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Glaciers and Climate Change
Chapter 13
Geology Today
Barbara W. Murck
Brian J. Skinner
Mount Fairweather, Glacier Bay, Alaska
N. Lindsley-Griffin, 1999
Glaciers
Glaciers are permanent bodies of ice
(recrystallized snow) that show evidence of
movement due to gravity.
Fig. 14.11, p. 410
N. Lindsley-Griffin, 1999
Types of Glaciers
Ice sheets are continent-sized glaciers
that overwhelm nearly all the land
within their margins.
Antarctica, one of two present-day ice sheets (the other is Greenland)
N. Lindsley-Griffin, 1999
Types of Glaciers
Ice caps cover mountain highlands,
or low-lying land at high latitudes
Vatnajokull
N. Lindsley-Griffin, 1999
Ice caps in Iceland (Fig. 14.11, p. 410)
Types of Glaciers
Types of glaciers are
determined by their
size and location Confined to a valley?
Spread over mountain
tops?
Cover a continent?
Fig. 14.11, p. 410
N. Lindsley-Griffin, 1999
Types of Glaciers
Valley glaciers flow down valleys.
Pressure of ice at higher elevations
pushes them down below snowline.
Denali National Park, Alaska (Fig. 14.11, p. 411)
N. Lindsley-Griffin, 1999
Types of Glaciers
Piedmont glaciers form large
lobes where valley glaciers come
together and flow onto lowlands.
Gorner Glacier, Swiss Alps (Fig. 14.11, p. 410)
N. Lindsley-Griffin, 1999
Types of Glaciers
Fjord glaciers occupy fjords: glaciercarved troughs in bedrock that fill
with seawater as the glacier retreats
Southwestern Greenland (Fig. 14.11, p. 411)
N. Lindsley-Griffin, 1999
Types of Glaciers
N. Lindsley-Griffin, 1999
Cirque glaciers are confined to cirques:
bowl-shaped depressions where snow
and ice accumulate on mountains.
Denali National Park, Alaska (Fig. 14.11, p. 410)
How Glaciers Form
1) Fresh snow is fluffy and porous. The delicate crystal points
evaporate; their vapor fills pore spaces.
2) The ice crystals gradually become smaller, rounder, denser.
3) Successive snowfalls bury and compact the ice crystals until
they recrystallize into a metamorphic rock - glacier ice
Fig. 14.13, p. 413
N. Lindsley-Griffin, 1999
Alpine or valley glaciers
form on high mountains
at all latitudes, wherever
snow remains all year.
Snow
granular ice
glacial ice (fused, massive)
Glacier Budgets
Positive budget - glacier grows
Negative budget - glacier shrinks
Accumulation zone - snow builds up to form glacier ice at the head
Zone of ablation - snow melts and evaporates at the terminus
Lutgens & Tarbuck; N. Lindsley-Griffin, 1999
See Fig. 14.14, p. 414
Glacier Budgets
Glacier will advance
if more ice is added
to head than is
removed at terminus
Glacier will retreat
if more ice is
removed at terminus
than is added at
head.
Fig. 14.14, p. 414
N. Lindsley-Griffin, 1999
Glacier
Movement
Ice crystals move by
internal creep.
Stress imposed by
weight of overlying
ice aligns the crystal
axes.
Internal cleavage
planes slip past each
other like a deck of
playing cards.
N. Lindsley-Griffin, 1999
Fig. 14.15, p. 415
Glacier
Movement
Glaciers flow by
internal creep at the
center, away from
the sides.
Flow is slower along
sides and base,
where they are
abrading.
Shallow ice is brittle, it
cracks to form crevasses
under tensional stress.
Fig. 14.14, p. 414
N. Lindsley-Griffin, 1999
Glacier
Movement
Basal sliding, in
which the glacier
slides along its bed,
occurs when
meltwater lubricates
the base of the
glacier.
Basal sliding may be
one cause of glacial
surges - very rapid
advances.
Fig. 14.14, p. 414
N. Lindsley-Griffin, 1999
Glacial Landscapes
The landscapes that result depend on the type of glaciation.
Ice sheets and ice caps override nearly everything in their
reach - they smooth out the landscape.
Valley or alpine glaciers carve valleys deeper and wider, and
leave sharp ridges and peaks between the valleys.
N. Lindsley-Griffin, 1999
Glacial Erosion
Lutgens & Tarbuck; N. Lindsley-Griffin, 1999
Ice abrades on the upstream side;
plucks on the downstream side.
Ice flow on uneven bedrock surface
Glacial Erosion
Abrasion smooths rock surfaces to
form glacial polish; scrapes surfaces
to make grooves, striations
Striated cobbles, Peyto Glacier, Alberta
Glacial polish, Sierra
Nevada Range, California
N. Lindsley-Griffin, 1999
Erosion by
Pleistocene
ice sheets:
Scooped out
Great Lakes
and the
Finger Lakes
of New York
U.S.G.S., N. Lindsley-Griffin, 1999
Erosion by Pleistocene ice sheets:
Produced the smooth, scoured topography of the
Canadian Shield
U.S.G.S., N. Lindsley-Griffin, 1999
Glacial Erosion
Landscapes shaped by valley glaciers
= Alpine glaciers
Horn
Arete
Tarn Cirque
Hanging
Valley
V shape
U shape
Steep,
straight
valley walls
U shape
Houghton-Mifflin, 1998; N. Lindsley-Griffin, 1999
Aretes
Icefall
Cirque
Cirque
Crevasse zone
Arete
U.S.G.S., N. Lindsley-Griffin, 1999
Glacial Erosion
Alpine glaciation is signaled by:
glacial horns and aretes
glacial grooves and striations
U-shaped valleys
Pilot Peak, WY/MT
The Matterhorn, Switzerland
N. Lindsley-Griffin, 1999
Glacial Erosion
N. Lindsley-Griffin, 1999
U-shaped valleys carved by glaciers
have broad, flat floors and steep walls.
Beartooth Range, Montana
Glacial Erosion
N. Lindsley-Griffin, 1999
Yosemite Valley was carved by a
glacier. The famous waterfalls are
streams that flow down hanging
valleys and fall to the valley floor.
Yosemite National Park, California
Glacial erosion may indicate flow direction
Striations -- parallel grooves and scratches gouged into
bedrock by rock fragments embedded in the glacier
N. Lindsley-Griffin, 1999
Roche moutonee Ice Flow
N. Lindsley-Griffin, 1999
a glacially carved rock knob that is smooth
on the upstream side, steep and rough on
the downstream side.
“Stone walls do not good neighbors make...”
Glacial Deposits
The stone walls of New England, immortalized by poet Robert
Frost, were built by European settlers clearing glacial boulders
from their fields. (The surface layer is till with little or no soil.)
N. Lindsley-Griffin, 1999
Mt. Chocorua, White Mountains, NH
Till - a heterogeneous mixture of
Glacial Deposits
crushed rock deposited by a glacier.
Poorly sorted: boulders, cobbles, pebbles, sand, silt, rock flour
No layering; may be angular or rounded; striated or grooved.
Deposited far from source; may rest on striated surface.
Pavement
outcrop and
till, central
Maine
N. Lindsley-Griffin, 1999
Glacial Deposits
Moraines - ridges or piles of debris
deposited along glacier edges.
Lateral moraine
Medial
moraine
Moraine
Terminal moraine
N. Lindsley-Griffin, 1999
Glacial
Deposits
Terminal moraines form at the ends of
glaciers as they retreat.
Terminal
moraine left
behind by
retreat of
Lobuche
Glacier
(Fig. 14.17D,
p. 418)
Lateral
moraine
N. Lindsley-Griffin, 1999
Glacial
Deposits
N. Lindsley-Griffin, 1999
Lateral moraines are deposited along the
sides of valley glaciers.
Grand Plateau Glacier, St. Elias Mts., Glacier Bay National
Park, Alaska (Fig. 14.17, p. 418)
Glacial
Deposits
N. Lindsley-Griffin, 1999
Lateral moraines are deposited along the
sides of valley glaciers.
Grand Plateau Glacier, St. Elias Mts., Glacier Bay National
Park, Alaska (Fig. 14.17, p. 418)
Glacial
Deposits
N. Lindsley-Griffin, 1999
Medial moraines form where two valley glaciers
merge, joining their lateral moraines into a stripe
of debris in the middle of the glacier.
Grand Plateau Glacier, St. Elias Mts., Glacier Bay National
Park, Alaska (Fig. 14.17, p. 418)
Glacial
Deposits
N. Lindsley-Griffin, 1999
Medial moraines form where two valley glaciers
merge, joining their lateral moraines into a stripe
of debris in the middle of the glacier.
Grand Plateau Glacier, St. Elias Mts., Glacier Bay National
Park, Alaska (Fig. 14.17, p. 418)
Glacial Deposits
Besides moraines, retreating glaciers
leave behind kames and kettles,
drumlins, eskers, outwash plains
Drumlin
Outwash
Plain
Kame
Esker
Kettle
Braided stream
N. Lindsley-Griffin, 1999
Glacial
Deposits
N. Lindsley-Griffin, 1999
Drumlin - streamlined, elongate hill of glacially
deposited sediment, parallel to ice flow. Blunt
end is upstream, tapered end points in direction
of ice flow.
Drumlin field, Alaska
Glacial Deposits
Esker - ridge of sand and gravel
deposited by a subglacial stream.
Kettle-Moraine State Park, Wisconsin (Fig. 14.17B, p. 418)
N. Lindsley-Griffin, 1999
Glacial Deposits
Glacial erratics, isolated boulders
deposited by glaciers, are different
than the underlying bedrock.
Denali National Park, Alaska (Fig. 14.17 A, p. 418)
N. Lindsley-Griffin, 1999
Kame and kettle topography, Alaska
U.S.G.S.; N. LindsleyGriffin, 1999
Kame - mound of stratified drift deposited by
water under or within glacial ice
Kettle - depression formed when a buried ice
block melted after the glacier retreated
Glacial Deposits
Glacial Outwash - stratified
sediments deposited by
pools or streams of glacial
meltwater.
Sorted by size; layered.
Look like other alluvial or
lacustrine deposits
Glacial outwash, Vermont
N. Lindsley-Griffin, 1999
Glacial Deposits
Varves tend to form in glacial
meltwater lakes with seasonal
fluctuations in sediment supply
and wintertime freezing.
Each varve consists of: one light sandsilt layer (deposited in summer when
streams are active)
one dark clay layer (deposited in
winter when lake is frozen and quiet)
1 varve = 1 year
N. Lindsley-Griffin, 1999
Glacial Deposits
Glacial outbursts occur when
an active volcano erupts under
an ice cap or sheet.
Lava melts ice, meltwater
forms large pool under ice cap.
Explosive eruption of volcano
splits glacier open, releases
water in a catastrophic flood.
Vatnajokull volcano, Iceland - 1996
(Frontispiece p. 397)
N. Lindsley-Griffin, 1999
Periglacial Landforms
Ice wedges form in regions of permafrost when surface
meltwater seeps into open cracks in the ground and freezes.
Wedges grow wider each season as more meltwater flows in
during the summer and freezes in winter.
N. Lindsley-Griffin, 1999
Fig. 14.18, p. 420
Periglacial
Landforms
Individual ice wedges join
together to form polygonal
patterns.
After hundreds of seasons
patterned ground results.
Patterned ground, Alaska
(Fig. 14.18, p. 420)
N. Lindsley-Griffin, 1999
Periglacial
Landforms
Pluvial lakes are formed by
increased rainfall in outlying
regions adjoining large ice
sheets.
In the Western U.S., during
the cooler and wetter climate
of the late Pleistocene, Lake
Bonneville and Lake
Lahontan were the two largest
pluvial lakes.
N. Lindsley-Griffin, 1999; Lutgens & Tarbuck, J.R. Griffin , 1999
Periglacial
Landforms
Lake terraces (wave-cut benches)
formed by wave action when Lake
Lahontan was at high water levels
during the Pleistocene.
Pluvial lake terraces, Nevada
N. Lindsley-Griffin, 1999
Causes of
Climate Change
Three major factors:
Tectonic plate motion
Long term cyclical
variations in solar
radiation
(Milankovitch cycles)
Changes in Earth’s
atmosphere
N. Lindsley-Griffin, 1999
Causes of Climate Change
Plate motions: Continents at
high latitudes favor growth
of ice sheets :
Ice sheets on Gondwana
when located over
South Pole 276 m.y.a.
Ice sheets on Eurasia
and North America
during Pleistocene
Evidence of Gondwanaland
ice sheets found on
southern continents today
Houghton Mifflin 1998; N. Lindsley-Griffin, 1999
Causes of Climate Change
Temperatures normally cycle from warm to cold and back
again. Glaciation occurs when global temperatures drop a few
degrees and remain low long enough for ice sheets to form.
N. Lindsley-Griffin, 1999
Fig. 14.19, p. 421
Global Climate Change
Astronomic basis for climatic cycles (Milankovich cycles):
1) variations in Earth’s orbital distance from the sun, 2) tilt of Earth’s axis
varies slightly, 3) Earth’s axis wobbles slowly like a spinning top
All 3 act on different time scales that combine in a complicated
way to alter amount of solar energy reaching Earth’s surface
N. Lindsley-Griffin, 1999
Fig. 14.21, p. 425
Pleistocene Ice Age
Maximum extent of
glaciation in the
Northern Hemisphere
during the ice age
Lutgens & Tarbuck, J.R. Griffin , 1999
Pleistocene Ice Age
North America Coastline during maximum Pleistocene glaciation
Location of coast line if all the present ice sheets melt
Tarbuck & Lutgens, J.R. Griffin , 1999
Pleistocene Ice Age
Northern Midwest
Blue line = Maximum extent of latest
period of glaciation (Wisconsin)
Red = Maximum extent of earlier
Pleistocene glaciation
W. Wayne, J.R. Griffin, N. Lindsley-Griffin, 1999
Pleistocene Ice Age
Blue line = Maximum extent of latest period of glaciation
(Wisconsin) in Nebraska
Red = Maximum extent of earlier Pleistocene glaciation in
Nebraska
W. Wayne, J.R. Griffin, N. Lindsley-Griffin, 1999
NE Conservation & Survey, J.R. Griffin , 1999
Light green area -Glaciated area
Loess, wind-deposited silt, is common near
glaciers because of abundant rock flour
In Alaska, loess is
forming today
Loess, Indian Cave State Park, NE
Thick Pleistocene loess, AK
NE Conservation & Survey, J.R. Griffin , 1999
Olive green - Area covered by Loess