Chapter 16, Problem 1FQ

To determine

The way in which firn differ from snow.

Answer to Problem 1FQ

The firn and snow differ on the basis of density. Snow is soft, hexagonal ice crystal with low density and firn is packed ice crystals, denser than snow.  Snow has a significant amount of air in it. Firn is formed by closely packing of snow granules and then, coalescence of these granules as a result of compression until it attains approximately half the density of water.

Explanation of Solution

“Glaciers form when snow accumulates over an extensive period of time. The transformation of snow to ice involves a complex sequence of progressive burial, compression, partial melting and re-freezing. The transformation takes place progressively, as younger snow buries older snow.”

The process begins as snow starts falling as hexagonal ice crystals that form snowflakes containing beautiful intricate forms. Snow contains up to 90% air. The density (mass per unit volume) of the snowflake is low of about 0.1 grams per cubic centimeter (0.06 oz/in.3).

With progressive burial, compression, partial melting, and re-freezing take place, the snow first turns to firn and then, to ice (solid mass of interlocking ice crystals). Firn is the denser crystalline material formed by the compaction of granular snow due to the weight of the overlying material from the progressive burial of snow. Firn only contains about 25% air. The deeply buried frozen material becomes glacial ice when it achieves a density of 0.9 grams per cubic centimeter (0.52 oz/in.3).