Presentation on theme: "Isostatic rebound Ice moves in, occupies the area for certain period of time. Then moves out again to leave a depression that has to rebound/restore to."— Presentation transcript:
Isostatic rebound Ice moves in, occupies the area for certain period of time. Then moves out again to leave a depression that has to rebound/restore to the original height.
Pleistocene glaciation eCordilleran ice sheet - 30,000 - 25,000 C yr ago, single body at 20,000 C yr ago. -divide crest of the Coast Mountains shifted to a position over the British Columbia interior. -Transition mountain ice sheet to continental ice sheet.
Area Covered eAll of southern British Columbia, and into northwestern United States. eSource: Coast, Selkirk and Columbia Mt’s.
eGrowth and decay of the Cordilleran ice sheet. A)Mountain area at the beginning of a glacial cycle. D) Decay of ice sheet by downwasting; upland areas are deglaciated before adjacent valleys.
Growth eData from wood, peat and shell samples. eWithin 4000-yr ice increased 2 to 4 fold. eTime period 18,000 to 14,000 C yr BP
Retreat eTerminus of the Puget lobe 100km back past Seattle within 200-300 yrs. -was helped by calving into proglacial lakes + sea. -Retreat was coincided with top- down deglaciation. Became segmented into ice tongues. -Within 4000 yrs southern BC deglaciated. By 9500 C yr BP, things are like today (in our area)
eThe weight applied to the crust is dispersed throughout the lithosphere. The lithosphere is so rigid (like plastic rock) that the weight is transferred across the crust (laterally) resulting in a depression and a bulge in a different place. The lateral displacement of mantle material from below the center of ice sheet loading results in the formation of an area of slight uplift beyond the depression. We can see this by sea-level changes.
The Land ePeak of Glaciation most of British Columbia was depressed due to loading by glacier ice. eLateglacial shoreline occur on mainland coast up to 200m above present sea level. Depression of 400-500m
The Sea ePlaces like Parksville show that sea level fell from 108 to 52 m in few hundred years. eBy 7000 C yr BP; sea level 5m below present. eCalving embayment in Strait of Georgia
Uplift eIsostatic uplift migrated in step with retreating glacier margins. It would start as soon as the ice was thin enough. eDuring Pleistocene and early Holocene. eSome uplift may have been by fault slip.
References eJohn J. Clague, Thomas S.James. History and isostatic effects of the last ice sheet in southern British Columbia. Quaternary Science Reviews. 2002 eD. P. Dethier, Fred Pessl, R. F. Keuler, M. A. Balzarini, D. R. Pevear. Late Wisconsinan glaciomarine deposition and isostatic rebound, northern Puget Lowland, Washington. GSA Bulletin, 1995. eTom Rothhamel. Isostacy. 2000. http://www.homepage.montana.edu/~geol445/ hyperglac/isostasy1/