Glaciers Lecture Notes

GeoClassroom Physical Geology Historical Geology Structure Lab

Physical Geology

Glaciers

- Alpine glaciers: moving ice streams in the mountains that occupy valleys

- Ice sheets: occupy large areas of land and flow out in all directions from a central point. Also called a continental glacier.

- Ice caps: remnant continental glaciers. Covering large areas of land in Norway, Iceland, and some Arctic islands.

Alpine Glaciers:

- streams of ice that originate in the snowfields of high mountains ranges.

- two essential parts of the system: zone of accumulation, and zone of ablation.

- more snow means an active, moving glacier

- more melting means a retreating glacier

- boundary between the 2 is the snowline

- below the snowline, the surface is irregular and pitted with crevasses

- crevasses are very dangerous

- can be hidden in the winter by snow bridges

- areas where the stream of ice cracks and buckles in response to bending forces.

- if accumulation = ablation, then the terminus of the ice remains stationary

- ice still flows to the terminus and melts though.

- Glacial erosion produces very unique landforms:

U-shaped valleys: glaciers modify a V-shaped valley to U-shaped

cirques: head of glacier is enlarged by plucking and grows headward toward the mountain crest

hanging valleys: where tributary glaciers meet the main valley glacier

horns: where 2 or more cirques meet at the crest of a mountain

outwash plains: where the great volume of water melting from a glacier is released. Deposits lots of sediment

Lateral moraine: Along the margins of a glacier

Medial moraine: Where 2 laterals join

End moraine: A broad arc in the shape of the retreating ice.

Continental Glaciers:

- Also an area of net gain through snowfall and net loss thru melting

- much larger areas, very inhospitable.

- ice depresses the land surface cuz of its weight

- causes the land to slope inward to it, forming large lakes on its margins.

- can block pre-existing drainage systems.

- In rugged terrane, direction of ice movement is influenced by the mountains, and the ice moves thru mountain passes in large streams called outlet glaciers. These are similar to alpine glaciers, except that they move faster, as much as 1 m per hour, enough to see.

- Erosion greatly modifies the original landscape:

- soil is removed along with several feet of bedrock

- This material is transported by the glacier large distances

- pre-existing drainage pattern is disrupted and sometimes permanently changed.

- sediment is deposited at the terminus of the glacier as a terminal moraine, an melt water washes away on the outwash plain.

- Beneath the ice is sediment called till ( a mix of stuff), and this till is called the ground moraine.

- Ground moraine can be reshaped as ice advances and retreats over and over again to form streamlined hill called drumlins. These are pointed in the direction the ice advances.

- Sinuous eskers remain where sediment was deposited by subglacial streams.

- Kettles are areas where ice chunks broke off the retreating glacier and subsequently melted to form a depression.

How glaciers erode:

- plucking: the lifting and removal of rock pieces

- process involves ice wedging, where meltwater beneath the glacier gets into the bedrock and does the freeze/thaw thing, weakening the rock loose.

- abrasion: where the pieces of rocks grind together and over bedrock, polishing and smoothing the surface of the bedrock

- Striations: left from this grinding, pointed in the direction of travel.

- both processes form roches moutonnee

- glacial plucking produces a ragged edge on the downcurrent side, and a smoother top with striations.

- Sediment is deposited as glacial drift (also called till)

- unsorted & unstratified material, quite different in that respect from well-sorted stream deposits.

Pleistocene Epoch:

- during the most recent ice age (100,000 year period), ice covered 30% of the Earth's land area.

- sea level was 200 feet lower than today cuz all the water was trapped in ice

- land in the north has been rising steadily as the ice burden retreated, around 2 cm/year

- Drainage systems were changed

- northern rivers like the Ohio and Missouri were part of a NE drainage system which flowed N, NE and drained into the Arctic and Atlantic Oceans.

- Ice sheets blocked this drainage, forcing the rivers to divert into the Mississippi.

- many, many pluvial lakes (from a cooler, wetter climate) formed in the west, along the margins of the glaciers. These have since dried up.

- One lake (Lake Missoula) helped to produce the Channeled Scablands in Washington.

- Ice in N Washington advanced and damned the N-flowing Clark Fork River, forming a temporary Lake

- As glacier receeded, the ice dam failed, and a catastrophic flood swept across the Columbia Plateau.