Which Way Did It Flow?
Glaciers and Pancakes (4:26)
Richard Ally: When snow falls, we expect it to melt. The ducks expect the snow to melt too. But what if the snow survived the summer? Even a few inches of squeezed ice under the next winter snow would give you a foot in a few years and 1000 feet in a few thousand years and you'd have a glacier.
A glacier is ice that flows. Here's a National Park Service time-lapse of a glacier flowing down Mount Rainier. It's pretty cool.
Suppose you make a pile like my dear wife Cindy making pancakes. All piles tend to spread under their own weight because the pressure under the pile is higher than the pressure outside the pile, giving a push. We met these physics earlier when one talked about how Death Valley and Nevada are spreading. When it gets stronger, the pile doesn't spread as fast. Like most buildings and a cooked pancake, if you hold the pile back, it doesn't spread as fast. Early Gothic cathedral builders didn't make the building strong enough to avoid spreading, and they had to invent the flying budgers to push back so that their masterpieces didn't spread and fall apart.
If you pour the pancake batter on something rougher or bumpier like a waffle iron, it doesn't spread as fast and it looks cool. Maybe you can imagine one of these flows as the ice that carved Yosemite Valley, or deposited Cape Cod, or Mendenhall Glacier in Alaska. This is the Juneau Ice Fields outside the city of Juneau. The airport is down there in the lower left. Snow is accumulated on the cold mountains and makes a pile that spreads across this waffle iron, making glaciers such as Mendenhall Glacier, which you see here. Up on top of the Juno Ice field, it looks like this with Penn Staters Kaia Riverman and Don Voigt teaching about glaciers.
And Greenland and Antarctica are just ginormous piles of old snow squeezed to ice and spreading under their own weight. Researchers working with NASA measured the flow of Antarctica's ice using satellites, and then they color-coded it with the magenta going fastest in the orange slowest. So you can see the pile spreading under its own weight and funneling into the low troughs of the bedrock waffle iron down to miles underneath the ice. It's pretty cool.
Sometimes you'll have a glacier on just one side of the mountain because the other side is too sunny, too windy, or too steep to keep the snow. But that's the ice will still be flowing down the mountain the way glaciers do. If the climate warms and the glacier doesn't get as far down the mountain before all the ice melts, it still flows down the mountain, away from the center of the pile.
So if you look at this Landsat, NASA view of Mendenhall Glacier from 1984, the ice flowed down the mountain and it ended here. Go back in 2023. The front of the ice had retreated from where it ended before because the climate warmed. But the ice was still flowing down the mountain, spreading away from the pile up on top. So now you know how glaciers flow, and maybe you'll have pancakes for breakfast tomorrow.
A glacier is a mass of snow or ice that deforms and moves. Glaciers form wherever snowfall exceeds melting over enough years to make a pile big enough to flow. In places with extremely high snowfall, this can occur where average temperatures are near or even slightly above freezing, such as on the mountains of the Olympic Peninsula. In dry places, glaciers may be absent even if average temperatures are well below freezing. Such places have frozen ground instead, called permafrost (because the frost is permanent).
A glacier is a mass of snow or ice that deforms and moves. Glaciers form wherever snowfall exceeds melting over enough years to make a pile big enough to flow. In places with extremely high snowfall, this can occur where average temperatures are near or even slightly above freezing, such as on the mountains of the Olympic Peninsula. In places with very little precipitation, glaciers may be absent even if average temperatures are well below freezing. Such places have frozen ground instead, called permafrost (because the frost is permanent). We will spend a little while looking at how glaciers flow, in part because glaciers are important, and in part, because they reveal processes that are important in many other settings—pancakes, landslides, swimming pools, huge cathedrals, and glaciers, and many other things have some behaviors in common.
If you pour water onto a tabletop to make a pile, the water will spread out across the table and eventually drip off onto the floor. Pour pancake batter onto a griddle, and the pile will spread, although the spreading will be slower and the pile will be thicker than for water. All piles tend to spread under their own weight, because the pressure at the bottom of the pile is higher than the pressure outside the pile, giving a net push outward. Spreading may be avoided if the pile is strong enough—a hot griddle cooks the pancake batter, making it stronger and stopping the spreading. Most buildings are strong enough to resist spreading, too. However, builders of early cathedrals faced the problem that the roofs tended to cave in as the walls bulged out because the “pile” of the cathedral was spreading, requiring the invention of flying buttresses to support the walls and prevent pile-spreading collapses.
The diagram illustrates the spreading of a pile, with water or ice or pancake batter moving away from where the upper surface is highest. This occurs because the pressure at point A (the weight of the material above point A) is larger than at point B because there is more ice above A than above B. The higher pressure at A gives a net push from A to B. Thicker or steeper piles give larger pushes, and tend to spread faster. Typically for glaciers, ice thicker than about 50 m (150 feet) will deform and flow fast enough to be easily measured, making a glacier.
If you make a pile of pancake batter on a waffle iron, rather than on a flat griddle, some of the batter may flow along the low grooves and then move up to cover the bumps, but the flow will still move away from the place where the upper surface of the pile is highest. In the same way, ice can flow up a hill in the bedrock if the flow is going in the “down” direction of the upper surface. (If you want more detail on this, just for fun or some other class, we recommend the text The Physics of Glaciers, by brilliant Penn State grad Kurt Cuffey, which is available at the Penn State library and in many other libraries) For example, farmers in northeastern Pennsylvania grow food in soils made of pieces that were brought across Lake Ontario and New York by the glaciers. The bottom of that glacier climbed out of the low spot that now is the lake basin, driven by the upper surface of the ice sloping down from Canada to the U.S.
If you have ever slipped on the ice, you know how slippery ice can be, and you won’t be surprised by the second part of the figure. Where glaciers are thawed at the bottom, they generally slide over the rocks or soil beneath (shown in the figure), and if the material is loose soil, it often deforms in a sort of slow landslide that lets the glacier go even faster. And, all glaciers deform internally, like your slow pancake batter spreading on the griddle. A vertical hole drilled in a glacier will deform as shown in the figure. The stresses are the largest, causing the most intense deformation (the permanent bending of the hole shown in the figure), in the deepest ice. The upper ice rides along on the deeper ice, so the velocity is the fastest at the top of the ice.
Recall that rivers adjust to move sediment and water from one place to another. So do glaciers. Frozen water is supplied where snowfall exceeds melting in the accumulation zone. The frozen water flows to where melting exceeds snowfall, called the ablation zone, or else flows to where icebergs break off (called calving) and drift away to melt elsewhere. For ice sheets covering continents or for smaller ice caps covering plateaus or mountain tops, the ice forms a dome and spreads out in all directions. For glaciers on the sides of mountains, the ice flows down the mountain.
When melting decreases or snowfall increases, a glacier generally thickens and advances—its terminus, where it ends on land or calves icebergs, moves over land or water that did not have ice before. When melting increases or snowfall decreases, the terminus retreats and the glacier gets smaller. Notice that ice almost always continues flowing in the same direction, from the accumulation zone through the ablation zone to the terminus, whether the glacier is advancing or retreating.
Some people find it strange that we can walk on glaciers (being careful not to fall into crevasses!) and even land airplanes on glaciers—which clearly are solid—yet the glaciers flow. We have met something similar before, though. Like the soft rock of the asthenosphere down in the mantle, or the soft chocolate bar in a hot pocket, or the red-hot horseshoe in the blacksmith’s shop, ice in all glaciers on Earth is nearly warm enough to melt, and so can flow slowly. As a general rule, materials heated more than halfway from the coldest possible temperature - absolute zero - to their melting point can flow slowly, and flow becomes easier as the temperature increases closer to the melting point. For ice, the coldest yearly average temperature on Earth is about eight-tenths of the way from absolute zero to the melting point, so ice at the Earth’s surface is always “hot” and can flow. For more on this, and on the occurrence of crevasses as well as flow, see the Enrichment.