KEFJ Alaska Glacier Resource Flashcards
Glaciers require three conditions to form:
Abundant snowfall, cool summers, and the gravitational flow of ice. Large amounts of snowfall, combined with cool summers and gravity, form multiple, connected glaciers over time, known as an icefield.
Tidewater glaciers
If a glacier is fed by enough snow to flow out of the mountains and down to the sea, we call it a tidewater glacier—the type many people come to Glacier Bay and Kenai Fjords to see. These types of glaciers will break off or calve into saltwater at sea level, and a few others that reach the sea at high tide only. The show can be spectacular. As water undermines some ice fronts, great blocks of ice up to 200 feet high break loose and crash into the water. When tidewater glaciers calve icebergs into the marine environment, they serve as pupping and molting habitat for some of the largest seasonal aggregations of harbor seals in Alaska. Although tidewater glaciers are naturally dynamic, advancing and retreating in response to local climatic and fjord conditions, most of the ice sheets that feed tidewater glaciers in Alaska are thinning and, as a result, many of the tidewater glaciers are retreating. Climate change models predict rapid loss of glacier ice with unknown impacts to seals that rely on tidewater glacial habitat.
Glaciers and Climate Change
Most of Alaska’s glaciers have been retreating over the last century, and research shows that the rate of recession has increased significantly in recent years. The effects of melting glaciers impact freshwater flow, vegetation, and coastal marine ecosystems. Glacial meltwater from tidewater glaciers has chemical properties that can exacerbate ocean acidification. Freshwater inputs and cold water tend to decrease ocean pH and increase acidification. This acidification can have adverse effects on marine wildlife habitat and populations.
Ocean Acidification
Ocean acidification refers to a reduction in the pH of the ocean over an extended period time, caused primarily by uptake of carbon dioxide (CO2) from the atmosphere. For more than 200 years, or since the industrial revolution, the concentration of carbon dioxide (CO2) in the atmosphere has increased due to the burning of fossil fuels and land use change. The ocean absorbs about 30% of the CO2 that is released in the atmosphere, and as levels of atmospheric CO2 increase, so do the levels in the ocean. When CO2 is absorbed by seawater, a series of chemical reactions occur resulting in the increased concentration of hydrogen ions. This increase causes the seawater to become more acidic and causes carbonate ions to be relatively less abundant. Carbonate ions are an important building block of structures such as sea shells and coral skeletons. Decreases in carbonate ions can make building and maintaining shells and other calcium carbonate structures difficult for calcifying organisms such as oysters, clams, sea urchins, shallow-water corals, deep-sea corals, and calcareous plankton. Some plankton, terapods, are already exhibiting signs of stress from ocean acidification. Read more about how the Gulf of Alaska is being impacted now by ocean acidification. These changes in ocean chemistry can also affect the behavior of non-calcifying organisms as well. Certain fishes ability to detect predators is decreased in more acidic waters. When these organisms are at risk, the entire food web may also be at risk. Ocean acidification is affecting the entire world’s oceans, including coastal estuaries and waterways. Many economies are dependent on fish and shellfish and people worldwide rely on food from the ocean as their primary source of protein.
Ablation
the retreat and degradation of glaciers
Advance
glacier flow exceeds ablation and the terminus extends beyond its previous point
Calve
process of ice breaking off at a glacier’s terminus
Crevasse
a large crack in the surface of a glacier produced by the stress of glacial flow
Hanging
a glacier that clings to the side of a steep mountain or one that terminates at a cliff
Moraine
a deposit of rock debris shaped by glacial flow and erosion
Tidewater Glacier
a glacier that terminates in the sea and discharges icebergs and other small pieces of ice
Terminus
the lower end, or snout, of a glacier
Glacial Formation
The formation of a glacier requires three conditions: abundant snowfall, cool summers, and the gravitational flow of ice. All of these conditions are met in Kenai Fjords. Moist air moving off the Gulf of Alaska in the winter drops, on average, 60 feet of snowfall on the Harding Icefield every year. These prevailing weather systems from the Gulf also ensure cool (and wet) summers in which much of the winter snow does not melt. As the snow accumulates, the weight of overlying layers causes the snowflakes to degrade and compact. This process, called firnification, is the first step in the transition from airy snow into dense glacial ice. A first year snowfall is approximately 80 percent air. As the snow degrades and compacts, it passes through stages defined by air content: firn is 50 percent air, névé is 20-30 percent air, and eventually glacial ice is less than 20 percent air. In Kenai Fjords, the entire process takes about 4-10 years.
Glacier speed of movement
Dense and heavy glacier ice begins to flow downhill. Extremely thick glaciers, which form in areas of especially high snowfall, tend to flow faster than thinner glaciers, as their greater mass is more affected by gravity. Similarly, glaciers with steeper gradients flow faster than glaciers spread across gentler slopes.
Three types of glacial movement
There are three main types of glacial movement: basal slippage, pressure melting, and plastic deformation. Basal slippage occurs when the ice slides or slips over the underlying bedrock. This process is facilitated by meltwater flowing at the base of the glacier which reduces friction between the ice and the bedrock. In temperate regions where there is high melting, such as Kenai Fjords, basal slippage can account for up to 90 percent of overall movement. Because meltwater plays such an important role in basal slippage, glaciers flow faster in summer than winter. From the Edge of Glacier Trail at Exit Glacier, you can hear the meltwater rushing beneath the ice and eventually gushing out across the outwash plain. When the underlying bedrock is particularly rough or a large obstacle such as a ridge or boulder is present, pressure melting begins. As the weight of the glacier bears down on the obstacle, the ice on the uphill side is subject to increasing pressure, which causes the ice to melt. The meltwater then flows around the obstacle and refreezes on the downhill side, facilitating the movement of the glacier downhill. Plastic deformation occurs when the ice itself flows as a viscous solid. As the ice responds to gravity, layers within the ice slide over one another along layers or planes of weakness in the ice. This is referred to as plastic deformation because bonds between ice crystals are stretched or altered, rather than broken. Thicker glaciers are more likely to move by plastic deformation than thinner glaciers, as this type of movement is in response to weight and pressure from overlying ice.
