Glaciers

definition of a glacier

a large mass of ice on land that shows evidence of motion.

how can ice be viewed as a igneous, sedimentary, and metamorphic rock?

• igneous - forms from melt that has undergone cooling and recrystallization

• sedimentary - deposition of snow in layers

• metamorphic - Ice recrystallizes under pressure and deforms by plastic flow.

unique properties of ice

• Freezes at a low temperature compared to silicate rocks

• Less dense than liquid water → floats in water

• Much weaker than most rocks → flows under stress

• Can deform plastically over geologic timescales

Describe the steps of the formation of glaciers

1) Snow falls and accumulates in layers

2) Pressure causes recrystallization

3) Pressure from overlying ice causes air to be squeezed out

4) Ice becomes dense enough to flow like a viscous fluid

describe the compaction stages and the proportion of air at each stage

• loose snow (~90% air)

• Granular snow (~50% air)

• Firn (~25% air)

• Fine-grained ice

• Coarse-grained ice

conditions required for glacier formation

• Cold local climate (requires polar latitudes or high elevation)

• adequate snowfall: snow accumulation must exceed melting rate. polar regions tend to be dry; wet (windward) sides of mountain ranges

• Snow must not be removed by avalanches or wind

Valley (aka Alpine, Cirque, or Mountain) Glaciers

• Flow down mountain valleys

• Confined by topography

• Flow like rivers of ice

• Carve landscapes between mountains

Continental Glaciers (ice sheets)

• Cover large areas of land

• Flow outward from thickest center

• Two modern examples:

  • Antarctica

  • Greenland

ice caps

cover mountain peaks and ridges

piedmont glaciers

large, fan-shaped lobes of ice that form when valley glaciers spread out from confined mountain valleys onto a broad, flat plain

ice shelves

• Floating extensions of continental ice sheets

• Still connected to land-based ice

Tidewater glaciers

• Valley glaciers that terminate in the ocean

• Produce icebergs via calving

sea ice

• sea ice is NOT glaciers

• Forms from frozen seawater

• Does not contribute to sea-level rise when melted

accumulation

ice growth by snowfall

ablation

shrinkage by melting, iceberg calving, sublimation, wind erosion

glacial budget

glacial budget = accumulation - ablation

if accumulation > ablation: advance, erosion of landscapes, cooling climate

if accumulation < ablation: retreat, deposition of sediment, warming climate

if accumulation = ablation: ice front remains in the same position

Advance

More snow falls and compacts into ice than melts away; the glacier flows forward, extending its terminus.

Retreat

More ice melts than is gained; the terminus recedes, even though ice still flows from the accumulation zone down the glacier like a conveyor belt.

what is the typical speed of glacial motion per year?

ranges from ~10 to 300 meters per year

what factors does glacial flow speed depend on?

• ice thickness: thicker ice creates more pressure and greater basal shear stress → faster movement

• slope: steeper slope → faster flow speeds

• temperature: temperate glaciers move faster due to basal slip

• meltwater at the base: the presence of liquid water at the glacier's base acts as a lubricant enabling basal slip

compare glacial flow in temperate (warm) glaciers vs. cold glaciers

temperate ice deforms more easily and has a lower viscosity than cold ice → enables basal slipping.

Cold ice is more rigid and deforms much more slowly, with movement primarily due to internal plastic flow

two main mechanisms of glacial flow

1. Basal slip

2. Plastic flow

basal slip

• Pressure from overlying ice lowers the melting point of ice

• Basal ice melts, producing water

• This water acts as a lubricant, reducing friction

• The glacier slides over the underlying rock

where does basal slip dominate?

• Temperate (warm) glaciers

• Valley glaciers

• Regions where summer melting is common → flow rate often increases during summer

plastic flow (internal deformation)

the slow movement of ice caused by deformation and recrystallization of ice crystals within the glacier. occurs throughout the ice mass.

• Ice crystals deform under stress

• Crystals slip along microscopic planes

• Recrystallization allows ice to change shape without breaking

• Ice flows like a very viscous fluid

glaciers are frozen to the bed at their base

where does plastic flow dominate?

• Cold, dry regions (where basal slip is impossible because they are frozen to the bed at their base)

• Continental ice sheets (e.g., Antarctica)

what is the equilibrium line?

 separates the zone of accumulation (above) from the zone of ablation (below).

what happens to the equilibrium line when the glacier is advancing vs. a glacier retreating?

• dvancing glacier → equilibrium line moves down

• Retreating glacier → equilibrium line moves up

Abrasion

rock fragments scrape bedrock

plucking

ice pulls away fractured rock

glacial erosion landforms

• U-shaped valleys

• Hanging valleys

• Cirques

• Arêtes

• Horns (e.g., Matterhorn)

• Roche moutonnée

• Striations (flow indicators)

• Fjords (drowned U-shaped valleys)

describe the glacial “conveyer belt”

• Sediment falls on ice

• Sediment is plucked from bed

• Transported to toe of glacier

identify 4 types of deposits

1. till: unsorted debris dropped by ice

2. stratified drift (outwash): sorted by meltwater into layers

3. erratics: large, out-of-place boulders

4. varves: annual sediment layers (e.g. mud deposited in glacial lakes) that act like natural calendars because the summer and winter layers are distinct

Moraines

long, rough ridges of rocky debris (till)

• lateral moraines: Run along the sides of a valley glacier

• medial moraines: Run down the middle of a glacier, forms where two glaciers merge

• terminal moraines: Found at the downhill end of a glacier

Drumlins

• elongated hills of till

• steep and blunt, the other end has a gentle slope

• all drumlins in an area point in the same direction

Esker

• Eskers are long, narrow, winding ridges of sand and gravel

• look like a river made of sediment

Kettle Holes or Kettle Lakes

• Shape is typically circular or bowl-like; Formed when buried ice blocks melt

• Dry kettle: shallow pit or hollow

• Kettle lake: depression filled with water

what does Eustatic Sea-Level Change mean?

global changes in sea level that affect all oceans worldwide. These changes occur when the total volume of water in the oceans changes

3 controls on Eustatic Sea-Level

1. Ocean Water Quantity (glacial ice volume): When more water is stored as glacial ice on land, less water is in the oceans

2. ocean water volume (thermal expansion): Water expands when it warms and contracts when it cools Warmer ocean water has lower density and takes up more space

3. ocean basin volume (plate tectonics): Changes in seafloor spreading rates or basin shape can alter how much water the oceans can hold

how can sea levels rise without any water being added?

Even without adding water, warming oceans cause sea level to rise because warmer water causes ocean water volume to expand. Water expands when it warms and contracts when it cools

Principle of Isostasy

• Earth’s crust “floats” on the mantle, similar to how ice floats on water.

• It is based on Archimedes’ principle: The weight of a floating solid (Earth’s crust) equals the weight of the fluid (Earth’s mantle) it displaces.

• The immense weight of ice sheets pushes the crust down into the mantle

• When glaciers melt, the crust rebounds upward (uplift)

glacial isostatic adjustment (GIA)

process of when glaciers melt, the crust slowly rebounds upward (uplift)

what is relative sea level (RSL) and what 2 factors does it depend on?

Relative sea level (RSL) is the sea level measured relative to the land at a specific location.

RSL is determined by 2 factors:

1. global eustatic sea-level changes caused by ice volume, ocean temperature, and basin size

2. local land elevation changes caused by glacial isostatic adjustment.

equation for relative sea level

RSL  = global sea level (eustatic fluctuations) — the local land elevation changes (isostatic fluctuations)

*Sea level can rise globally but fall locally if land is uplifting