Glaciers by 8x7c5J

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									Glaciers
• Glaciers are piles of snow that flow under their own weight.
• They are capable of enormous erosion and deposition, mainly because of the near-lack of
  limits on transportation and their close association with freeze-thaw cycles that produce much
  physical weathering.
• Many Northern landmasses were sculpted by glaciers.




    v 0023 of 'Glaciers' by Greg Pouch at 2011-02-21 16:48:10 LastSavedBeforeThis 2011-02-21 16:47:08
Glaciers
Processes
  3 Glacial Processes > Rheology
  4 Glacial Processes > Formation
  5 Glacial Processes > Flow
  6 Causes of Glaciers
Products
  Erosional
    7 Erosion > Continental
    8 Erosion > Alpine
    9 Diagram of alpine erosional features
    10 Paternoster and Rosary
  Depositional
    11 Deposition > Materials
    12 Deposition > Features
    13 Continental Deposition Figures II
14 Meanwhile, back in the tropics…
Glacial Processes > Rheology
• A glacier starts moving when
  somewhere in the glacier,
  stresses exceed the plastic limit
  of snow at that P-T.
• The upper part of the glacier is
  elastic and brittle. Crevasses
  form in the upper part and do
  not continue into plastic part
• The lower part of the glacier
  deforms ductilely (plastically
  or viscously) in response to
  ice-top elevation.
• Glaciers are plastic or very
  viscous, and can transport
  enormous quantities of
  sediment, without regard for
  the size of the particles.
Glacial Processes > Formation
• Stacks of snow (glaciers) form where more snow falls than
  melts/evaporates. Glaciers are favored by abundant snowfall
  and low evaporation and melting rates, as is found at high
  elevations and in polar regions, especially near oceans. Most
  glaciers are not terribly cold.
• Snow and Ice When snow falls, there are usually intricate
  crystals with lots of air (powder). During diagenesis and
  lithification, pressure solution of points and compaction result
  in coarse granular corn snow. More pressure solution and
  cementation result in firn. Finally, once most of the air is
  driven out and crystals starts getting bigger, it becomes glacial
  ice, which is a lot like schist.
• The region where more snow falls than melts is the zone of
  accumulation (clean, white, snow), and the region where more
  snow melts than falls is the zone of ablation (dirty snow). The
  line separating these two zones is called the snow line, and this
  shifts with the weather.
• Glacial ice is mainly compact grains, and often has rock
  particles entrained. It is a lot like snow-banks in a parking lot,
  especially after a few days of warm weather.
Glacial Processes > Flow
• Snow is like mashed potatoes: a small bit of
  it is very much a solid, but in a big enough
  pile, it flows like a fluid or plastic. (By the
  way, rocks are also like mashed potatoes in
  this sense.)
• A glacier is like mashed potatoes being
  dumped over some area (analogy in trouble)
  and oozing out from there, with mice eating
  them. How far the potatoes get is
  determined by how fast the potatoes are           Fig. 12.9b

  coming in and how fast the mice eat them.
  If there are lots of mice, the outermost part
  of potato glacier recedes, even though the
  potatoes are moving away from the potato-
  source-region. Similarly, a glacier is a
  balance of snow accumulation and ablation.
  Even when the glacier's terminus is
  retreating, the ice itself is advancing from
  the zone of accumulation.
• The main force driving movement is height
  of the upper surface of the glacier. Ice flows
  from points of high ice-top to low ice-top,
  which might not be related to ice-bottom.
  Pressure solution as well as crystal
  deformation are significant in ice flow.
Causes of Glaciers
                                                            http://en.wikipedia.org/wiki/Milankovitch_cycles
• In the recent past, there have been four
  major ice ages with warm inter-glacials
  between. This happens with surprising
  regularity. There have been a three other          http://eob.gsfc.nasa.gov/Library/
                                                     Giants/Milankovitch/milankovit
  glacial epochs further in the past we know         ch_3.html


  of. (The odds of preservation seem low, so
  there could be others we don't know about.)
• Milankovitch cycles (variations in
  insolation due to perturbations in earth's tilt,
  ellipticity and timing of solstices relative to
  perihelion) cause observable variations in
  sedimentation through much of the rock
  column. They correspond well with ice
  ages, but don't explain the lack of glaciation
  during most of earth history
• Circum-polar seas Presently, there is a
  nearly land-locked polar ocean, and circum-
  polar continent. Other glacial ages also
  seem to have had a land-locked polar ocean.
  This seems to be the other necessary
  condition. This is probably related to the
  efficiency of circulating heat on a planetary
  scale. (During times when there was easy
  circulation from poles to equator,
  temperatures seem to have been fairly even.
Erosion > Continental
 • Continental glaciation
 This refers to glaciers that cover big areas (ice caps) or huge areas (ice sheets) and are not confined by
     channels. They flow under their own weight with little regard to underlying topography. They do not leave
     spectacular scenery.
        –Polished surfaces are due to abrasion by fine glacier sediment
        –Striations are grooves cut by large, resistant chunks that are transported by glacier.
        –Chatter marks are crescent holes that look like a chip was knocked out with a hammer.
        –Removal of regolith and soil and transportation towards terminus.
 These erosional features are also common in alpine glaciation.
 • Because glaciers eventually retreat, the erosional areas often have later depositional features, so erosional
     features with no depositional features are rarely seen.
 • Continental glaciation erosion features are common in most of Canada, WI, MN, UP of MI…
Fig. 12.13
Erosion > Alpine
• Alpine Glaciation refers to glaciers that form at high altitude in mountain
  chains. Under these conditions, the glaciers move in stream-like fashion
  down channels, widening and deepening them.
   –Cirque: a semi-circular depression formed in the upper-most reaches of
    glacial valley. It looks like an amphitheater, or the hole left by an ice-
    cream scoop. This is a basic features in alpine glacial terrain.
   –Glacial Valley/U-shaped valley: a steep-walled, flat bottomed valley
    formed by glacial erosion.
   –A truncated spur results where a ridge gets cut off by a valley glacier.
    Glaciers don't zigzag as easily as water, and tend to sort of bulldoze
    through hills rather than follow the path of least resistance.
   –Arête: a sharp ridge separating two glacial valleys. The rock left after a
    bunch of cirques are removed
   –Côl: a low pass in an arête. Two cirques intersecting through an arête.
   –Horn: a sharp peaked formed where cirques have cut away.
   –Tarn: a rock-bottomed lake occupying a depression caused by glacial
    erosion. A tarn is found in a cirque.
   –Paternoster lakes: a string of usually-circular lakes connected by streams
    in a glacial valley.
   –Hanging valley: a valley whose elevation is higher than the main valley it
    empties into. (Glaciers flow according to ice-top elevation, not ice-bottom.)
   –Fjord/fiord: a U-shaped valley that is now filled with seawater, occurs
    where a glacial valley has been submerged. Common in Norway and the
    UK and Canada’s maritime provinces..
Paternoster and Rosary
•"Pater noster" is Latin for "Our Father". Medieval Swiss thought that paternoster lakes looked like the Our Father/Mystery beads on a rosary.
Figures from http://www.yoyita.com/como_rezar_el_rosario.htm
Deposition > Materials
• Materials                                               Mommy Rock, Daddy Rock and Baby Rock at Yosemite


   –Till is unsorted debris deposited by a glacier, and
    is largely unstratified. The co-occurrence of
    pebbles and clay, and rather dense, massive clay,
    are indications of till.
   –Rock flour is the finely-pulverized material in
    till: It often becomes wind-deposited loess. (High
    winds and vast exposed floodplains are common
    with glaciers: the ice melts vary seasonally and
    diurnally, and the temperature contrast causes
                                                           Fig. 12.22




    high winds, so eolian re-working of glacial
    sediments is very common.)
   –Any rock that is moved far from its point of origin
    by a glacier is a glacial erratic These can be as
    big as buildings, or as small as pebbles, although
    the term is usually reserved for boulders and
    bigger.
   –Outwash is the coarse material resulting from re-
    working of till by meltwater. The fines are carried
    off by water or wind. Outwash is coarse,
    permeable, and stratified. The streams of
    meltwater are usually braided and often leave
    extensive outwash plains.
   –Because glacial streams and lakes are strongly
    influenced by weather and melting, they deposit
    sediments with strong annual variations in
    composition and size known as varves.
 Deposition > Features                                                         Fig. 12.25




• Any deposit of till on, or left by, a glacier is a moraine.
    –Lateral moraines form at the sides of valley glaciers. When they
     merge, they become medial moraines alpine
    –An end moraine is a hill that results when the glacier’s advance is
     steady for several years. A terminal moraine is the one furthest
     out, and all the ones inside of that are recessional.
    –A ground moraine is a thin, extensive deposit, also known as a till
     plain. These have rolling topography, and most of the Midwest is
     till plain covered with loess.
    –Sometimes, a chunk of ice melts slowly and gets separated from
     the glacier and deposition occurs around it before it melts. The
     hole that is left behind is known as a kettle. Most ponds and
     small lakes in the Midwest are kettles
• Drumlins are hills that are steep in the up-ice direction, symmetrical
  left-ice to right-ice. They look soft of like blue-whales pointing
  northish. They are composed of till (the drumlins, not the whales)
• In-Glacier streams There are streams underneath, in, and on top of
  glaciers, mostly underneath.
    –The meandering, sandy channel deposits are known as eskers:
     this is basically a sandy ridge that meanders like a stream.
    –If there is depression in the top of a glacier, sediment deposits in it
     and the resulting hill left when the glacier melts is a kame.
Continental Deposition Figures II
Meanwhile, back in the tropics…
• Ice ages have profound geological effects, even far away from the glaciers. The climate is generally wetter
  and a bit cooler and more even.
• Huge quantities of water are tied up in the ice, resulting in decreases in sea level and exposure of much
  continental shelf as dry land. Many continental shelves were exposed during the last ice age, and
  continental sediments are found offshore.
• An ice sheet weighs a lot and depresses the underlying plate. When it melts, the plate gradually rebounds.
• Many areas that are currently desert received much more rainfall, resulting in extensive lakes, such as Lake
  Bonneville in Utah (Great Salt Lake is the only remnant). Many rivers that are fairly minor now carried
  huge amounts of rainfall, and much water infiltrated into aquifers that now receive little recharge, such as
  Egypt’s Nubian Sandstone.
Glaciers                                           From http://gsc.nrcan.gc.ca/landscapes/details_e.php?photoID=670




 •   Glaciers provide some of the most breathtaking (rugged erosional) and some the most
     uninteresting (flat and bumpy depositional) landscapes in the world. Many Northern
     landmasses were sculpted by glaciers. The recent glaciations locked up enough water to
     significantly affect sea-level, and so had major effects away from the glaciers.
 •   Glaciers generated a huge variety of landforms, both erosional and depositional.
 •   They are capable of enormous erosion and deposition, mainly because of the near-lack of
     limits on transportation and their close association with freeze-thaw cycles that produce
     much physical weathering. Glaciers have moved truly amazing amounts of material
     (100+m thick till in most of the Midwest) in a 'short' time

								
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