Why does the densification of snow slow after it reaches a density of 500 to 600 kilograms per cubic metre?

The densification of snow slows significantly after reaching a density of 500 to 600 kilograms per cubic meter because the initial mechanisms driving compaction—such as the rounding of ice grains, sintering, and grain rotation—become increasingly ineffective.1

As the snowpack reaches this density, these early processes are superseded by recrystallization under the stress of overlying snow. During this phase, grains change in size and shape to minimize internal stress, typically resulting in larger or more favorably oriented grains growing at the expense of others. While this process continues to increase the density of the mass and the average grain size, it proceeds at a slower rate than the earlier stages of transformation. This transition continues until the air spaces between grains are eventually sealed off at a density of approximately 830 to 840 kilograms per cubic meter, marking the shift from firn to glacier ice.1