[MUSIC] In this lesson, we focus on glaciers. Their physical composition and processes. How they form and move. And how they modify the landscape. We'll also examine how our changing understanding of glaciers have shaped the way peoples have engaged with mountain landscapes over the past few centuries. Let's begin with a basic definition. A glacier is a mass of relatively slow moving ice created by the long term accumulation of snow. In mountain regions, glaciers form wherever snow accumulation during the winter exceeds that which is removed by melting during the summer. And what's key to this process is the gradual buildup of successive annual layers of residual snow. The weight of accumulating residual snow eventually begins to convert the lower layers to ice as it's compressed and made denser. If the accumulating snow is lying on a slope, it will eventually get thick enough that it will start to deform and move or flow down slope under the force of gravity. And this is what makes a glacier really unique. Glaciers flow, very much like slow moving rivers. Glaciers can come in all sorts of different sizes. Some are as small as football fields. While others can grow to be dozens or even hundreds of kilometers long. Presently, glaciers occupy about 10% of the world's total land area. With most located in the polar regions, in Antarctica, in Greenland, or the Canadian Arctic. Glaciers can be thought of as remnants from the last ice age which ended about 11,700 years ago. At that time, ice covered over 30% of the planet. The glaciers that lie within mountain ranges, however, show evidence of a much greater glacial extent from ice ages throughout the past two million years. And more recent indications of a rapid retreat. Let's begin by taking a closer look at how glaciers are formed. The transformation from snow to glacial ice involves several steps. And it begins with the compaction of the surviving snow under a mass of new snow. And this compaction causes the expulsion of air bubbles within the buried layers. Freshly fallen snow can be over 90% air. And so its density is fairly low. Somewhere around 50 to 200 kilograms per cubic meter. For comparison, the density of water is 1000 kilograms per cubic meter. After about two winters, residual snow turns into what's called firn. An intermediate state between snow and glacier ice. This usually occurs once its density is around 400 kilograms per cubic meter. The snow is now well on its way to becoming glacial ice. As compression continues. When the firn density reaches 600 to 700 kilograms per cubic meter. Any air that hasn't been squeezed out is now trapped as bubbles as the snow pack is slowly sealed off. Finally, firn becomes glacial ice when its density hits around 850 kilograms per cubic meter. Any remaining air bubbles and are now completely isolated from one another, and locked into the ice. Now, the difference between firn and glacial ice is not always clearly marked. But aside from density, color is often a good indicator. If there are air spaces between the individual ice crystals. And the ice has a whitish color when viewed en masse, it's probably firn. If the material looks devoid of air spaces, and is reflecting and transmitting a bluish color, it's glacial ice. We'll revisit the trapped air bubbles later in the course. It's one of the ways that glaciers provide us with valuable information about past climates. There are several processes that act on individual snow crystals and cause them to change from snow into ice. Snow crystals tend to have very complex shapes with intricate arms or branches when they fall from the sky. Once on the ground, these structures come into contact with one another. And their arms connect and lock into place, leaving pore spaces between them. At each contact point, the mass of each snow crystal is shared with others, creating points of higher pressure. Melting occurs here first. Melt water flows into the spaces between the crystals where the pressure is lower and the freezing point is higher. Here, water refreezes, binding snow crystals together and enlarging the individual grains. This process is called sintering. And so, with time and increased pressure, the snow crystals tend to slowly interlock with each other and grow in size. At the same time, the volume of pore base is gradually reduced, increasing the overall density. Snow turns into firn, and firn into ice. If melting takes place in the snow's surface, water will inevitably drain down into the snow pack. If that snow temperature is at or below the freezing point, the water will refreeze. And this speeds the whole process up. But the transformation of snow into glacier ice can occur in the absence of water as well. Particularly, in what's called dry snow zones. Dry snow zones exist at some of the world's highest elevations, and on the largest polar ice sheets. These are places where melting rarely occurs, if ever. It's just too cold. Under these conditions, the transformation of snow to glacier ice, instead, results from the mechanical breakdown of snow crystals as they're blown around by the wind. And broken into smaller rounder, grains. Smaller rounder grains pack together much more efficiently. Further compaction leads to tighter assemblage of crystals, crystal growth, and sintering. So, how long does it take for this whole process to occur? [SOUND] It depends on climate. The range of air temperatures throughout the year. And the amount of precipitation. In a cold, dry place like the Baffin Mountains of Canada's eastern Arctic. Where there is little summer melting. And the rates of precipitation in the winter is low. The process can take hundreds, even thousands of years. In warmer areas, like the Coast Mountains of Canada's west coast. Where there is both melting in the summer and heavy snowfall in the winter. It may take only three or four years for glacier ice to form.
