glaciers and glaciation - · PDF filedistribution of modern glaciers! mostly limited to polar...

glaciers and glaciation!

Photo credit: G. Mattioli

owe much of our current landscape to glaciation…!• Cape Cod, Yosemite Valley, Great Lakes!

• coastlines--Memphis might be a seaport!

• Missouri, Mississippi would flow north to Arctic!

map of world if sea level rose 200 m!

glaciers and glaciation!

last glacial maximum during Pleistocene (~18,000 years ago)!…ice covered 30% of Earth and was 4 km thick in places!

currently in interglacial period of warming that began ~10,000 years ago!

…glaciers at high altitudes and high elevations at mid-latitudes…!

distribution of modern glaciers!

mostly limited to polar regions!northern hemisphere sea ice is seasonal!Greenland ice sheet is “permanent” (10%)!Antarctic ice sheet is “permanent” (85%)!

glaciers at high elevations in mountains (5%)!

most of Earth’s fresh water is tied up in ice!~ 75% of the total fresh water budget!

northern hemisphere polar ice: maximum extent !

image source: NASA Jet Propulsion Lab!

Arctic Sea Ice Growth

Images taken 9 days apart. RadarSAT scenes are 60 x 80 miles. Brighter features are older ice and darker features are newly formed ice.

Baufort Sea, north of the Alaskan coast

glacier!large long-lasting mass of ice, formed on land! that moves down hill under its own weight!

• part of hydrosphere (tie up fresh water)!• with sea ice, form the cryosphere!

…begins with snowfall and accumulation of snowflakes…!…hexagonal shape of snowflakes keep them apart…!

initial snowpile is ~90% space (density is 0.1 g/cc--water is 1 g/cc)!

snow is compacted!and recrystallized!first into firn…!

firn: well-packed snow that!survives summer melting!

(density is 0.4 g/cc)!

formation of glaciers!

second into!glacial ice!

(density of 0.9 g/cc)!

glacial ice!

from: http://www.gly.uga.edu/railsback/FieldImages.html!

only form when snow accumulation > snow melting!

…cool summers likely more important than cold winters…!

transformation of snow to glacial ice may take!a few decades in snowy regions!

(high snowfall and re-melting to ice)!or!

thousands of years in cold polar regions!(little snowfall and little re-melting to ice)!

glacial budget!

positive = growing and advancing!negative = shrinking and retreating!

formation of glaciers!

difference between annual gain and annual loss!

Zone of accumulation

Glacier ice

Glacier ice ablated during melting season

Zone of ablation

Snow and firn

Equilibrium line

equilibrium line separates zones of ablation and accumulation!…may change location from year to year…!

formation of glaciers!zone of ablation!where melting and !calving occurs!

zone of accumulation!where snow is!

added!

collapse of glacier on!Mount Kilimanjaro!

glaciers also lose mass!through collapse!

calving!collapse of pieces into the sea…forms icebergs!

types of glaciers!• Alpine glaciers!

occur in valleys of mountainous regions: valley glaciers!

• continental ice sheet!flow freely and cover large expanses!(Antarctic ice sheet: 4.3 km thick)!

Alpine glaciers!

Alpine glaciers!

Greenland continental ice sheet!

Antarctica!

Trans-Antarctic Mountains: !mountains are ~ 15,000 feet tall, !but buried by > 10,000 feet of ice!

continental ice sheet development !

because continental ice sheets are white….!….dark meteorites are easy to see!

meteorites are moved with glacier and !concentrated along the Transantarctic Mtns.!

“South Pole”!

pole erected 1956 - !movement of ice sheet has displaced it from!

current geographic south pole several km away!

basal sliding and plastic flow!

glacier flow!all glaciers move down slope in response to gravity, regardless of whether they are receding or advancing!

bottom of glacier slides on film of water and !lower part flows!

rigid zone!upper part of glacier goes along for the ride!

crevasses: cracks that form in rigid part of glacier!

glacier flow!

basal sliding and plastic flow!

rigid zone!crevasses!

note that velocity is NOT THE SAME at the bottom and top!…friction slows velocity at the bottom (and along sides)…!

glacier flow animation!

explains why crevasses are rarely > 60 m deep!

…above 60 m depth, ice is brittle and can crack!…below 60 m depth, ice is plastic and flows (can’t crack)!

glacial crevasses!

glacial erosion!glaciers erode underlying rock by !

• plucking of rock fragments !• abrasion as fragments are dragged along !

what the base of the glacier looks like!iceberg tilted and fell into the sea…!…showing the bottom of the glacier !

abrasion polishes underlying rock, grinds some into rock flour!(fine grained material) and scratches bedrock, making!striations!

glacial erosional landscapes!erosional features produced by valley (Alpine) glaciers!

Cirque

U-shaped valley

aretes: ridges separating!glacial valleys!

cirques: steep bowls!carved by glaciers at!their heads!

horns: sharp peaks that!form when cirques are!have formed on 3 sides!of the mountain!

Arete

U-shaped valley: shape of valley that!glacier carves as it moves down slope!

cirque!

horn! arete!

U-shaped !valley!

Grand Teton!horn!

U-shaped valley!(Yosemite)!

erosion by one glacier!and then a second can!produce a “hanging valley”!

glacial erosional landscapes!continental scale!

long, deep troughs !fill with water!

Great Lakes!

Loch Ness! Finger Lakes, NY!

glacial deposition!till!

unsorted, unlayered glacial sediment!

moraines!accumulation of till on sides and fronts of glaciers!lateral moraines: long, low mounds on sides of glaciers!

end moraines: !ridges piled up at !front of glacier!

medial moraines: !lateral moraines trapped !between ice sheets!

recessional moraines: !successive end moraines!left by retreating glacier!

lateral moraine!

medial moraine!

glacial valley and moraines!

lateral moraine!

end moraine!

glacial deposition!liquid water!

flows over, beneath, and away from ice at glacier end!glacial outwash-- sediment deposited by this water!

streams that emerge !are braided!

braided glacial streams!

outwash features!

drumlin: moraine overridden !by a glacier to make a hill !

esker: sinuous ridge that forms beneath glacier as it recedes !

kettle: glacial lake !

outwash features!

drumlins!

“tunnel” in glacier!in which!

water flows!

fills up with!sediment!as glacier!recedes!to form!

sinuous ridge!

esker!

glacial erratic: large boulder eroded from one type! of bedrock and transported to another!

images from: http://mail.rochester.edu/~kl001i/landerk2.html!

drumlin and glacial erratic!

deposits are loess !(German for loose)!

glacial outwash!glacial material can be transported far from source!

glaciers reached Iowa; Mississippi River transported sediment south!

wind transports fine grained glacial material over wide area!

past glaciation !glacial ages!

• glacial events repeat through geologic time!

• most recent period peaked 18,000 years ago!

• Antarctic glaciated > 20 million years!

indirect effects of past glaciation!• pluvial lakes (lakes form during wet periods)!

• lower sea level (canyons on continental shelf)!

• coastal fjords!

extent of Pleistocene glaciers (brown lines) !

purple !are !

pluvial !lakes!

humid, cool climate of glacial period yields more rainfall!

precipitation >> evaporation!

lakes form !in landlocked basin!

…e.g. Lake Bonneville !in Utah, Nevada…!

…Great Salt Lake !is small remnant…!

pluvial lakes!

Varved Sediments

Seasonal Deposition Light color = silt (summer) Dark color = clay (winter)

fjords!

sea level lower during glacial period!…glaciers eroded onto continental shelf…!

glaciation in the geologic past!tillite: ancient till deposits!

" "--oldest is 2.2 billion years!

additional glaciations:!"500 million years ago!" "--northern Africa!"245-286 million years ago!" "--Australia, India, South America, South Africa!

no glaciation during Mesozoic!"--period of warming and movement of continents!" "toward equatorial regions!

at those times those continents sat on South Pole!(strong evidence for plate tectonics)!

Mechanisms for Past Glacial Periods •  Pleistocene glacial and interglacial episodes are correlated

with variations in the Earth’s orbit at 21 ka, 41 ka, and 100 ka. Discovered by M. Milankovitch in 1921.

•  General absence of glacial periods in the past is NOT explained by Milankovitch cycles.

•  Other mechanisms: –  Changes in CO2 content in the atmosphere (large volcanic

eruptions can lower global temperature). –  Movement of the continental landmasses by tectonics. –  Changes in oceanic circulation as a result of both atmospheric and

tectonic processes.

Milankovitch Cycles in Earth’s Orbit

Image Source: Scott Rutherford

Snowball Earth & Carbon Cycle

Figure from: Hoffman and Schrag (1999)

what will happen in the future?!• Pleistocene records suggest interglacials last ~10,000 years!

"--have been in interglacial about 10,000 years--!

what about global warming?!

increases in !atmospheric CO2 !

since !Industrial Revolution!

“greenhouse effect”!

are we on the verge of cooling?!

Receding South Cascade Glacier

Photo credits: U.S. Geological Survey

1957 1980 Loss of 18.7 M m3 of ice - Due to global warming?