Question

Outline the major steps in the metamorphism of newly fallen snow to glacial ice.

Short answer

Snow transforms into glacial ice through compaction and recrystallization, passing through stages of snow, firn, and finally glacial ice.

Step-by-step solution

Snow Accumulation

Newly fallen snow accumulates on the ground. Initially, the snow is fluffy and has a low density, with a lot of air trapped between the snowflakes. The snow is characterized by a crystalline structure.

Compaction and Recrystallization

As more snow falls, the layers beneath are buried and subjected to pressure. This pressure causes compaction, pushing the snowflakes closer together and reducing the air content. Over time, the edges of snowflakes begin to melt slightly and refreeze, which leads to recrystallization, forming a denser layer called firn.

Transition to Firn

The snow further transforms into firn, a granular, intermediate form between snow and glacial ice. Firn has a higher density compared to initial snow because of the continuing compaction and recrystallization. This stage can take several years.

Formation of Glacial Ice

Eventually, the continued pressure and recrystallization cause the firn to metamorphose into glacial ice. This ice is much denser than both snow and firn and forms as individual ice crystals become interconnected. The spaces between the crystals are minimized, giving the ice its characteristic solidity and bluish tint.

Key concepts

Snow Compaction

As new snowflakes fall, they accumulate in layers, each layer pressing down on the one beneath it. Initially, snowflakes have a fluffy and open structure with low density because there is a lot of trapped air between them. As more snow accumulates, these fluffy flakes start to press tightly together. The weight of the snow above exerts pressure, causing compaction. This is the first step toward transforming the fresh snow into something denser. During compaction:

• The air pockets between the flakes reduce significantly.
• The fluffy structure begins to flatten.

This process signifies the beginning of metamorphism, where snow is not just fluff but a more consolidated mass.

Firn Formation

Firn is a critical stage in the transformation from snow to glacial ice. As snow compacts, it turns into firn, a dense and grainy form of snow that falls between regular snow and full-fledged glacial ice. The transformation to firn occurs over several years and involves the continuous effect of compaction and occasional melting. With firn:

• The density increases, making it much compact than fresh snow.
• It has typically survived at least one summer, giving it time to undergo significant changes.

While firn maintains some granular characteristics of snow, it is much closer to the density of glacial ice, indicating ongoing metamorphism.

Pressure and Recrystallization

Pressure plays a vital role in the transformation process of snow into ice. With the increase in depth of snow and its gradual transformation, the pressure on lower layers also increases. This pressure causes the edges of snowflakes to melt and refreeze. This process is known as recrystallization. During recrystallization:

• The crystals rearrange themselves into a tighter, more compact form.
• The free-flowing water between grains allows crystals to bond more effectively.

Eventually, snow ceases to remain as discrete flakes and turns into a continuous and denser structure, which moves closer to glacial ice.

Glacial Ice Density

The final stage of snow metamorphism is the formation of glacial ice. This results when firn transforms further under persistent pressure and recrystallization. Glacial ice is characterized by its high density, which is achieved when most of the air pockets are eliminated. A few key points about glacial ice density include:

• It is much denser than both snow and firn.
• The ice appears bluish not because of any pigment, but due to its density and the way light travels through it.

With a density approaching about 830 to 910 kg/m³, glacial ice becomes a solid mass that is quite stable, completing the metamorphic process started by snow.