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Lecture 3

ESS 101 Lecture 3: ESS 101 Plate Tectonics Notes


Department
Earth and Space Sciences
Course Code
ESS 101
Professor
Swanson Terry
Lecture
3

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January 13th, 2017 Plate Tectonics (Ch. 1,
9 & 10)
Lithosphere floats o the partially
melted asthenosphere
o Lithosphere and
asthenosphere are in isostatic
equilibrium
When a load, such as an ice sheet
was placed on the lithosphere it will
isostatically depress relative to the
asthenosphere
o When the weight is lifted (the
ice melts) the lithosphere
isostatically rebounds
Plate Tectonics
Plate tectonics can explain:
o The presence of volcanic mid ocean ridges
o Volcanic mountain chains around the Pacific
Ocean basin
o The matching coastlines of Africa and Europe and
South America and North America
As well as matching ancient rock
assemblages on adjacent continents
Paleontological evidence supports plate tectonics and
plate motions
Fossil evidence of land animals and plants that lived
during the Paleozoic Era are now found on disparate
continents separated by 1000s of miles of ocean
Plate tectonics can explain the spatial distribution of volcanoes around the world,
including those around the rim of the Pacific (called the Ring of Fire)
Plate tectonics can explain the spatial distribution of seismic activity around the world
including the depth of their respective foci
Geothermal activity often occurs near tectonic plate boundaries and is less common on
the interior of continents
o Some geothermal activities occur in the interiors of continents, however they
can still be explained in a tectonic context
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Interaction of the Asthenosphere and Lithosphere
The solid lithosphere is composed of continental and ocean crust in the uppermost
mantle
The plastic asthenosphere is composed of the upper mantle and is at or near the
melting temperature for the upper mantle rock
Plate tectonics are driven by convection within the asthenosphere
o Convective upwelling of the asthenosphere along divergent margins produces
new ocean lithosphere (sea floor spreading) and is again subducted along
convergent margins
Basaltic ocean crust is produced by partial melting of the garnet-peridotite
asthenosphere
o As the basaltic magma rises it becomes more of a fluid think of the slope of the
melting temperature curve with decreasing pressure versus the actual
temperature curve
Types of Boundaries
Divergent margins
o Form under the continental crust because of insulating properties
o Eventually the upwelling basaltic magma will completely melt through the
overlying continental crust and an incipient ocean basin will form as the two
segments of continental crust rift apart
o Ocean floor becomes wider as the new lithosphere is produced causing the
name sea-floor spreading
o East Africa is an example of a continental rift zone the Red Sea is the resulting
ocean basin, dark basalt can be seen in satellite imagery of the area
Magnetic stripes were discovered by
geophysicists in the 1950s and 60s
o Represent the fluctuation in the
polarity chages of the Earths
magnetic field in preserved ocean
crust
o The earths agetic sigature is
preserved through iron bearing
minerals because as the magma or
lava crystallizes, the magnetic
minerals will be aligned with the
prevailing magnetic field
o Distance between these polarity
stripes can provide information on
the age of the ocean floor as well as the rate of seafloor spreading
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