Warm Up 11/4
Why are subduction zones not commonly found at convergent
continental-continental boundaries?
a. Oceanic lithosphere is too buoyant to be forced down into the
mantle.
b. Continental lithosphere is too dense to be forced down into the
mantle.
c. Continental lithosphere is too buoyant to be forced down into
the mantle.
d. Subduction zones are never found at convergent boundaries.
2. Which of the following is a geographic example of a transform fault
boundary?
a. the Andes Mountains
c. the San Andreas Fault
b. the East African Rift valley
d. the Mid-Atlantic Ridge
3. The Red Sea is theorized to be the site of a recently formed ____.
a. convergent boundary c. divergent boundary
b. hot spot
d. transform fault boundary
Answers: 1) c. 2) c. 3) c.
1.
Testing Plate Tectonics
Chapter 9, Section 4
Paleomagnetism
Paleomagnetism – the study of changes
in Earth’s magnetic field, as shown by
patterns of magnetism in rocks that have
formed over time
 Some rocks contain iron-rich minerals that
will align with Earth’s magnetic field at the
time of their formation, creating a time
capsule of the magnetic field at that time
in time
 When the rock moves, or the magnetic
pole changes position, it will still retain its
original alignment
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Apparent Polar Wander
Paleomagnetism Cont.
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Normal Polarity – the same direction of
magnetism as the present day
Reverse Polarity – the reverse direction of the
magnetic field today
The discovery of strips of rocks of alternating
polarity, which lie as mirror images across the
ocean ridges, provided strong evidence of
seafloor spreading
Ships towed instruments called magnetometers
across the sea floor, revealing alternating high
and low magnetism running in parallel bands to
the oceanic ridges
As new basalt is added to the ocean floor, it gains
the magnetism of the current magnetic field, and
when the field changes over time, the basalt
records the changes
Paleomagnetism
Time Scale of Earth’s Polarity
Magnetic Reversals in Seafloor
Spreading
Earthquake Patterns
Scientists found a close link between
deep-focus earthquakes and ocean
trenches. Also, the absence of deep-focus
earthquakes along the oceanic ridge
system was shown to be consistent with
the new theory
 Scientists have found that intermediate
and deep focus earthquakes occur within
the subducting plate as it goes into the
mantle
 Shallow-focus earthquakes are produced
as the descending slab reacts with the
lithosphere around it
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Earthquake Distribution
Distribution of Earthquake Foci in
Japan Trench
Ocean Drilling
Some of the best evidence has come from
deep-sea drilling into the sediments on the
ocean floor
 The data on the ages of seafloor sediment
confirmed what the seafloor spreading
hypothesis predicted
 The youngest oceanic crust is at the ridge
crest and the oldest oceanic crust is at the
continental margins
 No sediment older than 180 million years
has been found in the ocean basins
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Hot Spots
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Mapping revealed that there was a chain of
volcanic structures in the middle of the Pacific
Ocean ranging from the Hawaiian Islands to
Midway Island and then north to the Aleutian
trench
Hawaii is the youngest and the islands get older
the further from Hawaii you are
Hot Spot – a rising plume of mantle material
that melts as it nears the surface, creating a
volcanic area
As the Pacific plate moves over this spot, the
island chain gets longer
The age of each volcanic island indicates when
that part of the Pacific plate was over the hot
spot
Hot spot evidence supports the idea that the
plates move over Earth’s surface
Hot Spots Trace Plate Motion
Directions and Rates of Plates