Glaciers and Ice Ages

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Assignments

Chernicoff, Glaciers and Ice Ages, ch. 15 (all).

Audubon Society Field Guide to the Weather, pp. 534-537.

Reminders

The Planetary Scavenger Hunt has been cancelled.

The Mars Show will be presented at 10 a.m. on Friday May 8.

Rock collections due Wednesday May 6.

Lesson Outline

Read the assigned portions of each chapter. Study the illustrations. Focus your attention on the vocabulary terms found in the following notes.

I. Introduction

Today, glaciers cover about ______ % of the world's land surface.

II. Glacier Formation and Growth

A moving body of ice that forms from the accumulation and compaction of snow is called a(n) ____________.
Glaciers flow downslope or outward under the influence of __________ and the pressure of their own weight.
__________ is the packed snow that survives a summer melting season.
1. Make a snowball of light snow, then pack and compress it as hard as you can. The inside core, if cut open, would become granular ice very similar to firn.
Types of glaciers
1. Confined glaciers. __________ glaciers are confined by surrounding bedrock highlands, and occur in three types:
  1. __________ glaciers create and occupy semicircular basins on mountainsides, usually near the heads of valleys.
  2. __________ glaciers flow in pre-existing stream valleys.
  3. Ice _________ form at the tops of mountains.
2. Partially confined glaciers
  1. A _____________ glacier originates as a confined alpine glacier but flows onto an adjacent lowland where, unconfined, it can spread rapidly.
  2. A piedmont glacier that reaches seawater is a ____________ glacier.
3. The only completely unconfined glacier is a _______________ ice sheet.
  1. Greenland
  2. Antarctica: 3 miles thick, 1.5 times the area of the USA.
The glacial budget
1. In the zone of _________________ more snow is added every year than is lost.
2. In the zone of ablation more snow is lost than is added.
  1. ______________ is the process by which chunks of ice are broken off by wave or tidal action; it is the source of icebergs.
3. Budget = accumulation minus ablation
Glacial flow: Like a down escalator
1. Although the terminus of a glacier may be either advancing, receding, or remaining stationary, the glacier is always flowing forward.
2. In internal ________________, a glacier's ice crystals deform under pressure from overlying ice and slip past one another.
  1. Make a bar of ice about 18 x 2 x 1 inches. Suspend the bar from its ends in a horizontal position inside a refrigerator. Place a weight in the middle of the bar. After a few hours, the ice bar will be curved as a result of internal deformation.
3. In __________ sliding, warmer glaciers thaw at their bases, producing a film of water on which it slides.

III. The Work of Glaciers

Erosion
1. Abrasion of underlying bedrock
  1. Ice carrying coarser rock fragments can cut long ______________, or scratches, into the bedrock surface. These markings are oriented in the same direction as the ice flow.
2. Glacial quarrying occurs when a glacier lifts masses of bedrock dislodged by frost wedging.
3. Cirques are deep alpine basins created by glacial abrasion and quarrying.
4. When cirque glaciers melt, a cirque lake or __________ forms in the basin.
5. When two cirque glaciers on opposite sides of a mountain erode headward and converge, they form a steep, narrow, spine-like ridge of erosion-resistant rock at their top and sides called a(n) ____________.
6. The peak remaining after three or more cirque glaciers erode a mountain is called a(n) __________.
7. Valleys carved out by rivers are V-shaped; glaciated valleys are ___-shaped.
8. When seawater submerges a glaciated valley it forms a deep, saltwater-filled _________, which is an estuary in a drowned glaciated coastal valley. They are common in Scandinavia, Alaska, and (to a lesser extent) Maine. Their existence reflects the rise of sea level associated with melting after the last ice age.
9. ___________ valleys result from the faster flow of the main glacier, often with a tall, beautiful waterfall.
10. U-shaped troughs (continental ice sheets)
  1. Finger Lakes, Great Lakes, Loch Ness
Transport and Deposition
1. All glacial deposits are called glacial ___________.
2. Drift that is deposited directly by ice is called glacial ______________ .
  1. Unsorted, nonstratified
  2. An exceptionally large, glacially transported rock that has been eroded from one type of bedrock and deposited on another is called a(n) _____________.
    1. These provided one of the chief evidences cited by early 19th century geologists such as William Buckland in defense of _______________, a theory which interpreted them as effects of a global flood.
    2. For an example, see the misunderstood picture in Lyell, p. 260.
  3. Moraines:
    1. If the terminus of a glacier is stationary, a moraine (continues | does not continue) to accumulate at the terminus.
    2. When a glacier recedes, it leaves a ______________ moraine marking the farthest advance of ice at its lowest elevation or southernmost latitude.
    3. Proglacial lake: a lake formed behind a terminal moraine (which serves as the dam).
    4. Recessional, lateral, median moraines
  4. ________________ are gently rounded, elongated hills created by ice sheets overriding and reshaping moraines. They are till deposits shaped into streamlined, nearly teardrop-shaped "drumsticks." Their long-axes are parallel to the direction of glacial advance. Steeper slopes are upstream, gentler slopes downstream.
3. Meltwater streams
  1. Stratified and sorted
  2. Outwash is deposited downstream of the glacial terminus (braided streams, plain). It consists of finer, stratified sediment deposited in a relatively flat area, carried by streams issuing from glaciers.
  3. Loess are deposits of silt carried by ___________ from exposed, drying outwash. Loess deposits provide fertile soil in much of the midwest, including northern Kansas and Missouri's rolling hills.
  4. Sinuous ridges formed beneath the ablation zone are called _____________. These may be found in rocks of many different periods.
  5. Kames: Mounds of gravel left at the end of streams flowing underneath the ice.
4. Kettle holes or ponds: vacuities in glacial areas left by chunks of ice which have melted. Many of Minnesota's 15,000 lakes are kettles.
Other effects
1. Near continental ice sheets
  1. Ice dams: alters drainage patterns
    1. Missouri River once flowed north to _________ Bay
  2. Weight: crustal depression and rebound
    1. Scandinavia __________
    2. Netherlands __________
2. Remote from glaciated areas
  1. Climatic changes: Widely contrasting air temperatures cause abnormally cloudy, cool, rainy weather.
  2. Pluvial lakes, landlocked basins in which additional precipitation accumulates.
    1. The Great Salt Lake is a small remnant of the huge Lake _________________, which covered much of Utah, eastern Nevada, and southern Idaho.
  3. Decreased sea levels due to increased water contained in ice.
    1. Extended coastlines
    2. Exposed land bridges

IV. Ice Ages

Glacial periods
Interglacial periods
We are in the midst of the ________________ Ice Age.
Two requirements for glaciation:
1. Sizable landmasses at or near the poles
2. Land surfaces with relatively high elevation
The Earth's glacial past
1. Did ice ages occur in geological periods other than the Pleistocene? Or are they relatively rare, geologically speaking? If they are common, how common are they?
2. Tillite, moraines and eskers
3. Precambrian (3)
4. Paleozoic Era: Sahara at south pole
5. Permian Period: Gondwana at south pole (Australia, India, South America, South Africa)
6. None in the Mesozoic Era, age of dinosaurs (18 degrees F warmer than today)
Pleistocene Epoch glaciation
1. Pleistocene: The most recent geological period in which major ice ages occurred. See the geological column for an overview.
2. Quaternary ice age (1.6 million years ago)
  1. 5-10 degrees C (9-18 degrees F) colder during glacials
  2. About current temperatures during interglacials
3. Four continental ice sheets during Quarternary Period
4. Height of Pleistocene glaciation occurred 18,000 years ago
  1. Ice covered 30% of the planet
  2. Laurentide and Cordilleran ice sheets in North America
  3. Boston as cold as Anchorage
5. How far south in the United States did glaciers reach?
6. Glacial retreat began 13,000 to 12,000 years ago
7. Little Ice Age: 1200 to 1850
  1. By 1920 there was virtually no floating ice south of the Arctic circle.
8. Derek Ager argues that "The evidence clearly indicates that, for most of [its] history, the earth was a milder and more pleasant place to live than it is at the present day. We are living in the aftermath of the Pleistocene catastrophe and, even without the concern of the ecologists about man burning fossil fuels, there is no doubt--on purely geological grounds--that global warming must come. With it there will be melting of the polar ice sheets, renewed marine transgressions across the continents and the renewed deposition of platform carbonates." The New Catastrophism, 1993, pp. 63-64.
  Does Ager's argument make sense? Is there another possible position? How do we know that the multiple glaciations of the Pleistocene have ceased? Might we be poised between recurring episodes of a continuing ice-age, in which we could expect a global cooling associated with a return of glaciers across America and Europe?
  
  What kind of evidence would help you decide between these two positions?

Reading Focus Questions

Be prepared to answer any of these questions should they occur on a quiz or on the final exam.

During an interglacial period:
1. glaciers increase in size
2. more glaciers are created than at any other time
3. glaciers thaw and retreat
4. there are no glaciers
Currently the Earth
1. is in an interglacial period
2. is not in an ice age
3. has only alpine glaciers left
4. has only ice sheets remaining
The major difference between alpine and continental ice sheet glaciers is that:
1. alpine glaciers form only in regions of trees
2. continental glaciers always flow into the oceans
3. alpine glaciers are much larger
4. one is confined, the other is not confined
The budget of a glacier
1. refers to the depth of the ice
2. indicates the boundary of the glacier
3. is the difference between the glacier's annual gain of snow and ice and its annual loss
4. is balanced when accumulation exceeds ablation
An arête is:
1. formed by glacial quarrying
2. a distinctive asymmetrical form produced by a combination of abrasion and quarrying
3. formed by two cirques on opposite sides of the same mountain that erode headward and converge
4. a U-shaped valley that becomes flooded with seawater
Tarns
1. contain marine water
2. are rock ridges left by glacial erosion
3. are cirque lakes
4. create hanging valleys
Glacial drift
1. refers to the accumulation of snow that forms a glacier.
2. represents the heaps of ice on a glacier
3. is the rock material at the base of an eroded mountain
4. refers to all glacial deposits
Erratics are
1. isolated glacial lakes
2. large rocks found far from their bedrock origin
3. small mountain glaciers
4. small moraines
Loess
1. is windblown silt
2. frequently forms eskers
3. forms erratics
4. is a deep glacial lake
True or False? A firn is a slow-moving glacier.
True or False? During the late Pleistocene epoch, ice covered about 30% of the Earth's surface.
True or False? Even when a glacier's terminus is receding, ice within the glacier flows from the accumulation zone toward the ablation zone.
True or False? Till is unstratified and unsorted.
True or False? Tidewater glaciers can carve tarns to increasingly greater depths.
True or False? Utah's Great Salt Lake is a remnant of a pluvial lake.
True or False? Ice Ages have occurred only in the recent past because in earlier geological ages the originally molten Earth had not yet cooled to the extent required for glaciers to develop.
True or False? Tillite is tightly packed, light snow.
True or False? All alpine glaciers are confined.