Glacial Geomorphology Lecture 1: Glaciers & Glacial Environments. GGY 166: Geomorphology of Southern Africa

Transcription

1 Glacial Geomorphology Lecture 1: Glaciers & Glacial Environments GGY 166: Geomorphology of Southern Africa

2 Relevance in Southern African Context South African landscape has been influenced by glacial action in the past Marion Island had, until recently, a glacier Sanae IV surrounded by glaciers

3 Glacier? Def: A thick mass of ice originating on land from the compaction and recrystallization of snow that shows evidence of past or present flow Forms part of two cycles: o Hydrologic Cycle o Rock Cycle Types of Glaciers o Valley Glaciers o Ice Sheets o Ice Shelves o Ice Caps o Outlet Glaciers o Piedmont Glaciers

4 Definitions of Glacier Types Valley Glaciers (aka Alpine Glacier) o A glacier confined to a mountain valley which, in most instances, had previously been a stream valley Ice Sheets o A very large thick mass of glacial ice flowing outward in all directions from one or more accumulation centres Ice Shelves o Forming where the glacial ice flows into bays, it is a large relatively flat mass of floating ice that extends seaward from the coast but remains attached to the land along one or more sides Ice Caps o A mass of glacial ice covering a high upland or plateau and spreading out radially Outlet Glaciers o A tongue of ice normally flowing rapidly outward, usually through mountainous terrain, from an ice cap or ice sheet to the sea Piedmont Glaciers o A glacier that forms when one or more alpine glaciers emerge from the confining walls of mountain valleys and spread out to create a broad sheet in the lowlands at the base of the mountains

5 Valley (Alpine) Glacier Flow in valleys originally occupied by streams Slow advance (cm/day) Stream of ice Flows down-valley from an accumulation centre Generally widths are short relative to lengths

6 Ice Sheets Exist on a large scale (Antarctic Ice Sheet = 13.9 mil km 2 & nearly 4.3km thick) Flow out in all directions from one or more snowaccumulation centers Obscure all but the highest underlying terrain (Nunnataks) Underlying topography influences behaviour Guide flow in certain directions Create zones of faster/slower movement

7 Ice Shelf Large relatively flat masses of ice Thickest on landward side but becoming thinner seaward Sustained by ice from adjacent ice sheet + snowfall + freezing seawater to their bases

8 Formation of Glacial Ice Snow is raw material o Thus glaciers grow where more snow falls in winter than melts in summer Conversion of snow to ice o Snowflake Granular Snow Firn Glacial Ice 1. When temp remains below freezing following snowfall the fluffy accumulation of hexagonal crystals (i.e snowflakes) soon changes 2. Air infiltrates and extremities of snowflakes evaporate and water condenses near the centres of the crystals (Granular Snow) 3. Through melting, freezing and pressure from burial underneath snow, an intermediary stage between flakes and ice (Crystalline granular stage) is reached which is termed Firn 4. As snow accumulates pressure increases at depth 5. When thickness of ice and snow > 50m, weight is sufficient to fuse firn into a solid mass of interlocking crystals

9 Movement of a Glacier Referred to as Flow Two basic types o Plastic flow Involves movement within the ice Ice behaves like a brittle solid until the overlying pressure is equal to approx 50m of ice when it starts to behave like a plastic material and flow begins Movement due to molecular structure of ice o Basal slip Entire ice mass slipping along the ground With the exception of some polar glaciers most glaciers are believed to move via this process Water acts as a hydraulic jack and lubricant helping ice over the rock Source of liquid water related to the relationship between pressure & melting point

10 Movement of a Glacier Not all the ice flows at the same rate o Frictional drag with bedrock o Upper part of glacier (above 50m) not under enough pressure to exhibit plastic flow (Zone of Fracture) o Ice in Zone of Fracture carried piggy-back by underlying ice o Underlying ice moves over irregular terrain tension in zone of fracture formation of cracks (Crevasses) Rates of Glacial Movement o Glacial flow not as obvious as streamflow o Rates vary between glaciers, but can be up to several meters per day o Advance of some glaciers characterised by periods of extremely rapid movements (surges)

11 Movement of a Glacier

12 Budget of A Glacier Glaciers are constantly gaining/loosing ice Zone of Accumulation o The part of a glacier characterised by snow accumulation and ice formation o Addition of snow thickens glacier and promotes movement Snowline o Lower limit of perennial snow and therefore the outer limit of the zone of accumulation o Elevation varies greatly Zone of Wastage o The part of a glacier beyond the snowline where annually there is a net loss of ice o In addition to melting glaciers waste due to calving

13 Budget of A Glacier Glacial budget o The balance, or lack thereof, between accumulation at the upper end of the glacier and loss at the lower end Ablation o A general term for the loss of snow or ice from a glacier Therefore: o If accumulation > ablation the glacial front advances until the two factors balance o If a warming trend increases ablation and/or a drop in snowfall decreases accumulation, the ice front will retreat

14 Budget of a Glacier

15 Glacier Advance and Retreat

16 Iceberg

17 Glaciations and Ice Ages Ice Age o A prolonged period of colder climatic conditions during which snow and ice covered large areas of the Earth Advancing ice: Glacial phase Retreating ice: Interglacial o Earth had several large ice ages: Early Proterozoic Late Proterozoic Permian Pleistocene (most recent ending ±11000 years ago) Little Ice Age (1300 AD to mid 1800 AD) o Causes: Plate tectonics (Gradual happening over millions of years) o Joined plates move to higher latitudes (e.g Gondwana Glaciation) Variations in Earth s orbit (Explains alternating glacial & interglacial events of Pleistocene) o Eccentricity: changes in the shape of Earth s orbit around sun o Obliquity: changes in the angle of Earth s axis relative to orbital plane o Precession: wobbling of Earth s axis

18 Present Extent of Polar Ice Caps Northern Hemisphere Southern Hemisphere

19 Pleistocene Glaciation

20 MilankovitchCycles Variations in orbit around the sun (Eccentricity) Changes in Earth s tilt (Obliquity) Wobbling of Earth s axis (Precession)