CH 2 Plate Tectonics Review
Name: _____________________________ Mods: _______
2.1 From Continental Drift to Plate Tectonics
Discuss the view that most geologists held prior to the 1960’s regarding geographic positions of the
ocean basins and continents.
Most geologists thought that the ocean basins were very old and were fixed in place (static)
2.2 Continental Drift
What was the first line of evidence that led early investigators to suspect that the continents were once
connected?
The South American and African continents looked like they fit together like puzzle pieces.
Explain why the discovery of the fossil remains of Mesosaurus in both South America and Africa, but
nowhere else supports the continental drift hypothesis.
The Mesosaurus was a small aquatic freshwater retile of the Permian period, if it was able to make the
journey across the vast Atlantic Ocean, therefore its remains should be more widely distributed. These
fossils are not widely distributed so we can make the assumption that S.A. and Africa must have been
connected or close to one another during their existence.
How did Wegener account for the existence of glaciers in the southern landmasses at a time when areas
in North America, Europe, and Asia supported lush tropical swamps?
By positioning the continents to form Pangea the southern land masses were located in the southern
hemisphere near the South Pole and the northern land masses were positioned over the equator which
would have allowed for the existence of glaciers in the southern land masses and lush tropical swaps in
the northern land masses.
List and explain the evidence presented by Wegener to support his continental drift hypothesis.
1. Fit of the continents, specifically Africa and South America
2. Continental fossil organisms in rocks of similar age in Africa-South America-Antarctica--India
(Glossopertis Fern, Mesosaurus Reptile) continents that are currently widely separated by
oceans. Wegener accounted for this by placing the continents together in their predrift
positions (Pangea)
3. Modern mountain belts and ancient rocks, if continents were once together then rock types and
other geological features (mountains) should match those continents that once were adjoined.
4. Ancient climates recorded in sedimentary rocks, including glacial evidence in the southern land
masses of Pangea and the tropical coal swamps that account for the coal deposits in North
America, northern Africa, Europe, and Southern Asia.
CH 2 Plate Tectonics Review
Name: _____________________________ Mods: _______
2.3 The Great Debate
What two aspects of Wegener’s continental drift hypothesis were objectionable to most Earth
scientists?
1. Wegener proposed tidal forces were responsible for the motion of the continents and he
implied that the continents just plowed their way through the oceanic crust.
2.4 The Theory of Plate Tectonics
Research conducted during WWII led to insights that helped revive Wegener’s hypothesis of continental
drift specifically the exploration of the sea floor revealed what previously unknown features?
1. Mid ocean ridge system, the young age of the oceanic crust compared to the age of the
continental crust, relatively thin sediment accumulations found in deep ocean basins.
How were the plate boundaries first established by scientists?
1. By mapping locations of volcanoes and earthquakes
Write a brief description of the three types of plate boundaries and explain what makes them described
as constructive, destructive, and conservative plate margins.
1. Divergent = constructive margins where two plates move apart resulting in the upwelling of
magma to create new sea floor.
2. Convergent = destructive where two plates move together resulting in oceanic lithospheric plate
being subducted beneath the overriding plate and eventually being melted back into the mantle
or the collision of two continental plates.
3. Transform – conservative where two plate slid by each other without the production or
destruction of the lithosphere.
2.5 Divergent Plate Boundaries and Sea Floor Spreading
What is the average rate of sea floor spreading? How about the Mid Atlantic Ridge? How about the
East Pacific Rise?
1. 5 cm/yr on average
2. Mid Atlantic Ridge 2 cm/yr
3. East Pacific Rise 15+ cm/yr
List 4 facts that characterize the oceanic ridge system.
1. Most divergent plate margins are associated with oceanic ridges
2. Oceanic ridge systems occupy 20% of the Earth’s surface and winds through all major ocean
basins.
CH 2 Plate Tectonics Review
Name: _____________________________ Mods: _______
3. Along the crest of the ridge system separating the two plates a deep canyon like rift valley is
found.
4. The mechanism responsible for creating new sea floor is called sea floor spreading
Describe the process of continental rifting and the eventual formation of new ocean. Where is this
occurring today?
Continental rifting occurs when:
1. Plate motion produce tensional forces that tins the lithosphere and promotes upwelling magma
2. Stretching of the crust causes the brittle crust break into large blocks and sink (normal faults
form) forming a rift valley. (East African Rift and Iceland Rift)
3. Continual spreading along the rift valley forms a long narrow sea. (Red Sea, btw Arabian
Peninsula and Africa)
4. Eventually continued spreading forms a deep ocean basin (Atlantic Ocean basin)
2.6 Convergent Plate Boundaries and Subduction
Explain why the rate of lithospheric production roughly balances with the rate at which it is destroyed.
Our planet is not expanding or growing larger, a balance between the subduction of oceanic crust and
the production of new oceanic crust along oceanic ridges allows the total surface area of the Earth to
remain constant.
In addition to the Himalayas, name several other major mountain systems have formed the same way.
The Alps, Appalachian, and Ural mountains all formed from C-C collisions.
2.7 Transform Plate boundaries
Describe the relative motion of transform plate boundary and be able to locate several examples on a
plate map.
Lithospheric plates sliding past one another horizontally without the production or destruction of the
lithosphere.
2.8 Testing the plate tectonics model
Multiple lines of evidence were obtains by the Deep Sea Drilling project, what were they?
1. Age of the sea floor increases with distance away from the spreading center
2. The thickness of the sediment increases with distance from the spreading center (older crust has
more time to accumulate sediment)
CH 2 Plate Tectonics Review
Name: _____________________________ Mods: _______
3. Magnetic reversals recorded as minerals in magma cooled on the ocean floor aligned
themselves with the orientation of the Earth’ magnetic field. This produces a symmetrical and
parallel pattern of these magnetic reversals on either side of the spreading center.
Describe how Fred Vine and D.H. Matthews related the seafloor-spreading hypothesis to magnetic
reversals.
Vine and Matthews suspected that we should see the pattern of magnetic stripes (size and polarity) to
be symmetrical on either side of the spreading center
What is a Hot Spot and what evidence do they provide to determine relative plate motion on which they
are forming?
A cylindrical upwelling of hot rock called a Mantle Plume rises through the mantle causing a drop in
pressure which triggers partial melting (decompression melting). This activity generates a hot spot at
the surface. The hot spot (mantle plume) remains stationary as the plate moves; this generates a line of
volcanoes that increase with age as distance away from the hot spot increases. This record of motion
can be used to determine relative speed and direction of plate motion.
Locate Figure 2.25 on page 64; Based on the location of the Easter Island hot spot and its associated
volcanic island chain describe the apparent direction of motion and the change in direction of motion of
the Nazca Plate.
Nazca plate was moving in a NE direction and is most recently moving almost due east.
Magnetic minerals such as magnetite align themselves with Earth’s magnetic field as rock forms. These
fossil magnets act as recorders of the ancient orientation of Earth’s magnetic field. In what 2 ways is
this information useful to geologists as evidence of plate motion?
1. Geologists can interpret a sequence of rock layers and their changes in relative position to
magnetic poles through time.
2. Evidence of sea floor spreading by the symmetrical pattern of magnetic stripes parallel to the
axis of the mid ocean ridge.
2.9 Measuring Plate Motion
Describe two methods researchers use to measure the relative motion of the plates.
GPS satellites
Transform plate boundaries point in the direction the plate is moving.
Data from the ocean floor can be used to determine rate and direction of plate motion
Sediments with diagnostic fossils and radiometric dates of igneous rocks
CH 2 Plate Tectonics Review
Name: _____________________________ Mods: _______
2.10 Tectonic Plate Motions Driving Mechanisms
Describe slab pull and ridge push. Which of these forces appears to contribute more to plate motion?
Ridge push; as the plate slides down from the peak of the mid ocean ridge if exerts a small outward
directed force.
Slab pull; as a slab of dense oceanic crust is subducted into the asthenosphere gravity pulls the slab
downward generating a tugging force on the plate (slab pull appears to have greater effect on the
motion of the plate)
What role are mantle plumes thought to play in the convective flow within the mantle?
Convective flow in the mantle generates a friction drag on the underside of the plates that is another
force that influences the rate and direction of plate motion.
Construct a cross section of the Earth that reflects the driving mechanisms responsible for the motion of
the tectonic plates. (see Figure 2.34 for help)