Geology of the National Parks

Main Topics, Unit 7

When we try to pick out anything by itself, we find it hitched to everything else in the Universe.
— John Muir, My First Summer in the Sierra, 1911, p. 110

Overview of the main topics you will encounter in Unit 7.

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

  • A glacier is a pile of ice and snow that flows.
  • It forms if snowfall exceeds melting by enough, for long enough, to make a big enough pile.
  • A glacier takes water (as ice) and sediment from the accumulation zone (where snow accumulates faster than it melts) to the ablation zone (where melting, also called ablation, exceeds snow accumulation) or to calve icebergs.
  • A glacier flows in the 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

  • A glacier moves by deformation within the ice, and if the bed is warmed to the freezing point, by sliding over the glacier's bed or deforming the sediment there.
  • Most deformation in the ice of a glacier is deep, but the top of a glacier moves fastest because it rides along on the deeper layers.
  • The ice of a glacier deforms under stress because the ice is almost hot enough to melt.
  • Glaciers erode by plucking rocks loose, sand-papering the bed, and through the actions of subglacial streams.
  • Glaciers with thawed beds, especially those with surface meltwater reaching the bed, change the landscape more rapidly than is typical for landscapes shaped by streams, wind or mass movement.

Ages of Ice

  • Recent (about 20,000-year-old), features known to be made by glaciers and no other processes are observed now across broad areas of the Earth where glaciers do not occur, suggesting that we have had an ice age or ice ages in the past.
  • The hypothesis of past ice ages predicts that the land now should be rising where the ice was, and sinking just beyond where the ice was, and these are indeed observed.
  • The hypothesis of past ice ages also predicts that sea level was lower when the ice was big, and indeed we observe dead shallow-water corals of that age in growth position in deep water on the sides of oceanic islands, flooded river valleys, etc.
  • The success of the ice-age hypothesis in predicting things that have since been observed, and the failure of other hypotheses to do so, give us high confidence that ice ages did occur.

Ice-Age Records

  • Isotopically lighter water evaporates more easily.
  • An ice sheet is formed from water that evaporated, mainly from the ocean, and then snowed, so during an ice age the water remaining in the ocean should be isotopically heavier than observed today.
  • And isotopically heavier water gives isotopically heavier shells
  • The history of the isotopic composition of shells recovered from cores of mud from the ocean floor shows that the ice grew and shrank, with the biggest ice every 100,000 years, and smaller wiggles in ice size about 41,000 and 19,000 years apart.
  • These exact timings were predicted by Milankovitch decades before they were observed, because they are the timings of features in Earth's orbit that control the distribution of sunshine on the planet.
  • Ice has grown globally when the far north was getting relatively little sunshine, especially in midsummer.
  • The rest of the world has cooled when ice grew in the north, even though parts of the world were getting extra sunshine, and the world has warmed when ice melted in the north, even when some places were getting less sunshine.
  • This seemingly bizarre behavior occurred because the changing ice and other features of the climate changed atmospheric CO2, which rose when ice melted and fell when ice grew, and the CO2 controlled global temperatures.

Bear Meadows

  • Ice sheets today cover about 10% of the land area; at the height of ice age the ice sheets covered about 30% of modern land; central PA was just beyond edge of Canadian ice.
  • The high parts of Rocky Mountain and the coastal parts of the NE Greenland National Parks are among the places that have permafrost—soil at some depth is frozen year-round.
  • Permafrost regions are especially affected by freeze-thaw processes that break rocks, and by rapid downhill soil creep (during the summer, the meltwater can't drain downward through the frozen soil beneath, so the soil gets very soggy and creeps easily).
  • These characteristics of permafrost regions contribute to formation of distinctive features, which are observed in places such as central Pennsylvania that were just south of the ice-age ice sheets but which are not forming there today.
  • This shows that central Pennsylvania, and other such regions just south of the ice-age ice sheets, were really cold during the ice age.

Glacier Tracks

  • Abrasion (sandpapering) under ice makes striae (scratches) and polishes rock.
  • Abrasion smooths the upglacier sides of bedrock bumps, while the glacier plucks blocks loose from the downglacier sides of bumps.
  • Glaciers erode valleys to give them “U”-shaped cross-sections, often with the floors of valleys coming in from the side left "hanging" above the floor of the main valley; streams make “V”-shaped valleys without hanging valleys.
  • Glaciers gnaw bowls called cirques into mountains.
  • Glaciers deposit till, which contains rock pieces of all different sizes.
  • Melting glacier ice also feeds streams that deposit outwash (because it is washed out of the glacier); the pieces in outwash are sorted by size (mostly sand here, mostly gravel there).
  • Till and outwash often form ridges called moraines that outline the glacier.