Before the era of universal air travel, which commenced less than half a century ago, few of the world’s population had seen a glacier. I suspect that majority of class members in this course have seen a glacier—if not close up, then out of a jetliner window. In the Canadian Rockies, you can drive to within almost a stone’s throw of the terminus of the Athabasca Glacier, a classic active valley glacier. In many other parts of the world, valley glaciers are accessible to even casual day hikers. The great ice sheets of the world, in Antarctica and Greenland, remain much less accessible.
In the broad context of geologic history, the Earth is in an “icehouse” time, with recurrent major ice-sheet advances across the Northern Hemisphere continents. (There have been several other such icehouse periods in Earth history, separated by long intervals of ice-free times, called “greenhouse” periods, with no evidence of glaciation.) The Earth has only recently emerged from the latest episode of continental glaciation. Does it surprise you to learn that a mere twenty thousand years ago the Boston area was beneath a mile of glacier ice moving slowly southward toward its terminus south of what is now the south coast of New England?
The Earth is in many senses a glacial planet:
• 10% of the Earth is covered with glacier ice (about 15 million square kilometers).
• About 40% of the Northern Hemisphere in winter is covered with solid water at any given time (land and sea).
• 75% of the Earth’s fresh water is in glaciers.
• Surficial deposits by glaciers cover a large percentage of the Earth’s land surface.
• Glaciers have a profound effect on the Earth’s climate (as well as being in turn controlled by climate).
And glaciers are not without practical importance:
• Role of ice sheets on climate
• Control on sea level
• Potential source of fresh water
Disciplines: in the US, people who deal with glaciers or their products are usually allied to geology; in other English-speaking countries, usually to geography. Disciplines relevant to this chapter:
- glacial hydrology
- glacial geology
What is a glacier?:
a body of ice and recrystallized snow (plus refrozen meltwater), on land (or, if floating, then connected to land) and moving by deformation under its own weight.
Why are there glaciers?
A necessary and sufficient condition is an excess of snowfall over snowmelt at some locality for a long enough time to build up ice thick enough for it to flow under its own weight.