Chapter 9
Plate Tectonics
Section 9.4 & 9.5
Testing Plate Tectonics
&
Mechanisms of Plate Motion
Testing Plate tectonics
Evidence supporting plate tectonics:
1.
•
•
•
•
Paleomagnetism:
The natural remnant magnetism in rock bodies; the
permanent magnetization acquired by rock that can be
used to determine the location of the magnetic poles at the
time it became magnetized.
When certain rocks containing iron-rich minerals
(magnetite) are heated above a certain temperature, they
lose their magnetic properties (convergent boundary).
When they are again cooled, they become magnetized in
the direction parallel to the existing magnetic field
(divergent boundary).
Rocks formed millions of years ago show the location of the
magnetic poles at the time of their formation.
Testing Plate tectonics
• Geophysicists have learned that Earth’s magnetic field periodically
reverses polarity.
– The north magnetic pole becomes the south and vice-versa.
• When rocks show the same magnetism as the present magnetic field,
they are described as having normal polarity.
• Rocks that show the opposite magnetism are said to have reverse
polarity.
• A relationship was discovered between the magnetic reversals and the
seafloor spreading hypothesis.
• Ships towed instrument called magnetometers across segments of the
ocean floor and discovered alternating strips of high and low-intensity
magnetism that ran parallel to the ridges.
– The strips of high-intensity magnetism are regions where paleomagnetism is
of the normal type. The low-intensity strips represent regions where the ocean
crust is polarized in the reverse direction, and therefore weaken the magnetic
field.
• The discovery of strips of alternating polarity, which lie as mirror images
across the ocean ridges, is amongst the strongest evidence of seafloor
evidence.
Testing Plate tectonics
Testing Plate tectonics
According to the property of
paleomagnetism,
A. Iron-rich rocks show the location of the
magnetic poles at the time of their
formation.
B. All rocks, regardless of when they are
formed, have the same polarity.
C. All rocks have a reversed polarity.
D. Rocks do not possess magnetic
properties.
Magnetic reversals,
A. Cause the movements of tectonic plates.
B. Confirmed the existence of subduction
zones.
C. Provide strong evidence for seafloor
spreading.
D. Have never occurred during geologic time.
Strips of alternating magnetic
polarities found in rocks in the
ocean basins
A. Conflict with the theory of plate tectonics.
B. Provide evidence that Earth’s magnetic
field has never reversed polarity.
C. Indicate changes in Earth’s gravitation
field.
D. Provide evidence for seafloor spreading.
What do the strips of low-intensity
magnetism represent on the ocean
floor?
A. Areas where there is no magnetism.
B. Areas where the rocks have a normal
polarity.
C. Areas where the rocks have a reversed
polarity.
D. Areas of different types of rock.
Testing Plate tectonics
2. Earthquake Patterns:
• Scientists found a close link between deepfocus 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.
• When the depth of earthquake foci and their
locations within the trench system are plotted,
a pattern emerges.
Testing Plate tectonics
Testing Plate tectonics
• In the plate tectonics model, deep-ocean trenches
are produced where cool, dense slabs of oceanic
lithosphere plunge into the mantle.
• Shallow-focus earthquakes are produced as the
descending plate interacts with the lithosphere
above.
• As the slab descends farther, deeper-focus
earthquakes are produced.
• No earthquakes have been recorded below 700kilometers.
– At this depth, the slab is heated enough to soften.
Testing Plate tectonics
Testing Plate tectonics
3. Ocean Drilling:
• The Deep Sea Drilling Project from 1968 to 1983 used the
drilling ship Glomar Challenger to drill hundreds of meters
into the sediments and underlying crust.
• When the oldest sediment from each drill site was plotted
against its distance from the ridge crest, its was revealed that
the age of the sediment increased with increasing distance
from the ridge.
• 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 old was found.
– Which means that the oceanic crust is relatively young as compared
to some continental crust which has been dated at 4.0 billion years
old.
The age of the rocks in the ocaen
basins was determined by
A. Ocean drilling.
B. The fit of continents across ocean
basins.
C. The depth of earthquake foci.
D. The amount of magnetism in the
rocks.
How does the age of seafloor
sediments change with increasing
distance from the ocean ridge?
A.
B.
C.
D.
Age decreases.
Age stays the same.
Age increases.
Age varies without a pattern.
Testing Plate tectonics
4. Hot Spots:
• Mapping of the Pacific seafloor revealed a chain of
volcanic structures that extended from the Hawaiian
Islands to Midway Island, and then north to the
Aleutian Trench.
• Dates of volcanoes in this chain showed that the
volcanoes increase in age with increasing distance
from Hawaii.
– Suiko Seamount is 65 million years old, Midway Island is
27 million years old, and the island of Hawaii formed less
than a million years ago and is still forming.
Testing Plate tectonics
• A rising plume of mantle material is located below
the island of Hawaii.
• Melting of this hot rock as it nears the surface
creates a volcanic area, or hot spot.
• As the Pacific plate moves over the hot spot,
successive volcanic mountains have been created.
• The age of each volcano indicates the time when it
was situated over the hot spot.
– Kauai is the oldest of the large islands in the Hawaiian
chain.
• Hot spot evidence supports the idea that the plates
move over Earth’s surface.
Testing Plate tectonics
The Hawaiian Islands were formed
when the Pacific Plate moved over
A.
B.
C.
D.
A subduction zone.
An ocean ridge.
The Aleutain Plate.
A hot spot.
The formation of the Hawaiin
Islands is associated with
A.
B.
C.
D.
A divergent plate boundary.
A convergent plate boundary.
A transform fault boundary.
No plate boundary of any kind.
Mechanisms of Plate Motion
• Scientists generally agree that convection occurring
in the mantle is the basic driving force for plate
movement.
– Warm, less dense material rises, and cooler, denser
material sinks.
• This motion is called convective flow.
• These movements are driven by the unequal heat
distribution of Earth’s heat.
• The heat is generated by the radioactive decay of
elements in the Earth’s mantle and crust.
– Example: Uranium
Mechanisms of Plate Motion
• One mechanism of plate motion is called the
slab-pull.
• This occurs because old oceanic crust sinks
into the asthenosphere and “pulls” the trailing
lithosphere along.
• Slab-pull is thought to be the primary
downward arm of convective flow into the
mantle.
Mechanisms of Plate Motion
• Another mechanism of plate movement is
ridge-push which results from the elevated
position of the oceanic ridge system.
• Ridge-push causes oceanic lithosphere to
slide down the sides of the oceanic ridges.
• The downward slide is the result of gravity
acting on the oceanic lithosphere.
• Ridge-push, although active in some
spreading centers, is probably less important
that slab-pull.
Mechanisms of Plate Motion
The main source of downward
convection flow in the mantle is
called
A.
B.
C.
D.
Ridge-pull.
Slab-pull.
Slab-push.
Ridge-push.
The downward sliding
characteristic of ridge-push is the
result of
A.
B.
C.
D.
Gravity.
Uneven heat distribution.
Paleomagnetism.
Continental rifting.
The thermal convection that drives
plate motion if caused by
A.
B.
C.
D.
Seafloor spreading.
An unequal distribution of heat.
Gravity.
Subduction.
Mechanisms of Plate Motion
•
Another mechanism for plate movement is mantle
convection.
• Most models suggest that hot plumes of rock are
the upward flowing arms in mantle convection.
• These rising mantle plumes sometimes show
themselves on Earth’s surface as hot spots and
volcanoes.
• There are two types of mantle convection:
1. Whole-Mantle Convection
2. Deep-Layer Model
Mechanisms of Plate Motion
Whole-Mantle Convection:
• In this model, slabs of cold oceanic
lithosphere descend into the lower mantle.
• This process provides the downward arm of
convective flow.
• At the same time, hot mantle plumes
originating near the mantle-core boundary
move heat toward the surface.
Mechanisms of Plate Motion
Mechanisms of Plate Motion
Deep-Layer Model:
• In this model, heat from Earth’s interior
causes the two layers (upper mantle and
lower mantle) to slowly swell and shrink in
complex patterns without much mixing.
• A small amount of material from the lower
layer flows upward as mantle plumes,
creating hot-spot volcanism at the surface.
Mechanisms of Plate Motion
Which one of the following has not
been proposed as a mechanism of
plate motion?
A.
B.
C.
D.
Slab-pull.
Ridge-push.
Mantle convection.
Crust-core convection.
According to whole-mantle
convection,
A. Small amounts of material from the lower
mantle move upward to the surface.
B. Slabs of cold oceanic lithosphere move
down and into the lower mantle.
C. Large chunks of continental crust are pulled
down into the lower mantle.
D. Material from the inner core rises into the
mantle to form super hot plumes.
Mechanisms of Plate Motion