As described in section 4.1 of your textbook, There have been many glaciations during Earth’s history (Figure 4.1.1), and some of the processes that have led to those are significant changes in the composition of the atmosphere (caused by biological processes or enhanced rock weathering), changes in the positions of the continents (that have led to albedo differences or changes in ocean currents), or some combinations of these. The factors that led up to the current glacial cycle are summarized on Figure 3.2.4, and the changes over the last 5 million years of that process are illustrated on Figure 4.1.5.
The following is an interactive version of Exercise 4.1 in the Environmental Geology textbook.
The figure below shows the past 500,000 y of global temperatures from the same data set shown in Figure 4.1.5 in the textbook. The last five glacial periods are marked with snowflakes. The most recent one, which peaked at around 20 ka, is known as the Wisconsin Glaciation.
The current interglacial (Holocene) is marked with a red circle. Identify the five previous interglacial periods.
The focus of this part of the course is on the erosional effects and the depositional products of glaciers in general, but especially those of the Pleistocene glaciations.
Section 4.2 provides a summary of the main different types of glaciers (continental versus alpine glaciers). Although there are currently only continental glaciers on Antarctica and Greenland (Figure 4.2.2 and 4.2.3), huge ice sheets covered much of North America (Figure 4.1.7) and a good part of Europe during the height of the Wisconsin Glaciation.
The mechanics of glacial ice movement are explained on Figure 4.2.7, and it is important to understand what is going on there. Although the diagram depicts an alpine glacier, the principles also apply to a continental glacier. The key point of 4.2.7 is that while ice is normally brittle, it will become plastic—and therefore can deform and flow—when it is under pressure because of the thickness of the ice above it and because of the slope of the surface of the ice. The cutoff for deformation is about 100 kPa, which is defined by the red dashed line. Below that the ice will deform (flow), and the deformation of each “piece” will be passed on the ice immediately above it (which is also deforming) and so the rate of flow increases upward to the red line. Above the red line the ice only moves along with the ice beneath it.
Another important factor is whether or not the base of the ice is warm enough for liquid water to be present. If it is warm, and the ice-rock interface is wet, the glacier can also slide along its base (in addition to moving by flow, as described above). If it is cold, the glacier will be frozen to its base, and it can only move by internal flow (Figure 4.2.9). A sliding glacier moves faster in most cases, and tends to erode its base much more effectively than one that is stuck to the rock.
Completing Exercise 4.2 in the textbook should help you understand how glaciers move and the role of melting ice at the front of a glacier.
Glaciers move due to the force of gravity, and so they move from mountains down into valleys, or, in the case of continental glaciers, from areas where the ice is thickest to where it is thinner. Glacial erosion takes place because of the forward movement of the ice and of the water derived from the glacier. While glaciers cannot move up-slope, nor backward, the front end of a glacier can advance, or stay in the same place, or retreat. That is because there is a balance between the rate of forward motion and the rate of retreat due to melting at the front. Almost all glaciers in the world are retreating at present because of anthropogenic climate change, but the ice of those glaciers is still moving forward, and still eroding the base as a result of that forward motion. Completing Exercise 4.2 Moving Ice should help you understand these processes.
Glacial erosion is covered in section 4.3 of the textbook, and as described at the start of that section, while ice alone doesn’t have much erosive power, ice with embedded sedimentary grains is very effective at eroding rocks.
In areas of continental glaciation—like most of Canada and some of the northern US—glacial erosion has left relatively flat terrain, but in mountainous regions glacial erosion has contributed to the formation of very steep terrain, with sharp peaks and ridges and with wide steep-sided (U-shaped) valleys. These types of features are illustrated on Figures 4.3.2 through 4.3.9 in the textbook. In general, glaciated mountains have much steeper slopes that non-glaciated mountains, and as we’ll see in Unit 2, that has implications for slope stability.
If you live in almost any part of Canada or the northern part of the US, you are likely surrounded by glacial sediments, although they are not easy to see. They make up lodgement- or ablation-till blankets that cover millions of square km. They are present in end-moraines that extend for hundreds of km, and exist as large sand and gravel deposits from glacial meltwater rivers, or silt and clay deposits that were deposited on the beds of glacial lakes.
The depositional settings for some of these types of deposits are illustrated on Figures 4.4.1 and 4.4.2 in the textbook, and examples of what they look like are provided on Figures 4.4.3 through 4.4.9. Glacial sediments are important to environmental geology for many reasons, including the following:
- They include material with a range of grain sizes that can be used for construction of anything from buildings, to dams, to roads, and many of the glacial deposits of these materials are situated in places that can make their exploitation less environmentally damaging than gravel deposits associated with existing rivers.
- Many (but not all) glacial deposits are sufficiently porous and permeable to be useful aquifers for drinking and industrial water supplies.
- Unconsolidated glacial sediments are commonly present on the steep slopes created by alpine glaciers, and this can worsen an already serious slope stability situation.
Glaciers and Climate Change
Glaciers are obviously sensitive to climate warming. Not only are they receding quickly in all parts of the Earth (as exemplified by Figure 4.5.1 in the textbook), but that melting has many other effects on the environment as summarized in the list following Figure 4.5.2.
The points in the list that follows Figure 4.5.2 of the textbook are summarized in the left-hand column of the table below. For each of those processes, indicate whether or not it represents a positive feedback, and if so, what is the feedback mechanism. The first one is done for you.
|Shrinking glaciers expose bare rock
|Bare rock absorbs more solar energy and so leads to more heating and more melting.
|Sea level rise
|Dilution of ocean water because of glacial melting
|Proglacial lakes get larger
|Slopes no longer supported by ice
|Reduction in human water supply
|Reduction in ecosystem water
|Reduced appeal for tourism