Glaciers and Glacial Mechanics The Coolest Geomorphic System Matanuska Glacier, Alaska. Lachniet (1997)

What is a glacier? “Glaciers are masses of ice and granular snow formed by compaction and recrystallization of snow, lying largely or wholly on land and showing evidence of past or present movement.” Matanuska Glacier, Alaska. Foto: Lachniet (1997)

From snow to ice Snow falls as crystals onto glacier surface, – σ (density) = 0.07 to 0.18 g/cm 3. Firn: Crystals lose shape to become coarse snow via melting of edges – and pack more closely – σ = 0.55 g/cm 3 Glacier Ice forms as firn is buried and begins recrystallization – porosity decreases – metamorphic ice crystals increase in size – σ = 0.8 to 0.9 g/cm 3.

Snow Metamorphism Time of metamorphism varies by climate Cold, polar glaciers with low accumulation = slower periods (up to decades) Warmer, temperature glaciers with higher accumulation = faster (weeks)

Glacier Mass Balance Mass balance is the sum of all mass inputs (+) and outputs (-) – positive – the glacier expands – negative – the glacier retreats Equilibrium Line – Zone between accumulation and ablation is the – Equilibrium Line Altitude (ELA) rises with negative mass balance and vice versa

Accumulation Accumulation is concentrated at high elevations and more polar latitudes – Snowfall – Avalanches – Rain that freezes on glacier – Superimposed ice: re-frozen snowmelt – Blowing snow

Ablation Ablation is the loss of mass – Melting – Evaporation (sublimation) in semi-arid climates – Wind erosion – Calving of large blocks of ice into lakes and ocean

Example: Mass Balance

Ablation Matanuska Glacier, AK. Lachniet (1997)

Calving Childs Glacier, near Cordova, Alaska. Lachniet (1998)

Icebergs and Calving Icebergs at Bering Glacier, near Cordova, Alaska. Foto: Lachniet (1998)

Copyright © Charles Higgins 2002 Glacier ZonesELA Ablation zone Accumulation zone Southwestern coast of Greenland

Primary Stratification Snow accumulates in annual layers Visible in stratigraphy by changes in porosity, dirt content Quelccaya Ice Cap - Peru 0/20_401_bslide.html

Glacier Movement Glacier ice deforms under its own weight and gravity to ‘flow’ downvalley Glaciers move via – 1) Internal deformation via plastic flow (aka “creep”) – 2) basal sliding – 3) bed deformation

Creep Creep is the process by which ice crystals are dislocated along shear planes Happens more rapidly in warmer glaciers

Foliation Secondary feature Formed from ice shear Alternating clear blue and bubble-rich white ice Note unconformity The Ice Palace in the Aletschgletcher, Jungfraujoch, Swiss Alps, Lachniet 2011

Basal Sliding Some glaciers move by slipping along basal water films Water source – Meltwater, geothermal and pressure melting – Not enough water – no sliding – Just right water – sliding – Too much water – development of subglacial drainage

Bed Deformation Ice Movement Deforming Bed Bering Glacier – Fenster site, Lachniet (1998)

Subglacial Sediment Deformation As much as 90% of down- valley movement in some glaciers results from bed deformation Subglacial sediments are very wet with high pore water pressures (low effective stress)

Velocity = Creep + sliding + bed deformation Figure 9-11A B C

Glacier Classification

Cirque Glacier Cirque and glacier, Rocky Mountain National Park, Denver, Colorado. Foto: Lachniet (2002)

Valley Glacier The Aletschgletcher, Jungfraujoch, Swiss Alps, Lachniet 2011

Piedmont Glacier From Benn and Evans. Glaciers and Glaciation, Arnold.

Bering Glacier piedmont Bering Glacier, near Cordova, Alaska. Foto: Lachniet (1998)

Copyright © Jeff Munro 2002 Vatnajokull Ice Cap, Iceland. Ice Cap

Subpolar Glacier From Benn and Evans. Glaciers and Glaciation, Arnold.

Extending and compressive flow Compressive flow – where base is concave up Extending flow – where base is convex up

Compressive flow: Terminus shear planes Shear planes dipping upglacier. Terminus of the Matanuska Glacier, AK. Foto: Lachniet (1997) Flow direction

Compressive Flow: Ice folding Matanuska Glacier, AK. Foto: Lachniet (1997) Flow direction

Figure 9-28 Ogives Form due to seasonal differential melting as ice passes over an ice fall – Summer = more melting, low spot

Crevasses Crevasses are cracks in the glacier ice – Form at low confining pressures – near the glacier surface ice is brittle Seracs are a form of crevasses that commonly form at icefalls

Crevasses Figure 9-26

Crevasses

Copyright © Matthias Jakob 2002 Marginal Crevasses

Splaying Crevasses Terminus of the Muir Glacier, Glacier Bay, Alaska. Lachniet (1997)

Transverse and Splaying Crevasses Martin River Glacier, AK. Lachniet (2000) Ice Flow

Transverse Crevasses Ice Flow Taku Glacier, near Juneau, Alaska. Lachniet (1999) Formed at an Icefall on the Taku Glacier

Copyright © Lynne Beatty 2002 Seracs Blocks of ice isolated by melting along intersecting crevasses Are dangerous to climbers because they can topple Famous seracs on the Khumbu icefall of the Khumbu glacier approach route to Mt. Everest

Complex crevassing Bering Glacier, AK. Lachniet (2001)