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 Composition Strength/Rigidity Mantle Core Inner and Outer Core Mesosphere

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

Reference point: sea level, but as sea level changes and 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

Surface uplift – raising elevation of the land surface eg Isostatic uplift – due to removal of eroded material eg. Tectonic uplift 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

US = E [(ρc / ρm) – 1] US = 1* [(2.7 / 3.3) – 1] US = 0.818 m 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] ρ c = 2.7 g/cm3 ρ m = 3.3 g/cm3 Assume: E = 1 m US = 1* [(2.7 / 3.3) – 1] 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?