XII) THE ICE AGE AND GLACIERS (CH. 19)
A) The Quaternary Period (the most recent "Ice Age"):
roughly 1.6 million years ago (mya) to the present
1. Quaternary Climate Change
a. Global
climate has been cooling since about 60 million years ago; rapid cooling
since 3 mya.
b. at
least 18 glacial/interglacial cycles have occurred since about 2 mya (the
Pleistocene Epoch)
c. glacial maxima
occur roughly every 100,000 yrs - last maximum was about 18,000 yrs ago
d. earth has been
in a warm interglacial period (the Holocene Epoch) since about 10,000
yrs ago
2. Causes of Quaternary climate change
a.
Milankovitch Cycles: changes in the shape of earth's orbit, tilt of axis,
and direction of axis tilt.
b. Movement of land masses into high latitude positions, increasing planetary
albedo (Arctic/Antarctica).
c. Tectonic development of high plateaus, increases albedo and fixes CO2
(Himalayas, Colorado Plateau).
d. Closure
of the Panama isthmus, diverting warm ocean water into high latitudes about
3 mya
3. Evidence for Quaternary Glaciation
a. Landforms & sediments created directly by ice sheets - glacial till,
moraines, etc. (collectively known as "drift")
b. Climate zones and Biomes shifted towards the equator - tundra in Appalacian
Mts., pine/spruce forest in southeast US.
c. Periglacial
activity - evidence of permafrost and frost weathering in near-glacier regions.
(too warm for such features today)
d. Sea level change - global sea level was about 400 ft lower at last glacial
max., rose rapidly as glaciers melted
e. Lakes - Large lakes existed in dry valleys due to decreased evaporation
or glacial meltwater (such as Great Salt Lake, Death Valley)
f. Rivers
- most river valleys in the northern US are filled with "outwash" - sand
and gravel from period of carrying glacial meltwater
g. Aeolian
deposits - large areas of active sand dunes and wind-blown dust ("loess")
dating to glacial periods
h. Archaeology
- evolution of homo sapiens, migration around the globe, development
of civilization occurred during the Quaternary.
B) Glacier Formation & Movement
1. Requires zone of accumulation,
snowfall > melting over many years (high latitudes, high elevations).
2. Snow accumulates, compresses
to ice, and removes air bubbles, about 90% as dense as water.
3. Ice at depths >60 ft begins
to deform under its own weight, flows as a plastic
4. Glacier bed is often melted
by the weight of overlying ice, providing lubrication to speed glacier flow.
C) Continental Glaciers
1. Erosional Features
a. Abrasion
features - grooves/striations carved in bedrock by debris carried along
glacier bed.
b. Plucking
features - bedrock hills abraded on up-ice side, rocks plucked from down-ice
side.
2. Depositional Features
a. Till
- material laid down along the bed of a flowing glacier; mixture of sand,
gravel, boulders.
b. Moraine
- hummocky ridge of material deposited at ice margin
i. terminal moraine records the farthest advance of the ice
ii. recessional moraines record stable margin during ice retreat.
c. Drumlins
- fluted/elongated hills and linear valleys aligned with direction of ice
flow.
d. Glaciofluvial
features - formed by glacial meltwater
i. eskers: gravelly linear ridges deposited by sub-glacial streams
ii. outwash plains: material carried away from the ice by meltwater streams
iii. kettle holes: lakes that formed where ice blocks melted in outwash
D) Alpine Glaciers
1. Erosional Features
a. Cirque
- bowl-shaped depression at head of a glacial valley, often occupied by
a tarn lake.
b. Horns
and aretes - jagged mountain peaks and knife-edge ridges, scoured by ice
on each side.
c. Glacial
troughs - U-shaped valleys scoured to cross section;
d.
Hanging valleys with waterfalls where smaller glacier joined a larger valley
glacier
e.
Fjords arm of the sea where a glacial trough was flooded by sea level rise.
2. Depositional Features: less
extensive than continental glaciers
a. moraines
along the lateral, medial, and terminal ice margins
b. outwash
plains filling valleys