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11/5/2015
Earth’s Internal Structure
Earth’s Layers and Zones
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GEOL 110: GEOLOGY
Ch. 12: Earth’s Interior
3 major layers based on chemical
composition:
• Crust: granitic/basaltic
• Mantle: peridotite
• Core: iron
4 zones based on physical properties:
• Lithosphere (crust and upper mantle):
rigid
• Asthenosphere: weak, viscous deforming
region, convection occurs
• Outer core: liquid
• Inner core: solid
Not for Distribution. For study purposes only for related class.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Earth’s Internal Structure
How do we study Earth’s layers?
Earth’s Internal Structure
How do we study Earth’s layers?
• Meteorites: similar to earth composition
• Seismic waves
• Look at rocks brought up from interior (xenoliths,
volcanic eruptions)
Stony: olivine &
pyroxene, silicates
(lithosphere)
Iron: nickel & iron
(core)
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Earth’s Internal Structure
What Caused Layers to Form?
Density:
Number of molecules per cm3
1 cm
What Caused Layers to Form?
Gravity and Density
• Gravity: force that attracts a body towards the
center of the earth
Earth’s Internal Structure
1 cm
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
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Earth’s Internal Structure
What caused layers to form?
Earth’s Internal Structure
Earth’s Interior
Gravity and Density
• Gravity: force that attracts a body
towards the center of the earth
• The densest material (iron) sinks to
the center of the planet (eg: bottle
filled with air, water, clay, iron)
• Gases rise to atmosphere
Water: 1.0
Andesite: 2.5 - 2.8
Basalt: 2.8 – 3.0
Peridotite: 3.1 – 3.4
Iron: ~8.0
Nickel: ~9.0
© 2014 Pearson Education, Inc.
Probing Earth’s Interior
How do we detect the
different layers?
© 2014 Pearson Education, Inc.
Probing Earth’s Interior
Seismic waves in
Homogenous Material
Seismic velocities
Seismic Waves
• Most knowledge comes from
the study of earthquake
waves
• Seismic rays “fan” out from
earthquake source
• Generally, travel faster through denser
materials. Eg: billiards
• Faster through cold (denser) vs. warm
• Travel faster through rigid materials
• Travel times of P and S waves vary
depending on the properties of the
materials
• Higher velocity in Ocean Crust than
Continental Crust
http://serc.carleton.edu/eet/seismicwave/going_further.html
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Probing Earth’s Interior
Probing Earth’s Interior
Seismic Waves in Heterogeneous Material
• Reflected wave and refract waves as they pass
through the different layers of Earth
• Generally, velocities increase with depth
• Do not travel in a direct path
• Reflect and refract as they pass through the different
layers of Earth
• Reflect: mirror
• Refract: light through prism
• Gives a layer-by-layer understanding of Earth’s
composition
Faster velocity =
denser material
© 2014 Pearson Education, Inc.
Slower velocity=
less dense material
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P and S wave Shadow Zones
Shadow Zones
Seismic Waves: How we
detect layers
• P-wave shadow zone: P-waves interact with the lowvelocity liquid iron and the outer core, causing rays to be
refracted downward, creating a shadow zone.
• S-wave shadow zone: S-waves cannot pass through
liquids.
Inge Lehmann 1888-1993
• Discovered solid inner core and
liquid outer core based on seismic
waves
• 1929 New Zealand earthquake; p
waves reflected off a boundary
and recorded at other stations.
• Lehmann boundary: between inner
and outer core
• Confirmed in 1970
http://serc.carleton.edu/eet/seismicwave/going_further.html
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Earth’s Layers
The Variable and Unusual D Layer Lies at the
Base of the Mantle
2 additional Earth Layers
•Crust: Ocean and Continental
•Moho: boundary between crust and
mantle; quick change in velocity
•Upper Mantle
•Lower Mantle
•D”: boundary between mantle and
outer core; source of deep mantle
plumes and remnants of subducted
plates
•Outer Core
•Inner Core
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Press and Seiver,
2002
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