Glaciers are the largest moving objects on earth. They are massive rivers of ice that form in areas where more snow falls each winter than melts each summer. The glaciers that form the ice cap covering Greenland hold enough ice to cover the entire Earth to a depth of 17 feet! The glaciers of Antarctica are so heavy that they actually change the shape of the planet. And, perhaps most importantly, 75% of the world’s total supply of freshwater is frozen in glaciers. There’s a good chance that the landscape you live on today was shaped by glaciers thousands of years ago, during so-called ice ages when glaciers covered three times as much area as they do now.
A glacier is basically a collection of snow that lasts for more than a year. In the first year, this pile of snow is called a névé. Once the snow stays around for more than one winter, it’s called a firn. As more snow piles up over the years, the weight of the snow on top starts to press down on the snow on the bottom and turns the snow into ice. This process continues for dozens, hundreds, or even thousands of years, adding more and more layers on top and adding even more weight. Eventually most of the air is forced out of the ice. This is what causes glacial ice to look blue.
Once the glacier becomes heavy enough, it starts to move. There are two types of glacial movement, and most glacial movement is a mixture of both:
- Spreading occurs when the glacier’s weight becomes too much for it to support. The glacier will gradually expand and “spread out” like cookie dough baking in the oven.
- Basal slip occurs when the glacier rests on a slope. Pressure causes a small amount of ice at the bottom of the glacier to melt, creating a thin layer of water that causes the glacier to slide down the slope. Loose soil underneath a glacier can also cause basal slip.
When a glacier moves, it is like a flowing river. That’s because the layers of ice are very flexible under great pressure. The upper layers are more brittle. This is why it’s so dangerous to walk on a glacier – the upper layers can fracture and form huge cracks that sometimes get covered by fresh snow.