Lecture 15 Cold Climates
Glaciers and Ice Ages
http://www.aad.gov.au/asset/webcams/mawson/
• Glacier: a large, long-lasting mass of ice,
formed on land that moves under the
influence of gravity
• Glaciers form by accumulation and
compaction of snow
– Packed snow becomes firn
– Then refreezes to ice
Davidson Glacier near Haines, Alaska
Formation of Glacial Ice from Snow
snowball
iceball
Alpine Glacier: it is just a frozen river
Types of Glaciers
Continental
Alpine
Alpine glaciation: found in mountainous regions
Continental glaciation: exists where a large part of a continent is covered by glacial ice
Cover vast areas
Alpine Glaciers– Cirque Glacier
Mount Edith Cavell, Jasper National Park, Canada
Alpine Glaciers – Valley Glacier
Lateral and medial moraines
Tongas National Forest, Alaska
Types of Glaciers – Icecap and Continental
http://www.nmm.ac.uk/server/show/conWebDoc.1221
Sentinal Range, Antarctica
•Antarctica is the broadest high place on Earth, the ice cap is up to 4km thick
and covers the continent
•Antarctica is a desert, with only 15 cm (6 inches) of snowfall a year around
the South Pole. The air is too cold to hold much moisture.
•The lowest recorded temperature is -89.2 °C.
•There is no life in Antarctica except near the coast
http://www.aad.gov.au/asset/webcams/mawson/
Types of Glaciers – Piedmont & Tidewater
Piedmont: Originally confined alpine, spread at foot of mountains
Source: Jim Wark/Peter Arnold, Inc.
Calving
Iceberg Calving –
Hubbard Glacier, Wrangell-St. Elias National Park, Alaska
A Glacier’s Budget
• Budget = Gain – Loss
• Gains snow in zone of accumulation
• Loses ice in zone of ablation
• Budget can be positive (net growth)
• Static
• or negative (net melting)
A Glacier’s Budget
Year round Snow
Summer Rain
Note that a glacier is a river. Even if the terminus doesn’t advance, it still flows downhill
Mechanics of Glacial Flow
Internal deformation
Ice crystals slide past one another
Basal Sliding Entire glacier slides downhill on a thin film of
meltwater at its base.
Glacier always flows toward zone of ablation
Erosion 1: Glacial Abrasion in Bedrock
Glaciers cause erosion
Rock embedded in Ice
Source: Tom Bean
Glacial Erosion – Roche Moutonée
2. Quarrying
FROST WEDGING
Glacial Erosion – Roche Moutonée
Yosemite NP, Calif
Erosion by Glaciers (cont.)
• Alpine glaciers erode mountain slopes
into horseshoe shaped basins called
cirques
– Melting forms cirque lake (tarn)
• Erosion of two or more cirques erodes
intervening rock
– Horns :pointy peaks made by trios
– Arêtes: long serrated ridges by pairs
– Cols: passes through the arêtes
Alpine Glacial Erosion
Cirque and valley glaciers form in river valleys, cut U-shapes
Alpine Glacial Erosion
Cirque glaciers erode uphill, widen
Valley glaciers enlarge
Arêtes, Horn, Col, HANGING VALLEYS
Yosemite Falls from Hanging Valley
U-Shaped Glacial Valley in Southeastern Alaska
Valley glaciers erode a large quantity of bedrock and sediment
Convert V-shaped stream valleys into U-shaped glacial valleys.
Seawater Flooded U-Shaped Valleys: Fjords
Bela Bela Fjord, BC
Sea Level rose as glaciers melted
Erosion by Continental Glaciation
• Erosional Landforms much larger in scale than
alpine glaciers
–Whalebacks – huge Roche Moutonée
–Huge U-shaped troughs
– Finger Lakes, Great Lakes, Puget
Sound, Loch Ness were all once stream
valleys excavated by Ice Sheets
Erosion by Continental Glaciation (Great Lakes, Finger Lakes)
Superior
M
i
c
h
i
g
a
n
Huron
Ontario
Erie
Source: U.S. Dept. of Interior, USGS Eros Date Center
Glacial Deposits - Drift
• Collectively called Glacial Drift
• TYPE 1: UNSORTED
• Glacial Till: unsorted, unstratified
sediments deposited by melting ice.
– May contain glacial erratics
– Often accumulates at glacier’s terminus as a
Terminal Moraine: hills of sediment left by a
glacier’s retreat.
– Moraines may be reshaped by a later glacial
advance into Drumlins: rounded elongated
hills
Advance & Retreat: Moraines
Note moraine,
no matter
direction
Analogy:
Escalator
Discussion: Advance & Retreat of
Glaciers and Terminal Moraines
Large Granite Erratics
Favored by climbers with families
Medial Moraines
Wrangell-St. Elias NP, SE AK
Medial Moraines – Kennicott Glacier
The Origin of Drumlins
Glacier retreats, leaving a terminal moraine. Then it advances
again, and redistributes the drift as a spoon shaped hill called a
drumlin.
Drumlins Rochester,NY
Glacial Deposits - Drift
• TYPE 2: SORTED
• Outwash: sorted stratified sediments deposited
by meltwater streams
• Loess: wind erosion of drying outwash silt.
• Eskers: sinuous meltwater deposits of sand
and gravel underneath ice
Origin of Eskers
Eskers and Kettles in South Dakota
Effects of Glaciation
• Change Climate – increase precipitation
pluvial lakes
• Depress continents & lateral rebound
• Drop sea-level: alter coastlines
• Moraines form Dams – Proglacial Lakes
– Divert streams – Ohio and Missouri rivers
Formation of Terraces due to Crustal Rebound
Lowered Sea-level - Land bridge
Bering
Glacier grows, sea-level drops
Lowered Sea-level exposed continental shelf
Massive
extinctions of
shallow-water
marine
organisms
The Creation of Glacial Lake Missoula
Purcell Lobe blocks Clark Fork River
The Draining of Glacial Lake Missoula
Repeated many times, last time 13000 kya
Giant Ripples of the Missoula Flooding
Flood kills everything in its path, 26 times
Giant Ripples
Country road for scale
Max Glacier Distribution 20,000 ya
Maximum glaciation occurs at coincidence of three astronomical cycles
if high land in polar latitudes
Causes of Ice Ages
• Plate Tectonics
Moves Continents to Poles
Raises mountains above snowline
Albedo increases, colder, spread
• Orbit distance, Axis Tilt and Wobble
– Moderates solar radiation past 65o Latitude
– Croll-Milankovitch Cycles ~ every 100,000 years
– With many smaller cycles between
– Need low summertime radiation past 65o Latitude
less melting, glaciers expand
Milankovitch Cycles
http://en.wikipedia.org/wiki/Milankovitch_cycles
The energy of solar radiation drops off as the square of the distance
Further away
100,000 years
Not as hot
High latitude getting more sunlight in winter
Warm Wet Winter
Cool Summer
Snow doesn’t melt
Glaciers grow
High latitude getting less sunlight in summer
Cold Dry Winter
Hot Summer
Snow melts
Glaciers shrink
High latitude getting more sunlight in summer
41,000 years
at Perihelion
at Perihelion
Determines which hemisphere gets conditions suitable for glaciation
25,700 years
One More Point On This
• The orbital affects that Milankovitch suggested
as a partial cause for ice ages each have a
different period.
• They combine at irregular intervals
• Many glaciation peaks are about 100,000 years
apart, but that is ONLY an average. This
suggests that orbital ellipticity is important. The
worst glaciations occur when minimum tilt
coincides with maximum ellipticity.
Earth’s Past Ice Ages
Cenozoic
Mesozoic
Paleozoic
Paleozoic
• Tertiary to Quaternary cooling – Pleistocene
• None in Mesozoic
• Late Pennsylvanian & Permian
in southern continents (Gondwana)
• Ordovician glaciation (Gondwana)
– Area that is now the Sahara at South Pole
PreCambrian
• PreCambrian Tillites (Lithified Till)
– Three, maybe four,episodes
– Oldest 2.8 bya
– 750 mya ice from poles to tropic “Snowball Earth”
Permian Glaciation – Gondwana Tillites
Cenozoic Cooling
Southern Ocean forms
Central America Forms
Foraminifera
tests
Ice
Age
Warm
Cold
Wisconsinan
Illinoian
About 30 pulses in
4 or so major
groups
Kansan
Nebraskan
3. Also spiral direction &
diversity depends on Temp.
1. Evap. water and CO2
removes 16O from oceans
18O left in oceans
used to make shells
2. Ice traps CO2
and water with light
oxygen
Continuous Ice Sheet 20 kya
Scoured 30 M below sea-level
Global Temperatures Cycle, largest
100,000 Year (orbit eccentricity)
Smaller signals about 25K and
Interglacials get really WARM
41K
http://en.wikipedia.org/wiki/Little_Ice_Age
The Holocene
• Latest retreat began 10,000 years ago
• Climate varied. Hot 8000 – 4000 ya
• Medievil Warm: 800 – 1200 AD
•
•
•
•
Little ice age 1200 AD until 1850
Widespread Famine
1253 Pueblo cliff-dwellings abandoned.
1340s AD Black Death Bubonic Plague
Holocene Temperature Variations
Mann’s Hockey Stick
+1
2004
o
C
Water
freezes
-1
12
6
Thousands of Years BP
0
http://en.wikipedia.org/wiki/Image:Holocene_
Temperature_Variations.png
Sustained warming since 1850
Athabaska Glacier, Columbia Icefield,
W. Canada
2005 Greenland glacier
retreated 9 miles!
Antarctic Glaciers started
melting this season