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Chapter 8 Notes
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Geology Notebooks.
The lithospheric plates of the Earth are
rigid and move about on the
asthenosphere.
The shapes of South America and Africa’s
coastlines led to early suggestions they
had been joined at one time.
Strong evidence that two continents were
once joined might include similarities in
similar coastal rock formations.
Wegener’s continental drift hypothesis
was not widely accepted because
Wegener could not explain how the
continents moved.
The theory of plate tectonics helps explain the
locations of earthquakes and volcanoes.
The lack of geologic activity is not a characteristic
feature of at least one type of plate boundary.
Plate boundaries are defined by:
• Earthquake activity
• Volcanic activity
• High heat flow
Click Here to See the
Locations of these Activities.
Minerals in igneous rocks can show the
direction of Earth’s magnetic field at the
time the rocks were formed.
With increasing distance from a midocean ridge, sea-floor rocks become
older.
The rift valley along a mid-ocean ridge
marks a boundary between two plates.
The Mid-Atlantic Ridge is associated with
hydrothermal vents, new rocks being
formed there, and many earthquakes.
Both collision boundaries and subduction
boundaries are types of convergent
boundaries.
When two ocean plates converge, the one
that subducts is the one that is made of
denser material.
A chain of volcanoes commonly forms
when an oceanic plate subducts.
If two plates carrying continental crust
share a convergent boundary, a likely
result is that the continents will
eventually become welded together.
The fracture zones that offset portions of
mid-ocean ridges are examples of a
transform boundary.
Mantle convection, ridge push, and slab
pull are all plausible causes for plate
movements.
The breakup of Pangea is a process that
is continuing today.
Only a portion of the North American
craton is visible because the rest has
been buried beneath other materials.
Deep-sea sediments most likely
contribute to continent growth by being
scraped off in a subduction zone and
left at the surface.
A terrane is completely bounded by major
faults, has different fossils than those in
the surrounding area, and the magnetic
record of rocks in the terrane is different
than the magnetic record of rocks in
surrounding areas.
• The diagrams show two types of plate
boundaries. Draw an arrow above each plate
to indicate its direction of relative motion.
Circle the area(s) where molten rock is likely
to erupt. (4 points)
Essay:
How are the properties of the lithosphere
and the asthenosphere different? How
is this difference important to plate
tectonics?
The lithosphere is the outermost layer of
Earth. It is rigid and brittle, and it is
composed of Earth’s crust and
outermost mantle. It is broken into
plates. Because they are rigid, these
plates move as blocks that interact
primarily at their boundaries. The plates
move over the asthenosphere.
The asthenosphere is a layer in the upper
mantle. The rocks there are so hot they
deform and convect slowly, though they
remain solid. Convection in the mantle
allows the overlying plates of the
lithosphere to move over the
asthenosphere.
Essay:
Compare the elevation and the age of the
sea floor at divergent boundaries with
the elevation and the age of the sea
floor at subduction boundaries. Explain
any differences.
At divergent boundaries the seafloor is
being formed, so the rocks there are
quite young. In this region there is high
heat flow due to rising magma. This
heat causes the rocks to expand and be
elevated.
At subduction boundaries the seafloor has
traveled far from its origin at a divergent
boundary, and the rocks are old. The
rocks are cold and dense. The greater
density causes the seafloor to be
deeper. As it plunges into a deep-sea
trench, the seafloor reaches its greatest
depths of anywhere in the ocean basin.