glacial landforms and the ice age glaciers alpine glaciers ice sheets of the present sea ice and...

Glacial Landforms and the Ice Age

•Glaciers

•Alpine Glaciers

•Ice Sheets of the Present

•Sea Ice and Icebergs

•The Ice Age

•Landforms Made by Ice Sheets

•Investigating the Ice Age in the PNW

Glaciers

when snow accumulates to a great thickness, it can turn into flowing glacial ice

alpine glaciers form in high mountains, while ice sheetsform on continental interiors at high latitudes

Alpine glaciers can flow into ice sheets and add to overall ice flow

Glaciers

glacial ice accumulates when the average snowfall of the winter exceeds the amount of snow that is lost in summer by ablation

the term ablation means the loss of snow and ice by evaporationand melting

when winter snowfall exceeds summer ablation, a layer of snow is added each year to what has already accumulated

as the snow compacts by surface melting and refreezing, it turns into a granular ice and is then compressed by overlying layers into hard crystalline ice

when the ice mass is so thick that the lower layers become plastic, outward (ice sheet) or downhill (alpine glacier) flow starts, and the ice mass is now an active glacier

Glacial Ice Formation

• Recent snow exposed to freezing and thawing, plus metamorphosis

• Granular ice neve’ forms

• Neve’ buried, pressure of snow and ice above changes it to firn

• Under pressure firn changes to into glacial ice

http://en.wikipedia.org/wiki/Image:Glacial_ice_formation_LMB.png

Alpine Glaciers

snow collects at the upper end in a bowl-shaped depression, the cirque

the upper end lies in a zone of accumulation

layers of snow in the process of compaction and recrystallization are called firn

the smooth firn field is slightly bowl-shaped in profile

flowage in the glacial ice beneath the firn carries the ice down-valley out of the cirque

Strahler and Strahler, Figure 19.3, p. 632

Alpine Glaciersmotion of glacial ice moves most rapidly on the glacier’s surface at its midline - movement is slowest near the bed, where the ice contacts bedrock or sediment

Strahler and Strahler Figure 19.4, p. 633

Alpine Glacierslandforms produced by alpine glaciers

(a) before glaciation sets in, the region has smoothly rounded divides and

narrow, V-shaped stream valleys

(b) after glaciation has been in progress for thousands of years, new erosional forms are developed

(c) with the disappearance of the ice, a system of glacial troughs (U-shaped valleys) is exposed

Strahler and Strahler Figure 19.5, p. 634

(b)

(a)

(c)

Alpine Glaciers(a) during glaciation, the U-shaped trough is filled by ice to the level of the small tributaries (b) after glaciation, the trough floor may be occupied by a stream and lakes (c) if the main stream is heavily loaded, it may fill the trough with alluvium (d) should the glacial trough have been deepened below sea level, it will be occupied by a fiord

Strahler and Strahler Figure 19.7, p. 635

(a) (b)

(c) (d)

Ice Sheets of the Present

the Greenland Ice Sheet

has an area of 1.7 million km2

(about 670,000 mi2) and

occupies about seven-eights of the entire island of Greenland

Strahler and Strahler Figure 19.9, p. 638

http://upload.wikimedia.org/wikipedia/commons/4/4f/Greenland_ice-sheet_hg.jpg

Ice Sheets of the Present

the Antarctic Ice Sheet covers 13 million km2 (about 5 million mi2)

Strahler and Strahler Figure 19.10, p. 639

http://upload.wikimedia.org/wikipedia/commons/f/f2/Antarctic_profile_hg.png

Sea Ice and Ice Bergstwo types - sea ice and icebergs

sea ice is formed by direct freezing of ocean water (< 5m thick) - pack ice is sea ice that completely covers the sea surface

icebergs are bodies of land ice that have broken free from glaciers that terminate in the ocean (may be hundreds of meters thick)

Strahler and Strahler Figure 19.11, p. 639

an ice age includes cycles of glaciation, deglaciation, andinterglaciation

at present, we are within an interglaciation of the Late-Cenozoic Ice Age, following a deglaciation that set in quiterapidly about 15,000 years ago

in the preceding glaciation, called the Wisconsin Glaciation, ice sheets covered much of North America and Europe, as well as parts of northern Asia and southern South America

the maximum ice advance of the Wisconsin Glaciation was reached about 18,000 years ago

The Ice Age

the maximum extent to which North America and Europe were covered during the last advance of the ice

The Ice Age

Strahler and Strahler Figure 19.13, p. 641 Strahler and Strahler Figure 19.14, p. 641

Why Ice Age Cycles?

• Positive and Negative Feedback

• Atmospheric Composition

• Milankovitch Cycles

• Tectonic Plate Movement

• Supervolcanoes

Positive Feedback

• Positive Feedback– Albedo– Fewer forests (>Albedo)– Ice-free Arctic Ocean?

• Ice no evap. or sublimation drier earth less snow in other areas

• Open more evap more precip more snow higher albedo

Negative Feedback

• Glacial erosion reduces land surface area

• Lower sea level reduces reflective area (< albedo)

• Glaciers tie up moisture, less precipitation for snow accumulation

Atmospheric Composition

• Carbon dioxide, methane, sulfur, sulfur dioxide and particulates affect greenhouse effect

• Variations over time in concentrations affect amount of heat as radiation retained from earth

• Greater retention warmer• Less retention colder• Snowball earth

http://en.wikipedia.org/wiki/Image:SnowballSimulations.jpg

Snowball Earth

• 850 – 630 mya• Cryogenian period• Equatorial Continent

distribution high weathering of rocks

• Absorbs CO2 and earth cools

• Not a generally accepted Theory (or theory)

Milankovitch Cycles

• Variation in Earth’s Orbit

• Sometimes cycle reinforce variation, other times negate

• Cycles don’t match up well to times of ice ages

Milankovitch Variables

Eccentricity

Axial Tilt (Obliquity)

Precession (wobble)

Variation in Solar Output

• Solar output varies with time

• Astronomers believe output increases 10%/billion years

• Shorter term variations would affect glaciation/ice ages– Sunspots and sunspot cycles

Continental Movement

• Location and distribution of continents affects sea and atmospheric circulation

• Heat transfer on earth is affected or controlled by sea and atmospheric circulation

• Mountain building, orographic effect also affects precipitation patterns and related atmospheric effects

Volcanoes

• Additional particulates might initiate global cooling

• However, unlikely particulates would stay suspended long enough to initiate on their own

• Undersea volcanoes could release enough methane and methane clathrates (methane molecules trapped in sedimentary rocks/molecules)

Landforms Made by Ice Sheetsice sheets are highly effective eroding agents

the slowly moving ice scraped and ground away much solid bedrock, leaving behind smoothly rounded rock masses

these bear countless abrasions trending in the general direction of ice

movement

Strahler and Strahler Figure 19.15, p. 642

Landforms Made by Ice Sheets

the term glacial drift describes all types of rock debris deposited in close association with glaciers

stratified drift includes sediment laid by water, while till is deposited directly by ice

Strahler and Strahler Figure 19.17, p. 643

Strahler and Strahler Figure 19.17, p. 643

Landforms Made by Ice Sheets

moraines are built of rock debrisdeposited at the edges of a meltingglacier or ice sheet

terminal moraines mark the limitsof glaciation

Strahler and Strahler Figure 19.17, p. 643

Landforms Made by Ice Sheets

an outwashplain is formed of stratified drift left by braided streams issuing from the ice

the plain is built of layer upon layer of sands and gravels

Strahler and Strahler Figure 19.17, p. 643

Landforms Made by Ice Sheets

after the ice has gone, the position of a former ice tunnel is marked by a long, sinuous ridge of sediment known as an esker (stream-bed deposit remains, forming a ridge)

Strahler and Strahler Figure 19.17, p. 643

Landforms Made by Ice Sheets

a drumlin is a smoothly rounded, oval hill resembling the bowl of an inverted teaspoon (consists oftill) - lie in a zonebehind the terminal moraine (commonly occur in groups)

Strahler and Strahler Figure 19.20c, p. 647

Landforms Made by Ice Sheetsthe long axis of each drumlin parallels the direction of ice movement

drumlins are typically steeper at the broad end, which faces oncoming ice

Strahler and Strahler Figure 19.17, p. 643

Landforms Made by Ice Sheets

between moraines, the surface overridden by the ice is covered by glacial till

where thick and smoothly spread, the layer forms a level till plain

Landforms Made by Ice Sheets

marginal glacial lakes formed along the ice front

streams of meltwater from the ice built glacial deltas into these marginal lakes

when the ice withered away, the lakes drained, leaving aflat floor exposed

Landforms Made by Ice Sheets

former glacial deltas become isolated, flat-topped landforms known as delta kames, composed of well-washed and well-sorted sands and gravels

Investigating The Ice Age

several theories have been proposed for the cause of the Ice Age: change in the position of the continents, increased volcanism, decrease in the Sun’s output of energy

the timing of glaciations and interglaciations is determined by variations in insolation produced by minor cycles in the Earth’s orbit and the Earth’s axial rotation (the Milankovitch mechanism)

Strahler and Strahler Figure 19.22, p. 649

Milankovitch curve

Investigating The Ice Age in the PNW

The elapsed time span of about 10,000 years since the Wisconsin Glaciation ended is called the Holocene Epoch Glaciers in the PNW were a mix of Continental and Alpine (of vast extent)Alpine glaciers predominated in High Cascades and OlympicsAlpine Glaciers in BC contributed to continental ice sheet, the Cordilleran Ice Sheet, to flow into Puget Lowlands East of Continental Divide was the Laurentide Ice Sheet

http://en.wikipedia.org/wiki/Image:Northern_icesheet_hg.png

From Orr and Orr, 2005, p. 17

From Orr and Orr, 2005, p. 17

http://en.wikipedia.org/wiki/Image:Rainiersourdoughridge.jpg

http://upload.wikimedia.org/wikipedia/en/d/d7/Mount_Olympus_Washington.jpg

http://www.nps.gov/archive/olym/glacier.htm

http://www.extremeicesurvey.org/