What is permafrost?

People living in the north have this experience: in winter, when the temperature drops below zero degrees Celsius, if you go outdoors to dig the soil, you will find that the originally soft soil has now become very hard, and a shovel is often left with only a white spot.

Careful people will find that there are some small ice crystals in these hard soils, and if you don't get discouraged and continue digging, you will find that this layer of hard soil is not very thick, and there is still relatively soft soil underneath it. This layer of soil containing ice crystals is frozen soil.

Figure 1: Frozen zone

(Photo source: veer photo gallery)

Why does permafrost occur?

The soil contains more or less water. When the temperature drops to zero degrees or below zero, the water in the soil will solidify into ice, freezing the soil, thus creating permafrost.

But why do we see only a layer of permafrost instead of it being completely frozen?

The earth beneath our feet has a very special property: on its surface, the temperature decreases with increasing depth, but at a certain depth, its temperature no longer decreases, but remains basically constant all year round. Scientists call this layer the isothermal layer, and the temperature of the soil layer below the isothermal layer gradually increases as it gets closer to the center of the earth.

In this way, we can conclude that permafrost is a special kind of soil that is below zero degrees Celsius and contains ice.

Therefore, permafrost is different from ordinary loess, black soil and red soil. It is a kind of frozen soil. The frozen soil can be black soil or loess. Of course, there is less frozen red soil because red soil mostly exists in the south, and the temperature in the south is not often below zero degrees Celsius.

Figure 2. Land temperature changes with depth

(Image source: Reference [1])

There is more than one type of frozen soil

The permafrost we mentioned above is just a type of permafrost. When the weather warms up, this permafrost will melt. We call this permafrost seasonal permafrost.

In addition, in some places there is a kind of frozen soil that can last for many years without melting, that is permafrost . In the Arctic or the Qinghai-Tibet Plateau, because the temperature there is below zero degrees Celsius all year round, the frozen soil will not melt all year round. Even in warmer years, only a small layer of the surface will melt.

Figure 3 Permafrost

(Image source: Wikipedia)

Distribution of frozen soil in my country

my country's permafrost is divided into high-latitude permafrost and high-altitude permafrost.

High-latitude permafrost is mainly concentrated in the Greater and Lesser Khingan Mountains, with an area of ​​380,000 to 390,000 square kilometers. High-latitude permafrost is the southern edge of permafrost in Eurasia, and its plane distribution follows the latitude zonality, that is, the higher the altitude, the larger the permafrost area and the thicker the thickness.

High-altitude permafrost is distributed in the Qinghai-Tibet Plateau, Altai Mountains, Tianshan Mountains, Qilian Mountains, Hengduan Mountains, Himalayas, and some mountainous areas in the east, such as Changbai Mountain, Huanggangliang Mountain, Wutai Mountain, Taibai Mountain, etc. The formation and existence of high-altitude permafrost is controlled by the local altitude.

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Figure 4 Himalayas

(Image source: Wikipedia)

Permafrost Tour

Now let us start from Nenjiang, a city in the north of the Songnen Plain, and conduct an investigation of the world's permafrost from south to north.

In Nenjiang, we found many frozen areas, indicating that we have reached the southern boundary of permafrost. As we continue northward to the northernmost end of my country's railway, we will find that the frozen areas that originally appeared in blocks have become one piece, and the thickness of the frozen soil has increased to 50-120 meters.

Continue going north and enter the Arctic Circle. We will find that the original towering ancient trees are gone, and what comes with it is a tundra landscape. Here, the thickness of the permafrost has increased to 200~600 meters, and in some places it even reaches 1,000 meters.

Going further north, you will reach the end of this journey - the coast of the Arctic Ocean. The climate here is extremely cold, and the permafrost layer is 400 to 900 meters thick. This is the place with the thickest permafrost in the world, with the thickest point reaching 1,400 meters.

Of course, in addition to these places, there are many other places with permafrost. Because the southern hemisphere has less land area, permafrost is mainly distributed in the Eurasian continent and the northern part of North America. We also know that the higher you go, the lower the temperature is, so the temperature on some high mountains is also below zero degrees Celsius all year round, so permafrost also exists on high mountains and plateaus in mid- and low-latitudes, such as the Andes Mountains in America and Mount Kilimanjaro in Africa.

Figure 5 Global cryosphere, where the last three items in the diagram are the global permafrost distribution

(Image source: United Nations Environment Programme)

Conclusion

In life, we don't have many opportunities to learn about frozen soil, but thick underground ice is the main reason for the damage of various engineering buildings. The ecology of frozen soil areas is very fragile and easily disturbed and damaged by the outside world. Therefore, studying frozen soil is a prerequisite for maintaining the ecological safety of frozen soil, promoting the construction of major projects in frozen soil areas, and preventing disasters. Protecting frozen soil requires your support.

References:

[1] Ashrafian, Touraj & Moazzen, Nazanin & Haghlesan, Masoud. (2011). Human Comfort in Underground Buildings.

Produced by: Science Popularization China

Author: Earth Story Team

Producer: China Science Expo