Earth

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• Chapter 17
• Earths interior

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• Earths interior structure
• Earth is composed of three shells
• Crust
• Mantle
• Core

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• Earths interior and seismic waves
• The study of earths interior can be done by
• Direct sampling
• Indirect methods

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• Indirect methods
• Meteorites
• Seismic waves (P and S)
• P-waves can pass through all materials (solids
  and fluids)
• S-waves can only pass through solids.

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• The crust
• Outermost layer of the earth forming a thin skin.
  Divided into 2 parts
• Continental crust
• Oceanic crust

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• Continental crust
• 10-70 km thick, thickest beneath Himalayas.
• Density 2.7 g/cc
• 0.374 of Earth's mass.
• Consists of (felsic) granitic rocks below and
  sed. Rocks above.
• Seismic wave velocity is 6 km/sec.

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• Oceanic crust
• 5-8 km thick.
• Density 3 g/cc
• 0.099 of Earth's mass
• Consists of mafic rocks (basalt and gabbro).
• Seismic wave velocity is 7 km/sec.

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• Mantle
• upper mantle and
• lower mantle
• Upper mantle and crust constitute Lithosphere
  where the rocks are in rigid form.

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• Lower mantle is also called Asthenosphere where
  the rocks are in plastic state.

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• The boundary between the crust and mantle is
  called the Moho.

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• The mantle
• 67 of Earth's mass
• Density 3.3 g/cc
• depth of 70-2900 kilometers.
• Consists of olivine, pyroxene and garnet
• Seismic wave velocity is 8 km/sec.
• The boundary between the crust and mantle is
  called the Moho.

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• The Core
• Is there really a core?
• Combined Density of the crust and mantle is less
  than that of the whole earth.
• S-waves shadow zone

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• The Core
• Core is divided into 2 parts
• The outer core is about 2,250 km thick, composed
  of molten iron and nickel.
• The inner core is probably solid iron and nickel
  at about 5000 C.

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• What is isostasy?
• Its a balance or equilibrium of the earths
  crust floating on the dense mantle.
• Can be compared with wooden blocks floating on
  water. The higher the wood block appears, the
  deeper its root.

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• Erosion of continental crust shows isostatic
  equilibrium.

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• Erosion of glacial ice shows isostatic
  equilibrium in the form of crustal rebound.

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• Physical properties of Earth
• Gravity
• Magnetism
• Heat flow

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• Gravity gravitational attraction exists among
  all bodies in the universe

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• Gravity meter is used to measure gravity.

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• Gravity anomalies abnormal gravity readings.
• Positive anomaly gravity reading higher than the
  regional gravity readings
• Negative anomaly gravity readings lower than the
  regional gravity readings

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• Magnetism the earth acts like a giant dipolar
  magnet.
• Magnetic poles oriented 11.5 from the
  geographic poles.
• North pole
• South pole
• Magnetometer is used to measure magnetic strength.

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• Source of magnetism
• the convection currents in outer liquid core
  produce electric currents and any electric field
  is associated with a magnetic field.

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• Normal and reversed polarity

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• Paleomagnetism the study of ancient magnetism
  preserved in rocks.

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• Magnetic anomalies
• Positive
• Negative

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• Heat flow the rate at which temperature
  increases within the earth is called Geothermal
  gradient.
• 25 C/km at shallow depths (crust) but drops to
  1C/km within the mantle.

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• Sources of heat inside the earth
• Radioactivity
• Rising magma bodies
• Gravitational energy from accretion.

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