Wrap Up

Review of the main topics and ideas you encountered in Unit 7.

Ice Is Nice: Yosemite, Glacier, Rocky Mountain, Bear Meadows, and NE Greenland

• Glacier = pile of ice and snow that flows
• Forms if snow exceeds melt enough to make a pile
• Takes water (as ice) and sediment from accumulation zone (snow exceeds melt) to ablation zone (melt exceeds snow) or to calve icebergs
• Flows in downhill direction of the upper surface (where ice meets air), even if that means the bottom flows uphill
• Think of pancake batter flowing on a waffle iron

Slip Sliding Away

• Glacier moves by deformation within ice, and if bed warmed to freezing point, by sliding over substrate or deforming sediment there
• Most deformation deep, but top fastest because rides along on deeper layers
• Ice deforms because almost hot enough to melt
• Glaciers erode by plucking rocks loose, sand-papering bed, and by subglacial streams
• Thawed-bed glaciers, especially those with surface meltwater reaching the bed, change landscape more rapidly than streams, etc.

Ages of Ice

• Recent (about 20,000-year-old), unique glacier tracks across broad areas now far from ice suggest past ice age(s)
• Ice-age hypothesis predicts land rising where ice was, sinking around, and that is indeed observed
• Ice-age hypothesis predicts sea level was lower when ice big, and indeed observe dead shallow-water corals of that age in growth position deep, flooded river valleys, etc.

Ice-Age Records

• Isotopically lighter water evaporates more easily
• Bigger ice-->isotopically heavier ocean and shells
• Shell-isotopic history from ocean-mud cores shows biggest ice every 100,000 years, smaller wiggles about 41,000 and 19,000 years apart
• Predicted by Milankovitch before observed--these are wiggle-spacings in Earth’s orbit
• Ice grows globally when little northern sunshine
• Orbitally changing sun controls northern ice, which affects CO², which controls southern ice

Bear Meadows

• Ice sheets today cover about 10% of land area; at height of ice age covered about 30% of modern land; central PA just beyond edge of Canadian ice
• Rocky Mountain, coastal NE Greenland National Parks have permafrost--soil at some depth frozen year-round
• Permafrost freeze-thaw and enhanced creep (summer melt can’t drain down, so soil soggy and creep easy) make distinctive features
• Those features exist but are not forming in central PA
• So, we were really cold in the ice age

Glacier Tracks

• Abrasion (sandpapering) under ice makes striae (scratches) and polishes rock
• Smooths upglacier, plucks downglacier sides of bumps
• Glaciers make valleys with “U”-shaped cross-sections, often with side-valley floors hanging above main-valley floor; streams make “V” shape without hanging valleys
• Glaciers gnaw bowls called cirques into mountains
• Glaciers deposit all-different-size-pieces till and washed-by-meltwater outwash, often in outlining ridges called moraines