Are glaciers motionless? In fact, they are constantly flowing and moving.

Author: Xu Yadong (China University of Geosciences (Wuhan))

Source: Knowledge is Power Magazine

In the movie "Frozen", there are crystal clear glaciers with a faint blue glow. In "Ice Age", there is a white ice and snow world where ferocious and strange prehistoric beasts live. Do you want to explore these unique ice and snow landscapes?

The fairy tale world of "from snow to ice"

First, let us look at how snow turns into glacier ice step by step.

How snow and ice change over time

Snowflakes are small ice crystals formed by condensation of water vapor in the sky (the phenomenon that matter skips the liquid state and changes directly from gas to solid). They are mostly hexagonal in shape and are also known as Weiyang Flower and Liuchu.

Affected by time and surface radiation heat, the tips of the snow melt and gradually solidify into larger snow grains; both snowflakes and snow grains have a large porosity (>50%), allowing air to flow freely between the snow grains.

As the snow layer thickens to 20-50 meters, the bottom layer of snow condenses into white granular ice crystals due to the pressure of the overlying snow layer, the infiltration and freezing of meltwater.

When the air porosity of ice crystals gradually drops to 20%-30%, after more than ten years of long-term compaction, the ice crystals are tightly connected or recrystallized to form blocky glacial ice; its porosity is less than 20%, and air cannot circulate.

The transformation of snow into glacier ice goes through a series of processes such as compaction, freezing and recrystallization, which is similar to compacting loose snowflakes into hard ice sculptures. When they "grow" into glacier ice, they will undergo plastic deformation (deformation that cannot be self-recovered). The plasticity of glacier ice is related to the thickness of the ice layer. The thicker the ice layer, the greater the plasticity, and the easier it is for the ice layer to slide and plastic creep. Under the pressure and gravity of the upper layer of ice and snow, the ice layer creeps in a certain direction and becomes a glacier.

Therefore, the glacier is a blue and white fairy tale world composed of ice and snow that undergo dynamic and static changes.

Glaciers: “Walking” Recorders

Glaciers seem to be very "cold" (high altitude, high latitude and high cold), so we misunderstand them and think that they stay in the same place forever and never move. In fact, the name of glaciers implies that ice and snow flow and move constantly, but they are huge and move very slowly. The changes in glacial landforms can tell us about the vicissitudes of life here.

Based on their climate, topography, size and morphological characteristics, glaciers can be divided into two major families: continental glaciers and mountain glaciers.

Morphological characteristics of continental glaciers (ice sheets) (stratification, layered structures produced by vertical changes in materials)

In high-latitude watersheds and polar plateaus, the snow line is low and the snow-covered area is wide. The ice layer is often several thousand meters thick and up to 10 kilometers wide, resembling a shield-shaped cover, namely a continental glacier, also known as an ice sheet. Under the action of gravity, the ice sheet flows in a tongue-like manner from the thicker ice layer to the surrounding areas in the form of an extrusion flow. The bottom ice of the glacier slides with the bedrock at the base, causing the bedrock to be eroded and damaged and concave, so the ice sheet is shaped like a convex lens, and the stretching movement to both sides makes the blue and white stratification thicker on the top and thinner on the bottom.

Morphological characteristics of mountain glacier landforms

In the mid- and low-latitude alpine regions, the shape of glaciers is restricted by the mountainous terrain. The snow line is high, the scale is small, the ice layer is thin, and it is often distributed in a linear shape. It is called a mountain glacier. Mountain glaciers have the gene of being tall and thin. All modern glaciers in China are mountain glaciers, mainly distributed around the Qinghai-Tibet Plateau.

Before the development of glaciers, the landforms were mainly arc-shaped peaks and ridges, and the high mountain valleys were mostly "V"-shaped with a flat valley bottom (as shown in Figure A above).

During the glacial development period, glaciers eroded along the valleys, deepening the valley bottom and widening the valley sides. The mountain mouth was eroded to form a truncated mountain mouth due to blocking the flow of the glacier. The glacial valley is often straight and wide, forming a "U" shape with steep walls, which is called a "U"-shaped valley.

A semicircular depression with steep walls on three sides caused by erosion is called a cirque, which is shaped like a chair. Cirques are often developed near the snow line and are often arranged along the direction of the hillside, so they can be used as a mark to identify the location of the ancient snow line. The valley walls of adjacent cirques retreat due to erosion, forming a ridge with steep slopes and thin ridges called a blade ridge (or fin ridge), which is like a knife blade or a fish fin. A mountain with steep rock walls surrounded by more than three cirques is called a horn peak, which is shaped like a pyramid. The famous Mount Everest is a horn peak. At the front end (or end) of the glacier, a protruding tongue is often formed due to the faster flow rate in the middle part and the slower flow rate on both sides due to friction with the bedrock. It is called an ice tongue (as shown in Figure B above).

During the glacier ablation period, the depressions can accumulate meltwater to form glacial lakes, which are mostly in the shape of beads. Sometimes, the cirque can also accumulate meltwater to form cirque lakes (as shown in Figure C above). After the front end of the ice tongue disappears, the large amount of debris it carries will accumulate into various moraine dykes (the deposits on both sides of the glacier channel are called lateral moraine dykes, and the location where the glacier disappears is the terminal moraine dyke).

Relics of the Ice Age

Glacial geological relics

Affected by global climate change, the growth and decline of glaciers can leave typical geological relics as they move. From the geological relics of the Ice Age, we can explore the temporal and spatial laws of landform evolution and climate evolution.

When the glacier expands, the plastic flow of the solid glacier can be divided into two mechanical modes: pushing and carrying. Pushing refers to the front end of the glacier pushing the rock debris on the ice bed forward with huge thrust, similar to a "bulldozer". Carrying refers to the rocks frozen in the ice or falling on the ice surface being transported with the movement of the glacier, similar to a "conveyor belt".

As the glacier retreats, the snow line rises, the ice at the front of the glacier melts, and the materials carried by the glacier and the meltwater accumulate together in a collective called moraine, in which rock fragments of different particle sizes are mixed together, resembling "concrete".

If the climate conditions are stable, the front of the glacier is stagnant, and the amount of ice melting and replenishment is basically balanced, the debris carried by the glacier will accumulate in large quantities at the front of the glacier, often forming a long arc-shaped dam outside the ice tongue, that is, the terminal moraine. Ice slides and ice scratches are common on the slopes on both sides of the "U"-shaped valley.

The bedrock (sheepback rock) affected by the undulations of the ice bed within the terminal moraine can form an elliptical hillock, whose long axis is often parallel to the direction of glacier flow, called a drumlin.

The moraine accumulated on the ice bed is called bottom moraine, among which flying stones and mortars can also be seen according to the distribution of large boulders and the erosion pattern.

Glacier environmental scientific research is on the way

The ecological environment of the glaciers on the Qinghai-Tibet Plateau is fragile and sensitive, and has a significant impact on the climate and ecological environment of China and even the world. In June 2021, a team from the Shenyang Institute of Automation of the Chinese Academy of Sciences completed a field test of the Climber research station robot on the Kuqionggangri Glacier on the Qinghai-Tibet Plateau (as shown below). China's first high-altitude glacier scientific expedition by a ground robot was successfully completed.

Climber conducts radar ice exploration on the Kuqionggangri Glacier

Against the backdrop of global warming, based on the "natural archives" of long-term climate change provided by the Qinghai-Tibet Plateau, scientists have found that the plateau is warming and humidifying as a whole, vegetation is increasing, the ecosystem is improving, the area of ​​lakes is expanding significantly, and the lake water is becoming clearer. However, since 1989, the Dongkemadi Glacier in the Tanggula Mountains has been retreating at an accelerated pace and its thickness has been reduced. The main reasons are global warming and glacier surface pollution, the latter of which mainly comes from black carbon emitted by humans. In addition, ice avalanches occur from time to time, accompanied by secondary disasters.

In response to glacier changes and disaster risks, the Chinese scientific expedition team has established a Chinese glacier enhanced monitoring network and successfully issued an early warning when the Yarlung Zangbo River ice avalanche blocked the river. This also proves that scientific investigation and research will have a very important impact on promoting the sustainable development of the Qinghai-Tibet Plateau, advancing the country's ecological civilization construction, and promoting global ecological environmental protection.

Acknowledgements: We would like to thank Songzhu Gu from China University of Geosciences (Wuhan) and Ning Zhu from Natural Resources Geological Data Center of China Geological Survey for providing some glacier photos. We would like to thank the China Geological Survey for co-funding (DD20190811, DD20190370)

(Editor-in-charge: Wang Jiaying; Art editor: Zhou You)