Approximately 2 million years ago, the North American continent, in essentially the same global position it is in now, entered an ice age. When we say ice age, we mean that ice covered a significant percentage of the Earth’s surface. In the case of this past ice age, at its peak, ice covered about 32% of the land surface and 30% of the oceans, significantly more than is covered today. While the cause of an initiation of an ice age is still under scientific investigation, when climatic conditions are just right, glaciers will form and behave in a well documented manner.
The climatic conditions that support the growth of glaciers are high winter snow fall, combined with cool summer temperatures. You see, glaciers, whether they form in the mountains or in the middle of a continental land mass, are formed from snow and only snow. The climate must be cool enough, at least regionally, that precipitation falls as snow, in at least the winter. The summers, if we can call them that, must be cool enough that all that snow doesn’t go away. At its simplest, a glacier starts as a multi year accumulation of snow.
The glaciers that define an ice age are continental glaciers, meaning they form large sheets of ice that cover virtually everything on a continent. Think of the ice coverage of the continent of Antarctica today. They form when it snows in the winter, and that snow doesn’t melt in the summer, over and over again, on a very large scale. After a few years, that multi year snow transforms physically to something called firn (f I r n). As snow ages it undergoes metamorphism, the snow crystals, or flakes, break down and become more rounded, and start to bond together-that’s firn, an intermediate stage between fresh snow and ice. More snow piles on top, and the weight of that snow presses down on the older snow below and accelerates this process.
As the firn snow gets more and more compressed, by the weight of the accumulating snow on top of it, the air pockets trapped by the original snow fall (think fluffy powder snow) get more and more compressed, and are slowly forced out of the compacting snow pack. It may take up to one hundred years, but this compaction gradually changes the original snow (fluffy, white!) into a solid blue material we would all look at and recognize as ice.
We think of ice as hard and solid, especially when we are first learning to ice skate, but in reality, ice is more malleable, and nowhere do we see this more clearly than in glaciers. Just like the atmosphere has mass and weighs upon us here on the surface of the earth (otherwise known as the bottom of the atmosphere), and the water of the ocean has mass weighs down on the bottom of the sea, the snow that accumulates on a glacier has mass. As that snow accumulates it gets heavier, it piles higher, it literally builds up. Once the ice gets thick enough, once enough snow has accumulated and weighs down on the snow underneath it, the ice that is formed starts to deform; in geological terminology we say the ice has become plastic.
Gravity doesn’t like it when some things (any things) are higher than other things. Gravity wants everything to be in equilibrium, in other words, at the same level. When the things that are higher than other things are solid, like mountains, gravity can’t do anything about it except wait for erosion. But if the things that are higher than their surroundings are fluid, or plastic, they are capable of flow, and will yield (albeit slowly, in the case of a glacier) to the power of gravity. Which is to say, when a continental glacier gets big enough, and thick enough, it will start to flow outward in all directions, sliding on its base, where it is in contact with the land below, and internally deforming (or squishing) in between the surface and base. The glacier will continue to spread as long as snow keeps falling on the interior, and more snow accumulates than melts each year. A glacier is said to be in retreat, when it is melting faster than it is forming. In this way the size of the glacier is directly related to climate, which is the primary reason that so many climate scientists, including several world class ones here in Maine, study the dynamics of the world’s remaining ice sheets in their pursuit of the keys to climate change.
We’ll leave it off there for today, but join us in the coming weeks as we look into the details of the past 2 million years of glacial advance and retreat, and what that has meant for the landscape we see around us today.
Caldwell, D. W. Roadside Geology of Maine
The National Snow and Ice Data Center (yes there is such a thing!) All About Glaciers!
When seen from above, it is much easier to see a glacier's fluid nature. From NASA’s Earth Observatory website (a must visit—they have a weekly email list serve for serious nerds, of which I am one). http://earthobservatory.nasa.gov/IOTD/view.php?id=4710