Chapter 22-glaciers questions

Explain how the balance between ablation and accumulation determines whether a glacier advances or retreats.

If accumulation of new snow to feed a glacier exceeds ablation (loss of ice due to melting or sublimation), the glacier will grow and advance; if the reverse is true, the glacier will retreat (disappear).

How was the world different during the glacial advances of the Pleistocene Ice Age? Be sure to mention the relation between glaciations and sea level.

Earth was much colder, and much of northern North America was covered in glacial ice; in the northern hemisphere, climatic zones shifted southward. Just to the south of the glaciers, conditions were wetter than normal, but the tropics were unusually dry. Because so much water was bound into glacial ice, sea level was considerably lower than it is today.

Describe the transformation from snow to glacial ice.

Fluffy snow, when sufficiently buried, packs together and melts in places due to pressure from above. The liquid refreezes to produce more tightly packed firn (1/4 air by volume) and ultimately a solid mass of interlocking ice (interrupted by bubbles).

Describe the various kinds of glacial deposits. Be sure to note the materials from which the deposits are made and the landforms that result from the deposition.

Glacial materials include erratic boulders and unsorted tills dropped directly from the glacier, in addition to outwash sands and gravels from meltwater streams, glacial lake sediment, and windborne loess. Landforms include end moraines, dropped ridges of till at the frontal margin of a glacier that form when a glacier ceases to advance and melts away; lodgment till, the smeared remains of an end moraine steamrolled by an advancing glacier; and ground moraine, a plain of till released over the broad area of glacial recession. Drumlins are asymmetric hills of till shaped by glacial ice (with a steeper slope in the upstream direction). Kettle holes are depressions formed when till buries ice, which later melts and flows outward. Meltwater streams deposit outwash consisting of stratified sediments, and varved sediments may accumulate at the bottom of meltwater-fed lakes. Eskers are sorted sediments that reflect the course of a stream that existed on the bottom surface of a glacier.

How fast to glaciers normally move? How fast can they move during a surge?

Glaciers normally move from 10 to 300 m per year. During a surge, they can move as fast as 110 m per day.

Describe the mechanisms that enable glaciers to move, and explain why they move.

If the bottom surface along which a glacier is traveling is wet, it may slide along the surface. Generally, movement is accomplished through the plastic deformation of internal ice crystals. At the surface, expansion and travel are accommodated by fracture. Glacial movement is ultimately driven by gravity.

How does a glacier transform a V-shaped river valley into a U-shaped valley? Discuss how hanging valleys develop.

: Glaciers and their abrasive sedimentary load not only carve into the floor of a valley but also hollow out the sides. Valley glaciers carving through a tributary to a major stream will not cut as deeply as the floor that is cut by the glacier in the main stream valley. Tributary glaciers contain smaller volumes of ice than do the main glaciers into which they feed. This smaller ice volume limits the depth to which tributary glacial valleys are eroded. The floor of the tributary glacial valley is thus elevated above the valley floor of the main glacier. After the glacial ice has melted away, the tributary valley can be seen as a hanging valley suspended above the main glacial valley.

What evidence did Louis Agassiz offer to support the idea of an ice age?

Agassiz noticed that many European deposits contained boulders, which water could not carry, and were unsorted (water produces sorted deposits because carrying capacity is a function of velocity).

Explain how arêtes, cirques, and horns form.

Arêtes are residual, elongate ridges between cirques, formed by glacial erosion and mass wasting. Cirques are bowl-shaped depressions scoured by mountain glaciers and the sediments they carry. Horns are residual, pointed peaks at the intersection of three or more arêtes. They are the highest erosion remnants of a mountain that has been glaciated.

How do mountain glaciers and continental glaciers differ in terms of dimensions, thickness, and patterns of movement?

Continental glaciers are thicker, much more expansive sheets. Mountain glaciers flow downhill as a result of gravity acting on the mass of ice. Continental glaciers move in response to pressure from the weight of material in their thick midsections.

How can a glacier continue to flow toward its toe even though its toe is retreating?

Downhill flow of ice is driven by gravity and is not interrupted by melting that occurs elsewhere. The toe is not truly retreating uphill but is merely melting or sublimating.

What are some of the long-term causes that lead to ice ages? What are the short-term causes that trigger glaciations and interglacials?

The proportion of carbon dioxide (a greenhouse gas) and plate-tectonic activity are the major factors affecting long-term climate. Active mid-ocean-ridge volcanism can increase the greenhouse effect by adding carbon dioxide to the atmosphere. Similarly, rapid ridge volcanism increases the volume of the ridges and can push sea level over the continents. The configuration of continents is also important; large continental masses near the poles favor the possibility of terrestrial glaciers. Short-term advance and retreat are related to Milankovitch orbital variation. Variation in the eccentricity of Earth's orbit affects the evenness of global insolation through the year. Additionally, Earth travels more slowly when it is more distant from the Sun, spending proportionately more time near aphelion in a more eccentric (elliptical) orbit. Variation in tilt affects seasonality. A small tilt means the poles will not receive a warm summer and favors glacial advance. Precession affects the timing of equinoxes and solstices. Summer will be cooler in the northern hemisphere if it occurs in conjunction with aphelion. Milankovitch variation is reinforced by positive feedback. A glaciated Earth has a high albedo, reflecting much sunlight back to space.

How was the standard four-stage chronology of North American glaciations developed? Why was it so incomplete? How was it modified with the study of marine sediment?

There were four distinct layers of till, of differing ages, separated by soils with fossil remains suggesting returns to warmer climate. The record of advances and retreats, like so much of the terrestrial sedimentary record, is vastly incomplete due to erosion of many glacial deposits. Marine sediments can be tested for oxygen isotope ratios that serve as gauges of past ocean temperatures; these sediments also contain distinctive fossil

Were there ice ages before the Pleistocene? If so, when?

There were ice ages in the early Proterozoic, late Proterozoic, early Paleozoic, and late Paleozoic.

How do the crust and mantle respond to the weight of glacial ice?

Thick glacial ice may cause local subsidence of the lithosphere, which presses down into the softer asthenosphere below. When the glacier melts, the lithosphere rebounds to its prior elevation, and asthenosphere flows in underneath the rising lithosphere.