Half Dome above Yosemite Valley' A glacier flowing to the lower right deepened and widened the valle

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Half Dome above Yosemite Valley. A glacier
flowing to the lower right deepened and widened
the valley see the next slide for a modern
example.
2
Glacier draining Greenland Ice Sheet into head of
Scoresby Sund, NE Greenland National Park. The
scale is similar to that in the previous picture.
Yosemites glaciers ended on land this one
calves icebergs into the fjord.
3
Glacially truncated cliff. The ridge on the left
was cut off by a glacier that reached at least as
high as the sunlit peak, and flowed over where
the photographer stands. Glacier National Park.
4
Glacially carved paternoster lakes, Glacier
National Park.
5
Glacially striated and polished bedrock, east
Greenland. Ice flowed up the cliff from the
lower right.
6
Glacier Bed

• Glacier moves by deformation within ice, and by
  ice moving over whatever is below
• Basal motion 0 to 100 of total
• Fast motion and rapid changes in motion (surges)
  mostly involve basal motion
• Almost all landscape modification at bed
• So, must understand bed to understand ice.

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The Glacier Bed

• If frozen to bed (much of big ice sheets, plus
  some small, thin, high mountain glaciers) not
  much happens--only very slow erosion and sliding
• If thawed at bed (some of big ice sheets, most
  mountain glaciers) much more action
• Happy to talk about basal T another time

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Thawed-bed, how move?

• Regelation/pressure-melting
• Enhanced creep (Weertman, 1957)
• Subglacial sediment (till) deformation (mostly
  newer work).

Ice flow
Regelation
Enhanced Creep
Rock
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AIR UP HERE
Drill a vertical hole
MODES OF ICE MOTION
ICE HERE
How hole looks later
Water where till meets ice
TILL HERE
ROCK BELOW TILL
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Regelation

• Higher pressure--gtlower melting point
• Upglacier side of bump has high pressure, low
  melting point, so melts
• Water flows around bump to low-pressure/high-melti
  ng-point side, freezes
• Heat from refreezing conducted through bump to
  upglacier side for more melting, so works well
  for little bumps with good heat conduction
• Can hang weights on a wire over an ice block,
  wire will go to bottom and leave a single block!

11
Enhanced creep

• Drop a pebble in water, and the pebble sinks
  water flows around the pebble as it sinks
• Push ice against bump, ice creeps around the
  bump, solid ice acting as a slow liquid (can talk
  about that sometime, too!)
• Big bumps affect much ice, little bumps only a
  little ice, so faster for bigger bumps
• Regelation fast for little bumps, enhanced creep
  for big bumps, in-between (few cm?) is probably
  hardest for ice to handle.

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Near ice-sheet edge, NE Greenland National Park.
The folds show that ice flows. Blue at top is a
meltwater lake such lakes may drain to the bed.
13
Corridoren Glacier, Greenland. The stripes are
medial moraines, rock debris picked up from
ridges where two tributary glaciers join.
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Till Deformation

• So far, measurements at beds of at least 10
  glaciers/ice sheets with till
• ALL show deformation of till
• Some deformation deep, some shallow, some
  continuous, some discontinuous
• So existence and behavior of subglacial sediments
  matters a lot
• Difficult we know more about ice than about
  sediments, and sediments are more variable.

15
Thixotropic clay. On the left, a core of clay is
clearly solid, supporting much weight. On the
right, a sample of the same clay has been shaken
WITHOUT ADDING ANY WATER AT ALL, and clearly
behaves in a very different way. Most soils
dont behave this extremely, but soil/till
mechanics is not easy.
16
http//landslides.usgs.gov/html_files/landslides/s
lides/slide11.htm Photograph by Terry Taylor,
Colorado State Patrol. Landslide near McClure
Pass, Colorado, 1994. The driver did not see this
nighttime slide in time to stop, but fortunately
was not injured.
17
http//landslides.usgs.gov/html_files/landslides/s
lides/slide21.htm Photo by R.L. Schuster, U.S.
Geological Survey. Spring, 1995 landslide at La
Conchita, California, south of Santa Barbara
along highway 101. Many homes were destroyed, but
no one was injured.
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Water water water

• Heat from Earth melts few millimeters of ice per
  year, ice-flow friction heat a bit more (but some
  heat may be conducted into ice so not
  melting)--less water to move sediment than
  anywhere else on Earth
• Heat from air melts few meters of ice per year in
  ablation zones--more water to move sediment than
  anywhere else on Earth
• Things a bit different where surface melt reaches
  bed than in other places.

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Zwally effect does matter to ice-sheet future (Par
izek Alley, 2004)
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Why water matters

• Lots of meltwater in ablation zones washes
  sediment away, so little or no till, so little or
  no till deformation (and, if till keeps ice away
  from rock, little erosion)
• Water floods bed bumps, smoothing bed and
  allowing faster ice motion
• Higher water pressure makes till softer, allowing
  faster deformation and so faster ice motion.

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Where is the water?

• If too little water, may escape downward from the
  glacier as groundwater flowing totally within
  rock and out of contact with the glacier (but
  this is not usual for ice sheets, which are so
  long that groundwater will run into a
  waterproof layer somewhere, and be forced back
  up to contact the ice)
• Water in contact with the ice must
• Have water pressure equal to local ice pressure
  or
• Have water pressure lower than the local ice
  pressure, but have ice melting away as rapidly as
  the ice creeps into the low-pressure region.

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Ice-pressure fluctuations

• Remember that flow over bumps creates
  low-pressure and high-pressure regions
• Water tends to the low-pressure regions
• Water pressure must be high enough to separate
  enough of the ice from the rock to allow the
  water to interconnect and drain (steady or
  nonsteady)
• This is probably within one atmosphere or so of
  the ice pressure (the weight of a 100-m-thick
  glacier is almost 10 atmospheres, of a
  3000-m-thick ice sheet is almost 300 atmospheres,
  and the water will float all but about 10 m of
  that).

23
Ice-melting

• Where water flow fast, turbulence heats
• When glacier bed nearly horizontal or falling in
  the direction of water flow, heat is more than
  enough to warm the water as the pressure-melting
  point rises, and the excess heat goes into
  melting ice (Röthlisberger channels)
• Lots of water can lower pressure by few
  atmospheres (surface melt, outburst floods)
• Not enough water from steady drainage of basal
  melt to affect pressure much.

24
Data support theory

• Mountain glaciers can have water pressures as low
  as a few atmospheres less than the ice pressure
• Where water contacts ice sheets, water pressure
  typically within tenths of an atmosphere of the
  ice pressure (Byrd, ice streams B, C, D,
  NGRIP)
• Change of a tenth of an atmosphere will matter to
  ice motion (till properties, etc.), but such
  accurate data almost impossible using normal
  techniques
• Seismic evaluation of till properties best
• And models valuable.

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Glaciers may modify land faster than rivers

• 3 m/yr (10 ft/yr) erosion of sediment under Taku
  Gl. (Nolan et al., 1995)
• 30 m/yr (100 ft/yr) sedimentation in front of
  Riggs Gl. (Powell, 1990)
• Sediment discharge equal to 1 cm/yr (1/2/yr)
  averaged over whole glacier common (Hallet et
  al., 1996)
• Many more big numbers.

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Glaciers can also do nothing to landscape, or
nearly so

• In places, Canadian ice-age ice blocked cosmic
  rays, and not much else
• Glaciers are seen retreating from undisturbed
  permafrost-patterned ground
• At 1 cm/yr erosion, Antarctic ice sheet would be
  into middle mantle by now instead, it rests on
  poorly lithified sedimentary rocks in many places.

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What controls these rates?

• If frozen to bed, erosion very slow (not zero, a
  little sliding, but very slow)
• If thawed at bed but with only basal meltwater,
  erosion still slow
• If surface meltwater supplied to bed, erosion can
  be very fast.

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Why is surface meltwater critical?

• If erosion occurs without sediment removal, till
  (fault gouge, sawdust) will develop and nearly
  stop erosion
• Earths heat melts only few mm/yr of ice (many
  years to melt an inch)
• Sun melts meters/year (lots of feet per year) on
  top, which flows to bed and washes it clean to
  let ice erode.

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Glaciers erode by

• Subglacial streams cutting rock (rare usually
  just sweep rock clean)
• Abrasionsandpapering by rocks held in basal ice
  (common)
• Plucking blocks loose (most important moves lots
  of rocks, and provides tools for abrasion).

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Debris-bearing basal ice (red arrow) or a glacier
in south Greenland. Rocks in such ice sandpaper,
or abrade, the bedrock beneath as the ice moves.
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Rock ptarmigan on glacially striated granite
(striae are faint lines on rock, a few of many
are shown by blue arrows), east Greenland.
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Ice Flow
Ice
Stress concentration
Rock
Water
Bedding/Jointing
Geometry for Iverson, Hallet plucking model
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Moraines around retreating glaciers, Alpe Fjord,
NE Greenland Natl. Park.
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Moraines around retreating glaciers, NE Greenland
Natl. Park.
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LOTS more to do!

• If water squeezed out of a subglacial hole, drop
  in pressure allows a little freezing, clogs
  channels, stops till removal in streams, which
  allows till to stop erosion
• So glaciers can dig only so-steep holes
• Matters to mountain belts, etc.

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LOTS more to do!

• The holes glaciers erode can trap lakes
• Which can eventually discharge catastrophically
• Carving canyons, changing climate, etc.

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LOTS more to do!

• Water flows to glacier beds down
  streams/holes/moulins
• But first a crevasse opens and then a stream
  forms along the crevasse
• Waiting for surface warming to warm the bed of
  Greenland may take 10,000 years
• Waiting for a new fracture to take water down to
  thaw the bed may take 10 seconds.

45
Calving front, S. Greenland glacier, with
meltwater refrozen in crevasses.
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LOTS more to do!

• To predict glacier changes, must know the
  distribution and properties of till, water, and
  basal roughness
• Roughness, water from radar just becoming
  practical(?)
• Seismics for till great, but more to do--heroic
  field work!
• Lab work on till ongoing including PSU
• A bit more modeling needed, too.