Visualizing Physical Geography
by Alan Strahler and Zeeya Merali
Chapter 14
Glacial Landforms and the Ice Age
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Chapter
Overview
Glaciers
Alpine Glaciers
Ice Sheets and
Sea Ice
The Ice Age
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Glaciers
Glacier: large natural accumulation of land ice affected by
present or past flowage
• Glacial ice has shaped many landforms in middle and high latitudes
• Glacial ice sheets affect global climate
• Glaciers reflect sunlight
• Glacial ice affects global heat transport
• Volume of glacial ice affects sea levels
• Large bodies of ice are plastic because of
pressure on ice at bottom of mass
• Large body of ice can flow in response
to gravity
• Ice on a slope can slide downwards
Ice sheet: large thick plate of
glacial ice moving outward in all
directions
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Glaciers
Glacial ice builds up when snowfall in winter exceeds
snow melted in summer
• Each annual layer of snow, melts, refreezes, forms ice
• Weight of ice compresses the lower layers into hard crystalline ice
• When the ice mass becomes thick enough, the lower layers flow
plastically
• Glacial ice forms where temperatures are low and snowfall is high
Alpine glacier: long,
narrow mountain glacier
occupying the floor of a
trough-like valley
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Glaciers
Glaciers contain rock of all sizes
• Glaciers and ice sheets erode and
deposit great quantities of sediment
• Glacial abrasion: rock fragments in a
glacier scrape and grind the bedrock
• Plucking: moving ice plucks loosened
rock material and carries it away
• Sediment is carried, then deposited when
the ice melts
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Alpine Glaciers
• Upper part of the glacier is brittle
• Lower part is plastic
• Zone of accumulation: glacier is
growing
• Zone of ablation: glacier is
evaporating/melting
• Alpine glacier can slide downhill on
meltwater and mud
• Rate of movement: few
centimeters/day to several meters/day
• Surge: rapid movement of alpine
glacier, up to 60 m/day for several
months
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Alpine Glaciers
Landforms Made by Alpine Glaciers
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Alpine Glaciers
Landforms Made by Alpine Glaciers
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Alpine Glaciers
Landforms Made by Alpine Glaciers
• Cirque: valley head enlarged and hollowed out
by glaciers, producing bowl-shaped valley
• Arête: sharp ridge formed between two cirques
• Horn: steep-sided peak formed by glacial
erosion from three sides
• Col: natural pass or low notch in an arête
between opposed cirques
• Moraine: accumulation of rock debris carried by
an alpine glacier and deposited by the ice to
become a depositional landform
• Tarn: small lake occupying a rock basin in a
cirque
• Hanging valley: stream valley that has been
truncated by glacial erosion so as to appear in
cross section in the upper wall of a glacial
trough
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Alpine Glaciers
Landforms Made by Alpine Glaciers
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Alpine Glaciers
Landforms Made by Alpine Glaciers
• Glacial trough: deep, steepsided rock trench formed by
alpine glacier erosion
• Fiord: narrow, deep ocean inlet
partially filling a glacial trough
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Ice Sheets and Sea Ice
Ice Sheets of the Present
Antarctic Ice Sheet
13 million sq km
4000 m thick
Greenland Ice Sheet
1.7 million sq km
3000 m thick
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Ice Sheets and Sea Ice
Sea Ice and Icebergs
Sea Ice: floating ice of the oceans
formed by direct freezing of
ocean water
• Always <5 m thick
• Pack ice: sea ice that completely
covers the sea surface
• Ice floes: individual patches of sea
ice
• Upper part fresh water; lower part
salty
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Ice Sheets and Sea Ice
Sea Ice and Icebergs
Iceberg: mass of glacial ice floating
in the ocean that has broken off a
glacier that extends into tidal water
• May be hundreds of meters thick
• About 5/6 of the iceberg is submerged
• Always fresh water
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Ice Sheets and Sea Ice
Landforms Made by Ice Sheets
Ice sheets scrape off regolith and abrade bedrock
Grooves, scratches, and polished rock show evidence of ice sheets
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Ice Sheets and Sea Ice
Landforms Made by Ice Sheets
Ice sheets pick up rock fragments,
transport them, and deposit them
when the ice evaporates or melts
Glacial drift: general term for all varieties
and forms of rock debris deposited by ice
sheets
Stratified drift: layers of sorted clays, silts,
sands or gravels, deposited by meltwater
streams or lakes
Till: unstratified mixture of rock fragments
of all sizes, deposited directly by the ice
Ground moraine: thin, even cover of till
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Ice Sheets and Sea Ice
Esker: narrow, often sinuous
embankment of coarse gravel and
boulders deposited in the bed of a
meltwater stream in a tunnel
within stagnant ice in an ice sheet
Drumlin: smoothly rounded, oval
hills of glacial till, formed by the
moving ice
Terminal Moraine: glacial till that
accumulates at the farthest
advance of the ice sheet
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Ice Sheets and Sea Ice
Landforms Made by Ice Sheets
Moraine: accumulation of rock
debris carried by an ice sheet
and deposited by ice to become
a depositional landform
Outwash Plain: formed from
stratified drift left by streams
issuing from the ice
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Ice Sheets and Sea Ice
Landforms Made by Ice Sheets
Recessional Moraine: moraine
formed when the ice paused in its
retreat
Kame: steep-sided mound of
stratified sand and gravel
deposited at or near the terminus
of a glacier
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Ice Sheets and Sea Ice
Landforms Made by Ice Sheets
Pluvial Lakes: lakes formed during a moister climate, during the ice
ages
• Many pluvial lakes formed in the western U.S.
• Lake Bonneville, in western Utah, was about the size of Lake Michigan
• Pluvial lakes shrank or dried up as the climate became warmer and drier
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
The Ice Age
Glaciation: single episode or
time period in which ice sheets
formed, spread, and
disappeared
Glaciation occurs when
temperatures drop or snowfall
increases
55 million years ago
There have been many
glaciation episodes in Earth’s
history
5 million years ago
Recent ice ages
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
The Ice Age
Late-Cenozoic Ice Age: series of glaciations, deglaciations, and
interglaciations experienced during the late Cenozoic Era
Maximum Glaciation in North
America and Europe
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
The Ice Age
Investigating the Ice Age
Scientists study the glacial history of the Ice Age:
•
•
•
•
Take sediment core samples on the deep ocean floor
Use evidence of magnetic reversals to find the age of the sediments
Study composition and chemistry of the cores
Create a record of dates, temperatures, biology, and chemistry
using the core samples
Deep-sea cores show alternating glaciations and
interglaciations going back 2-3 million years
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
The Ice Age
Possible Causes of the Ice Age
1. Motions of tectonic
plates brought landmass to high latitudes
2. Volcanic activity
produced dust that
blocks solar radiation
3. Decrease in Sun’s
energy output
4. Change in atmospheric
composition; reduction
in greenhouse gases
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
The Ice Age
Possible Causes of Glaciation Cycles
Astronomical hypothesis:
explanation for glaciations and
interglaciations based on cyclic
variations in the solar energy
received at the Earth’s surface
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
The Ice Age
Holocene Environments
Holocene Epoch: last epoch of geologic
time, commencing about 10,000 years
ago and including the present
Three major climate periods in Holocene:
• Boreal Stage: boreal forest vegetation in
midlatitude regions
• Atlantic Stage: warmer temperatures, about
8000 years ago
• Subboreal Stage: cooler, about 5000 years
ago to 2000 years ago
During past 2000 years:
• Warm from AD 1000 to 1200
• Little Ice Age from AD 1450 to 1850
• Global temperatures increasing at present
Fossil pollen and spores
preserved in glacial bogs show
changes in vegetation over
time
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Ice Sheets and Global Warming
Disintegration of Larsen B ice shelf, Antarctica, 2002
31 Jan 2002
23 Feb 2002
3 Mar 2002
5 Mar 2002
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Ice Sheets and Global Warming
Polar sea ice has been decreasing in the last 50 years
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.