Topography of the earth’s surface
Topography of the earth’s surface
Depth to the Moho under north america and environs
Seismic structure of Greenlandmargin, and a related interpretivecartoon.
Topography in continental mountain chains and plateaus
Seismic structure beneath Himalayas
First-order topography of the ocean floor
Seismic velocity at 100 km depth
Fast (blue) = stiff and dense ~ coldSlow (red) = soft and low-density ~ warm
Seismic structure near a mid ocean ridge
Moho is hiding hereat ~6 km
Fast (blue) = stiff and dense ~ coldSlow (red) = soft and low-density ~ warm
Topography near ocean island chains
Seismic structure of the deep mantle near hawaii
• High topography = thick crust or warm mantle, and visa versa
• Often crust is thick and mantle cold, and topography is still fairly high; Thus crustal thickness effect ‘trumps’ mantle temperature effect
These observations reflect the role of isostacy in controlling topography
[chalk board notes on isostacy and orographic cycle]
WM Davis and the Geographic Cycle
Incision
Erosion
Mature
Isostatic ‘event’increases elevation
(‘Uplift’)
Heat flow at the earth’s surface
Measurements from a geothermal area in Iceland The archetype for the outer 300 km of the Earth
dT/dz ~ 1˚/40 meters, on average, near Earth’s surface
Temperature gradients near the earth’s surface
[chalk board notes on heat production and conduction]
Note that conduction also leads to a change in rheology between interior and outer shell
Rayleigh number = Buoyancy
Viscous drag XMomentum diffusivity
Thermal diffusivity
acceleration Thermal expansion
Kinematic viscosity Thermal diffusivity
Length scale
Temperature contrast
If > ~1000, convection ensues. The mantle is ~106
What are the dynamics of the hot, viscous (fluid like) interior?
A numerical model of whole-mantle convection in a2-D earth
Lord Kelvin’s measurement of the age of the earth
Take 1: a proof was presented in his Ph.D. thesis, but he burned his writings on this work after his thesis defense. It has never been recovered or reproduced.
Lord Kelvin’s measurement of the age of the earth
Take 2: directly determine age of the Earth by inverting the conductive temperature profile observed in its outer few km of crust
T (˚C)
Radial distance
1500
‘pinned’ by radiative balanceof surface
t0t1t2
0
dT/dt = k d2T/dx2
k = thermal diffusivity ~ 5x10-3 cm2/s (= ‘conductivity’/(densityxCv))Solution not simple, but is approximated by x = (kt)0.5, where x = distance from surface to mid-point in T profile.
x ~ 30 km; t ~ 20 million years
Melting point of rock
Jheat = k(dT/dx)
Lord Kelvin’s measurement of the age of the earth
Take 3: determine the age of the Sun using principles of gravitation and thermodynamics; infer this to be the maximum age of the Earth.
I: Measure flux of energy at earth’s surface (best above atmosphere directly facing sun) =1340 Js-1m-2
II: Integrate over area of a sphere with radius equal to distance from earth to sun (assumes sun emits energy isotropically) area = 4π(1.5x1011)2; power = 3.8x1026 Js-1
If dJ/dt is a constant:
(dJ/dt)xAge ≤ mass of sun x initial energy content (‘E’, in J/Kg))Age ≤ (2x1030 Kg)/(3.8x1026) x E Age ≤ 5000 x E
Lord Kelvin’s measurement of the age of the earth
Take 3, continued:
Age of sun ≤ 5000 x initial energy content of sun in J/Kg
Case 1: If sun’s radiance is driven by a chemical reaction, like combustion, then it’s highest plausible initial energy content is ~ 5x107 J/Kg
If the sun is a ball of gasoline, it is ≤ 2.5x1011 s, or 8000 years, old
Case 2: Sun’s radiance is dissipating heat derived from its initial accretion:
Potential energy of pre-accretion cloud…
converts to kinetic energy when cloud collapses…
turns into heat if collisions between accreting material are inelastic
Case 3: Sun’s accretion, continued:
Age ≤ 0.5MsxV2
3.8x1026 J/s
Age ≤ 1015 s ~ 30 Million years
Potential energy = -GMimj
Rji
Total mass M at center-of-masslocation, i
Component particle mass mat location j
Rji
Solution depends on the distribution of mass and velocity in the cloud before its collapse to form the sun
One simple solution supposes all constituent masses arrived at the sun with a velocity equalto the escape velocity from the Sun today:
(plus any contained in rotationor other motion of cloud)
V = (2GMs/R)0.5 = 618 km/s
i0.5miv2 = 0.5Ms(6.18x105)2
Q.E.D.: Physicists rule; geologists drool