The topics covered in this chapter can be summarized as follows:
17.1 Glacial Periods in Earth’s History
There have been many glaciations in Earth’s distant past, the oldest known starting around 2,400 Ma. The late Proterozoic “Snowball Earth” glaciations were thought to be sufficiently intense to affect the entire planet. The current glacial period is known as the Pleistocene Glaciation, and while it was much more intense 20,000 years ago than it is now, we are still in the middle of it. The periodicity of the Pleistocene glaciations is related to subtle changes in Earth’s orbital characteristics, which are exaggerated by a variety of positive feedback processes.
17.2 How Glaciers Work
The two main types of glaciers are continental glaciers, which cover large parts of continents, and alpine glaciers, which occupy mountainous regions. Ice accumulates at higher elevations — above the equilibrium line — where the snow that falls in winter does not all melt in summer. In continental glaciers, ice flows outward from where it is thickest. In alpine glaciers, ice flows downslope. At depth in the glacier ice, flow is by internal deformation, but glaciers that have liquid water at their base can also flow by basal sliding. Crevasses form in the rigid surface ice in places where the lower plastic ice is changing shape.
17.3 Glacial Erosion
Glaciers are important agents of erosion. Continental glaciers tend to erode the land surface into flat plains, while alpine glaciers create a wide variety of different forms. The key feature of alpine glacial erosion is the U-shaped valley. Arêtes are sharp ridges that form between two valleys, and horns form where a mountain is glacially eroded on at least three sides. Because tributary glaciers do not erode as deeply as main-valley glaciers, hanging valleys exist where the two meet. On a smaller scale, both types of glaciers form drumlins, roches moutonées, and glacial grooves or striae.
17.4 Glacial Deposition
Glacial deposits are quite varied, as materials are transported and deposited in a variety of different ways in a glacial environment. Sediments that are moved and deposited directly by ice are known as till. Glaciofluvial sediments are deposited by glacial streams, either forming eskers or large proglacial plains known as sandurs. Glaciolacustrine and glaciomarine sediments originate within glaciers and are deposited in lakes and the ocean respectively.
Questions for Review
1. Why are the Cryogenian glaciations called Snowball Earth?2. Earth cooled dramatically from the end of the Paleocene until the Holocene. Describe some of the geological events that contributed to that cooling.
3. When and where was the first glaciation of the Cenozoic?
4. Describe the extent of the Laurentide Ice Sheet during the height of the last Pleistocene glacial period.
5. In an alpine glacier, the ice flows down the slope of the underlying valley. Continental glaciers do not have a sloped surface to flow down. What feature of a continental glacier facilitates its flow?
6. What does the equilibrium line represent in a glacier? Explain.
7. Which of the following is more important to the growth of a glacier: very cold winters or relatively cool summers? Explain.
8. Describe the relative rates of ice flow within the following parts of a glacier: (a) the bottom versus the top and (b) the edges versus the middle. Explain.
9. What condition is necessary for basal sliding to take place?
10. Why do glaciers carve U-shaped valleys, and how does a hanging valley form?
11. A horn is typically surrounded by cirques. What is the minimum number of cirques you would expect to find around a horn?
12. A drumlin and a roche moutonée are both streamlined glacial erosion features. How do they differ in shape?
13. Four examples of glacial sediments are shown here. Describe the important characteristics (e.g., sorting, layering, grain-size range, grain shape, sedimentary structures) of each one and give each a name (choose from glaciofluvial, glaciolacustrine, lodgement till, ablation till, and glaciomarine). [SE photos]
14. What are drop stones, and under what circumstances are they likely to form?
15. What types of glacial sediments are likely to be sufficiently permeable to make good aquifers?