[MUSIC] The first known depiction of a mountain glacier might well be this watercolor from the Tyrol region of Western Austria, in 1601. The painting depicts an ice-dammed lake. Icebergs float on the left, with a heavily broken up glacier tongue, or the Vernagtferner Glacier on the right. The glacier lake formed several times in the early decades of the 1600s, and the drainage was usually catastrophic, involving a large sudden release of water and ice. It was a constant menace to the people living down valley. You’ll remember the discussion of glacier lake outburst floods from your lesson on water towers. This lake is a striking example. The painting also serves to remind us that glaciers are dynamic, they flow, they can advance, they can retreat, they can even slip and slide. To the English speaking world, it would be another three-quarters of a century before glaciers and their dynamics begin to earn a distinctive reputation, rumors of the outlandish phenomena, first circulated in the 1670s throughout the Royal Society, England's foremost intellectual institution. One report from 1673, characterized Swiss glaciers in this way. >> The mountain itself is very high and extends itself every year more and more over the neighboring meadows, by increments that make a great noise of cracking. There are great holes and caverns which are made when the ice bursts, which happens at all times, but especially in the Dog-days. Hunters do there hang up their game they take during the great heat, to make it keep sweet by that means. When the sun shined, there is seen such a variety of colors as in a Prism. >> Well, the English speaking world of the 1600s knew very little about mountain glaciers. >> Indigenous mountain cultures in stark contrast, had a sophisticated knowledge of glaciers and their dynamics. You may have gleaned as much from that last quote. Recall the hunter from the high alps, who used glaciers during the warm summers to store meat. Examples like this can be found all over the world, North American first nation's people living on either side of the Saint Elias Mountains. Tlingit people on the Gulf Coast of Alaska and the inland speakers of the Athabascan languages, came to know this glaciated landscape over successive generations. And their experiences reinforce division that humans in nature make and usually maintain a habitable world, a view now echoed by environmental historians. Here's University of Alberta historian, Professor Liza Piper. >> In Athabascan and Tlingit oral traditions, glaciers are living actors, far from inanimate objects. This is essential to northern indigenous history. That humans in nature occupy a shared social world, in which they have significant, ethical relationships. Julie Cruikshank describes this really well in her book, Do Glaciers Listen? For the people who'd lived and travelled amidst the Saint Elias Mountains, glaciers were an active part of that landscape. There were strict prohibitions, for instance, against cooking with grease. which could offend the glaciers, leading to catastrophic surges or glacial advances. This is an excellent example of how people and glaciers communicated back and forth, and how relations with glaciers influenced social behavior. >> In this section, we're going to look at some of the ways that glaciers move. Let's start with the basics, by discussing mass balance and glacier zonation. Glaciers only form when the amount of material or snowfall added in a given year, is greater than the amount removed. Material is primarily removed by surface melting and evaporation, but also by glacier calving. We refer to the material added as the annual input and the material removed is the annual output. The difference between the input and the output is the annual net balance, or mass balance. From a climatic point of view, a glacier's state of health can be determined by an analysis of its mass balance. Positive mass balance will cause the glacier to grow and advance, while a negative mass balance means the glacier will shrink and retreat. In a steady state, the mass balance over the course of a year equals glacier's equilibrium and the glacier remains roughly the same size. Thinking about annual inputs and outputs, allows us to consider two main zones on a glacier, an accumulation zone and an ablation zone. Snowfall often occurs at the greatest quantities at higher elevations where the temperatures are coldest. We call this area the accumulation zone. Here, annual inputs exceed outputs, and the mass balance is positive. Further down the glacier, where it's warmer at lower elevations, outputs in the form of melting and evaporation exceeds the inputs. This is the ablation zone. Here the annual mass balance is negative. Now between these two areas, a balance is reached where snowfall equals snow melt and the glacier is said to be an equilibrium. The average height where this occurs is called the Equilibrium Line Altitude or the ELA. In a given mountain region where conditions favored glacier advance, the ELA tends to be relatively low. Where conditions favor retreat, the ELA is relatively high. Glaciers are dynamic, remember. But if they weren't static, if they weren't actually able to move, let's try imagining what would happen based on what we've just learned. Because of accumulation, the area above the ELA would get thicker and thicker over time while the area below would thin out and decay due to ablation. The glacier then would get steeper and would get shorter, it would bulge out of the top and it would start wasting away at the bottom. And over time, it would start to look a bit odd, wouldn't it? In fact, that's exactly what happens, and in part, it's what drives glacier flow. The first recorded observation that glaciers flowed, can be attributed to the Icelander Sveinn Palssen in 1794. In that year, he climbed a big ice covered volcano and looking down onto one of it's outlook glaciers, observed the prominent arcuate or bow-shaped bands that have since come to be known as ogives. This prompted him to the remarkable observation of the time, that the glacier ice without actually melting has some kind of fluidity, like several resins. Glaciers flow in large measure because of gravity. Think back to our static model, if the area above the ELA gets increasingly thicker while the area below thins out, it results at an increase to the stress exerted on the ice by gravity. Something eventually has to give and it's that downward pull that eventually causes the ice to deform, and begin to flow downwards. Mass that accumulates above the ELA is then transferred into the ablation zone, and replaces the mass being lost by melting. In a stable climate, this process acts to maintain the shape, the surface slope, and the thickness distribution of the glacier. But stable climates are the exception rather than the rule, and over time, glaciers tend to be continually changing in length, in area, and surface slope. In a climate that's cooling, or snowfall is increasing, the ELA lowers, the accumulation zone grows, and the ablation zone shrinks. Mass transfer from the accumulation zone by flowing increases, and the glacier grows. This normally occurs by an advance to the glacier's terminus or the end of the glacier or its snout. On the other hand, in a warming climate, the melt increases and the ELA rises. The ablation zone then grows and the accumulation zone shrinks as the amount of snowfall that survives the year is no longer sufficient to replace that which has been removed from melting. The glacier develops an overall negative mass balance that, if sustained over many years, will promote glacier retreat. Now an important point to emphasize here is that when a glacier is in retreat, the ice itself is still flowing like a great conveyor belt down valley towards the glacier's terminus. What's retreating is the position of the terminus, which is rising up valley. In the warming scenario, melt removes ice faster from the terminus than the down valley flow of the glacier can replace it. When the opposite is true and the flow is delivering more ice to the terminus than melting can remove, the glacier terminus moves forward and the glacier advances. Another major mechanism involved in glacier movement is basal sliding, which involves the slippage of ice on mass over the rock surface at its base. If you've spent enough time in or near glaciated terrain, you might have noticed the abrasions or the striations on the bedrock near the glacier's terminus. The distinctive marking are evidence of this type of movement. The important controls for basal sliding are the temperature of ice at the base, and the presence of water to serve as lubricant. Basal sliding does not generally occur in polar glaciers since the ice is frozen to the underlying rock surface. In other regions, where the temperature of the glacier rises higher and water may be present along its base, sliding over the bed surface is much more pronounced. If the bed surface underneath the glacier isn't solid rock, if it's loose sediments or soils for example, the presence of water can substantially weaken the bed. And this makes it easier for the glacier to deform the sediment beneath it rather than to move along on top of it. And this introduces a third mechanism that contributes to glacier flow, bed deformation. Most of the deformation takes place in the bed surface and the ice is simply carried along top like a deforming carpet. Flow rates due to ice deformation are relatively constant. Remember, ice deformation is driven by gravity, by the thickness of the ice, and by the slope angle. And if all of those remain constant, the flow rate remains roughly the same. But once you introduce water into the system, either by melting at the bed or by surface water finding its way down to the bed, we get a much greater variability in the flow rates. In the early 1700's, glaciers and their dynamics in the European Alps was a cause of great concern, especially for those living near or beneath them. Glaciers for many, were seen as an enduring plague. And part of that concern, may have stemmed from the popularity of Johann Scheuchzer's travelogue, Itinera Alpina, published in 1708. Scheuchzer was a Swiss scholar and his massive book outlined his observations made during several trips to the Alps of that same decade. And its fame was due mostly to a chapter that reported stories of encounters with dragons. Other stories reported snakes with limbs and faces almost human, or with two tails and two tongues. Glaciers and mountains were the abode of fantastical beasts, frightful and best to be avoided. There were other reasons, too, to worry. Advancing glaciers in the Western Alps had actually crushed the small French hamlet of Bon Annes near Chamonix in 1644. Amateur science and the emergence of a mountain tourism industry, changed popular perceptions of glaciers in Europe, especially during the 19th century. Gone were the musings about dragons, but science had embraced another chilling thought, one that equally stirred the imagination. Scientific discoveries revealed that an ice age was something that had actually happened at least once in the history of the Earth, and suggested that it actually might happen again. [SOUND] It's difficult now for us to understand just how drastically the idea of an ice age re-wrote the 19th century view of the world. In fact, in almost every scientific discipline, natural history, chemistry, physics, and others, glaciers became big news. Chamonix, for example, became so popular for it's glaciers. Then in 1821, the Compagnie Des Guides De Chamonix was formed to regulate the access to the slopes on Mont Blanc. The company was the world's first mountain running association and was part of a growing tourism infrastructure that would make Chamonix a prototypic alpine tourism destination. Throughout the century, the considerable space in newspapers devoted to glaciers radically increased. As the number of glacier goers from Europe to the Americas grew radically. Site seers were eager to witness first hand these masses of ice which had so shaped the surface of the world, and that might one day return to consume it. The great cultural shift in our understanding of glaciers and ice all happened during what we might call a glacier high tide in the Alps. It was a short period of time, a window when most of the world's glaciers were actually advancing. We now call this period the Little Ice Age. And it lasted from around 1500 to 1850, three and a half centuries in all. Ending abruptly during the height of the Industrial Revolution.
