Ice has been a major force in the Glacier Bay region for at least the last seven million years. The glaciers seen here today are remnants of a general ice advance – the Little Ice Age – that began about 4,000 years ago. True to its name, this advance in no way approached the extent of continental glaciation during Pleistocene times known as the Wisconsin Ice Age. The Little Ice Age reached its maximum extent here about 1750, when general melting began. The advance or retreat of a glacier snout reflects many factors: snowfall rate, topography, and climate trends. Today, glacial retreat continues on the bay's east and southwest sides, but on the west side several glaciers are advancing.
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 flows out of the mountains into the ocean, we call it a “tidewater” glacier – the type many people come to Glacier Bay to see. The park includes 11 tidewater glaciers that break off or “calve” either into saltwater or freshwater lakes near sea level, eight of which are located within the bay. 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. Johns Hopkins Glacier calves such volumes of ice that it is seldom possible for larger boats to approach its ice cliffs closer than about two miles.
Huge icebergs may last a week or more. They provide perches for bald eagles, cormorants, and gulls, as well as haul-outs for seals. When passing close by, kayakers can hear splashes and crackles as melting water drips and the ice deteriorates. The ice pops and sizzles as it releases ancient air first trapped between the delicate snowflakes and then frozen in under pressure – a phenomenon called “bergie seltzer.”
Colors betray a berg's nature or origin. White bergs hold many trapped air bubbles. Blue bergs are dense and are likely recently calved. Greenish-blackish bergs may have calved off glacier bottoms. Dark-striped brown bergs carry morainal rubble – rocks that the glacier acquired on its journey down the mountain.
How high a berg floats depends upon its size, the ice's density, and the water's density. Bergs may be weighed down or even submerged by rock and rubble. A modest-looking berg may suddenly loom enormous – and endanger small craft – when it rolls over. Boaters and especially kayakers should keep in mind that what one sees is "just the tip of the iceberg."
