Transcript Isostasy
But, classic Plate Tectonics do not explain everything… Large-scale uplift - Caused by tectonics (Orogeny) - But ultimately in most places controlled largely by crustal thickness, density and isostacy (epeirogeny) Earth’s Layers (a review) Lithosphere Crust Asthenosphere Mantle Core Inner and Outer Core Composition Mesosphere Strength/Rigidity Tectonic Geomorphology Plate Tectonics (driven by mantle flow), Dynamically Supported Topography (due to heat and density differences), Isostasy (response to loading and unloading) Uplift driven by these mechanisms generates erosion that sculpts the landscape Uplift or Depression by Isostasy - Caused by an equilibrium condition where Lithosphere floats on Asthenosphere Asthenosphere flows under Lithosphere due to Addition/removal of mass from a region Region of Isostatic Adjustment is broader than the Area being load Sea level or Geoid Reference point: sea level, but as sea level changes and needs to be projected onto continents, we use the Geoid Geoid – sea level epipotential surface (adjusted for gravity variations) and projected onto continents The geoid is the shape that the surface of the oceans would take under the influence of Earth's gravitation and rotation alone Large-scale uplift of a region due to Isostasy Example: Loading and unloading during glaciation Example: Erosion of a mountain after uplift Note forebuldge +200 km from LGM ice margin Depression of the crust under Greenland – Most of the bedrock surface below sea level today Lake water loading – Lake Bonneville Logan Wendover SLC Isostatic Adjustment due to Mountain Range or ice sheet Similar loading and uplift due to tectonic-driven deposition and loading of lithosphere Rebound following erosion MIS 5 meanderbelt Rittenour et al 2007 Differential elevation of continents and oceans due to rock density: Mountain ranges are high because: Pratt’s Hypothesis: Composed of less dense materials Why different elevation between ocean and Continental crust Airy’s Hypothesis: Thicker and have deeper roots Iceberg example: 90% of iceberg Below the surface Both in proposed in AD 1855 Surface Elevation differences therefore are a function of both Density Differences And Lithosphere Thickness eg. Tectonic uplift eg Isostatic uplift – due to removal of eroded material Surface uplift – raising elevation of the land surface Rock uplift – raising elevation rock unit, but not changing depth Exhumation – uplift of rocks relative to ground surf by erosion of surface rocks Surface uplift (US), Rock uplift (UR) and exhumation (E) are all related: US = UR - E Where, Rock uplift (UR) is the sum of tectonically driven rock uplift (UT) and isostatically driven rock uplift (UI) UR = UT - UI But, erosion (Exhumation) and rock uplift are not independent… Exhumation leads to unloading and isostatic compensation (surface and rock uplift), which results in less mean surface lowering than the depth of rock removed by erosion The magnitude of isostatic compensation is related to the density of the mantle (ρm) and crust (ρc) US = E [(ρc / ρm) – 1] US = 1* [(2.7 / 3.3) – 1] Assume: ρ c = 2.7 g/cm3 ρ m = 3.3 g/cm3 E=1m US = 0.818 m Or, for every 1 m of erosion, the isostatic response uplifts the underlying rock by 82 cm But erosion commonly leads to more isostatic uplift…. which leads to more erosion of valleys (erosional gradient)…. which leads to increase relief (valley to peak difference)… and higher peaks, but lower mean elevation also Climate-Uplift?