The Earth’s Structure

1
The Earths Structure

• series of layers or spheres which differ in
  density, chemistry (or composition) and physical
  properties.

2
Density

• is the relative heaviness of a substance
• defined as the mass per unit volume
• usually expressed in g/cm3
• EARTH HAS DENSITY STRATIFICATION

3
Evidence of the Earths Interior Composition

• Can only drill about 7.5 miles (earths radius
  4000 mi)
• Vents, volcanoes, variation in pull of gravity,
  etc.
• study of the shocks from distant earthquakes

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Earths Interior Is Layered

• What evidence supports the idea that Earth has
  layers?
• The behavior of seismic waves generated by
  earthquakes give scientists some of the best
  evidence about the structure of Earth.
• (a) If Earth were uniform (homogeneous)
  throughout, seismic waves would radiate from the
  site of an earthquake in straight lines.
• (b) If the density, or rigidity, of Earth
  increased evenly with depth, seismic wave
  velocity would increase with depth, and the waves
  would bend smoothly upward toward the surface.
• (c) If Earth were layered inside, some seismic
  waves would be reflected at the boundaries
  between layers while others were bent. Seismic
  evidence shows that Earth is layered.

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Compositional Layers of the Earth

• the Crust
• thin outermost layer
• the Mantle
• thick middle layer
• the Core
• densest inner layer
• composed mainly or iron (90)

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Figure 1.14
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Figure 1.16
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Continents and Ocean Basins Differ
2-3

• Continental crust
• is mainly composed of granite, a light colored,
  lower density (2.7 gm/cm3) rock
• thicker
• Oceanic crust
• is composed of basalt, a dark colored, higher
  density (2.9 gm/cm3) volcanic rock
• thinner

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Table 1.1
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Figure 1.17
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Physical Properties of the Layers

• Not determined only by chemical composition
• The behavior of the rock (brittle or plastic) is
  determined mainly by temperature and pressure

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Effects of Pressure and Temperature on Physical
State of Layers
2-1

• Increasing pressure raises the melting point of a
  material.
• Increasing temperature provides additional energy
  causing material to melt.
• Both pressure and temperature increase toward the
  center of the Earth, but at different rates.

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Divisions of the Earth Based Upon Physical State
2-1

• the Lithosphere
• cool, rigid outer layer
• comprised of continental crust, oceanic crust and
  the uppermost cool, rigid portion of the mantle
• the Asthenosphere
• hot, slowly flowing layer of the upper mantle
• the Mesosphere (Lower Mantle)
• rigid layer, similar chemically to the
  asthenosphere but very different physically
• the Outer core
• thick liquid layer
• the Inner core
• solid layer (due to tremendous pressure)

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Figure 1.14
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What Are the Sources of Internal Heat That Keep
the Asthenosphere Plastic?

• Radioactive decay (atoms of elements give off
  heat when their nuclei break apart)
• Internal heat moves to the surface by conduction
  slow migration of heat through a material by
  collision of atoms
• And by convection movement of heat in a fluid
  as it expands, becomes less dense and rises.

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Fig. 3-11, p. 57