Lithospheric%20Plates

1
Lithospheric Plates

• The lithosphere can be defined thermally by an
  isotherm at the base of the lithosphere which
  should be around 1350oC.
• Mantle rocks below this isotherm are cool and
  behave rigidly
• Rocks above this isotherm are hotter and may
  deform
• How are plates created ?

2
Heat Flow Through the Oceanic Lithosphere
Thermal Conductivity profiles in sediments

• Measuring heat flow on the seafloor

3
Heat Flow Through the Oceanic Lithosphere

• Heat flow measurements on the seafloor are found
  to decrease steadily with increasing distance
  from the spreading center.
• Deviations from a theoretical curve for heat
  flow indicate complications such as hydrothermal
  circulation.

4
Heat Flow Above the Oceanic Lithosphere

• Heat flow is highest for what age lithosphere ?
• How does heat flow vary with seafloor age ? Is
  it linear ?
• What kind of mathematical expression does this
  resemble ?

• On a log-log plot, the data falls on a slope of
  -1/2. How can you express this in an equation ?

5
Heat Flow Above the Oceanic Lithosphere

log Q log A -1/2

• Heat flow is inversely proportional to
  the square root of age (time).
• Lithosphere is defined by heat flow
• Why does heat flow decrease with age ?

6
Depth of the Lithosphere (Topography or
Bathymetry)

• Sonar techniques taken from ships passing over
  the ocean surface measure the water depth to the
  seafloor with excellent accuracy (at least within
  a few meters).

7
Depth of the Lithosphere (Topography or
Bathymetry)

• Seafloor bathymetry across a spreading ridge is
  shown here.
• Do you notice any differences between the
  Atlantic (A) and Pacific (E) spreading centers ?
• What causes these differences ?

8
Depth of the Lithosphere

• The depth to the seafloor (from sea level) is
  not constant...

• The seafloor is shallow at the rise axis and
  gets deeper away from it.
• Why is this ?

9
Depth of the Lithosphere

• Plotting seafloor depth versus the sqrt(Age)
  shows a slope of -1/3 regardless of spreading
  rate !
• How can we write the equation that describes this
  ?

d -1/3 sqrt(A) (1)
d is proportional to sqrt(A)
-or sqrt(time)
d sqrt(t)

• The lithosphere is described by it's depth in
  the ocean
• What does this mean ?

10
Depth of the Lithosphere

• The lithosphere can be described by conductive
  cooling
• Other factors have a smaller effect (upwelling
  volcanism at spreading axis etc...)
• How does thermal conductivity vary over time ?
• The time-dependent heat conduction equation
  (notes on in class and on board)

11
Class notes on board...
12
Time-Dependent Heat Conduction
dT/dt ? d2T/dx2
(Known as the Heat flow Equation)

• ?Where ??? k/?Cp is thermal
  diffusivity (m2/s).
• ? describes the diffusion of temperature or
  heat across a body of material

13
Time-Dependent Heat Conduction
dT/dt ? d2T/dx2

• ?Charcteristic diffusion time (t) can be
  described using ? where

t d2/?

• This gives the time for heat to diffuse across a
  distance, d.

14
Time-Dependent Heat Conduction
dT/dt ? d2T/dx2

• ?Charcteristic diffusion distance (d) can be
  described using ? where
• This gives the distance temperature will
  propogate through the material in a given time
  period.

d sqrt(??t?
15
Time-Dependent Heat Conduction
dT/dt ? d2T/dx2

• ?Charcteristic diffusion distance (d) can be
  described using ? where
• This gives the distance temperature will
  propogate through the material in a given time
  period.

d sqrt(??t?
16
Activity

• P wave tomography image of the Tonga trench
  subduction zone
• High velocity subducting slab is clearly visible
  (blue) extending down to at least 660 km depth.

17
Activity

• Seismic tomography image of the Pacific plate
  subducting beneath Japan.
• Scientists argue about whether subducting plates
  penetrate through the 660 km discontinuity into
  the lower mantle.

18
Activity

• Some authors say some slabs just rest at the 660
  and may thermally assimilate over time.
• Calculate how long it would take such a slab to
  thermally assimilate.
• Use the thickness of the slab you observe in the
  images above
• Assume thermal conductivity of peridotite, k
  3.0 Wm-1K-1, density 3250 kg m-3, and heat
  capacity, Cp 0.8 kJ/kg K