Glaciers What is a glacier? Formation of glacial ice Glacial mass balance Glacial flow Types of glaciers Glacial erosion and erosional landforms Glacial deposition and depositional landforms Ice ages—evidence and effects Causes of ice ages (glaciation)

Glaciers Store 85% of fresh water on earth Are sensitive to climate change--record past and present warming/cooling >800 glaciers in WA Melt produces 470 billion gallons of water each summer (decreasing) Important effects on our landscape-most areas here affected by glacial erosion or deposition

What is a glacier? Accumulation of ice on land By compaction and recrystallization of snow over a period of years Moves under its own weight, after about 50m thick Alpine glaciers: due to high altitude in mountains Continental ice sheets at poles, or during ice ages; can form at low elevation

Snow Line: Elevation Above Which Snow Snow Line: Elevation Above Which Snow Doesn’t Completely Melt in Summer

Valley Glacier Near Juneau, Alaska See Fig. 14.14

Through the Antarctic Ice Cap Mountains Sticking Up Through the Antarctic Ice Cap

Antarctic Ice Cap

Transformation of Snow to Glacial Ice Granular Ice (50% porosity (Like metamorphism) Snow 90% porosity (Like Sand) Granular Ice (50% porosity (Like Sandstone) Firn (20%) Glacial Ice (0% except bubbles) (Like quartzite)

Mosaic of Glacial Ice Crystals Air Bubbles

Mass Balance or glacial ice budget: 1) the rate of glacial accumulation at the upper end of the glacier verses 2) the rate of glacial ablation (loss) at the lower end of the glacier.

Glacial Budget Equilibrium line or snow line (a visible feature) Fig. 14.16

Snow lines on two glaciers, Mt. Elbrus region, Caucasus, Russia (end of summer season)

When glacial accumulation is greater than glacial ablation, the result is glacial growth and associated glacial erosion (by glacial plucking and abrasion). Equilibrium line moves down-valley, terminus advances

When glacial accumulation is less than glacial ablation, the result is glacial retreat and associated glacial deposition. Equilibrium line moves up-valley, terminus retreats But ice still flows downhill!

Successive Stages of Ice Shelf Retreat

Animations of glacial advance and retreat http://www.wwnorton.com/earth/egeo/animations/ch18.htm

Plastic Flow: Glaciers Move by Plastic Flow and Basal Slip: Individual ice crystals move microscopic distances over short periods of time (ductile) Valley glaciers move mainly by plastic flow below about 40 m depth; top 40 m is brittle Fig. 14.16

Movement of a rigid ice slab along a base lubricated by water Glaciers Move by Plastic Flow and Basal Slip: Basal Slip: Movement of a rigid ice slab along a base lubricated by water Continental glaciers move mainly by basal slip Fig. 14.16

Mt. Formidable, N. Cascades Crevasses: Cracks in the Glacier indicate brittle deformation near surface of ice Mt. Formidable, N. Cascades Crevasses tend to form where the valley curves or has a sudden change of slope (e.g. moves over steps in underlying bedrock

Ice plucks rock fragments, producing a rough surface Glacial erosion: Abrasion, plucking Ice + ground up rock polishes smooth surface Ice plucks rock fragments, producing a rough surface

Abrasion by glaciers: Glacial Polish, Striations, and Grooves Formed by Advancing Glacier and Exposed by Retreating Glacier in Glacier Bay National Park See Fig. 14.14

Erosional Landforms Associated with Valley Glaciation U-shaped valleys hanging valleys fjords horns cirques aretes

Glacial Erosion--Landforms Before Glaciation During Glaciation After Glaciation

U-shaped valley, hanging valley, Yosemite

Fjord: a Drowned Glaciated Valley

Horn, aretes Switzerland

Horn, aretes North Cascades Le Conte glacier, aretes, glacially polished surface Sahale Mt.

Cirque: an amphitheater-shaped hollow formed at the head of a valley glacier Fig. 14.14

Types of Glacial Deposits Glacial Drift: all deposits derived from a glacier Glacial Till: material deposited directly from melting ice (unsorted, not bedded) Glacial Erratic: large boulders within till Glacial Outwash: glacial deposits that have been reworked by meltwater streams (sorted, bedded)

All Material Derived from Glaciers Glacial Drift: All Material Derived from Glaciers

Depositional Landforms Associated with Glaciation Include: glacial moraines end moraine ground moraine lateral and medial moraine esker drumlin kettle lake outwash plain

Lateral Moraines Medial Moraines

Terminal (end) moraines and lateral moraines, Sierra Nevada Evidence of past glacial advance, retreat http://vesr.ucnrs.org/pages/snarlgeol.html

and landforms of continental glaciation Glacial Deposits and landforms of continental glaciation

Present extent of permafrost in the northern hemisphere Extent of glaciation in the northern hemisphere, based on the distribution of glacial drift Fig. 14.15

Effects of the last ice age Last Glacial maximum advance 18,000 yrs ago Sea level was lower (~120m) Crust depressed by ice (isostasy) Ecological changes--temp., food supply Pluvial lakes (wetter climate) Glacial lake Missoula, Great Salt lake Temp. was lower, but average only 2-6°C Landforms of continental glaciation

Extent and thickness of glacial ice and sea surface temperatures for a typical day in August, 18 ,000 years ago, during the last ice age. See Fig. 14.15

Evidence of Previous Ice Ages Late Paleozoic Glacial Deposits ~350 Million Years Old

Evidence of Previous Ice Ages Late Paleozoic Glacial Deposits ~350 Million Years Old See focus on 1.1 for plate tectonic-ice age connection

Late Precambrian Glacial Deposits ~700 Million Years Old

Oxygen isotopes in marine microfossils record numerous glacial and interglacial periods Focus on 14.3

The periodicity of glacial and interglacial cycles is best explained by cyclic variations in solar energy, governed by periodic variations in the Earth’s: Eccentricity of Earth’s orbit around sun Tilt of Earth’s rotation axis Precession (rotational “wobble”) These are called Milankovitch cycles

Orbital Eccentricity (~100,000 cycle) Focus on 14.3, fig 2

Orbital Tilt (~41,000 cycle) Focus on 14.3, fig 2

Orbital Precession (~23,000 cycle) Focus on 14.3, fig 2