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Chapter 7
Structure of the Solid Earth/
Plate Tectonics
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Outline for this and the next lecture:
• Anatomy of the Earth
(as derived from seismology)
– Seismic information
– Internal structure
• Movement of the solid Earth
– Theory of plate tectonics
– Evidence for plate tectonics
• Mid ocean ridges
• Magnetic reversals
– What drives plate tectonics
– Plate tectonics through earth history
• Plates, plate boundaries, and plate interactions
• The Wilson Cycle
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Alfred Lothar
Wegener
(1880-1930)
-- the originator of
the theory of
continental drift
His theory was first
dismissed but later
resurrected based on
evidence from seafloor spreading.
Photograph courtesy of the Alfred Wegener Institute
for Polar and Marine Research, Bremerhaven,
Germany
Kump et al (2000)
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Alfred Wegner’s
reconstruction of
the positions of the
continents in the
geological past.
“Pangea”
Based on:
• Similar fossils in S.
America and Africa
• Apparently contiguous mountain
ranges
• Similar shapes of
coastlines of S.
America and Africa!
Kump et al (2000)
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Perceived Problems with
Wegener’s Theory
• Wegner’s hypothesis was initially
rejected based on (wrong) theoretical
analyses and the lack of a physical
mechanism
• How could continents plow their way
through the solid rock of the seafloor?
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Answer to problem: The seafloor itself was
moving as a consequence of plate tectonics
We learned about this using evidence from
Earth’s magnetic field. Back to that later.
First, let’s consider what we can learn from
other phenomenon, e.g., earthquakes
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Earthquakes:
One way to learn about the Earth’s interior
Kump et al (2000)
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Effects of an
Earthquake
Loma Prieta earthquake
CA, October 1989
Magnitude 7.1 on Richter scale
Kump et al (2000)
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A simple
seismograph
Note: Rotating
drum data recorders
may now be found
in museums..
Kump et al (2000)
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Seismic
body waves:
P and S
waves
P-waves:
-Primary waves
-Series of compression
and expansions in the
direction of wave
movement
- much like a slinky
S-waves:
- Secondary waves
- Perpendicular
replacement
P-Waves
S-Waves
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How to remember them
• P waves
– Primary waves or pressure waves
• S waves
– Secondary waves or shear waves
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Where do P and S waves
propagate?
Consider the restoring force:
• For P waves it is pressure (as for sound
waves)
– Sound waves propagate through air. What
about water? Solids?
• For S waves it is tensile strength
– Do solids have tensile strength? Do
liquids?
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Where do P and S waves
propagate?
• P waves travel though both solids and
liquids
• S waves travel only through solids
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Outer core
(liquid)
P-wave
Inner core
(solid)
S-wave
Mantle
(solid)
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Internal
Structure
of the
Earth
Important features:
- Crust
- Mantle
- Outer core
- Inner core
Kump et al (2000)
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A
Internal
structure of
the Earth a second
overview
focusing
on material
strength
Kump et al (2000)
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Internal
structure
of the
Earth detail A
(“MOHO”)
Kump et al (2000)
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The interior
of the
Earth: A
first look
Tomographic
image of the
mantle’s (a) Pwave and (b) Swave velocity
Kump et al (2000)
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Locations of Earthquakes
Kump et al (2000)
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The lithosphere is divided into roughly 20 rigid plates
Plate move at speeds of a few cm per year.
What happens at the plate margins?
Kump et al (2000)
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Hawaiian
Island
Chain
(mantle
plume)
 The Pacific
Plate is drifting
northward and
westward
From: Skinner and Porter, The Blue Planet, 1995, p. 145
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