The topics covered in this chapter can be summarized as follows:
|4.1 Glacial Periods in Earth’s History
|There have been many glaciations in the Earth’s distant past, the oldest known starting around 2400 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, 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 the Earth’s orbital characteristics, which are exaggerated by a variety of positive feedback processes.
|4.2 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 down slope. 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.
|4.3 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 glacial erosion in 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 deep as main-valley glaciers, hanging valleys exist where the two meet. On a smaller scale, both types of glaciers form drumlins, roches moutonée and glacial grooves or striae.
|4.4 Glacial Deposits
|The deposits of glaciation are also quite varied, as materials are transported and deposited in various ways in a glacial environment. Sediments that are moved and deposited directly by ice are known as till. Till is present in moraines and drumlins and in blankets that cover wide areas. Glacio-fluvial sediments are deposited by glacial streams, either forming eskers or large proglacial plains known as sandurs. Glacio-lacustrine and glacio-marine sediments that originate within glaciers and are deposited in lakes and the ocean respectively.
|4.5 Glacial Response to Climate Change
|Glaciers around the world are receding because of climate change, and the results are already serious in some areas, and continuing to get worse. Enhanced melting affects sea-level rise, ocean currents, slope stability, water supplies, aquatic ecosystems and tourism and recreation opportunities.
Glaciers also play important roles in Earth systems, especially in the context of sea-level changes and erosion.
Answers for the review questions can be found in Appendix 1.
- Why are the Cryogenian glaciations called “snowball earth”?
- The Earth cooled dramatically over the 40 million years from the late Eocene until the Holocene. Describe some of the geological events that contributed to that cooling.
- When and where was the first glaciation of the Cenozoic?
- Describe the extent of the Laurentide Ice Sheet during the height of the last Pleistocene glacial period.
- 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?
- What does the equilibrium line represent in a glacier?
- Which of the following is more important to the growth of a glacier: very cold winters or relatively cool summers?
- 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.
- What condition is necessary for basal sliding to take place?
- Why do glaciers carve U-shaped valleys, and how does a hanging valley form?
- A horn is typically surrounded by cirques. At least how many cirques would you expect to find around a horn?
- A drumlin and a roche moutonée are both streamlined glacial erosion features. How do they differ in shape?
- Four examples of glacial sediments are shown below. Describe the important characteristics (e.g., sorting, layering, sedimentary structures) of each one and give each a name (e.g., glacio-fluvial, glacio-lacustrine, lodgement till, ablation till, glacio-marine).
- What is a drop stone, and under what circumstances are they likely to form?
- What is the likely implication of a strong glacial period on the overall salinity of the Earth’s oceans?