The Birth of the Earth's "Sculptor"

The movement of glaciers constantly "carves" the landforms along the way. From the birth of this "sculptor" to picking up the "carving knife", it is a slow and peculiar process.

The boundary of glaciers

In the clear summer, residents of the Tianshan and Qilian Mountains can clearly see a black and white line across the mountainside, and above the line is a world of glittering ice and snow. This line is the snow line. To be precise, the snow line is the lower limit of the year-round snow area. At the snow line, the snow that falls each year just melts in the same year.

The snow line in a place is not fixed. Seasonal changes can cause the snow line to rise and fall. Only in summer is the snow line relatively stable, returning to a relatively fixed height every year. For this reason, the snow line height is measured in the hottest month of summer.

Globally, the snow line decreases from the equator to the poles. The snow line is an important symbol that controls the development and distribution of glaciers. Only when the height of the mountain exceeds the snow line in that area will there be excess snow accumulated every year. Over time, it will become a region with permanent snow and glacier development.

Figure 1 Snow line

(Image source: Wikipedia)

The balance of glacier activity

Snow falling from the sky and snow sliding down the hillside tend to gather in low-lying areas. Since low-lying terrain is generally shaped like a basin, it is called a granular snow basin in glaciology.

The granular snow basin is the cradle of the glacier. How does the snow accumulated in the granular snow basin become an ice tongue?

Step 1: After a series of metamorphic processes, snowflakes gradually become granular snow. There are many airways between the snow granules, and these airways are interconnected, so the snow granule layer is as loose as a sponge.

Step 2: In some places, the granular snow is very thick. The granular snow at the bottom is compacted under the heavy pressure of the upper layer, and slowly settles and recrystallizes, connecting and merging with each other. At the same time, the meltwater on the surface of the granular snow basin seeps down and partially freezes, gradually closing the airway of the granular snow, and the air surrounded by the ice becomes bubbles. This kind of ice has a white color due to the high content of bubbles, so some people specifically call it granular snow ice.

Step 3: The granular snow ice is further compressed, expelling bubbles and turning into light blue glacier ice.

Step 4: The thick glacier ice flows plastically under the combined effect of its own pressure and gravity, passing over the exit of the snow basin and winding down to form ice tongues of varying lengths. The long ice tongues can extend to the lower valley or even outside the valley mouth.

Figure 2 Ice tongue

(Image source: Wikipedia)

Mature glaciers generally have a granular snow basin and an ice tongue. The granular snow basin above the snow line is the accumulation area of ​​the glacier, and the ice tongue below the snow line is the melting area of ​​the glacier. The two are like the two ends of a scale, jointly controlling the balance of the glacier and determining the activity of the glacier.

The Earth's Sculptor's Knife

Moraine is the main material transported and deposited by glaciers, and is also one of the evidences that glaciers change the face of the earth.

When water freezes into ice, its volume increases by about 9%. When the melted ice and snow water freezes again in the cracks of the rocks at night, it exerts a strong frost heave force on the surrounding rock mass, and many rocks are cracked in the face of such a strong frost heave force.

Frost weathering occurs not only in exposed areas on hillsides, but also in the glacier bed, because there is temporary pressure meltwater at the glacier bed, which seeps into the cracks in the rocks at the bottom of the valley and generates strong frost heave when it freezes.

Frost weathering constantly creates loose rock fragments on the hillside and the glacier bed. The debris on the hillside rolls down to the glacier under the action of gravity, and the debris on the bed is more easily carried along by the glacier. The rock fragments carried by the glacier are generally called moraine.

Figure 3 Moraine

(Image source: Wikipedia)

The rock fragments on the surface of the glacier are called surface moraine, those inside the glacier are called internal moraine, those at the bottom of the glacier are called basal moraine, and those on both sides of the glacier are called lateral moraine.

The lateral moraine is close to the hillside and has a rich source of rock fragments, so the lateral moraine is high and large, like two towering walls sandwiching the glacier on the left and right. At the front of the ice tongue, most of the lateral moraines on both sides converge together to form a circular terminal moraine. The terminal moraine is like a tall castle, guarding the glacier. People who climb the glacier must first climb the terminal moraine before they can approach the glacier.

Not all glaciers have terminal moraines. Fast-moving glaciers do not have terminal moraines. A glacier can only form a tall terminal moraine when it stops in one place for a long time. When two glaciers merge, the two adjacent lateral moraines merge into a central moraine. There are many central moraines on the surface of a dendritic valley glacier, and the entire glacier appears as black and white stripes.

Figure 4 Lateral Moraine, Terminal Moraine and Medial Moraine

(Image source: Morgan Stanley)

Conclusion

The formation of glaciers is a wonderful masterpiece of nature, and their continuous movement enables them to pick up the "carving knife" to carve nature. The existence of glaciers plays a vital role in the earth. In today's global warming, what reason do we have not to protect these lovely "carvers"?

Produced by: Science Popularization China

Author: Earth Story

Producer: China Science Expo