Glaciers and Glacial Landscapes

A glacier is an agent of erosion: Removal and transport of sediments

Glacier: mass of snow and ice that moves under its own weight. Glacial ice is deformed snow.snow.

What causes glaciers to form? Climate: Accumulation > melting for a very long time period when Summer temperatures do not melt all snow. Glaciers form at: High altitudes (alpine glaciers) and High latitudes (continental glaciers)

Glaciers grow by Accumulation and Diminish by Ablation: Ablation includes: Melting (ice to water) Sublimation (ice to water vapor) Calving Advancing glacier is growing and a Retreating glacier is getting smaller, but still moving forward!

Glaciers move by: Deformation: Ice under confining pressure is deformed, moves by plastic flow in response to overlying weight Sliding : Partial melting at base provides lubrication; mass of ice slides

Crevasses: Large cracks in ice; Evidence of movement Ice Fall : area of many crevasses

Glaciers are constantly moving forward, even if they are retreating, as in this diagram of a retreating glacier.

The material moved forward by the glacier is deposited as the ice melts. Where does this material come from? Sediment, rocks, boulders that have been “bulldozed” by the moving ice, and incorporated into the ice.

Mini-example in Duluth in spring: Sand and gravel forming dirty ice as the ice melts. Snow piles in Duluth are black with the sediment that remains behind as snow melts

Glacial deposition Glacial drift : term for all glacial deposits Till : deposited directly by ice Unsorted, non-stratified Stratified drift (outwash) :: depositeddeposited byby meltwatermeltwater fromfrom thethe iceice Sorted and stratified Glacial erosion Abrasion Polishing Plucking Roche moutonnée: Streamlined bedrock knob created by abrasion and plucking,( e.g. Half Dome, Yosemite)

Alpine Glaciers: High altitude; Low temperatures, high precipitation Accumulation in low areas (cirques and valleys) Glacial valley: U-shaped (Pre-glacial stream valley : V-shaped) Cirque: Bowl-shaped depression at top of mountain where ice accumulates; Source area for alpine glaciers; valley glaciers flow from cirques Cirque glaciers Hanging valleys: Main glacier carves deeply; leaves tributary valleys “high and dry”; often contain waterfalls Truncated spurs: steep bluffs on sides of glacial troughs between tributary (hanging) valleys Arête :: RidgeRidge betweenbetween 22 cirquescirques Horn :: PeakPeak carvedcarved byby 33 oror moremore cirquescirques Moraine : Accumulation of rock debris deposited directly by ice Composed of till

Lateral, Medial, Terminal moraines

Continental Glaciers (ice sheets) : High latitudes;latitudes; More extensive in area; 300-5000 m thick “Bulldozer” effect Net Erosion : near source region; Net Deposition: near terminus Ice-scoured plain :Vast:Vast flatflat toto rollingrolling plain;plain; strippedstripped ofof soil;soil; manymany lakeslakes andand swampsswamps (Canadian shield) Glacial grooves: Streamlined depressions in rock ; Parallel to ice movement Glacial striations: Scratches on rock surfaces; Result of abrasion Moraines: composed of till Irregular belt of knobby hills, depressions, lakes, wetlands “Knob and Kettle Topography” (Example: Kettle Moraine, Lab )

Ground moraine (till plain): blanket of till over previous surface Terminal moraine :: widewide beltbelt ofof tilltill markingmarking furthestfurthest extentextent ofof glacierglacier Recessional moraines: represent stages of retreat; lobe-shaped Kettle : Circular depression in moraine (or outwash) Process of kettle formation: At edge of ice sheet, debris is in rubbly heap; iceice bouldersboulders areare buriedburied inin till;till; asas iceice melts,melts, tilltill andand outwashoutwash subside,subside, leavingleaving round depression. Kettle lakes: water-filled kettle Drumlin: Streamlined hill of plastered till; Occur in swarms Esker : Long, winding, narrow ridge; Formed from subglacial meltwater stream Outwash plain :: TremendousTremendous amountamount ofof meltwatermeltwater carryingcarrying sedimentsediment fromfrom glacierglacier Plain similar to till plain, but sorted, stratified, usually sandy deposit Brainerd pitted outwash plain erratic gouges

Ice Ages Earth is approx. 4.7 billion yrs old Evidence is that for MOST of that time, no glaciers of any significance. In past 1 billion yrs., evidence of 5 “brief” periods (couple million years) of continental glaciation : Ice Ages Those 5 Ice Ages: First four: hundreds of millions of years ago Fifth: began 2.5 – 3 million yrs. ago Late Cenozoic Ice Age (Pleistocene Ice Age)

During Ice Age: Fluctuations in ice extent: Glacial Periods:  iceice sheetssheets expand,expand, grow,grow, thicken,thicken,  Alpine glaciers coalesce  Sea level drops  Cooling of global climate Interglacial Periods:  Intervening periods of warmth  Ice sheets retreat  Alpine glaciers disappear or shrink

We are in an Interglacial Period of an Ice Age now. Major Ice sheets today: Greenland, Antarctica Alpine glaciers today are relatively small, compared to what they were during glacial period. 4 most recent glacial periods: Nebraskan , Kansan, Illinoian, Wisconsin