T5 Lithosphere

• Lithosphere


  Represents only c.2% of the Earth, by mass
• Heat flow


  The rate at which heat flows from the Earth's interior through one square metre of the Earth's surface
• Heat flow units


  Watts per square metre (W/m2)
• Heat flow is uneven. It varies with depth and across the Earth's surface
• Geothermal gradient


  • Steeper slopes indicate smaller change in temp with depth, so smaller geothermal gradient
• Rock in the lithosphere is too strong and rigid for convection to take place


  Heat therefore moves through the lithosphere mainly by conduction
• Conduction is slow


  The geothermal gradient in the lithosphere is high (temperature changes rapidly with depth)
• Heat flow decreases more slowly with increasing age in continental lithosphere compared to ocean lithosphere
• Isotherm


  Line on a map\curve on a diagram joining points of equal temperature
• 1300 degree isotherm


  The base of the lithosphere is defined as the 1300°C isotherm
• Lithosphere


  • Includes the crust (continental & oceanic) and the uppermost part of the mantle
  • Thins to a few km at ocean spreading centres
  • Thickens to about 100-150 km under older parts of ocean basins
  • Is up to 250-300 km thick under continental shield areas
  • Mainly composed of mantle rocks

  
• The 1300 degree isotherm is a boundary for the mechanical division of the lithosphere and asthenosphere

  
• Lithosphere


  The outermost solid shell of a rocky planet, including the crust and uppermost part of the mantle
• Evidence for the lithosphere & asthenosphere

  • Asthenosphere
  • Upper Crust
  • Lower Crust
  • Upper Mantle
  • Mantle

  
• Seismic wave velocities change when the material they are passing through changes
• Increase in velocity of P and S waves through the crust due to an increase in rigidity and incompressibility
• Increase at the Moho- change in composition (to peridotite). Rigidity and incompressibility dramatically increase
• Decrease at the LVZ- this is where rocks are close to their melting points and behave in a ductile manner. Rigidity and incompressibility decrease
• Discontinuities


  Boundaries in the lithosphere that result in changes in seismic wave velocities as a result of changing rock properties
• Mohorovicic Discontinuity

  • Marks the position of the crust-mantle boundary
  • Sudden increase in seismic wave velocities as a result of increasing incompressibility and rigidity
  • Under oceans (average 7km)
  • Under continents (av. 35km and up to 90km)
• Mohorovicic Discontinuity


  1. Two pairs of P and S waves are received

  2. If far enough away the refracted waves arrive before those moving horizontally through the crust
• Conrad Discontinuity


  • Boundary between the upper and lower crust
  • Depth of about 15-20km
  • Found in some continental areas only
  • Exact nature of the boundary is unknown
• ρ is the density, k is the bulk modulus, u is the shear (or rigidity) modulus
• Calculating Seismic Wave Velocities

  vp^2 = k + 4/3 u

  vp^2ρ = k + 4/3 u

  vp^2ρ - 4/3 u = k

  vp^2ρ - k = u
• Lithosphere


  The outermost solid shell of a rocky planet, including the crust and uppermost portion of the mantle
• Most of the content draws on knowledge from the year 1 plate tectonics and structure of the Earth topic
• New content will be indicated on the slides with an easter egg
• The Geology of the Lithosphere

  1. Part 1: the Earth's heat flow

  2. Part 2: Seismological evidence

  3. Part 3: Ocean lithosphere

  4. Part 4: Continental lithosphere
• Ocean lithosphere


  • Has a layered structure: seismic layers 1, 2, 3 and 4
• Layers of ocean lithosphere

  • Layer 1: Sediments
  • Layer 2: Basaltic lavas, Sheeted dykes
  • Layer 3: Gabbro
  • Layer 4: Peridotite
• Layer 1


  Sediments - black shales, volcanic tuff, siltstones, sandstones. 0-10km thick, increases with distance from mid-ocean ridge (MOR), very little over MOR and very thick near continental areas
• Layer 2


  Mafic - Basalt, up to 1/2km thick. Pillow lavas, sheeted dykes up to 1km thick
• Layer 3


  Mafic - Gabbro, 5km thick, coarse-grained (slow cooling), magma chamber
• Layer 4


  Ultramafic - Peridotite, upper mantle - lithosphere, olivine-rich
• Evidence for structure of ocean lithosphere

  • Seismic
  • Deep sea drilling
  • Ophiolites
• Seismic evidence


  Layer 1: Increases as sediments become more consolidated

  Layer 2: Large increase due to change from sedimentary to igneous rocks, increases with depth due to fewer fractures and vesicles

  Layer 3: Gradual increase due to fewer fractures and less water

  Layer 4: Large increase where gabbro rests on peridotite - more rigid and less compressible
• Ophiolite complexes


  Sections of the Earth's oceanic crust and upper mantle that have been uplifted and deposited on continental margins, consist of 5 layers: sediment, pillow basalt, sheeted dykes, gabbro and peridotite, best direct evidence for the structure of the lithosphere
• Palaeomagnetism


  Basalt contains iron-rich minerals that align to the magnetic field when cooling, preserving the magnetic field at that time (remnant magnetism)
• Palaeomagnetism provides information on magnetic reversals, with an average of 4-5 reversals per million years, and the polarity is recorded on the seafloor as evidence for seafloor spreading
• Magnetic stripes on seafloor

  Not all the same width, varying with length of time during particular polarity and amount of lava erupted