixed in the soil be squeezed out.

Stone rings are often found on flat land not far from river beaches or at river exits from mountains, because these are the places where the conditions for the formation of stone rings are met.

The frost heave grass ring forms a polygonal or nearly circular turf on the ground surface, with rock debris and gravel exposed in between . This is a relatively rare periglacial landform in permafrost areas, with reddish-yellow in the middle and emerald green around it, which is dazzling. At present, scientists are still not very clear about its formation mechanism and process.

Frost heave grass ring. Image source: Reference [1]

03

"Gravel" in frozen soil

Due to the existence of cracks, when water freezes and expands, the rock breaks into many small pieces, or due to temperature changes, the minerals that make up the rock expand and contract unevenly, causing the rock to break . In this case, a large number of angular rocks and rock debris of varying sizes will be produced. Under the condition of flat terrain, most of the rock debris will remain in place, forming rubble covering the ground, which is the stone sea .

Stone Sea. Image source: Wikipedia

Stone seas are distributed in the Qinghai-Tibet Plateau, western high mountains and the northern permafrost zone of Greater Khingan Range. The development of stone seas requires not only hard and brittle rocks with many cracks, but also certain water and heat conditions. That is, the rocks must have a certain amount of water, and the temperature of the water must change greatly, and the temperature must fluctuate around 0℃ for a long time.

The material source and formation of rocky slopes are similar to those of rocky seas, but they appear in different geomorphic locations: rocky seas are often seen on gentle mountain tops, while rocky slopes appear on hillsides.

The rocks and debris on the rock flow slope are not only produced by frost weathering on the slope, but also rolled down from the mountain under the action of gravity. This determines that the composition of the rock flow slope is fine at the top and coarse at the bottom, with mostly small rock debris at the top and mainly large rocks at the bottom .

Stone flow slope. Image source: Wikipedia

Debris slope is a common periglacial landform in permafrost areas. It is widely distributed in the Greater Khingan Range and the high mountains and plateau permafrost areas in western my country and can be seen almost everywhere.

Under natural conditions, the surface materials are often a mixture of coarse and fine materials. Since the thermal conductivity of stones and soil is different, the freezing speed is also different. The thermal conductivity of crushed stone is large, so it will freeze first, and water will migrate to the vicinity of the stone and form ice near it. After the water forms ice, its volume will expand, causing the crushed stone to move, so that the coarse materials and the fine materials will be separated. Scientists call this phenomenon freeze-thaw sorting because it separates different materials like a sieve.

Stone strips are formed when the crushed stones on the rock flow slope gather at low places after repeated freeze-thaw and freeze-thaw sorting, and then extend downward along the slope under the action of gravity. Therefore, stone strips often exist at the same time as rock flow slopes, and the crushed stones and fine-grained materials are arranged in strips along the slope .

Stone strips, the "brother" of stone slopes. Image source: Wikipedia

The rocks and debris produced by frost weathering gather in the groove under the slope under the action of gravity. The gravel slowly moves downward along the groove to form a small river filled with stones, so it is named Shihe .

The stone sea, stone slope and stone river are brothers of the same clan, and there is an organic connection between them. They often appear at the same time in the same mountain. The mountain is often paved with huge rocks, the slopes are covered with gravel, and the foothills are covered with gravel forming streams .

Stone River. Image source: Wikipedia

Nature's "gravel stones" can only be formed when rocks are in a special permafrost environment. Although its form is strange, only if humans carefully protect the fragile permafrost environment can it bring continuous shock to humans.

References:

[1] Hjort, Jan. (2006). Environmental Factors Affecting the Occurrence of Periglacial Landforms in Finnish Lapland: A Numerical Approach.

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Submitted by: Computer Information Network Center, Chinese Academy of Sciences