The Structure of the Earth

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The Structure of the Earth
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Differentiation of the Earth

• When Earth was molten, denser material and
  lighter material separated gravitationally
• Differentiation is responsible for the structure
  of the Earth as we understand it

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Structure of the Earth
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How do we know what is inside Earth??

• Seismic data
• Waves from earthquakes travel at different
  velocities through different media
• Network of seismometers detects delays as the
  waves go through the Earth
• Infer different layers of different density and
  composition
• We will talk about this in great detail when we
  discuss earthquakes

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How do we know what is inside Earth??

• Density measurements
• Average density of the Earth is 5.5 g/cm3
• Measure speed of earths rotation, revolving
  around the Sun, calculate density
• Average density of the crust is 2.7-3.0 g/cm3
• Density must be much greater inside the earth to
  account for this

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The Crust

• Two types of crust
• Continental crust
• 30-50 km thick on average
• Avg. density of 2.7 g/cm3
• Rocks high in silica and aluminum
• Principle rock type is granite
• Oceanic crust
• 7 km thick on average
• Avg. density of 3.0 g/cm3
• Rocks high in silica and magnesium
• Principle rock types basalt/gabbro
  (extrusive/intrusive)

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Lithosphere

• Comprises crust and uppermost mantle
• Named due to its rocky (litho) nature
• Lithosphere comprises the plates in plate
  tectonic theory
• Oceanic lithosphere is about 70 km thick
• Continental lithosphere can be 125-250 km thick

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Asthenosphere

• Part of the mantle immediately below the
  lithosphere
• 70 km thick
• Possibly partially melted rock
• Liquid and crystal slush

Mantle

• Ultramafic rock that is able to flow via plastic
  deformation
• Thickness (from crust to core) of around 2,900 km

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Crust and Mantle
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Convection of Mantle

• Hot, less dense material rises, cool denser
  material sinks
• Slow process
• Drives motion of tectonic plates (lithosphere)

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Inner Core

• Solid probably Nickel-Iron alloy
• Radius of 1,200 km

Outer Core

• Liquid of similar or identical composition to
  inner core
• Thickness of 2,000 km

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Circulation of Outer Core

• The outer core is a thick layer of circulating
  molten iron
• Flow rate of several km per year
• Driven by heat source
• Radioactivity?
• Creates electric current dynamo effect
• Thought to be the source of the earths magnetic
  field

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Geothermal Gradient

• Describes the change in temperature with depth
  into the earth