Minnesota: Ice sheet

• What is an Ice sheet

  • An ice sheet is the largest accumulation of ice, being greater than 50,000 km^2.  
  • There are currently two in the world located in Antarctica and Greenland.
• Minnesota Key Facts:

  • Minnesota is a northern state that borders Canada and the Great lakes
  • Present landscape due to glaciation during the Quaternary period, from 2 million years ago to now
  • The Laurentide ice sheet spanned over 13 million km^2 and was up to 3000 m thick in some places
  • It covered most of Canada and Northern America through advancing and retreating
• Physical factor influencing landforms: Climate

  P: MOST SIGNIFICANT FACTOR

  Regional or National scale as it is responsible for the main input to the system - precipitation

  E: The ice sheet forms through the process of diagenesis

  E:

  • This created the Laurentide ice sheet between 95,000 to 12,000 years ago
  • Due to its large size and weight> isostatic lowering> many ellipsoidal basins formed (eg. Large: Hudson Bay, Small: the Great Lakes)
  • 
    

  L: Ice sheets form when diagenesis occurs at a large scale and forms more than 50,000 km^2 of ice over large distances
• Erosional landform examples in Minnesota:

  1. Ellipsoidal basins - Northern Minnesota

  • Due to the large weight of Laurentide Ice sheet ( over 1 km thick ) > isostatic lowering > forms ellipsoidal basins> Examples: Upper and lower Red Lakes in Northern Minnesota

  2. Striations

  • Abrasion from advancing lobes left striations in resistant igneous rock

  3. High mountains worn down by erosion > today the highest peak is only 500-700m
• Depositional landform examples in Minnesota:


  1. Drumlin fields from Wadena lobe
  2. terminal moraines from Rainy and Superior lobes
  3. Till deposits from Des Moines lobe
• Changes over time:

  Quaternary period - 2 million years ago to the present:

  • Ice sheet advanced and retreated
  • During colder periods> extended across the upper midwest USA

  Around 75,000 years ago:

  • A series of ice lobes extended from the main Laurentide ice sheet across Minnesota many times> transported and deposited till across the area

  Around 30,000-10,000 years ago - Lake Agassiz:

  • The edge of the ice sheet blocked the natural drainage in the area
  • Meltwater was trapped between the ice sheet and the south
  • Lake Agassiz formed> eventually flooded> left behind fertile land
• Physical factors influencing landforms: Geology 


  P: Local scale as it determines the type of landform: erosional or depositional

  E:

  • Northern Minnesota - ellipsoidal basins with alternating belts of igneous granite (BVG) and sedimentary sandstone rock ( Windermere )
  • Tectonic compression created mountains several km high in Northern Minnesota

  E:

  L:
• Ellipsoidal basins before glaciation

  Surface land comprised of alternating belts of igneous and sedimentary rock
• Ellipsoidal basins during glaciation


  The weight of the advancing lobes compressed and eroded the soft rock more than the hard rock. Therefore isostatic lowering took place due to the pressure created
• Ellipsoidal basins after glaciation


  Ellipsoidal basins form after glaciation with a lake in the deep large basin Eg. the Great Lakes, USA