A glacier is like a checking account. It has an income in the form of new snow, and there are expenditures in the billions of gallons of water it loses each season. Like a good accountant, a glaciologist keeps track of this exchange. A healthy glacier, such as the Blue, retains a large fraction of its income of winter snow on the upper reaches, while the snow on the lower half is spent through yearly melting.

Thus a glacier has two zones: the area of accumulation where snow is retained, and the ablation area where more ice melts than accumulates. The boundary between these two zones is called the equilibrium line and is usually measured in altitude. Glaciologists can quantify the health of a glacier by calculating the ratio between the area of accumulation and the entire glacier surface. The higher the ratio, or the more accumulation, the healthier the glacier. Unfortunately, you cannot see the equilibrium line on a glacier—its is something that must be computed.

Late in the summer a climber can see at least two distinct lines on the glacier: the snow line, and also, a few meters down slope and a little darker in color, the previous year’s snow line known as the firn line. The brighter blue below the firn line is bare ice. On Blue Glacier the snow line is usually around 1675 meters, just below the base of the icefall. At the end of the summer, the snow line may lie above or below the firn line depending on the severity of the current ablation season. If the snow line is below and obscuring the firn line, the glacier is growing, because accumulation is greater than ablation. On Blue Glacier the firn limit is exposed indicating that either expenses exceed income or the glacier is in equilibrium.

Because a new firn line is created annually and the glacier is flowing, a succession of firn lines—usually darkening in color—are visible in the ablation zone. These layers look like topographic lines on a map and are made more visible by the dust and debris that collect on the surface. When viewed in cross-section, on the wall of a crevasse for example, each firn line becomes a horizontal band known as an ablation horizon. Each band represents one year’s accumulation similar to the growth rings in a tree. If a summer season is extremely hot, multiple ablation horizons may melt away. A break in the record occurs as new snow lands on a much older ablation horizon and takes the place of the missing layers. This gap in deposition is known as an unconformity.



intro | terminology | accumulation | firn | blue | ablation | water | equilibrium | massbalance | movement | crevasse | structure | algae | moraine | debris | erosion
pdf version | glacier glossary | bibliography | about blue ice

Benjamin Drummond 2002