1. But, classic Plate Tectonics do not explain everything…

2. Large-scale uplift - Caused by tectonics (Orogeny)
But ultimately in most places controlled
largely by crustal thickness, density and isostacy
(epeirogeny)

3. Earth’s Layers (a review)
Lithosphere
Crust
Asthenosphere
Composition
Strength/Rigidity
Mantle
Core
Inner and
Outer Core
Mesosphere

4. 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

5. 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

6. 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

7. 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

8. Large-scale uplift of a region due to
Isostasy
Example: Loading and unloading during glaciation
Example: Erosion of a mountain after uplift

10. Note forebuldge +200 km from LGM ice margin

12. Depression of the crust under Greenland –
Most of the bedrock surface below sea level today

13. Lake water loading – Lake Bonneville
Logan
Wendover
SLC

15. Isostatic Adjustment due to Mountain Range or ice sheet

16. Similar loading and uplift due to tectonic-driven
deposition and loading
of lithosphere
Rebound following erosion

17. MIS 5 meanderbelt
Rittenour et al 2007

18. Differential elevation of continents and oceans
due to rock density:

19. 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

20. Surface Elevation differences therefore
are a function of both Density Differences
And Lithosphere Thickness

21. 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

22. 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

23. 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 = m
Or, for every 1 m of erosion, the isostatic response uplifts the underlying rock by 82 cm

24. 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?