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.

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.
  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)
  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

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)