In many ways, glaciers behave like ketchup. Once the layer of ketchup reaches a certain thickness, the ketchup will start flowing off your hot-dog to drip onto your newly washed white shirt. The same is true for glaciers. When more and more snow piles up, at some point in time, after it has been compressed to ice, gravity will make the ice move down the mountain slopes.
The further down the ice flows, the higher the temperature, and the more ice melts. So, even if the glacier does not change in length, ice is constantly flowing through the glacier from the top where the snow piles up to the bottom where ice melts away. When more snow piles up at the top than ice melts away at the bottom, the glacier will become longer and the glacier front will advance.
Gravity pulls on the ice all the time. Then, why doesn’t the ice move faster and faster down the mountain? Also, why is there at least some of the ketchup left on your hot-dog, after you’ve ruined your shirt? The reason is friction. It keeps both the ketchup on your hot-dog and the ice from accelerating down the mountain.
The friction acts where the ice touches the rock, or where the ketchup touches the hot-dog. That means, the ice is held back along the valley floor and sides. Ice flows fastest where it is furthest away from solid ground.
Differences in the ice movement because of friction are one cause for splits in the glacier. The splits are called crevasses and can be hundreds of meters long and up to meters about 50 meters deep. Friction is not the only reason for crevasses. For example, if the valley becomes steeper, the ice will flow faster and crevasses will appear. Similar things happen, if the valley is curved, or if it is getting wider. Each of the reasons creates crevasses with a distinct orientation. So, one can tell the cause of a crevasse just by its looks.
The ketchup dripping onto your shirt does not show any crevasses. But, in a few locations in the world, ice and snow drips down a cliff from what is called a hanging glacier to feed a child glacier at bottom of the cliff. Think of that the next time you eat a hot-dog!
[This post was developed and written for Turspor in collaboration with ClimateSnack at the ResClim science communication course in Finse 16-20th September, 2013]