[MUSIC] Glaciers come in various shapes and sizes, and size is one of the ways that we classify glaciers. Let's take a look at some different types from around the world. The largest glaciers on the planet are known as ice sheets. Today, there are only two places on the planet where we find them, in Antarctica and Greenland. And they're called ice sheets for a good reason. An ice sheet is a massive glacial land ice extending more than 50,000 square kilometers. And because of their great dimension, their flow is completely independent of the topography beneath. Take as examples the east and west Antarctic ice sheets. They essentially cover the entire continent, and are so massive that they actually submerge the underlying bedrock topography. The two ice sheets are separated by one of the longest mountain chains in the world, the Transantarctic Mountains. And these mountains reach an elevation of 4,500 meters above sea level, but are almost completely buried by ice. So much so, that they have have very little impact on the ice surface topography. Icecaps are miniature ice sheets covering less than 50,000 square kilometers. They form primarily in the polar and sub-polar regions that are usually high in elevation. And to see the difference between an icecap and an ice sheet, compare Iceland and Greenland from satellite imagery. The smaller mass of ice on Iceland, that's an icecap. Like larger ice sheets, icecaps are not constrained by topographical features, meaning that they lie over the top of mountains. The dome of an icecap is usually centered on the highest point of the massif, and ice flows away from this high point which is sometimes called the ice divide towards the icecap's periphery. The largest group of glaciers that we'll be focusing on this course are those can be found in high mountainous regions. Mountain glaciers, this broad category encompasses different types of glaciers, which range in size and characteristics but share one thing in common. Unlike ice sheets or icecaps, mountain glaciers are confined by the topography of the landscape in which they reside. The largest type of mountain glacier is an icefield. At first glance, an icefield looks a lot like an icecap. But there's one important difference. Unlike an icecap, the flow of an icefield is constrained by the underlying topographic features. This is the Columbia Icefield in the southern part of Jasper National Park. Here, the snow accumulation is so great that the glaciers flowing from some of Rocky's largest peaks have all amalgamated in this high basin, spilling over to create a network of glaciers that reach out in all directions, like spokes in a wheel. Take a look at the icefield from above. From this vantage, it's easy to see the network of long glaciers that fall away from the high basin and spill down the valley. These glaciers, which look very much like giant tongues, are called valley glaciers. Because they originate from an icefield, they're also sometimes referred to as outlet glaciers. Valley glaciers can be very long, often flowing down beyond the snow line. The Fedchenko glacier in Pamir Mountains of Tajikistan, for example, presently extends 77 kilometers and covers over 700 square kilometers. It's the longest glacier in the world outside of the polar regions. The valley glaciers flowing from the Columbia Icefield here in the Canadian Rockies aren't nearly as long. But they are big enough to form the headwaters of some Canada's greatest rivers, flowing west to the Pacific Ocean, north to the Arctic Ocean, and east to the Atlantic Ocean. It's what makes this icefield the hydrological apex of the North American continent. Valley glaciers can be further delineated as well. Here's a piedmont glacier. Piedmont glaciers occur when steep valley glaciers spill into a relatively flat plain, they fan out into bulblike lobes. The Malaspina Glacier in southeastern Alaska is one of the most famous examples of a piedmont glacier. It arises where several valley glaciers spill out of the Saint Elias Mountains onto a coastal plain, and it covers a whopping 3,900 square kilometers. Although it handily fills the plain, nowhere does it actually reach the water. So it doesn't qualify as the next type of glaciers that we'll look at. Tidewater glaciers are valley glaciers that flow far enough to reach out into the sea, like this spectacular glacier, the Columbia Glacier in the Chugach Mountains, which lie further north and west of the Saint Elias Mountains in southern Alaska. As the ice reaches the sea, pieces break off, or calve, forming small icebergs. Although not as imposing as Antarctic icebergs, the icebergs produced by tidewater glaciers can still pose problems for shipping lanes. Finally, when a major valley glacier system retreats and thins, sometimes tributary glaciers are left in smaller valleys high above the shrunken central glacier's surface. These are called hanging glaciers and these glaciers often terminate at or near the tops of cliff bands. Much smaller than valley glaciers are cirque glaciers. Cirque glaciers are named for the isolated bowl-like hollows or basins they occupy, which are called cirques. Typically, they're found high on mountainsides and tend to be wider rather than longer. Given their unique position, a cirque glacier usually can't survive solely on the accumulation of direct snowfall. Instead, its survival requires snow deposited by avalanches from the surrounding rock walls. The walls also provide the glacier with some shade, greatly reducing the direct solar radiation and limiting melting. Shading also limits the size of these glaciers, constraining the glacier to the shaded area. The various forms of mountain glaciers result from both topography and climate. Because environmental conditions fluctuate through time, the specific form that glaciers assume fluctuates, too.
