Transcript Testing Plate Tectonics & Mechanisms of Plate Motion

Warm Up 11/5
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 &
Mechanisms of Plate Motion
Chapter 9, Sections 4 & 5
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
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
 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
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Hot Spots Trace Plate Motion
Directions and Rates of Plates
Causes of Plate Motion
 Scientists
generally agree that
convection occurring in the mantle is
the basic driving force for plate
tectonics
 Convective Flow – the motion of
matter resulting from convection
 The slow movements of the plates
and mantle are driven by the
unequal distribution of Earth’s heat
from the radioactive decay elements
Convective Flow
Slab-Pull and Ridge-Push
Slab-Pull – old oceanic crust, which is
relatively cool and dense, sinks into the
asthenosphere and “pulls” the trailing
lithosphere along
 Slab-pull is thought to be the primary
downward arm of convective flow in the
mantle
 Ridge-Push – causes oceanic lithosphere
to slide down the sides of the oceanic
ridge as a result of gravity
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Mantle Convection
Mantle Plumes – hot plumes of rock that are
attributed to the upward flowing arms of
mantle convection
Mantle plumes sometimes show themselves on
the surface as hot spots and volcanoes
Whole-mantle convection is when slabs of cold
oceanic lithosphere descend into the lower
mantle, at the same time, hot mantle plumes
originating near the mantle-core boundary
move heat toward the surface
Another model is called the deep-layer model,
and can be likened to a lava lamp, heat from
Earth’s interior causes the layers to mix and
combine in swirling patterns
The unequal distribution of heat within Earth
causes the thermal convection in the mantle
that ultimately drives plate motion
Whole-Mantle Convection
Deep-Layer Model
Assignment
 Read
Chapter 9, Sections 4 & 5 (pg. 265-
270)
 Do Chapter 9 Assessment #1-30 (pg.
275-276)
 For Section 4: Do #’s 9, 13, 26, & 30
 For Section 5: Do # 24