What is a Glacier?
Glaciers form because snowfall in the high mountains exceeds snowmelt. Imagine a place high in the mountains that catches a vast amount of falling snow every year. This place is so high and so cold that none of the snow melts even in the summer. In fact, whatever precipitation that falls over the course of the year, falls in the form of snow. Over time, that snow pack builds. Soon the weight of the snowflakes in the upper layers of the snow pack presses down deforming the snowflakes beneath. The snowflakes in the pack first change to granular snow – round ice grains – and eventually morph into solid ice.
Glacier ice is different from the ice in your refrigerator. The ice crystals form slowly under pressure and individual crystals can grow to be the size of a football. Air trapped between the snowflakes is also frozen into the ice at pressure. Ice near the bottom of the glacier is under tremendous pressure, which allows it to flow almost like a plastic over the bedrock beneath. Friction between the glacier and the bedrock produces meltwater which further lubricates the bedrock allowing the ice to slide.
If a glacier is fed by enough snow to flow out of the mountains and down to the sea, we call it a "tidewater" glacier - the type many people come to Glacier Bay and Kenai Fjords to see. These types of glaciers will break off or "calve" into saltwater at sea level, and a few others that reach the sea at high tide only. The show can be spectacular. As water undermines some ice fronts, great blocks of ice up to 200 feet high break loose and crash into the water.
When tidewater glaciers calve icebergs into the marine environment, they serve as pupping and molting habitat for some of the largest seasonal aggregations of harbor seals in Alaska. Although tidewater glaciers are naturally dynamic, advancing and retreating in response to local climatic and fjord conditions, most of the ice sheets that feed tidewater glaciers in Alaska are thinning and, as a result, many of the tidewater glaciers are retreating. Climate change models predict rapid loss of glacier ice with unknown impacts to seals that rely on tidewater glacial habitat.