Isostasy and the
dynamics of the
Earth’s crust
Isostasy: the condition of
equilibrium, comparable to
floating, of the units of the
lithosphere on the
asthenosphere.
Lithosphere
Asthenosphere
Altitude (km)
Area of Earth’s surface (106 km)
Hypsographic Curve
for the Earth
Rigid mantle ρ = 3.3 g/cc
Depth (km)
continental crust
oceanic crust
mean altitude 823 m
mean depth 3729 m
Percentage of Earth’s surface
Mars
Earth
Venus
aesthenosphere
The physical state
(solid / partially solid / liquid)
of the different layers of the
Earth depends on which is
higher - the melting
temperature or the actual
temperature.
mantle
core
outer
rock
p.sol.
solid
liquid
A=M
A<M
A>M
inner
iron
Graph of Age vs. Depth of Ocean Floor
Mid-Ocean
Ridge
Trench
Why might the ocean floor sink as it gets older?
Mid-ocean ridge
time
Trench
Oceanic crust ρ = 3.0 g/cc
lithosphere
Rigid mantle
ρ = 3.3 g/cc
Asthenosphere mantle
ρ = 3.25 g/cc
ρlithosphere = ρcrust x %thicknesscrust + ρmantle x %thicknessmantle
Mid-ocean ridge
time
Trench
Oceanic crust ρ = 3.0 g/cc
Rigid mantle
ρ = 3.3 g/cc
Asthenosphere mantle
ρ = 3.25 g/cc
age = 0 my
ρlithosphere = 3.0 g/cc x 1.0 + 3.3 g/cc x 0 = 3.0
ρlithosphere << ρasthenosphere
Mid-ocean ridge
time
Trench
Oceanic crust ρ = 3.0 g/cc
Rigid mantle
ρ = 3.3 g/cc
Asthenosphere mantle
ρ = 3.25 g/cc
age = 10 my
ρlithosphere = 3.0 g/cc x .4 + 3.3 g/cc x .6 = 3.18
ρlithosphere < ρasthenosphere
Mid-ocean ridge
time
Trench
Oceanic crust ρ = 3.0 g/cc
Rigid mantle ρ = 3.3 g/cc
subduction
Asthenosphere mantle
ρ = 3.25 g/cc
age = 100 my
ρlithosphere = 3.0 g/cc x .06 + 3.3 g/cc x .94 = 3.28
ρlithosphere > ρasthenosphere
Mid-ocean ridge
time
Oceanic crust ρ = 3.0 g/cc
Rigid mantle ρ = 3.3 g/cc
Trench
Continent
ρ = 2.7 g/cc
ρ = 3.3 g/cc
Asthenosphere mantle
ρ = 3.25 g/cc
Can continents be subducted into the mantle?
Isostasy - equilibrium position of lithosphere
“floating” on asthenosphere.
continental lithosphere
oceanic lithosphere
Asthenosphere
oceanic lithosphere
Other ways to change the
isostatic equilibrium of the crust
• Heating / cooling of the lithosphere
– Changes the density of the crust = buoyancy
– Caused by passage of plates over mantle hot
spots
• Heating / cooling of the mantle
– Rising plumes of hot mantle displace the
lithosphere upward
– Sinking slabs of cold mantle pull the
lithosphere downward
Continental Crust
acrust
sea level
hcrust = acrust + dmantle
dmantle
tcrust
ρ = 2.65 g/cc
tmantle
(displaced)
ρ = 3.25 g/cc
Archimedes’ principle:
tcrust x ρcrust = tmantle x ρmantle
Example
acrust
sea level
dmantle
tcrust
ρ = 2.65 g/cc
tmantle
How much mantle
will be displaced
by 45 km of crust?
tcrust x ρcrust = tmantle x ρmantle
45 x 2.65 = tmantle x 3.25
ρ = 3.25 g/cc
Archimedes’ principle:
tcrust x ρcrust = tmantle x ρmantle
45 x 2.65 = tmantle
3.25
36.7 = tmantle
Example
acrust
sea level
dmantle
36.7 km = tmantle
45 km = tcrust
If dmantle = 8.0 km
tcrust
ρ = 2.65 g/cc
tmantle
How high above sea
level will the crust be?
acrust = tcrust - ( dmantle + tmantle)
ρ = 3.25 g/cc
Archimedes’ principle:
tcrust x ρcrust = tmantle x ρmantle
acrust = 45 - ( 8 + 36.7)
acrust = .3 km