Glaciers and Glacial Erosion –
GLY 2010- Summer 2012 -Lecture 20
• Ice Margin, Commonwealth Glacier,
Antarctica
1
Glacier
• Moving mass of ice that forms when snowfall
exceeds snowmelt over a long period of time
• Movement is downhill, due to gravity
• Form at or above the snowline, the lowest
altitude at which snow commonly forms in the
mountains
• As climates warm, the snowlines are expected
to move higher in elevation, and glaciers to
retreat
2
Types of Glaciers
3
Continental Ice Sheet
• Large ice masses that blanket a sizable
part of a continent
• Ice may be kilometers thick, and
movement is limited, occurring mainly in
local areas or very slowly over time
4
Present Day
Continental Ice
Sheets
• Greenland and
Antarctica currently
are occupied by
continental ice sheets
5
Snowfield and Ice Cap
• Large mass of snow and ice on a flat
surface, topped by recent snow
• Ice caps show little movement
• They occupy the tops of mountains
• Outlet glaciers may flow downward from
the ice cap
6
Snowfield and Icecap Above
Byrd Glacier, Antarctica
• Snow blankets and fills the valleys
between the nunataks in the foreground
7
Fox Glacier, New Zealand
• Outlet glacier fed by Ice Cap
8
Mountain Valleys
• High in the mountains, snow may accumulate
• Glaciers are formed in stages
 Snow
 Firn
 Glacial Ice
9
Snow
• Snow occurs in many forms, for example
wet or dry
• Snow accumulates with a great deal of air
trapped inside (you need to pack snow to
make a decent snowball)
10
Firn
• During the first summer following
snowfall, some of the snow melts
• Meltwaters trickles downward, helping to
compact the snow
• As winter approaches, resulting mass
freezes together to form firn
11
Formation of Glacier Ice
12
Glacial Ice
• Repeated years of thaw/freeze cycles, and
the weight of accumulated firn and snow,
transform the lower layers to ice
• There are twelve known structural forms
of water ice, at least half of which occur
in nature
13
Cirque Glacier
• Ice movement erodes a depression near the
head of the glacier
• Glacier occupies the hole for a prolonged
period, creating a bowl-shaped depression
under the glacier
• After the cirque glacier melts, the
depression remains
• Cirque may be filled with water, and is
called a tarn lake, or it may be dry
14
Cirque Glacier Photo
15
Cirque
• Photo by Dr. Michael Hambrey
16
Alpine or Valley Glacier
• Confined by surrounding bedrock
highlands
• Generally move down steep to very
steep surfaces
• As cirque glaciers expand, they flow
into pre-existing stream cut channels,
enlarging and changing the shape of
these valleys
• Stream valleys have V-shaped profiles,
whereas glacial valleys are U-shaped
17
Valley Glacier Photo
Valley glacier flowing through mountains in Alaska
18
Piedmont Glaciers
• One or more valley
glaciers flowing from the
confines of valley walls
and spread out to form
broad sheets
• Piedmont literally means
foot of the mountain
• Malaspina Galcier (Alaska) is a classic large
piedmont glacier that descends to tidewater from
several mountain sources
19
Tidewater Glacier, Alaska
• Tidewater glaciers flow into the sea,
calving icebergs
20
Calving Glacier Video
• Video was shot from a
small ship July 1993
about one-half mile
from the calving glacier
by the instructor
© David Warburton, 2006
21
Glacial Zones
•
•
•
•
Accumulation
Wastage (Ablation)
Fracture
Flowage
22
Glacial Anatomy
23
Zone of Accumulation
• Snowfall exceeds snowmelt, on average
over many years
• Upper portion of the glacier, at all depths
24
Zone of Wastage (Ablation)
• Snowmelt exceeds snowfall, on average
over many years
• Glacier will retreat unless gravitational
movement of glacier downhill replaces
glacial ice as fast as it melts
• 85% of world’s glaciers are currently
retreating
25
Glacial Cross-Section
26
Zone of Fracture
• Upper surface of glacier is under little
pressure
• Near toe of glacier, glacier moves faster,
and pulls ahead of the ice uphill
• Tensional cracks develop (crevasses)
• Ice behaves as a brittle solid
27
Zone of
Flowage
• Lower part of the glacier, except near the
toe, where glacier is thin
28
Glacial Advance and Retreat
29
Crevasses, Exit Glacier, Alaska
• Extensional
crevasses
• Note that they
have the same
shape as
normal fault
blocks
30
Inside a Crevasse
• Photos by Kristina
Ahlnas, University
of Alaska
• Glacial ice is so blue
because the dense ice
of the glacier absorbs
every other color of
the spectrum except
blue--so blue is what
we see!
31
Meltwater
• As glaciers melt, water trickles through
the glacier, getting under it
• May form under-ice stream channels
• Channels form tunnels under the ice
• Tunnels emerge at snout of glacier
32
Ice Cave
• Video of tidewater glacier, Prince William Sound, Alaska,
July, 1993 © David Warburton, 2006
33
Work of Glaciers
• Like rivers, glaciers alter the landscape
• They may erode the landscape, but they also
deposit large amounts of material
34
Glacial Erosion
•
•
•
•
•
•
•
•
Abrasion
Quarrying
U-shaped valleys
Hanging valleys
Cirque
Arěte
Horn
Roche moutonnée
35
Abrasion
• Rock fragments imbedded in the base of
the glacier scrap, and polish underlying
rock, and in some cases create long
striations (thin) or grooves (thicker) in the
bedrock
• Stria and grooves indicate the direction of
glacier movement
36
Kelley's Island Grooves, Ohio
• Grooves
may be of
glacial or
fluvial
origin
• View is in
direction of
ice flow
37
Glacial Striations
• Striations
are
smaller
than
grooves
38
Quarrying
• Large masses of bedrock are lifted away
from the bed after water from the glacier
seeps into cracks and refreezes (frost
wedging)
• Rock is incorporated into the glacier
39
New Zealand Glacier
• Block to the right has been partially quarried
40
U-Shaped Valleys
• Glacial ice follows previously cut stream
valleys
• Stream valleys have V-shaped profiles
• Glacial erosion changes the shape to a U
41
Glacial Valley Development I
• Typical, meandering V-shaped
river valley
42
Glacial Valley Development II
• Running water
erodes and deepens
the V - shape
43
Glacial Valley Development III
• Glacier fills the
river valley
• Channel is
widened and
straightened
44
Glacial Valley Development IV
• Melting of
glacier reveals a
U – shaped
valley
45
Glacial
Trough
• The U-shaped glacial trough seen here is in
Glacier National Park, Montana
46
Hanging Valleys
• Where tributary glaciers flow into the trunk
glacier, they are often unable to erode as fast
as the heavier trunk glacier
• When the ice melts, a hanging valley is left
47
Yosemite Valley
48
Waterfalls in a
Hanging Valley
• Yosemite Falls
occupies a glacial
valley
49
Cirque
• Bowl-shaped
depression near
the head of an
alpine glacier
• Cirques remain long after glaciers
disappear
• May fill with water to become tarn lakes
50
Tarn Lake
• Lake Ann, North Cascades National
Park, is a tarn lake, occupying a cirque
51
Arěte
• Knife-edged ridge
of land, formed
by parallel
erosion of two
alpine glaciers
• Another visible
sign of previous
glaciation
52
Horn
• A three or four-sided mountain
• Cut by glaciers flowing off an isolated mountain
in several directions
• Matterhorn in Switzerland is an excellent
53
example
Roche Moutonnée
• From the French,
meaning sheep rock
• Glacial abrasion
smoothes the slope
facing the oncoming ice
• Glacial quarrying plucks
rocks from the opposite
slope, steepening it
54
Liberty Cap,
Yosemite
National Park
• Liberty Cap
is a Roche
moutonnée
55
Alpine Glacial Erosion
56
Glacial Deposition
•
•
•
•
Till
Drift
Erratics
Rock flour
57
Glacial Till
• Deposits directly
from glacial ice,
with no sorting
• Light rocks are
cobbles and
pebbles
• Dark tan "matrix"
is a mixture of
sand, silt, and clay
58
Formation of a Till Deposit
59
Glacial Drift
• Includes glacial till, material dumped by
glacier when melting
• Deposits from meltwater flowing out
under a glacier
• Meltwater deposits are moderately to
well-sorted
60
Rock Flour
• Finally ground
sediment
produced under a
glacier
• Meltwater
streams carry the
flour to lakes like
this one in
Alberta, Canada
61
Moraines
• Masses of glacial drift left behind by
a glacier
• Types of Moraine
 Terminal
 Lateral
 Medial
62
Terminal Moraine
• Left at the end of a glaciers advance
• May act as a natural earth-fill dam
• Multiple terminal moraines may form a
series of hills, running parallel to a ridge
of mountains
63
Moraine Formation
64
Terminal Moraine Formation
• Terminal moraine is a more common name for what they call end moraine
65
Cape Cod From Space
• Cape Cod is a
terminal moraine
• It marks the farthest
point that the
glaciers reached
during the most
recent "ice age" in
North America
66
Southern New England Moraines
67
Lateral
Moraine
Photo
• Lateral
moraines
are visible
to either
side of the
glacier
Athabaska Glacier, Jasper National Park,
Canada
68
Medial Moraine Formation
69
Medial Moraine
Photo
• Wrangell National Park, Alaska
• Lateral moraines merging to form a complex of
medial moraines
70
Glacier Junction, Southern Alps,
New Zealand
71
Twentymile Glacier, Alaska
• Two tributaries meet in a complex of
medial moraines
72
Outwash, Tasman Glacier, New
Zealand
• Running water re-works the glacial gravel into
the outwash
• Note the car in the lower center of the image 73
Braided Outwash Channels, Toklat
River, Alaska
• Braided channels are constantly
changing
74
Glacial Erratics
• Large glacial erratic
• Ice is capable of carrying all sizes of material
75
Kettle Lakes
• Form when a large chunk of glacial ice is buried as
glacier passes over
• Later, it melts, and forms a lake
• Bear and Nymph Lakes, Rocky Mountain NP
76
Drumlin
Aerial View of
drumlin field
• Gently rounded hills
formed by a massive
ice sheet (thicker then
a glacier) overriding a
moraine
• Massive ice reshapes
the moraine into
elongated hills
77
Drumlin
Formation
78
Esker Formation
• Meltwater streams, flowing
within or under active glaciers
deposit sand and gravel in
curving channels
• When the glaciers melt the
eskers are exposed as
topographically positive
features
• Long, linear features,
sometimes in a network
79
Esker Photo
• Esker visible as a sinuous ridge (arrow) in
this aerial photo
80
Kame
• Formed when meltwater washes sediment into
openings in a stagnant wasting glacier terminus
81
Glacial Features
82
Glacial Subsidence and Rebound
• Continental ice sheets are so heavy they depress
the rock under them (subsidence, due to isostasy)
• When the ice sheets melt, the land begins to
slowly rise
• In the Canadian Shield region, the glacial rebound
rate is about one foot per century
• Rebound is sometimes visible along lake-shores,
where older beaches and wave-cut terraces are
now considerably higher then the current
lakeshore
83
Glacial Isostasy
84
Crustal Depression
• Crust bulges on either side of glacier
(isosastacy)
85
Bulging Produces Raised Beaches
86
Postglacial Effects
87
Creation
of Lake
Missoula
88
Ice
Dam
Breaks
89
Bretz Floods
• The resulting floods, which recurred
numerous times, are called Bretz floods
after J Harlan Bretz, who first realized
the significance of the gravel ridges
meters high, and sometimes with
wavelengths of a hundred meters, as
Ripple Marks!
90
Giant Ripple Marks
• Aerial view of giant ripple marks
91
Catastrophic
Flood
• Flooding occurred
about 13,000 to
15,000 years ago
92
Dry Falls
• Dry channels occur hundreds of feet above
the present river channels, and former
cataracts existed in many areas
• The best known of these, Dry Falls, is a 3.5
mile wide former cataract that dropped 400'
over vertical cliffs of basalt
93
Scablands
• Floods gouged and tore at the land
removing surface cover
• Left masses of basaltic lava as remnants,
like scabs on a wound
• Area is now called the channeled
scablands
94
Palouse Falls, Washington
• Note the very large channel behind the
falls
95
Pluvial Lakes
• During glacial times, the
climate was cooler, and
evaporation rates were
much lower in arid and
semi-arid regions
• Many pluvial lakes
formed from rainwater
which did not evaporate
• Pluvial comes from
Latin pluvia meaning
rain
96