Glacial and Periglacial Environments
Glacier classification: • Glacier: ➢ Large mass of ice resting on land or floating as an ice shelf in the sea adjacent to land • Form by accumulation of snow that recrystallizes under its weight • 2 general classifications of glaciers: ➢ Alpine ▪ Cirque ▪ Valley ➢ Ice sheet or Ice Cap
Alpine glaciers: • Form within mountain ranges formation in snowfield confined to bowl shaped recess- cirque ➢ Main source are cirques • Cirque glacier
Cirque glaciers: • Can study in the past landscapes ➢ Eg: past accumulations of ice had eroded out huge ‘bowl’ shaped areas
Valley glacier: • Several cirque glaciers may feed valley glaciers ➢ Formed when cirque glaciers come together
Ice caps and Ice Sheets: • A broad ice sheet resting on a plain or plateau and spreading outward from a central region of accumulation. • Huge plates of ice • Antarctica and Greenland • Antarctica = 92% of glacial ice • Present day Greenland and Antarctic Ice sheets • European ice sheet at last glacial ➢ Maximum around 20 ky ago when sea ➢ level was around 125 m lower than today
Putting this into context: • Glacier ice sheets store masses and masses of water • The ice sheets can be the depth of Everest
Mass balance of a valley glacier:
• Accumulation zone near the top/ source of the glacier, near the cirque • Ablation zone increases/ advances as the accumulation zone increases, until an equilibrium is reached in which the accumulation zone is equal to the ablation zone ➢ Temperature may change which would alter the balance between accumulation/ ablation eg: ablation may retreat with increased temperatures and so the accumulation would be greater than the ablation zone.
Deformation and movement: • Shear stress () exerted on the bed is: =DgS ➢ ρ = Ice density ➢ D = thickness of the glacier ➢ g = acceleration due to gravity ➢ S = sin of the slope angle n • The rate of deformation. Glen’s Law: Strainrate= A ➢ A = constant (temp dependent) ➢ n = constant (around 3) ➢ small increase in stress = big increase in strain • Deformation= the movement/ moulding of the ice due to the stress/ strain placed upon the ice
Thermal regime and drainage systems:
• Cold ice is well below the freezing point. ➢ Cold glaciers are only made of cold ice. • Warm ice co-exists with water at the pressure melting point (the freezing point is lowered with pressure under a column of ice). • Temperate glaciers are made only of warm ice, except for the surface 10-15 m in winter • Polythermal glaciers have both ice types (figure). • Water at the bed enables sliding of ice on its substrate, whereas cold glaciers move more slowly
Deformation and movement: → • The vertical velocity profile will also depend on the occurrence of basal sliding (A versus B). • Sliding is quicker for a smooth bed. • If there are sediments rather than rock under the glacier, they will tend to deform and allow the C glacier to move faster (C).
Glacial Erosion: • Abrasion: ➢ Rock fragments at the base of the glacier scratch the rock surface over which it moves • Plucking/Quarrying: ➢ Rocks are incorporated into the basal ice when water at the base of the
glacier freezes. ➢ Previously loosened rock fragments • Larger fragments of rock
Erosion landforms
➢ Produced by the erosion processes ➢ Small scale ➢ Striations ➢ Grooves ➢ Roche moutonee ➢ Ribbon lake ➢ Large scale ➢ Glacial Trough ➢ Cirques (cwm, corrie) ➢ Arete/Pyramidal Peak
Small-scale
Striations (Striae):
• Usually multiple, straight parallel lines • Movement of the sediment loaded base of glacier. • Microscopic size (seemingly polished rock) to obvious markings millimeters deep • Small scale→ small pieces of material producing groves in the glacier
Grooves:
• Movement of the sediment loaded base of glacier. • Large in size than striations • Large scale→ larger pieces of material abrading against the ice and making huge groves into the glacier
Rouche Moutonnee:
➢ Asymmetrical hill of exposed bedrock ▪ Upstream→ gently sloping ▪ Downstrea → abrupt, steep ➢ Use to measure the direction of movement of the glacier
Large scale
Glacial transport:
• Generally 3 ways: ➢ Supraglacial (surface) ➢ Englacial (within the ice) ➢ Subglacial (beneath the glacier) Glacial depositional landforms:
• Variety of different environments and processes occur near the snout of a glacier on land
Depositional features: moraines
• Moraines: ➢ Mounds of glacial debris (can origionate in difeereenty ways at the the large scale) • 2 types: ➢ With orientation: ▪ Terminal or end moraine ▪ Lateral (margin) ▪ Medial ➢ Without orientation] ▪ Lodgemnet till ▪ Ablation till ▪ Hummocky moraine
Moraines:
• (Think where is the material, where will it end up if the glacier were to melt?) • Terminal: ➢ Maximum extent of glacier ➢ Recessional ➢ When a glacier melts, the material is deposited- see the movement of ice within the glacier/ where the glaciers were breached / where the end point of the glacier is and how far do they propagate down the valley producing the terminal moraine • Medial moraines
Subglacial depositional features
• Not strictly moraines, as they have no direction – unsorted sediment let down onto the landscape by melting glacier or from overloading - from debris trapped beneath the ice. • Lodgement Till: scraped over the landscape. Deposited by the sole of the glacier in a subglacial environment • Ablation Till: collapses out during melting and let down. A Mass Wasting feature ➢ May have zones of higher concentration of material within the is zone of till • Hummocky Moraine: Ablation till let down into a series of mounds. Thought to be
the result of an ice-sheet stagnating in situ.
Proglacial and periglacial environments:
Proglacial:
• Downstream of the glacier • Fluvial environments important where glaciers terminate on land. • Rivers rework deposited sediment in glacial outwash plain. • Glacier-fed river flows strongly dependent on melt conditions, ➢ annual peak, ➢ daily flow maximum near glaciers
Periglacial:
• Periglacial environments: ➢ Areas of exposed ground characterized by the presence of permafrost. • Permafrost ➢ Permanently frozen ground (26% of Earth’s surface) lying between deeper rock warmed by geothermal heat and the surface active layer which melts in summer.
Summary:
• Glacier classification • Mass balance • Deformation and movement • Glacial erosion and associated landforms • Glacial transport • Sediment deposition and associated landforms • Proglacial and periglacial environments