Be careful! If you build a house here, it will "run away" by itself

In today's global warming, glaciers have become a major concern for everyone, but in addition to knowing that they are melting, did you know that they are also constantly moving?

01

Glaciers are also active

In 1827, a geologist built a stone hut on the Eagle Glacier in the Alps. Thirteen years later, the hut moved 1,428 meters downstream. The hut itself does not move, but the glacier on which it is based is constantly moving .

The movement of glaciers is due to deformation of the glacier ice and sliding of the bottom, both of which are caused by the glacier's own gravity .

The movement of glaciers is somewhat similar to that of water flow, with the center moving faster and the sides moving slower . If you insert a row of flower poles horizontally on a glacier, you will soon find that the flower poles in the middle move far ahead, and the originally straight row of flower poles will become arcs protruding downstream. Many marine glaciers have very strange arc-shaped arches, which are caused by the different speeds in the center and on the sides during the movement of the glacier.

Glacier movement. Image source: Wikipedia.

It’s strange to say, but glaciers can also form whirlpools like water flows, and some whirlpools on glaciers are very spectacular.

But glaciers are made of ice after all, and there is a big difference between ice and water.

The most obvious point is that there are often many cracks on the surface of glaciers , some of which are tens of meters deep. The existence of cracks shows that the glacier is brittle. However, after hundreds of years of investigation and observation, the depth of cracks on the glacier also has a certain limit, which shows that the lower part of the glacier is plastic and can "softly" adapt to various external forces without breaking. Therefore, the glacier can be divided into two parts. The layer on the surface that is easy to break is called the brittle zone , and the "soft" part below is called the plastic zone . The existence of the plastic zone is the fundamental reason for the movement of the glacier .

Glacier crevasses. Image source: Wikipedia.

02

How were the “two zones” of the glacier formed?

To answer this question, we have to start with the physical concept of deformation .

When an object is subjected to force, there are two ways of deformation in order to adapt to or eliminate the external force, namely elastic deformation and plastic deformation . For example, when a rubber band is subjected to a small force, it will produce elastic deformation; when the force exceeds the elastic strength, it will undergo plastic deformation and the rubber band will never return to its original shape. Of course, when the force on the rubber band is particularly large and exceeds its own breaking strength, it will be broken.

Elastic deformation (A), plastic deformation (B) and fracture (C). Image source: Clinical Gate.

For ice, it is easy to achieve internal sliding of crystals, which is conducive to plastic deformation. However, when the external force increases suddenly, it is easy to exceed the fracture strength of ice and break. Only when the load is added slowly and the force is applied for a long time can ice fully show the characteristics of plastic deformation.

We know that when an object is subjected to a long-term force, even if the force is small, it will produce plastic deformation. Due to the pressure of the upper ice layer and the thrust of the upstream ice layer, the lower part of the glacier is always under stress, which makes the plasticity of the lower ice layer more fully manifested. At the same time, due to the pressure, the melting point of the lower ice layer is lower than that of the upper ice layer, so the lower ice layer is closer to the melting point , and thus plastic deformation is easier to achieve. In this way, it is not difficult to understand the appearance of the plastic zone at the bottom of the glacier. However, the surface of the glacier lacks the important condition of long-term stress. When the external force suddenly increases, it often breaks directly and becomes a brittle zone.

Plastic zone and brittle zone. The dotted line in the figure is the boundary between the plastic zone and the brittle zone. Image source: Lauren Adams.

03

Most glaciers are slow

The speed of glacier movement is only a few centimeters per day at the slowest and a few meters per day at the fastest, so it is difficult for the naked eye to detect that the glacier is moving. Some glaciers in Greenland have the fastest movement speed in the world, but they only move more than a thousand meters per year. The annual flow rate of glaciers in other regions is only more than a hundred meters. Most of my country's glaciers are continental glaciers, with little accumulation of glaciers and slow material circulation on the glaciers, which leads to a relatively slow movement speed.

The speed of glacier movement varies with the seasons , being faster in summer and slower in winter. One of the reasons for this difference is the change in glacier temperature. When the glacier warms up, the viscosity of the ice decreases rapidly, and the decrease in viscosity increases the plasticity, thus speeding up the movement of the glacier. In summer, meltwater appears inside and at the bottom of the glacier, which is another reason for the rapid movement of the glacier.

Generally speaking, the speed of glacier movement is very slow, but some glaciers have a very strange "temperament". After a long period of slow movement, they suddenly advance forward in an explosive manner. The explosive advance occurs periodically on some glaciers and is a special form of glacier movement. People call this phenomenon the "fluctuation" of glaciers, and glaciers with fluctuating properties are called " dynamic glaciers ".

Glacier fluctuations often cause major floods. There is a glacier in the upper reaches of the Indus River that periodically enters the main valley. When it blocks the river, it forms a large lake. If the lake bursts, it will form a major flood and cause disasters. The periodic major floods in the Yarkand River in Xinjiang may also be related to the outburst of glacial lakes caused by glacial fluctuations.

A glacier in Svalbard, Norway, shows fluctuations. Image credit: HEÏDI SEVESTRE.

If glaciers move on a large scale, they can cause a series of geological effects, leading to the extinction and migration of organisms, thus changing the environment of the entire earth. Studying glacier movement can provide timely information on sea level changes, the transformation of water systems and hydrological conditions, the extinction and migration of organisms, etc. It seems that the importance of "active" glaciers to the earth cannot be underestimated.

Produced by | Science Popularization China

Author: Earth Story

Producer｜China Science Expo

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

The cover image and the images in this article are from the copyright library

Reproduction of image content is not authorized