by Lauren Krone and Samantha Morrow
Later in the week we looked at glacial dynamics and movement. Over time, as snow falls and layers on itself, it changes in shape and density. Snow begins as feathery and delicate flakes. As it settles and packs it changes to more hardened grain-shaped textures. It also continuously melts and refreezes. This increases density, changing from 200 kg/m3 to 840 kg/m3 over the course of about 120 years. In addition, as the ice increases in density air gets trapped in the glacier. This has recently been used to analyze the atmosphere’s composition over time.
There are three main types of glacial movement, depending on different characteristics of the glacier.
Internal deformation, or creep, occurs when the glacier is frozen to the bedrock, and this movement dominates for cold-based glaciers. Glaciers can also move by sliding across the bed, or basil sliding, within warm-based glaciers. The final type of movement is deformation or flow of underlying sediment sliding across till, which also occurs for warm-based glaciers. Furthermore, different parts of the glacier move at different velocities. The center of a glacier moves much faster than the edges due to less friction. The glacier can also be divided in to two parts. The top half is know as the accumulation zone, where snow and ice increase from snowfall, wind-drift, and avalanching. The bottom half is know as the ablation zone, where ice leaves the glacial system due to melting, calving, and sublimation. In the middle of these two zones is the ideal place to take ice cores for examining the atmosphere, because snow falls straight down at that spot, as opposed to curving away from the initial point.
At the end of the week we looked at how glaciers affected Stockholm in particular. The esker in the glacier covering Stockholm created hills, which used to be the bed of a river. As Stockholm was built, part of the esker was flattened and the material was used for building. In addition, water moves easily through the esker, and as it moves it is cleaned. This was convenient, because wells at the top of the esker would produce clean water, providing the city with a reliable resource. Another affect of the glacier on Stockholm’s landscape is the constant land-rise of the city, creating the appearance of sea levels falling. This is due to glacial isostatic rebound, occurring at about 3 mm per year. We were able to observe these effects in person. We visited King Carl IX’s fishing house from the late 1600s. While it used to be on the edge of the water, we had to walk several meters to reach today’s edge, providing us with a clear representation of the land being created from the isostatic rebound. We finished the week by discussing different periglacial processes and landforms, which we hope to see in the Arctic!