Glacial Rebound Glacial Rebound Studies depend on many factors. What are they ? Ice load History of the load Ocean water load on coastlines and globally.
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Glacial Rebound Glacial Rebound Studies depend on many factors. What are they ? Ice load History of the load Ocean water load on coastlines and globally Changes in ocean water volume Gravity mass distribution changes Lithospheric elasticity Mantle Viscosity Variations in mantle viscosity (lateral and vertical) An equation encompassing all this would be quite complicated! Taylor Glacier, Antarctica
Glacial Rebound Glaciers form and cause subsidence of Earth's crust When glaciers melt, weight is removed and crust starts to rebound
Glacial Rebound Full quantitative analysis is complicated ! Some processes are dominant We can simplify and obtain bulk estimate of the signal For example: The time scales of rebound is well approximated at the center of an ice sheet The depth of viscous response is roughly equal to the radius of the ice sheet (~1/2 the lateral extent) R glacier
Glacial Rebound R In Sweden (Angerman River), the depth of viscous response was estimated as far as 1400 km! Later Haskell inferred a viscosity of 10 21 Pas suggesting an average mantle viscosity to 1400 km depth
Glacial Rebound Later Haskell inferred a viscosity of 10 21 Pas suggesting an average mantle viscosity to 1400 km depth Large glaciers can cause mantle deformation or displacement deep in the Earth's interior
Glacial Rebound R Mitrovica showed that a layered viscosity could give the same average viscosity ( Figure 6.12 in your Davies' text ) Viscosity increases in the lower mantle (but gives the same overall average) 660 km 3 x 10 20 Pas 6 x 10 21 Pas
North American Ice Sheet The North American ice sheet was bigger than in Sweden We can obtain deeper estimates of viscosity up to 2000 km !
Load of Ocean Water We can measure rebound of the seafloor far from glaciers The rise and fall of sea level creates load changes on seafloor The seafloor responds by uplift or depression Does seafloor respond as quickly as sea level changes ? Why ?
Load of Ocean Water The seafloor and underlying mantle responds slower than sea level can change, so response is delayed Mantle viscosity is higher than water viscosity and flows much slower This delay causes water to flow back to coastlines and temporarily 'overrun' until seafloor relaxes See Figure 6.13 (Davies, pg. 162) Sealevel measured over time “overshoots” its peak Age(ka) Relative sea level (m) 10 60 -20 0 8
Load of Ocean Water We can measure the viscous response of the mantle by measuring sea level changes along coastlines over time This gives us a good estimate of the upper 1/3 and middle 1/3 of the mantle viscosity, but we still know very little about the viscosity of the deepest 1/3 of the mantle We must turn to laboratory studies of rock rheology and high pressure and temp for deeper viscosities. ?
Isostacy of The Earth's Crust The Earth's crust is in approximate hydrostatic balance on large horizontal scales This indicates that the Earth's interior is responding as a deforming fluid over time (not completely rigid)
Isostacy of The Earth's Crust At some depth (the compensation depth) below the crust, we can draw columns which represent blocks of equal mass Below this depth, crustal variations are hydrostatically balanced
Isostacy of The Earth's Crust ice crust mantle D Consider a glacial load acting on the crust Removal of the icea load generates stresses in the underlying mantle caused by a “pressure deficit”
Isostacy of The Earth's Crust ice crust mantle D C1 C2 We can estimate these stresses by considering the mass addition and removal of a representative column The mass of column 1 (C1) equals the mass of column 2 (C2) What are the masses which make up column 2 ? c g D + m g
Isostacy of The Earth's Crust ice crust mantle D C1 C2 What properties effect the rate of rebound ? c g D + m g Mantle viscosity Lithospheric rigidity
Isostacy of The Earth's Crust We can relate these stresses to the rate of rebound (strain rate) . See class notes