Glaciers are massive sheets of ice, sliding slowly down a mountain and carving enormous grooves in the land. They flow down to the lowest point where gravity can take them, often into the ocean. The normal school of thought for these "tidewater glaciers" said that due to their weight and compaction to the earth's surface, they were grounded on the sea floor, only to arise once disintegrated. However, there is one glacier that extends into the water, floating intact on the ocean waves.
The glacier is the Columbia Glacier in Alaska. It has lost its footing on the sea floor and is transitioning to a floating glacier. The difference between this and an ice shelf (think Antarctica) is that the ice shelf does not move like a glacier. Also, a glacier is renewed at its source high on a mountain, while an ice shelf grows at its end where it meets the water.
Floating glaciers display more erratic behavior than grounded glaciers according to a new study published in the journal Geophysical Research Letters. Floaters produce much larger icebergs at more unpredictable intervals. The process of forming icebergs is known as calving and is one of the largest sources for global sea level rise.
The study of the different calving process was undertaken by a US Geological Survey (USGS) team led by glaciologist Fabian Walter of the Scripps Institution of Oceanography at UC San Diego. According to Mr. Walter, the transition from grounded to floating glaciers will greatly aid in understanding calving in large-scale glacier models, essential in predicting sea level rise.
The USGS team believes the floating section of the glacier may have been caused by the rate the glacier is receding. The Columbia Glacier has retreated four kilometers since 2004 and 20 kilometers since 1980, one of the fastest receding glaciers in the world.
Their study was conducted by installing a seismometer to measure seismic waves produced by shifts in geologic formation which includes glacier calving. Measurements were taken while the Columbia Glacier was grounded in 2004-2005 and when it was floating 2008-2009 allowing an accurate comparison.
The grounded glacier had continuous calving but each iceberg was quite small. On the other hand, the floating glacier calved more erratically and had much larger icebergs. The calving was caused when cracks in the ice join together, causing a large chunk of ice to completely break off. Understanding this process will be critical in predicting sea level rise, which will be ever more important as global warming continues and the planet’s ice melts.