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Formation of Sea Pack Ice
Sea ice is formed when water on the surface of the ocean drops to the freezing point. The freezing point of saline water is slightly lower than that of fresh water -- about 29 degrees Fahrenheit compared to 32 for fresh water -- and therefore sea pack ice requires a lower temperature to form than glacial ice.
Formation of Glacial Ice
Glacial ice is composed entirely of fresh water and is formed by snowfall in places where the temperature very rarely exceeds 32 degrees Fahrenheit. As a layer of snow falls, some of it may briefly melt and then refreeze, turning into small ice crystals known as firn. As more layers of snow fall, the firn underneath is compacted into a sheet of ice, which slowly begins to move as the pressure above it increases.
Effect of Sea Pack Ice
The primary function of sea pack ice is as part of the ocean circulation process. The formation of sea pack ice expels salt from the water that freezes, which sinks into the water below it, making this water denser and causing it to sink lower. This forms part of the "great conveyor belt" which keeps the oceans circulating and prevents stagnation.
Effect of Glacial Ice
Glacial ice functions in a very different way from pack ice, mainly due to the conditions it exists in. A glacier, as it is on land and exerting immense force on the ground beneath it, alters the landscape it travels across. As it travels, the immense friction it causes carves out enormous fissures in the landscape. Evidence of this can be seen by the vast U-shaped valleys carved out by ancient glaciers.
Structure of Sea Pack Ice
As sea pack ice floats on the surface of the ocean, its structure is very different from that of glacial ice. Like icebergs, the majority of the mass of pack ice is below the surface. Sheets of pack ice can be up to 20 feet thick in the Arctic, although it is more common to find sheets between one and six feet thick. The distance from the top of the ice to the surface of the water is known as the freeboard, and the distance between the surface and the bottom of the ice is known as the draft. Sea pack ice is made up predominately of salt water and whatever organisms were caught up in the freezing water.
Structure of Glacial Ice
As has been noted, glacial ice is made up of enormous sheets of freshwater ice compacted tightly below looser, granulated ice on top. However, as the ice begins to move, a bottom layer forms of ice mixed with the churned up debris that has been produced as the glacier moves. This debris forms a wedge shape that thickens towards the front of the glacier as the debris is pushed forwards.