Plates, Isostasy, and
Planetary Topography
Lithosphere: Rigid outer layer
How do we know they are rigid?
Deform along their edges, undeformed internally.
Lithosphere: Rigid outer layer
Plate types
Oceanic lithosphere
6 km of basalt
Up to 100 km of upper mantle peridotite
6-7 km
Plate types
Oceanic lithosphere
~6 km of basalt, up to 100 km of upper mantle peridotite
Plate types
Oceanic lithosphere
Plate types
Continental lithosphere
Plate types
Continental lithosphere
Old in the center
Plate types
Continental lithosphere
Cold in the center
Plate types
Continental lithosphere
Thickness variations
Plate boundaries
Constructive
Plate boundaries
Constructive
Plate boundaries
Constructive
Ridges ~ 2 km above average ocean floor
Plate boundaries
Destructive
Plate boundaries
Destructive
Plate boundaries
Destructive
Plate boundaries
Conservative
Plate boundaries
Conservative
Great India Triangulation Survey (GITS)
1802 George Everest
Mountain don’t have a mass excess, they have a mass deficit.
Isostasy:
Equilibrium in the Earth such that the forces tending to elevate
landmasses balance the forces tending to depress landmasses.
Isostasy:
Equilibrium in the Earth such that the forces tending to elevate
landmasses balance the forces tending to depress landmasses.
Airy Isostasy
Pratt Isostasy
Airy-Pratt Isostasy
Isostasy
Force (F) = mass x acceleration
Pressure (σ) = F/unit area
Force/unit area (N/m2 )
= density (ρ, kg/m3 ) * height (m) * acceleration (g, 9.80665 m/s2 )
b
ρM =
2.75 gm/cm3
h
ρM = 3.3 gm/cm3
Weight
Weight
=
here
here
Isostasy
ρC =
2.75 gm/cm3
b
h
ρM = 3.3 gm/cm3
Weight
here
=
σM=ρ Mgb
Weight
here
Depth of lithosphere
σ C =ρ Cgh
ρC gh = ρ Mgb
ρC h/ρM = b
h = b(ρ M /ρ C )
Continental height: h-b = h- h(ρC /ρ M)
= h(1-ρ C/ρ M)
= h(1-2.75/3.3)
= h (0.17)
= 35 km (0.17) = 5.8 km
i
j
1
2.9
2.7
Σρighi = Σρjghj
3.3
3.3
Tectonic impacts
Shave a range
To destroy topography, must remove a huge amount of rock.
7 km removed to get rid of 1 km elevation.
Lithospheric support
If load is relatively small (i.e., has a diameter less than the
thickness of the plate, load will be supported rigidly.
Size is >50 to 100 km.
100 km
Lithospheric flexure
In reality, isostatic compensation doesn’t happen at a point,
with an exact mirror image of topography at depth. Often there
is regional lithospheric bending to partially accommodate the
load, as well as a root.
Glacial Isostasy
Mantle viscocity: 1021 x water
Thermal Expansion and Topography
Change in volume or density with temperature
α
( 1 / K)
= 1 ΔV = - 1 Δρ
V ΔT
ρ ΔT
Hotter rock, less dense: α = 3 x 10-5/k