Glacial geomorphology• Glacier: “a natural accumulation of ice that

is in motion due to its own weight and slope of its surface”

• Ice cores– Paleoclimate archive: high-resolution records

of climate change– Compared to deep sea core

Ice Ages Ice Ages Throughout Throughout

Geologic TimeGeologic Time

12°22° 17°Average Global Temperature (0C)

Ice Age

Ice Age

Ice Age

Ice Age

Figure modified after C.R. Scotese

PALEOMAP Project (www.scotese.com)

Quaternary

Karoo

Saharan

Sturtian: 750-700MyaMarinoan/Varangian: ended 635MyaGowganda: 2.3Gya

Pleistocene: 3Mya

C.R. Scotese, PALEOMAP Project, (www.scotese.com)

Last Glacial Maximum 18,000 years agoLast Glacial Maximum 18,000 years ago

Present vs. Past Glaciation

• Now – One major (Antarctica) and one minor (Greenland) ice sheets

• Then - At least three major (Antarctica, Laurentide, Fennoscandian) and numerous minor (Greenland, Cordilleran, Patagonian…) ice sheets

Milankovitch Cycles

Eccentricity90,000 to 100,000 years

Precession19,000 to 23,000 years

Obliquity (Axial Tilt) 41,000 years

Figures modified after Matt Beedle, Montana Sate University.

~5~5%%

~0%~0%

http://www.homepage.montana.edu/~geol445/hyperglac/time1/milankov.htm

How does snow become ice?• Deposition

– Reworked by wind• Destructive metamorphism

(orig. crystal becomes a rounded ball)

• Sintering (rounded grains fuse by freezing into larger crystals)

• Compaction/Cementation

How long does it take?

• It depends!– Antarctica – hundreds

of years– Greenland – 100 years– Temperate glaciers –

decades– Maritime glaciers -

years

As snow turns to ice, porosity decreases

Faster in wet snow

Ice density with no pore space = 917 kg/m3

Ice is derived from snow

Partially compressed/compacted snow is firn (snow/ice)

When a wedge of firn is thick enough to deform under its own weight and move downhill, it’s now a glacier

Snowline and Equilibrium Line Altitude

Mass Balance: Net Loss or Gain of Ice

x

Q

zWzb

t

H

1

H = ice thicknessW= glacier widthQ=ice discharge/unit widthb = local mass balance, m. of water/yrmass lost or gained over an annual cycle

Q represents losses due to ablation and sublimationacross the width of the glacierMore loss due to solar radiation than Earth’s heat

Mass Balance

if ablation dominates below the ELA, why are there glaciers below the ELA?

Ice in Motion

• If not in motion, not a glacier but a snowfield

• Motion by– Basal sliding– Internal deformation

• Q = U x H– Discharge per unit width– U, mean velocity– H, thickness

At steady state,

x

Q

zWzb

t

H

1

Ice Deformation: Internal Deformation (Force Balance for a Column of Ice)

W/A

Step 1: Resolve weight force into two components:

a normal force and a shear force

sinA

W

Body force of gravity acts upon the ice column

Step 2: Substitute for W, A, and sin

SzHg

gdxdy

dxdyzHA

gVolA

mgA

W

)(

sin)(

sin

sin

sin

Fluid Deformation

• Fluid deforms under its own weight

• As shear is applied, the ice is strained

• Strain rate is

0L

L

" viscosityeffective"dz

dUZ

U

xz

1

Strain rate

horiz vel

Ice Deformation

• How the ice responds to the stresses is determined by its rheology (rate and style of deformation under stress)

• Ice is non-Newtonian• As shear increases,

effective viscosity decreases such that ice is less “stiff” near the bed than near the surface

viscosity: resistance of fluid to deformation

Much more like “plug” flow

Ice Deformation: Highly Nonlinear

5

sin

sin

53

33

HgAQ

zHgAdz

dU

Ice discharge = f(slope and ice thickness)

S

Ice thickness

For a given slope, if the ice thickness increases by 15%, the ice discharge will DOUBLE

“flow law parameter”in Glen’s flow law

• Assumptions– basal shear stress = 0.8 bars = 8x104 Pa– glacier is wide enough that walls do not support ice– contours on the map show that the ice slope is 10m/km at

this location– density of ice = 918 kg/m3 and g = 9.8 m/s2

– 1Pa = 1

• Estimate the ice thickness H at this location• The basal shear stress is given by:

where i is the ice density, g is the acceleration due to gravity, H is the thickness of the ice, and S is the ice surface slope angle

• Rearrange:

H =8x104Pa

918 kg

m3( ) 9.8 ms2( )(0.01)

kgm

m2s2

gSH

gHS

i

i

=890 m.

Ice Deformation: Sliding

• Sliding occurs because high pressure promotes melting (in water)

• Down valley component of weight promotes motion– Resistance to motion due to pressure

variations from bumps in the bed

Pressure Melting

• For ice at PMP:– Movement increases pressure, thus melting,

on the up-ice side of an obstruction– Movement away from the obstruction causes

freezing on the down-ice side – “regelation”

meltmelt

Effects of Pressure Melting• High pressure is

experienced on the upice side of an obstruction.

• Pressure melt results

• Water migrates around/through obstacle

• Regelation occurs in low pressure zone

MELT REFREEZE

Regelation

Higher pressure on up-valley side of bumps than down-valley side of bumps

Ice melts on the stoss (high pressure) side, consuming energyMoves around bump as water filmRefreezes in the low pressure shadow (lee)Heat released by refreezing is conducted back to bump

Coupling of thermal and fluid mechanics

Recently deglaciated bed of Blackfoot Glacier,Glacier NP.Argillite (Belt rock) bed shows dissolution of limestoneon upstream side, and reprecipitation as calciteon downstream side (white areas)