Glaciers (Part 1)

Long term ice ages

Every 250 million years

Quaternary

Includes Holocene (includes today) and Pleistocene (Last Glacial Maximum)

Precambrian (Proterozoic)

Snowball Earth (global evidence for glaciation)

Weathering of silicate rocks

Removes CO2 which cools the Earth (out of snowball by volcanism)

Gondwana

In Permo-Carboniferous period, Supercontinent cluster at equator (glaciated)

Tillite

Has become a solid rock, fine grained with large rocks within, multiple rock types indicates glaciation

Cenozoic

Last 66my, early Eocene Climatic Optimum, return of Antarctic ice sheets, Includes Pleistocene

Climate changes due to:

solar variations, volcanic dust, aerosols, albedo, silicate rock weathering, continent location and elevation, tectonics

Tectonics

Considered cause of Cenozoic ice age (lots of fresh weathering material)

Ice ages

Long buildup of ice to an “instant” termination

Cause of ice ages

Orbital forcing

Milankovitch Cycles

Eccentricity (orbit shape, 100,000yr), Tilt (obliquity, 41,000yr), Precession (wobble around the axis, 19,000-23,000yr)

Problem with orbital cycles

Does not explain a global synchronous ice age, asymmetry (long buildup with fast termination)

James Croll

Wanted to combine the solar variation cycles, chose winter as most important month (wrong)

Milankovitch

Chose summer as most important, need to have a very cold summer in Northern Hemisphere for ice age

Oxygen Isotopes supporting ice ages

Heavier oxygen stays in ocean, lighter oxygen gets trapped in ice, causes high concentrations of oceanic heavy oxygen

Last Glacial Maximum

18000-24,000yr, global temps 6-7 degrees colder than now, huge ice sheets, Laurentide, Cordilleran, Inuitian, Barents-Kara Sea, Fenno-Scandinavian, Andes, New Zealand

Laurentide Ice Sheet

From Canada to Northern US, responsible for about 70 meters in sea level rise, two domes

Inuitian and Cordilleran Ice Sheets

Other LGM North America Ice Sheets

Katabatic winds

Extreme winds that come off a glacier, cold air piles up on ice sheet and flows down due to gravity

U-shaped valley

Evidence for past glacial extent, striped of sediments

Marine-based ice sheet

resting on continental shelf below sea level, very unstable due to warming waters

Moraine

Ridge formed from glacial deposits at edge of ice sheet, marks past glacial extents

LGM sea level change

120 meters of change (70 from Laurentide)

Little Ice Age

Biggest glacial expansion in Holocene, only 1 degree colder but huge effect, westerlies intensification, (1250-1870)

Healthy glaciers

big bulging front with evidence of advancement

Trimline

Erosion changes or change in vegetation (abrupt changes), marks past glacial extent

Little Ice Age causes

Solar variability (coincides with sunspot minima), ocean circulation, volcanism (only 1-2 year effect), people (contributed not cause)

Antarctic Ice Sheet Importance

controls most of global sea level

West Antarctic Ice Sheet

Marine-based, smaller and thinner, natural tendency to float, inland sloping bed, accelerated calving from higher sea levels and warmer water and less sea ice

Grounding Line

Where the ice sheet changes from being grounded to becoming an ice shelf (floating)

Ice streams

bodies of fast-moving ice, glaciers imbedded in ice sheet

Pinning Points

high areas of the bed that stabilize, causes retreat to stop or slow

Ice shelves

Floating in water, collapse of shelves=faster glacial retreat

Controls of glacier locations

elevation, latitude, precipitation, topography, aspect

Classification schemes

topography, geography, terrestrial or marine-based, temperature (thermal regime)

Topography classification

Unconstrained (flows over everything) and constrained (feels the bottom)

Unconstrained

ice sheets, ice caps, outlet glaciers, ice shelves, ice streams, domes, ridges

Domes and ridges

Domes (high areas with flow outward radially), Ridges (connects domes, ice divides, flow in different directions)

Ice streams

fast moving sheet flow, bound by surrounding ice

Outlet Glaciers

Fast moving sheet flow, bound by rock walls

Ice shelves

Floating portion of glaciers, very flat on top, unstable

Nunatak

islands surrounded on all sides by ice

Distributary lobes

side lobe of a glacier, little shootout

Valley glacier

Glacier in a valley

Cold bed glacier

not picking anything up, very clean glaciers since no debris

Polynya

permanently/semi-permanently open water, kept open by katabatic winds, extremely productive and formation of deep waters 

Ice caps

Unconstrained, smaller than sheets

Ice fields

Can see underlying topography

Fjord/Tidewater glacier

valley glacier that outlets into the ocean

Piedmont Glacier

coming out of a bottleneck in mountain, spreads out in a lobe

Alpine glacier

small glaciers on mountain tops

Cirque glacier

bowl shaped impression on top of mountain (also an alpine when flow down mountain)

Reconstructed glacier

other glacier avalanches/falls down to feed another seperated glacier

Glacier formation

Snow → firn (compacted to crystalys) → glacial ice

Mass Balance

difference between accumulation and ablation

Accumulation

adding mass, snowfall, rime ice, avalanche, drifting, basal freeze on (regelation), superimposed ice

Ablation

loss of mas, sublimation, melting (radiation) (basal melting from geothermal, pressure increases, warm ocean, friction), calving, wind scour, avalanche

Equilibrium Line Altitude

line where accumulation zone becomes ablation zone, very close to zero degree isotherm

Measuring mass balance

ablation stakes, snow pits, laser altimetry, satellites, radar, snowline at end of summer

Accumulation Area Ratio

Accumulation is generally about 60% of total area of a glacier (helps for reconstruction if U-shaped valley or moraines)

Glacial flow

requires 30-50 meters of snow

Extrusion flow

Max Demorest, like pancake batter flowing out from top (correct), thought that more pressure = more squeezing out (wrong)

Actual Interior flow

Deformation of ice on bottom, piggybacks off underlying on way up to surface, gets faster and faster, move faster at surface than bottom, do not typically reach velocity of zero at bed (unless frozen at bed)

Moulin

vertical shaft kept open by water, starts as a crevasse

Melange

ice with sea ice around it frozen together like a jigsaw puzzle

Calving

breaks off from rest of glacier at crevasse, causes berg to rise up and flip, can cause big earthquakes when hits rest of glacier

Sheer stress (curvy t)

force per unit areas that causes deformation, typically at bed, =(density)(gravity)sin(surface slope of glacier)

Surface slope of glacier

Inversely proportional to ice thickness (thicker=flatter) (thinner=steep)

Creep (reversed 3)

Internal deformation, due to stress, small changes in stress=big changes in creep

Glen’s Flow Law

creep=(thickness parameter)(stress)^creep exponent(3)

Ice hardness

warm ice (softer=easier to deform), cold ice (harder=difficult to deform)

Internal Deformation

creep, folding (creep can’t keep up), fracture (extensional, makes crevasses)

Enhanced creep

Encountering an obstacle=stress increases=higher velocity

Regelation

obstacle encountered → pressure increases → melting → water goes to low pressure cavity → refreezes onto glacier

Pressure Melting Point

temperature at which ice melts given a pressure (1km of overlying ice=PMP at 1 degree C)

Temperate Glacier

at PMP throughout entire glacier, water throughout, wet based

Polar Glacier

No water throughout, can be wet based

Cold based glacier

no water the bed, PMP more negative that required for melting

Typical thermal regime 

Wet at center (highest pressure, scouring), wet freezing on either side (still wet but less pressure, intense erosion → plucking), wet melting on warmer side (deposition of debris), frozen on cold side (nothing happens here)

Meltwater

erosion, causes faster flow (basal sliding), drinking water, mostly surface meltwater

Basal Water

eliminates adhesion, decreases friction, smooths bottom, lubricant, can lift glacier

R-channel

Subglacial tunnel, carries sediment, creates eskers

N-channel

Subglacial channel, creates river in bedrock

Super crustal melting

Clear top of ice allows for a greenhouse like effect, zone of water

Superglacial melt

on top of glacier, thin stream going through ice

Subglacial lakes

water at bed due to immense overlying pressure and geothermal heat, Lake Vostok

Ice dammed lakes

dammed water by glacial ice

Outburst floods

destruction of ice dam or moraine dam, rapid tunnel enlargement, volcanism, self dumping (stays in positive mass balance)