CHAPTER 11 Glaciers and Glaciation
284
Key Terms
ablation (p. 268)
abrasion (p. 270)
alpine glaciers (p. 263)
arêtes (p. 272)
calving (p. 267)
cirque (p. 272)
crevasses (p. 266)
drumlins (p. 277)
end moraine (p. 275)
eskers (p. 277)
fiords (p. 272)
firn (p. 265)
glacial budget (p. 268)
glacial drift (p. 274)
glacial erratics (p. 275)
glacial striations (p. 270)
glacial trough (p. 271)
glacier (p. 263)
ground moraine (p. 276)
hanging valleys (p. 272)
horns (p. 272)
ice caps (p. 264)
ice sheets (p. 263)
ice shelves (p. 263)
kames (p. 278)
kettles (p. 276)
lateral moraines (p. 275)
medial moraines (p. 275)
outlet glaciers (p. 264)
outwash plain (p. 276)
pater noster lakes (p. 272)
piedmont glaciers (p. 264)
Pleistocene epoch (p. 281)
plucking (p. 269)
pluvial lakes (p. 280)
roche moutonnée (p. 273)
rock flour (p. 270)
snowline (p. 265)
stratified drift (p. 274)
tarn (p. 272)
till (p. 274)
valley glaciers (p. 263)
valley train (p. 276)
zone of accumulation (p. 267)
zone of wastage (p. 267)
C H A P T E R
E L E V E N
Glaciers and Glaciation
in Review
A glacier is a thick mass of ice originating on the land from the compaction and recrystallization of snow, and it shows evidence of past or
present flow. Today, valley or alpine glaciers are found in mountain areas
where they usually follow valleys that were originally occupied by
streams. Ice sheets exist on a much larger scale, covering most of
Greenland and Antarctica.
Near the surface of a glacier, in the zone of fracture, ice is brittle.
However, below about 50 meters, pressure is great, causing ice to flow
like a plastic material. A second important mechanism of glacial movement consists of the entire ice mass slipping along the ground.
The average velocity of glacial movement is generally quite slow, but
it varies considerably from one glacier to another. The advance of some
glaciers is characterized by periods of extremely rapid movements called
surges.
Glaciers form in areas where more snow falls in winter than melts
during summer. Snow accumulation and ice formation occur in the zone
of accumulation. Its outer limits are defined by the snowline. Beyond the
snowline is the zone of wastage, where there is a net loss to the glacier.
The glacial budget is the balance, or lack of balance, between accumulation at the upper end of the glacier, and loss, called ablation, at the
lower end.
Glaciers erode land and acquire debris by plucking (lifting pieces of
bedrock out of place) and abrasion (grinding and scraping of a rock
surface). Mass-wasting processes also make significant contributions to
the load of many alpine glaciers. Erosional features produced by valley
glaciers include glacial troughs, hanging valleys, pater noster lakes, fiords,
cirques, arêtes, horns, and roches moutonnées.
Any sediment of glacial origin is called drift. The two distinct types of
glacial drift are (1) till, which is unsorted sediment deposited directly by
the ice; and (2) stratified drift, which is relatively well-sorted sediment
laid down by glacial meltwater.
The most widespread features created by glacial deposition are layers
or ridges of till, called moraines. Associated with valley glaciers are lateral
moraines, formed along the sides of the valley, and medial moraines,
formed between two valley glaciers that have joined. End moraines, which
mark the former position of the front of a glacier, and ground moraine, an
undulating layer of till deposited as the ice front retreats, are common to
both valley glaciers and ice sheets. An outwash plain is often associated
with the end moraine of an ice sheet. A valley train may form when the
glacier is confined to a valley. Other depositional features include
drumlins (streamlined asymmetrical hills composed of till), eskers (sinuous
ridges composed largely of sand and gravel deposited by streams flowing
in tunnels beneath the ice near the terminus of a glacier), and kames
(steep-sided hills composed of sand and gravel).
The Ice Age, which began 2 million to 3 million years ago, was a very
complex period characterized by a number of advances and withdrawals
of glacial ice. Most of the major glacial episodes occurred during a division of the geologic time scale called the Pleistocene epoch. Perhaps the
most convincing evidence for the occurrence of several glacial advances
during the Ice Age is the widespread existence of multiple layers of drift
and an uninterrupted record of climate cycles preserved in seafloor
sediments.
In addition to massive erosional and depositional work, other effects
of Ice-Age glaciers include the forced migration of organisms, changes in
stream courses, formation of large proglacial lakes, adjustments of the crust by
rebounding after the removal of the immense load of ice, and climate
changes caused by the existence of the glaciers themselves. In the sea, the
most far-reaching effect of the Ice Age was the worldwide change in sea
level that accompanied each advance and retreat of the ice sheets.
Any theory that attempts to explain the causes of glacial ages must
answer two basic questions: (1) What causes the onset of glacial conditions? and (2) What caused the alternating glacial and interglacial stages
that have been documented for the Pleistocene epoch? Two of the many
hypotheses for the cause of glacial ages involve (1) plate tectonics and
(2) variations in Earth’ s orbit. Other factors that are related to climate
change during glacial ages include changes in atmospheric composition,
variations in the amount of sunlight reflected by Earth’ s surface, and
changes in ocean circulation.
284
Key Terms
ablation (p. 268)
abrasion (p. 270)
alpine glaciers (p. 263)
arêtes (p. 272)
calving (p. 267)
cirque (p. 272)
crevasses (p. 266)
drumlins (p. 277)
end moraine (p. 275)
eskers (p. 277)
fiords (p. 272)
firn (p. 265)
glacial budget (p. 268)
glacial drift (p. 274)
glacial erratics (p. 275)
glacial striations (p. 270)
glacial trough (p. 271)
glacier (p. 263)
ground moraine (p. 276)
hanging valleys (p. 272)
horns (p. 272)
ice caps (p. 264)
ice sheets (p. 263)
ice shelves (p. 263)
kames (p. 278)
kettles (p. 276)
lateral moraines (p. 275)
medial moraines (p. 275)
outlet glaciers (p. 264)
outwash plain (p. 276)
pater noster lakes (p. 272)
piedmont glaciers (p. 264)
Pleistocene epoch (p. 281)
plucking (p. 269)
pluvial lakes (p. 280)
roche moutonnée (p. 273)
rock flour (p. 270)
snowline (p. 265)
stratified drift (p. 274)
tarn (p. 272)
till (p. 274)
valley glaciers (p. 263)
valley train (p. 276)
zone of accumulation (p. 267)
zone of wastage (p. 267)
C H A P T E R
E L E V E N
Glaciers and Glaciation
in Review
A glacier is a thick mass of ice originating on the land from the compaction and recrystallization of snow, and it shows evidence of past or
present flow. Today, valley or alpine glaciers are found in mountain areas
where they usually follow valleys that were originally occupied by
streams. Ice sheets exist on a much larger scale, covering most of
Greenland and Antarctica.
Near the surface of a glacier, in the zone of fracture, ice is brittle.
However, below about 50 meters, pressure is great, causing ice to flow
like a plastic material. A second important mechanism of glacial movement consists of the entire ice mass slipping along the ground.
The average velocity of glacial movement is generally quite slow, but
it varies considerably from one glacier to another. The advance of some
glaciers is characterized by periods of extremely rapid movements called
surges.
Glaciers form in areas where more snow falls in winter than melts
during summer. Snow accumulation and ice formation occur in the zone
of accumulation. Its outer limits are defined by the snowline. Beyond the
snowline is the zone of wastage, where there is a net loss to the glacier.
The glacial budget is the balance, or lack of balance, between accumulation at the upper end of the glacier, and loss, called ablation, at the
lower end.
Glaciers erode land and acquire debris by plucking (lifting pieces of
bedrock out of place) and abrasion (grinding and scraping of a rock
surface). Mass-wasting processes also make significant contributions to
the load of many alpine glaciers. Erosional features produced by valley
glaciers include glacial troughs, hanging valleys, pater noster lakes, fiords,
cirques, arêtes, horns, and roches moutonnées.
Any sediment of glacial origin is called drift. The two distinct types of
glacial drift are (1) till, which is unsorted sediment deposited directly by
the ice; and (2) stratified drift, which is relatively well-sorted sediment
laid down by glacial meltwater.
The most widespread features created by glacial deposition are layers
or ridges of till, called moraines. Associated with valley glaciers are lateral
moraines, formed along the sides of the valley, and medial moraines,
formed between two valley glaciers that have joined. End moraines, which
mark the former position of the front of a glacier, and ground moraine, an
undulating layer of till deposited as the ice front retreats, are common to
both valley glaciers and ice sheets. An outwash plain is often associated
with the end moraine of an ice sheet. A valley train may form when the
glacier is confined to a valley. Other depositional features include
drumlins (streamlined asymmetrical hills composed of till), eskers (sinuous
ridges composed largely of sand and gravel deposited by streams flowing
in tunnels beneath the ice near the terminus of a glacier), and kames
(steep-sided hills composed of sand and gravel).
The Ice Age, which began 2 million to 3 million years ago, was a very
complex period characterized by a number of advances and withdrawals
of glacial ice. Most of the major glacial episodes occurred during a division of the geologic time scale called the Pleistocene epoch. Perhaps the
most convincing evidence for the occurrence of several glacial advances
during the Ice Age is the widespread existence of multiple layers of drift
and an uninterrupted record of climate cycles preserved in seafloor
sediments.
In addition to massive erosional and depositional work, other effects
of Ice-Age glaciers include the forced migration of organisms, changes in
stream courses, formation of large proglacial lakes, adjustments of the crust by
rebounding after the removal of the immense load of ice, and climate
changes caused by the existence of the glaciers themselves. In the sea, the
most far-reaching effect of the Ice Age was the worldwide change in sea
level that accompanied each advance and retreat of the ice sheets.
Any theory that attempts to explain the causes of glacial ages must
answer two basic questions: (1) What causes the onset of glacial conditions? and (2) What caused the alternating glacial and interglacial stages
that have been documented for the Pleistocene epoch? Two of the many
hypotheses for the cause of glacial ages involve (1) plate tectonics and
(2) variations in Earth’ s orbit. Other factors that are related to climate
change during glacial ages include changes in atmospheric composition,
variations in the amount of sunlight reflected by Earth’ s surface, and
changes in ocean circulation.