Lecture 5-3 Glacial systems Flashcards
(36 cards)
What is a glacier?
A mass of ice (>.1 km2) that has its genesis on land and that represents a multiyear surplus of snowfall over snowmelt
- perennial ice covers about 10% of the land areas of earth
How do glaciers form?
Snow accumulation doesnt melt in the summer, recrystalization of snow to form large crystals of ice (rough and granular) called Firn
-Lower layers turn to Solid Ice under the weight of overlying firn and snow
What are some properties of ice?
Has a density of .9g/cc (it floats)
its a weak solid and will not hold a vertical cliff higher than 40m
ice will flow under its own weight
pressure can cause local melting
What are the 2 types of glaciers?
Alpine and Continental
What are Alpine Glaciers?
Long and narrow mountain glaciers, slow-moving, wedge-shaped streams of ice
- small= 1-2 km long, 100’sm wide, 100’sm deep
- Large= over 100km long x 100’sm deep
What are continental glaciers?
Very old (1000’s of years), thick (1000’s of m) mass of ice covering almost and entire land mass
- Greenland and Antarctic
- Ice cap: dome shped glacier which buries the landscape
- Ice sheet: <50,000km2, flattened dome in shpe, burrie underlying relief
What are Alpine cirques?
-Semi-cicular shaped bedrock feature created as a glacier scours back toward the mountain
Found at the head of glacial trough, hollows
they occur just below the high peaks and have shape that vary depending on:
-Pre-glacial topography at the valley head
-level of activity by the alpine glacier
-duration of glaciations
-composition and structure of the bedrock
What is ablation?
The loss of snow and ice by melting and evaporation. most melting occurs at the glacial snout
What is Glacier Mass balance?
The change in mass (diff between total accumulation and gross ablation) of a glacier over some defined interval of times, determined either as a value at a point, an average over an area, or the total mass change for the glacier
-provides measure of glaciers condition, is it gaining or losing mass?
What are the 4 parts of glacial zonation?
1) Accumulation zone: the upper part of the glacier
2) Firn: granular old snow that forms a surface layer in the accumulation zone
3) Equilibrium line: line that separates the accumulation zone from the ablation zone
4) Ablation zone: the lower part of the glacier
What makes glaciers move?
Weight: the weight of the overlying mass forces the ice to spread out
Flow: ice is slippery. flows over underlying rock/soil
Crevasses form along steep slopes
How far do glaciers move?
Below snowline: glaciers begin to melt and evaporate, thining out (lower elevation/latitude)
Ice front: The front edge of a glacier
Melting=movement: stationary ice front
Extended to the sea: break off (calving)
What is ice velocity dependant on?
Distance from bed and valley sides
accumulation vs. ablation zone
free water -temperate glaciers
Stresses cause the surface to crack (crevasses)
How does glacial erosion take place?
Ice push: the glacier acts as a bulldozer and may be capable of pushing a limited amount of loose rock debris
Plucking: involves removal of blocks of rock from outcrops. the glacier exploits rock weaknesses along fractures to pull the rock loose
What is Glacial Abrasion?
Glacially transported rock fragments that show polish, striations and grooves. both clasts and bedrock are likely to show these features
What factors influence abrasion?
- Amount of debris
- basal sliding velocity
- ice thickness to apply normal pressure
- hardness of bedrock
- basal water pressure (enough to increase sliding velocity but not so much that friction is needed)
- removal of rock material by melt water
What is Arete:
Steep-sided, sharp-edged bedrock ridge formed by two glaciers eroding away on opposite sides of a ridge.
What is a col?
Low areas, or passes, through a ridge that once had glaciers on either side
What is a horn?
3 or more cirques adjacent to one another (A jagged peak that has survived the glacial erosion all around it)
What is a tarn?
A small lake that occupies a rock basin in a cirque or glacial trough
What are Erratics?
A boulder-sized rock dumped by a glacier. the rock is usually of a different type than the surrounding rock
What are evidence for glaciers?
Unsorted materials, Erratics, Striations, polished bedrock
What are striations?
Grooves carved into the bedrock by pebbles and cobbles carried at the bottom of a glacier
What is Polished bedrock?
smooth rock surfaces created as glaciers flow over bedrock
