M6 Freezing and Metamorphism

Topic

The processes of freezing and metamorphism describe how the microstructure of ice and snow changes over time: the growth of constituent grains, the recrystallization of those grains, the sintering that bonds them together via vapor diffusion, and the formation of larger cryotic structures visible to the naked eye, such as pingos and palsas.

Snowpack metamorphism begins the moment snow is deposited and continues thereafter: temperature gradients within the snowpack create vapor pressure gradients, which drive water vapor from warmer regions of the snowpack toward colder ones, where the vapor deposits onto existing ice grains. When this process occurs under moderate gradients, it transforms the fresh, branched crystals of newly fallen snow into rounded grains; however, under much stronger temperature gradients, it produces angular depth-hoar grains instead.

Firn sintering is the gradual transformation of snow—first into firn and subsequently into true glacial ice—driven by the weight of accumulating overlying layers and the diffusion of water molecules within the solid ice itself. During this sintering process, density increases from a mere 300–400 kilograms per cubic meter in the uppermost firn to approximately 830 kilograms per cubic meter at the boundary where firn has transitioned into glacial ice; the rate of this density increase is highest at the top of the firn layer and slows progressively as sintering proceeds to greater depths. The depth at which air bubbles are finally sealed within the ice—when the firn density reaches that same 830 kilograms per cubic meter—means that the air trapped inside each bubble is always younger than the solid ice surrounding it; this is a critical age difference for the correct interpretation of an ice core.

Pingos are mounds of massive ice formed when pressurized water freezes beneath permafrost; there are two types: open-system pingos, formed by water intrusion beneath the thawed ground left after a lake drains, and closed-system pingos, formed in areas where groundwater is already under artesian pressure.

For a pingo to form, there must first be a zone of thawed ground within the permafrost; this zone progressively freezes from the edges toward the center. As freezing advances inward, it concentrates increasing amounts of water in the remaining unfrozen space, raising the pressure within that area; this mounting pressure eventually heaves the ground surface into the characteristic pingo dome, thereby completing the sequence of ice and snow transformations—ranging from the microscopic grain to the visible hill—with which this description began.

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