CS.2 - Laurentide - Landscape associated with the action of ice sheets

When was Laurentide’s last glacial advance

75,000 years ago, during the Pleistocene, ending 20,000 years ago

How long did this glaciation last

From 2 million years ago. Most of the landscape results from the Quarternary period

What is an ice sheet

A very large expanse of ice spanning a minimum of 50,000km²

What ice sheets currently exist?

There are currently two in the world: one in Antarctica and one in Greenland.

They possess 96% of the world’s ice

How big was Laurentide at its peak?

13 million km²

How deep was Laurentide on average

3km

How did the North and South parts of the ice sheet differ

• Was cold based at high latitude - bound by Arctic Ocean at about 80° N

• Acted more like a warm-based glacier further south - up to only 37° N at it’s max extent

What are the 4 lobes (‘tongues’) of Laurentide

• Wadena

• Des Moines

• Rainy

• Superior

Give two examples of the lobes’ colours

• Des Moines had a tan buff colour

• Wadena had red

  • Deposited the Alexandria moraine

    • A terminal moraine that reaches up to 107m

• This formed drumlins, moraines, till

Give an example of a glacial erratic (+data)

• The Green Mountain Giant

• Located in Vermont, plucked from the Green Mountains by the Laurentide ice sheet

• 12m tall

• 3400 tons

Give examples of drumlins

The Wadena Drumlin Field over the Wadena, Otter tail and Todd counties

• Almost 6,000 drumlins

• 2.83 km³ of sediment contained in these drumlins

Give example of a moraine: height and sediment

The Alexandria moraine, deposited by the Wadena ice lobe

170m tall

Total volume of sediment 70-110 km³

What physical factors influence the formation of landforms within this landscape system

• Geology

• Climate

• Latitude/Altitude

What is the majority of Minnesota’s landscape part of

The Canadian Shield

How are Minnesota’s oldest rocks laid?

Minnesota’s oldest rocks lie in alternating belts in the Northern half of the state and much of the Minnesota river valley

The belts are of volcanic (more resistant) and sedimentary (more easily eroded) rocks, and granitic rock materials lie in the area between the belts

What’s along the Minnesota river valley? [geology]

• Outcrops of metamorphic gneiss

• Date back 3600 million years

• These are folded/faulted

  • So they have weaker joints and are more vulnerable to erosion (plucking)

AO1 about the mountains formed by Gneiss rock

Mountains formed by gneiss rock were eroded at the top and reduced to 500-700m

• e.g. Eagle mountain is the highest point of Minnesota and is 701m

What are ellipsoidal basins

Deep, elongated depressions formed by subglacial activity beneath ice sheets

How did geology influence ellipsoidal basins

• Due to the sedimentary rock

• Ellipsoidal basins are particularly deep in relation to the surrounding area in the Arrowhead region

  • Due to earlier tectonic tilting of the landscape exposing weak shale rocks

  • e.g. Lake Vermilion, 23m deep

• Now contain lakes such as the Red Lakes in Minnesota

How was the climate in the US

During the Pleistocene in the Quaternary period, temps across the US were cold enough for precipitation to fall as snow in large quantities

→ diagenesis could occur - colder climate = denser firn → glacial ice

• Global temps ~21,000 years ago ~6 degrees colder than today

• This very significant as enabled the ice sheet to form at such a large scale and for such a long time + [AO1]

How did changes in climate over 1000s of years impact the landscape

• The 4 lobes constantly advanced and retreated, shaping landscape via both erosion and deposition

  • As climate temps increased, the lobes retreated exposing characteristic depositional landforms - e.g. Wadena lobe retreated ~30,000 years ago

• Formation of pro-glacial Lake Agassiz formed moraine which dammed natural drainage - climate formed moraine to dam and meltwater for lake

How are different landforms inter-related

• Erosion happens (e.g. tops of mountains plucked) and the colours of the till deposited will have the colours of these different rocks

• Ice sheet dammed natural drainage from going into Hudson bay then sea → proglacial lake formed - Lake Agassiz

How are glacial and fluvio landforms interrelated - Lake Agassiz

• The ice sheet was so large that it dammed natural drainage of water northward into the Hudson Bay → formed Lake Agassiz

• Not only the glacier blocked water, but the ‘Big Stone Moraine’ was a ridge of glacial debris that blocked the southern end of the lake

How large was Lake Agassiz

280,000km²

Up to 210m deep

When was Lake Agassiz formed?

When the ice sheet began to retreat ~13,000 years ago

What is a prominent example of a fluvial landform

The deeply engraved Glacial River Warren valley formed about 12,000 years ago by the overflow of Glacial Lake Agassiz.

How wide and deep is the Glacial River Warren valley

8km wide

76m deep

How does a high latitude impact ice sheets formation?

At high latitudes, the intensity of solar radiation is reduced as the sun’s rays are spread over a wider area,

→ resulting in reduced melting of the snow and therefore a greater chance of snow that falls over one year to remain frozen over the next year.

How does high latitude impact the action of ice sheets?

• High latitude → v. low temperatures → low erosional / fluvio-glacial imapct

• Rates of movement are so limited

  • So do not form as dramatic landforms as dynamic valley glaciers, e.g. corries and pyramidal peaks.

What happens over millennia to the ice sheet?

• The ice sheet advances and retreats over tens of thousands of years

• Huge volumes of ice during max advances [+ AO1] shape continents

• Long term climate fluctuations: Milankovitch cycles

What are Milankovitch cycles?

Long-term, periodic variations in Earth’s orbit and tilt that alter solar radiation - particularly in the Northern Hemisphere

These variations drive glaciation over millennia

How large are the variations in incoming insolation caused by Milankovitch cycles

Milankovitch cycles cause variations of up to 25% in the amount of incoming insolation at Earth’s mid-latitudes (30-60° N) over millennia

What glacial features are created over millennia

The vast majority of landforms formed over millennia

• Moraines - require a long period of stable ice margin (position at the edge of an ice sheet that remains in approximately the same location for a significant period)

• Drumlins - require consistent ice flow direction and pressure over long periods

• Glacially eroded basins require cumulative action of abrasion and plucking over multiple glacial cycles

What are the seasonal (annual) changes to the landscape?

• Summer meltwater increase

• More erosion and sediment transport

  • Aids the landforms formed over millennia

What landscapes are formed through seasonal changes

• Meltwater channels - erosional landforms created by flowing water originating from melting glacier ice, either subglacial, proglacial, or ice-marginal

  • Large networks of meltwater channels in Laurentide ice sheet HOWEVER during periods of ice retreat - so still over millenia

• Meltwater deposited extensive outwash plains

What landscapes are formed through short-term changes (seconds to days)

• Ice calving (chunks break off into proglacial lakes

• Glacial outburst floods - jökulhlaups

What jökulhlaups (glacial outburst floods) occured in Laurentide ice sheet

As Laurentide receded from maximum extent ~13,000 years ago, two significant meltwater rerouting events occured in eastern North America

Example:

• Glacial Lake Iroquois drained to the Atlantic in catastrophic Hudson Valley releases - 3 jökulhlaups occured.

• Large sediment deposit lobes on the continental shelf

• Glacial erratic boulders greater than 2m in diameter on the outer shelf