Question

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

Short answer

Snow undergoes compaction to become firn, which further compresses over time to form glacial ice.

Step-by-step solution

Snow Accumulation

Metamorphism of snow to glacial ice begins with the accumulation of snowflakes on the ground. These snowflakes are light and airy with a high volume of air relative to ice.

Compaction and Settling

As more snow accumulates, the weight of the overlying snow begins to compress the layers beneath. The individual snowflakes start to compact, reducing air content and causing the snow to settle.

Formation of Firn

Over time, the compacted snow transforms into firn, a granular intermediate state between snow and ice. Firn has a lower air content and denser structure than fresh snow, but it isn't as dense as glacial ice yet.

Firn Compaction

As more snow accumulates on top of the firn over successive seasons, the firn compacts further. The grains recrystallize and bind together, expelling trapped air and increasing density.

Formation of Glacial Ice

Eventually, as the firn compacts over many years, it transforms into dense glacial ice. This ice is characterized by its bluish tint and much lower air content compared to firn or snow.

Key concepts

Snow Accumulation

The journey of snow turning into glacial ice starts with snow accumulation. When snowflakes gently fall to the ground, they create a blanket of white. At this stage, snow is light and fluffy, consisting of intricate crystals and a lot of air. This high volume of air between the snowflakes makes fresh snow soft and airy. Over time, layers of snow build up during periods of snowfall.
This accumulating snow lays the groundwork for future transformation, as it begins to compress under its own weight. Without enough snow accumulation, the transformation process to glacial ice cannot even begin.

Firn Formation

As snow continues to stack up, a gradual transformation takes place, leading to firn formation. Firn serves as a transitional phase between snow and glacial ice. It's more compact than fresh snow but still contains some air. The important difference is that firn is grainy and has a denser structure.
This change happens because the piled-up snowflakes start to bond and settle. As they do so, the boundaries between the grains blur, and over time, the structure becomes more solid. Frequent cycles of melting and refreezing during this phase also contribute to further compacting the snow into firn.

Compaction Process

In this critical compaction process, the continuous layering and weight of new snow cause the underlying firn to compress even more. As pressure increases, the grains within firn begin recrystallizing, becoming bonded tightly together. This compaction forces out some of the air and results in firn becoming denser.
The compaction is an important step towards forming glacial ice because it reduces porosity and increases the mass. Think of the compaction as nature's way of squeezing firn until it is ready to become ice. This process is gradual, often taking decades, and requires sustained pressure to progress further.

Glacial Ice Characteristics

When firn has undergone sufficient compaction, it transitions into glacial ice. Glacial ice is a robust structure compared to its predecessors, snow and firn. One noticeable characteristic is its physical appearance; it's typically dense and often displays a bluish tint due to light refraction in the closely-packed ice crystals.
The glacial ice has such low air content that its density allows it to become ancient and powerful. This dense ice can slowly move and flow, creating glaciers that shape landscapes. Unlike snow, which is ephemeral, glacial ice is enduring, often remaining for hundreds or thousands of years, thus playing a crucial role in our planet's climate and water systems.