PERIGLACIAL PROCESSES & LANDFORMS

Transcription

1 PERIGLACIAL PROCESSES & LANDFORMS Periglacial processes all non-glacial processes in cold climates average annual temperature between -15 C and 2 C fundamental controlling factors are intense frost action & ground surface free of snow cover for part of year Many periglacial features are related to permafrost Permafrost table: Effects of frost action & mass movements enhanced by inability of water released by thawing active layer to infiltrate permafrost Temperature fluctuations in permafrost to about m depth; zero annual amplitude minimum annual T -10 mean annual ground T C permafrost table maximum annual T level of zero annual amplitude Permafrost base of permafrost 1

2 Slumping streambank underlain by permafrost, northern Brooks Range, Alaska Taliks: surface of ground active layer permafrost table (upper surface of permafrost) permafrost talik 2

3 Permafrost tends to mimic surface topography large river permafrost small lake large lake pf talik Effect of surface water on the distribution of permafrost taliks underlie water bodies Stream cutbank underlain by permafrost 3

4 Where annual temperature averages < 0 C, ground freezing during the winter goes deeper than summer thawing each year adds an increment & the permafrost grows until it is stopped by geothermal heat (~ 1 C/30 m decrease) Because permafrost accumulation rates are in cm/yr, thick permafrost can be tens of thousands of years old Permafrost underlies 26% of the Earth s surface, in both continuous and discontinuous (talik) types The southern limit of continuous permafrost Sporadic permafrost: 4

5 Periglacial zones are dominated by frost action & mass movement Frost action: formation of segregated ice talus slope, San Francisco Peaks, AZ alpine zone, Bighorn Mts., WY talus slope, Rocky Mountain N.P., CO 5

6 nivation hollows, Logan Pass Glacier National Park, MT southeastern Alaska avalanche scars, Cabinet Mts., MT Processes of Frost Action Frost wedging Frost heaving vertical displacement thrusting 6

7 Two hypotheses of explanation are i) frost-pull: ii) frost-push: Probably, both processes act together Surface form occurs as needle ice (piprake) ice crystals 1-3 cm long in unvegetated, loamy soils iii) frost cracking: needle ice, Nepal needle ice, n Sweden soil disturbed by needle ice, AZ needle ice, n Sweden 7

8 snow snakes, Snowy Mts., WY Two types of mass movements frost creep & solifluction dominate in cold regions Frost creep: Solifluction: gelifluction is a type when water is released from the active layer in summer, the permafrost prevents infiltration, & the saturated soils behave like viscous fluids, flowing on 1 slopes 8

9 Landforms associated with mass movements Solifluction lobes: central Alaska Blockfields: solifluction terraces, Glacier National Park, MT 9

10 Solifluction lobes on hillslopes in north-central Alaska Rock glaciers: Idaho 10

11 inactive rock glacier, Rawah Wilderness, CO mass movement/rock glacier, Indian Peaks, CO Other periglacial landforms Ice wedges & ice-wedge polygons ice wedges connect in polygonal pattern with diameters of meters to > 100 m; have some with ridges at boundary, others with troughs active ice wedges 11

12 Patterned ground in northern Alaska White Mountains, AZ 12

13 Hummocky terrain in northern Alaska Greenland 13

14 Pingos closed-system pingos open-system pingos pingos grow at 5 cm to > 1 m/yr pingos in the delta of the Mackenzie River, Canada 14

15 pingo on the coastal plain near Kotzebue, Alaska Thermokarst features ground ice is destroyed Thermokarst ponds on coastal plain near Kotzebue, Alaska 15

16 Patterned ground arrangement of surface materials into geometric shapes like polygons, circles, & stripes polygons: circles: stripes: polygons, n Sweden 16

17 Snowy Mts., WY central Alaska relict patterned ground, Trail Ridge, CO central Alaska Mount Katahdin, Maine 17

18 patterned ground on the coastal plain near Kotzebue, Alaska patterned ground, sw Brooks Range, Alaska 18

19 vegetation outlining patterned ground, sw Brooks Range, Alaska Patterned ground: pattern changes as slope increases from polygons through nets or steps into stripes origin; Permafrost presents special engineering difficulties 19

20 leaning high rise in Hay River, Canada Hay River, Canada utilidors, Inuvik, Canada bank erosion exposing permafrost, Mackenzie River delta, Canada 20