Lab 9 _Glaciers_LLL

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Feb 20, 2024

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Lab 9—Glacier dynamics and sea level changes 12 Questions Lab introduction and purpose Of the various spheres (the atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere) the cryosphere is perhaps the least familiar to many people. Not many people live at high altitudes or latitudes where glaciers tend to be, which can be cold and unpleasant places at times, although also stunningly beautiful. However, as a geologic force, glaciers have shaped major parts of our earth in significant ways. This is, in part, because glacial ice used to be so much more extensive during past Ice Ages, at times extending all the way from the interior of what is now Canada down into Kansas. But, the waxing and waning of the glaciers has affected all latitudes, mainly by causing changes in sea level, a topic we will look at in some detail. The waxing and waning of glaciers is also related to global climate change. Glaciers are a significant part of the water cycle. Sea ice also strongly influences weather and, in the long run, climate. Therefore, this lab course would be deficient if it did not introduce you to glaciers and associated earth system science behavior. This lab has several goals: 1) to familiarize you with glaciers, their attributes and behavior, 2) to learn how imagery can be used to track how glaciers have changed in recent times, and 3) to explore how glacier volume and sea level are connected. You will also gain additional appreciation for how even simple mathematical models that are approachable at the introductory college level can provide insights into how the earth behaves. As with previous labs, there is an explanatory and training section first, and then you will apply what you have learned. Image from NASA’s Earth Observatory of the Arctic area showing several important elements of the cryosphere – the sea ice covering parts of the Arctic Ocean, the Greenland continental ice cap, and snow cover on the land. Abundant smaller alpine glaciers also occur in this view but are too small to resolve. Image source: http://earthobservatory.nasa.gov/IOTD/view.php?id=45766 . What are glaciers? (1.5 pts) Some of you may be familiar with glaciers and maybe even fortunate enough to have walked on one, but most of the students in this class will not have been so fortunate, and it is reasonable to start with the simple question – what are glaciers? Where more snow falls than melts in a year, 1

on average, the snow starts piling up. The snow lower down gets compressed and refrozen into ice, and the “pile” of snow and ice begins to move. If the ice is in a mountain valley it flows down the valley forming an alpine glacier (such as are very common in Alaska). If the ice forms a cap that mantles the landscape, it flows outward from the central and thicker part of what is called an ice cap or continental glacier (e.g. Greenland and Antarctica). Probably one of the best ways to understand a glacier as a dynamic feature is to see one move. However, most glaciers move slowly enough that one would have to be very patient indeed to witness the movement, which is evident over time scales of days to months, not to mention the difficulty of getting to the glacier. Luckily several people have made time lapse movies available on the web. The event where a piece of the front of a glacier at the water’s edge falls off and into the water is known as a calving . This is one way that glaciers lose ice and of course icebergs result, which in turn can sink unwary ships. View these YouTube clips to get an idea of how glaciers behave (sorry for any associated ads). >Stunning examples of calving events around the world: https://www.youtube.com/watch?v=xLFWV0d3-d0 >Extreme Ice Video of Columbia Glacier in Alaska: http://www.youtube.com/watch? v=6dFbuaz130c >Time lapse of Chilean glacier: http://www.youtube.com/watch?v=hRhnLtFZxso >BBC Earth Lab on glacial features https://www.youtube.com/watch?v=ghC-Ut0fW4o > Extreme Ice video of massive calving event in Greenland filmed as part of James Balog’s efforts (this is spectacular footage): http://www.youtube.com/watch?v=hC3VTgIPoGU Given what you saw in these videos, describe the relationship between glacial ice movement forward and the rate of melting or calving at the glacier front, and describe how the front (or terminus) of the glacier changes in position over time. Question 1: Description of glacial movement: You may have noted in the videos dark stripes and patches on the glaciers. This is rock sediment that fell off the mountainsides onto the glaciers. You can also see that the moving ice of glaciers also carries a tremendous amount of sediment within and on top of the ice. As the ice melts, the rock debris gets deposited in front and on the sides of the glacier to form features called moraines . The mixture of boulders, sand, and mud that gets deposited is known as till . In some of the videos large open cracks known as crevasses can also be seen to form. The position and orientation of crevasses provides insight into how the glacier is moving. In the simplest case the ice is moving perpendicular to the crevasse length, but more complicated relationships also exist. On the surface, or at the bottom of glacier (beneath the ice), and continuing out in front, are a great array of melt-water streams and rivers . The more plentiful 2

these are, the greater the rate of ice removal from the glacier at that time. In some parts of the world, these glacial melt-water streams and rivers represent an important water resource for people downstream. Sometimes small ponds and lakes form on glaciers. Because they are darker and can absorb more sunlight, such ponds can localize heating and be self-perpetuating (a positive feedback loop). With time, as the ice moves and grinds over the rocks, the glaciers scour distinctive U-shaped valleys , and where the glaciers melt and retreat and the sea invades the valley, fjords are formed. Fjord walls in Norway are over a mile high to give you some idea of the size these features can attain. You will be asked to identify moraines, crevasses, and meltwater features in images provided later in this lab. Glacial features are labeled and described in the images below to help train your eye. Study them carefully – they are not simply pretty illustrations, but sources of information. Most of these images come from the Arctic archipelago of Svalbard (link to info on Svalbard: http://en.wikipedia.org/wiki/Svalbard if you want more info) in the extreme North Atlantic (where one of the authors of this lab has been lucky enough to spend a lot of time). Glaciers are very diverse in their characteristics, so you will need to generalize from these specific examples to other instances. Above is a view across a fjord in northwest Spitsbergen, Norway, of a rather typical mountain glacier. You can see the upper portion is smoother and whiter. This is where last year’s snowfall remains (this photo was taken in August), and will likely be covered by next year’s snowfall. Eventually (unless continued summer melting comes to dominate the entirety of the glacial surface), some of this snow will become part of the glacial ice. Further down the glacier you can see a rougher glacial surface with a grayish tint. This is where older glacial ice has been exposed by melting, and the roughness is caused by uneven melting of the ice. The glacier is losing mass over this portion of the glacier. Out in front of the glacier is a jumble of brown debris, which consists of a chaotic mix of boulders, sand, silt, and mud. This is where the glacier has dropped a lot of the rock debris it was carrying at a stable ice front position, forming what is 3

known as a terminal moraine. The ice front no longer reaches to this moraine because in the last century, melting at the front has dominated and the glacier’s ice front has retreated, leaving the moraine stranded behind. In between the ice front and the terminal moraine can be seen a brown and muddy lake, which, because of its position, is known as a proglacial lake.The mud was part of the sediment the glacier was carrying and was picked up by the melt waters that run into this lake. Such lakes quickly fill with sediment, often seasonally layered. The image above is another view in the same part of Svalbard. Here we are standing on the terminal moraine of one glacier, looking across the fjord waters to the front of another large glacier that has a calving front that ends in the seawater, and is several miles wide. The glacier that built the moraine in the foreground is to the viewer’s back. Note how the moraine material in the foreground includes large boulders and is quite uneven in its topography. This hummocky character is typical of moraines and produces lots of depressions and small lake and ponds, such as the pond seen here. The hummocky character is a result of the irregular way in which the sediment gets deposited as the ice melts. This moraine is old enough that vegetation has begun to establish itself. Looking at the larger glacier in the background one can also see how it is linked to and fed by a smaller trunk glacier in the center of the photo coming out of the mountains. This is similar to how a smaller stream feeds into a bigger one, and one can think about a glacier system of ice drainage with glacial tributaries. Other smaller glaciers can be 4

seen perched in small valleys cut into the flanking rocky ridges. These relatively sharp ridges are known as arêtes and this landscape is very typical of alpine topography carved by glaciers. The image above is a view from a boat on the fjord waters of the front of a calving and crevassed glacier front. All the small chunks of ice floating in the water are small icebergs. Look carefully and you can see a three-masted schooner right in front of the glacier to give you a sense of scale. You can also see the numerous crevasses on this glacier – these large cracks form as the glacier moves. The sharp rock ridges in the background are typical of alpine topography carved by glaciers. 5

The image on the left above shows an overview of the front of yet another glacier in the same area. The strip of distinctive grey rock debris on the far side is a lateral moraine, sediment that was carried and deposited by this glacier. Note how the moraine extends farther to the right and much higher than the present position of the glacier. This indicates that in the past this glacier extended much farther to the right and was also much thicker. The great majority of glaciers in Svalbard show clear signs of getting smaller, with their fronts retreating. The image on the right above is a close up of a glacier in summer with meltwater pools on the glacier, in this case located in an old crevasse. The ice is grayer, because as the glacier top melts rock debris stays behind and becomes concentrated. In some places this process occurs to such a degree that the glacier top is covered by rock debris hiding (and shielding from melting) the underlying ice. This is a close-up view of the same glacier as in the photos above. Degraded and partly snow- filled crevasses can be seen here. Note the slightly grey character of the ice. This is clearly a part of the glacier that has seen significant melting and ice loss. The sediment is exposed and accumulates on the glacier surface as the ice melts away, creating the darker streaks and coloration. The darkening of the glacial surface can promote even more melting. However, once 6

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