2. 4-2
Chapter 4
Earth’s Structure and Plate Tectonics
Amante, C. and B. W. Eakins, ETOPO1 1 Arc-Minute Global Relief Model: Procedures, Data Sources and Analysis, National Geophysical Data center, NESDIS, NOAA, U.S. Department of
Commerce, Boulder, CO, August 2008.
3. 4-3
Deformation of Rocks (1)
Elastic limit deformation becomes
permanent
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5. 4-5
Earth’s Interior
Seismic waves
• Travel at different speeds through different
materials
• Reflect and refract when density changes
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6. 4-6
Earth’s Structure
Crust –
• Lithosphere < 15 km
Mantle
• ~2900 km thick
• top is asthenosphere
Outer core
• liquid
Inner core
• solid Jump to long description
7. 4-7
Earth’s Magnetic Field
• Inner core is solid, rotates faster than
liquid outer core
• Generates magnetic field
National Geophysical Data Center/NOAA
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8. 4-8
Earth’s Internal Heat
• Geothermal gradient
25°C/km
• Heat from radioactive
decay of uranium,
thorium, and potassium
• Friction and pressure
• Convection transfers
heat
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9. 4-9
Continental Drift
Frank Taylor 1910 suggested the
continents were once joined
Alfred Wegener 1922 book on theory of
continental drift
• More evidence than Taylor
• No mechanism
11. 4-11
Developing Theory of Plate Tectonics
Mapping ocean floor
• Mid Atlantic ridge
Magnetic studies
• Polar reversals
Map location and depths of earthquakes
• Fall on plate boundaries
Polar wandering
• Moving poles and continents supported data
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16. 4-16
Plate Tectonics and Earth’s
Systems
Plates
• Rigid slabs in lithosphere
Plate Boundaries
• Plates move, slide past
• Override
• Tear
• Smash into each other
25. 4-25
Plate Boundaries and People
Natural hazards
• Earthquakes, volcanic eruptions
Natural resources
Climate
Development of life
USGS
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26. 4-26
The shifting position of continents affects
Earth’s climate by altering the circulation of
heat and water in the oceans.
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29. Deformation of Rocks (1) Long Description
A two-dimensional representation of the deformation (strain) that would result from different types of stress
acting on a square.
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30. Deformation of Rocks (2) Long Description
Rocks will deform elastically up to a point, beyond which deformation becomes permanent. Ductile materials
deform permanently by flowing plastically, whereas brittle materials fracture. Rocks near the surface are
typically brittle and will fracture, but when buried, the higher temperatures and pressures cause them to
become ductile and deform plastically.
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31. Earth’s Interior Long Description
Seismic waves generated by earthquakes and human-made explosions will reflect and refract when
encountering layers of different density. Recording instruments measure the waves that return to the
surface, enabling scientists to determine the depth of different layers all the way to Earth’s core.
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32. Earth’s Structure Long Description
The Earth has a layered structure consisting of a high-density metallic core surrounded by a less dense rocky
shell of silicate minerals, called the mantle. Near the top of the mantle the silicate minerals are close to their
melting points, creating a weak and plastic zone called the asthenosphere. The outermost silicate shell is
called the crust, which has the lowest density of all the layers. Geologists refer to the crust and
uppermost mantle as the lithosphere since they behave as a single, rigid slab that moves over the
asthenosphere. Note that the lithosphere in continental areas contains granitic crust, whereas basaltic
lithosphere lies beneath the oceans.
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33. Earth’s Magnetic Field Long Description
Earth’s strong magnetic field is believed to be the result of the metallic ions in the outer core circulating as
the planet rotates. The magnetic field is important to the biosphere because it helps shield the Earth from
harmful radiation streaming from the Sun.
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34. Earth’s Internal Heat Long Description
Soon after Earth formed, enough heat was generated to cause the planet to become molten, with only a thin
crust. Convection cells developed and began transporting both heat and matter to the surface, where some
of the heat was lost to space by conduction—similar to that of a pot of boiling soup. Earth soon developed
its layered structure as molten iron and nickel sank to the core and lighter elements tended to rise,
eventually forming the crust and mantle.
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35. Plate Tectonics Long Description
a) The distribution of unique plant and animal fossils on different continents (A) supports the idea of a single
supercontinent. The matching fossil assemblages between South America and Africa were first noted by
Alfred Wegener. The supercontinent called Pangaea (B) was originally proposed by Wegener, who
suggested that the continents slowly drifted to their present position. Modern data show that Pangaea
began to break up approximately 225 million years ago.
b) The mismatch of glacial deposits and present day climates on several continents helped support the idea
of continental drift.
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36. Developing Theory of Plate Tectonics Long Description
A near-perfect fit of Africa and South America was obtained when the two landmasses were reassembled
using the edge of their continental shelves as opposed to their coastlines.
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37. Mapping the Ocean Floor Long Description
Modern map showing the topography of both the land and seafloor. One of the striking features of the
oceans is the extensive network of mid-oceanic ridges that circle the globe, shown here in white. The oceans
also contain narrow trenches that reach depths of nearly 7 miles (11 km).
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38. Magnetic Reversal Long Description
a) Illustration showing the development of magnetic striping by seafloor spreading. Magma rises up through
mid-oceanic ridges and then cools to form basaltic rocks whose magnetite grains record the orientation
and polarity of Earth’s magnetic field. As the seafloor continues to spread and new rocks form, a
symmetrical pattern of reverse and normal polarity develops on opposite sides of the ridge .
b) Map showing the age in years before present (BP) and topography of the Atlantic seafloor. Note how the
seafloor gets progressively older away from the mid-oceanic ridge, with the oldest seafloor being about
200 million years old.
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39. Earthquake Locations Long Description
Map showing the location of earthquake epicenters from 1963 to 1998. Note how the earthquakes are not
randomly distributed. Inset (A) illustrates how rising magma beneath mid-oceanic ridges generates
earthquakes whose epicenters lie in a relatively narrow zone at the surface. Inset (B) shows how epicenters
near ocean trenches are spread over a wider area due to the way the subducting slab generates earthquakes
in an inclined zone.
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40. Polar Wandering Long Description
When paleomagnetic studies assumed that the continents remained fixed (A), the position of the magnetic
North Pole appeared to wander over time. Moreover, each continent showed a different wandering path for
the pole. When seafloor spreading data were used to reposition the continents at different times in the
geologic past (B), a single location for the pole emerged.
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41. Plates Long Description
Map showing the distribution of lithospheric plates. Note how the North American plate is covered by both
continental and oceanic crust, whereas the Pacific plate is covered entirely by oceanic crust.
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42. Plate Boundaries Long Description
Illustration showing how divergent, convergent, and transform plate boundaries are under tension,
compression, and shear forces, respectively. Note how mid-oceanic ridges and mountain chains are features
that form parallel to plate boundaries. Volcanic hot spots occur away from plate boundaries and are believed
to be related to hot plumes of material that rise from deep within the mantle.
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43. Movement of Plates Long Description
a) Relationship between mantle convection cells and plate boundaries. Note how the plates move in the
direction of the convection cells.
b) A thick layer of snow on this car provides an example of the ridge push mechanism. Gravity caused the
snow to push downward on the sloping surface of windshield, forcing the rest of the snow to slide over
the hood. Note that the snow layer behaved as a rigid plate, allowing it to buckle, forming a fold. Also
note that unfrozen water was present on the surface of the car, creating a weak layer that facilitated the
movement of the overlying snow.
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44. Surface Features & Plate Boundaries Long Description
Map showing the types of movement taking place at the boundaries of Earth’s major plates.
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45. Divergent Plate Boundaries Long Description
Sequence of events showing the development of a midoceanic ridge system. A rising convection cell (A)
pushes upward against the brittle lithosphere, creating tensional forces that cause the plate to fracture. This
also creates a zone of lower pressure, causing melting of the asthenosphere and formation of magma.
Continued tension (B) leads to faulting and the development of a rift valley where rising magma forms layers
of basaltic rock. As the rift widens, it becomes flooded (C), forming a new ocean with a spreading center in
the middle.
Map showing the location of a young spreading center in eastern Africa. The northern portion of the rift zone
has opened to a point where it has flooded, forming the Red Sea and Gulf of Aden. To the south is the much
younger Great Rift Valley. Inset is a satellite photo showing the Sinai peninsula located at the top of the Red
Sea. This peninsula is bounded on the left by the spreading center and by a transform boundary on the right.
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46. Convergent Plate Boundaries (1) Long Description
a) The Aleutian Islands off the Alaskan mainland (A) are a narrow string of volcanic islands that represent a
young island arc system. The much larger islands of Japan (B) represent a more mature island arc. Note
the deep ocean trench in both examples, caused by the subducting plate on the ocean side of the island
arc.“ "The Andes Mountains are a continental arc system associated with an oceanic-continental
boundary. The boundary of these plates is marked by the position of the ocean trench.
b) The Andes are the result of continued convergence and subduction of the oceanic plate, which has
produced volcanic activity and numerous earthquakes.
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47. Convergent Plate Boundaries (2) Long Description
a) The Himalaya Mountains are a modern example of the collision between two continental plates. As India
moved toward the Asian landmass, the subduction of oceanic crust created a volcanic arc (A). After the
two landmasses collided, the subduction zone eventually shut down and volcanic activity ceased. The
collision continues to cause uplift and deformation today (B).
b) Subduction does not occur along convergent boundaries involving two continental plates composed of
relatively low-density material. Instead, the collision creates a thick zone of highly faulted and deformed
mountains where the rocks undergo regional metamorphism. Photo shows the folded Zagros Mountains
in Iran.
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48. Transform Plate Boundry Long Description
The San Andreas fault is a transform boundary associated with the East Pacific Rise spreading center. The
San Andreas forms the boundary between the Pacific and North Amercian plates. Note that just north of San
Francisco, the western edge of North America changes from a transform to a convergent boundary that
includes a subduction zone and volcanic arc.
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49. Plate Boundaries and People Long Description
Aerial photo showing the San Andreas fault in Southern California, which forms the boundary between the
North American and Pacific plates. Note how the stream channel has been offset due to recent movement
along this transform plate boundary.
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50. The shifting position of continents affects
Earth’s climate by altering the circulation of
heat and water in the oceans. Long Description
The shifting position of continents affects Earth’s climate by altering the circulation of heat and water in the
oceans. Climate is also affected by the uplift of mountains and by the rates at which heat-trapping gases are
released by volcanic activity and absorbed in the rock cycle.
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51. Allopatric Speciation due to a ocean trench Long Description
The Wallace Line in Southeast Asia represents the boundary between distinctly different groups of plant and
animal species. This boundary coincides with a deep ocean trench that forced species to evolve in isolation
from one another on opposite sides of it.
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