[MUSIC] While mountain glaciers don't contain actual dragons, they do contain a number of interesting features resulting from the movement of glaciers. And two features deserve mention, crevasses and moraines. Crevasses are deep cracks or fractures found in the glacier, as opposed to crevices which form in rock. Crevasses form due to tensional stress. So their distribution, their size and their arrangement, provide useful information on the flow behavior of ice. For instance, crevasses occur most often when the middle and the sides of the glacier move at different rates. They occur when ice curves around a bend. Or where the slope steepens, and the rate of movement increases. Crevasses can be caused by the ice flowing over bumps or steps in the bedrock. They're usually transverse or horizontal to the direction of flow, but can be orientated really in any direction. They're also largely restricted to the surface, where the ice is much more brittle and fractures more easily than down lower. The presence of crevasses increases the efficiency of rock transport. Remember that conveyor belt analogy? Rock debris that falls into crevasses becomes incorporated into the glacier, as often not seen again until the glacier releases the material at the terminus. Crevasses also hasten ablation by increasing the glacier's surface area, by the pooling of melt water and by the disaggregating the ice near the terminus. Crevasses pose a major hazard to anyone traveling on glaciated terrain. And this is especially true in instances where fresh snow has covered up or bridged the surface. Snow bridges can easily blend in with the surrounding landscape, often hiding crevasses below. Occasionally an old snow bridge over a crevasse can start to sag, which provides a little bit of landscape relief. This can't really be relied upon. It's the reason why, when traveling over glaciers, experienced mountaineers protect themselves with ropes and with harnesses, regularly practice crevasse rescue scenarios, or hire professional mountain guides. Another noticeable surface feature of mountain glaciers is the linear accumulation of rocky debris oriented in the direction of the flow. These are called moraines, and they're created when the glacier pushes or carries rocky debris as it moves. These features are nicely displayed on the Kaskawulsh Glacier in Kluane National Park, within the Saint Elias Mountains of Canada's Yukon Territory. Now this glacier here is the result of a convergence between two outlet glaciers, the south and the central arms. When one ice stream joins the other, it carries with it a load of debris along its edges. These are its lateral moraines. The lateral moraines become incorporated into the larger ice flow as vertical partitions between two ice masses. The material then is called the medial moraine on the main, now larger glacier. What we see is only the surface expression of the rock debris. It actually extends deep into the ice, frequently all the way to the bottom. The presence of moraines on the ice alters the mass balance. Since the rock material is dark in color and can absorb more of the sun's energy. On the other hand, if the rocky burden is thick enough, it may serve as an insulated cover and inhibit local melting of the underling ice. This leads to more rapid melting of ice on either side of the debris, leaving the moraine often with an ice core exposed as a high ridge. The glacier terminus is the end of the journey for the larger rock material. The finer debris however can be transported further through the actions of glacier melt in the water. Glaciers not only transport material as they move, they also sculpt and carve away the land beneath them. Here's University of Alberta glaciologist Martin Sharp. >> A glacier's weight combined with its gradual movement can drastically reshape the landscape over hundreds or even thousands of years. The ice erodes the land surface and carries the broken rocks and soil debris far from their original places, resulting in some interesting glacial landforms. We call this modification of the landscape glacial erosion. Glaciated valleys, cirques, erects, and horns are all erosional types of landforms created when glaciers cut away at the landscape. >> Common all over the world on a large scale, glaciated valleys are probably the most readily visible glacial landform. They're trough shaped, often with steep, near-vertical cliffs, where entire mountainsides were scoured by glacial movement. One of the most striking examples of glaciated valleys can be seen in Yosemite National Park in California's Sierra Nevada Mountains. Here, glaciers literally sheared away mountainsides, creating deep valleys with vertical walls. Cirque glaciers further erode mountainsides scouring into them and creating rounded hollows with steep uphill faces shaped like tilted bowls. Aretes are jagged narrow ridges created where the back walls of two glaciers meet, eroding the ridge on both sides. Horns are created when several cirque glaciers erode a mountain, until all that's left is a sharp pointed peak, with sharp ridge-like aretes, leading all the way to the top. The famous Matterhorn in the Switzerland seen here, displays all three types of glacial erosion. Here's an animated sequence of events, showing the headward erosion by cirque glaciers to create steep sawtooth ridges or aretes and glacial horns. Headward erosion is the erosion of the upper origin of the glacier, a sort of cutting backwards. When cirque glaciers develop on opposite sides of a ridge, they erode headward and eventually meet to create a saddle or notch in the ridge crest. This also tends to reduce the thickness of the ridge. The continuation of this process along the ridge creates narrow sawtooth arete ridges. The headward erosion of cirque glaciers on all sides of a summit gives us a pyramidal shape. The Matterhorn is a classic example, but such features are common in most glaciated mountains. The landscapes of glaciated mountains are among the most distinctive and striking on the planet. The features and forms created by ice sculpting are very different from those caused by running water. And glaciated mountains posses a ruggedness and a grandeur seldom achieved in unglaciated landscapes. For many of us, the visual image of high mountains is often typified by glacial landscapes with pyramidal peaks, jagged sawtooth ridges, amphitheater like basins and deep elongated valleys. It's a landscape largely inherited from the past, when the ice was much more extensive than it is today. Today, concerns about global climate change are giving glaciers new meaning for many people, who may have previously considered them eternally frozen or largely inert. Most of the world's glaciers are now melting rather than reproducing themselves. Becoming a new kind of endangered species and a weathervane for a potential, natural, and social upheaval. This is a theme that we'll return to in our last lesson, when we discuss the mountains of the future Earth. But for now, let's pause and return to your mountain world, this time along with some new mountain and mountain ranges. We have some of the world's great glaciers for you to locate on your map. Our mountain guide Matt Peter is back with a tech tip about managing hazards in the back country. And then there's your end of lesson quiz. So good luck, and we'll see you next time when we cover mountains in the popular imagination.
