Glaciers and ice ages (chapter 12)

Two main themes:

The first variety is found in mountain regions, is incised into and guided by the surrounding terrain
The second variety covers the entire landscape and has the potential to obliterate pre-existing toography; radial outward flow; maximum extent may be continental in scope and maximum thickness may be > 4 kilometers
subsidiary forms: ice caps and outlet glaciers, ice shelves, piedmont glaciers

accumulation of snow, persistent sub-freezing temperatures throughout year
gradual compaction and transformation of snow: granular snow to firn to glacial ice (final transformation only when thickness > 50 meters)

ability of glaciers to move depends on the mechanics of glacial ice, behaving partly as a solid and partly as a fluid: combination of basal slip and internal plastic flow (only when thickness exceeds 50 meters)
brittle upper zone (top 50 m) and crevasses
spatial patterns of flow and velocity distribution (p.332-334); changes in velocity over time include surges (normal velocity from <1 to several m/day; surge velocity from >10 up to >50 m/day)

concept of glacier budget and transition from zone of accumulation (above/uphill from the snowline) to zone of ablation or wastage (below or downhill from the snowline)
net growth occurs when volume of accumulation at upper end (above the snowline) is greater than volume of wastage or ablation at lower end
retreat of the glacier front occurs when melting in lower zone of ablation exceeds the supply of ice coming from the upper zone of accumulation
the ice itself always flows forward/downslope even when the glacier terminus is retreating

ice can freeze around an obstruction, pluck it out of bedrock, use it as a tool to gouge bedrock creating striations, grooves, chatter marks aligned with direction of flow
erosion occurs both by plucking and abrasion; how are they different?
erosional landforms:

formation of cirques - freeze-thaw of infiltrating meltwater, plucking and abrasion to form steep headwalls, rounded basins
retreating headwalls form horns and aretes (this type of topography forms only under influence of glacial ice, not running water)
a string of shallow depression in floor of glacial trough may be filled by a series of pater noster lakes
U-shaped glacial troughs are distinctive from v-shaped valleys formed by fluvial incision; fjords are formed by drowning of glacial troughs as sea-level rises, or by erosion of rock floor of trough by glaciers extending below sea level
convergence of valley glaciers at tributary junctions - formation of hanging valleys because valleys under the thickest part of the glacier may be eroded more deeply than under the thinner glaciers coming from tributary valleys
streamlined erosional knobs of rock are roches moutonnees; smoothed and abraded by ice flow over upstream side and plucked on downstream side
dramatic, sharp-edged topography caused by erosive action of valley glaciers; more subdued topography caused by combined scour and depositional action of large ice sheets

glacial drift is a generic term for all forms of glacial sediment no matter how or where deposited; areas affected by continental glaciers may have muted topography caused by burial of pre-existing landscape features underneath glacial sediment
rock flour formed by scour of bedrock
till (unsorted sediment transported by and deposited directly from glacier)

lateral and medial moraines, end moraines, terminal moraines;
ground moraines
glacial erratics: boulders transported and deposited in an area after being carried from original source area

outwash plains: formed by sediment transported and redeposited as glacial meltwater emerges from the glacier front

abrasion and striation of bedrock
removal of weathered mantle or regolith (e.g. broad areas of exposed bedrock in Canadian shield)
formation of till plains or outwash plains with streamlined landforms such as drumlins, molded by glacial ice, or eskers, formed by subglacial meandering streams
kames and kettles, formed by masses of stratified drift left behind along stagnant ice margins

late Cenozoic - Pleistocene development of alternating glacial and interglacial episodes
for an excellent review of Pleistocene climate history, see "Anatomy of a glacial age" from Carleton College
recognition of evidence of the glacial periods or Ice Ages by Louis Agassiz in 1800s
periodicity of glacial episodes: traditional view of only four distinct Pleistocene glacial episodes, now recognized to include >20 glaciations for entire Pleistocene, period of ~100,000 years for at least the past 800,000 years (based on continuous records from deep-sea sediment cores)

evidence from oxygen-isotope ratios in deep-sea sediments and ice cores [box 12.4, p.356]
trapped air bubbles preserving evidence of changing CO₂ levels in the atmosphere [box 12.4]

note maximum global extent of Pleistocene glaciation (p. 352); existence of Antarctic ice sheet predates the Pleistocene, extending back perhaps to 14 million years
eustatic changes in sea-level with advance and retreat of glaciers; exposure of continental shelves, Bering land bridge as a result of glacial advance and dropping sea level; vs. flooding of continental shelves and mouths of river valleys, formation of estuaries and drowned shorelines with melting of glaciers and rising sea level
subsidence of earth's crust under the weight of glacial ice and isostatic rebound when the weight of the the ice is removed
formation of pluvial lakes under cooler, wetter climates in parts of otherwise arid continental interior
occurrence of catastrophic glacial outburst floods - when large interior lakes trapped behind ice dams were suddenly released as the ice retreated and the dam broke (not mentioned in the book, but see the following web sites if interested):

Astronomical theory for Pleistocene climate fluctuations: Milankovitch cycles and the effects of variations in earth's orbit as indicated by changes in eccentricity, tilt or obliquity, and precession; effect on atmospheric balance between incoming solar radiation and outgoing longwave radiation, in turn affecting global temperatures; predicted periodicity matches record indicating changes in global temperature and global ice volume based on sediments from deep-sea cores
Long time interval between Pleistocene and previous time periods of continental glaciation (about 600 million and 2 billion years ago) may be related to rare occurrence of supercontinents at high latitudes as the tectonic plates move around like puzzle pieces on the globe (see p. 357, fig. 12.33)