PLATE TECTONICS AND THE SCIENTIFIC METHOD

I. California K-6 Standards for Plate Tectonics

II. Goals of This Module

III. Continental Drift

IV. Reasons Why This Early Hypothesis Was Rejected

V. Discovery of Paleomagnetism

VI. New Evidence From Paleomagnetism

VII. Hypothesis of Plate Tectonics

VIII. Testing the Hypothesis

IX. Implications of the Theory of Plate Tectonics

X. Driving Force Behind Plate Tectonics

XI. Possible Essay Questions

XII. Practice Questions

I. California K-6 Standards for Plate Tectonics -

1. Grade 6
   1. Plate Tectonics and Earth’s Structure
      1. Plate tectonics explains important features of the Earth’s surface and major geologic events. As a basis for understanding this concept, students know:
         1. the fit of the continents, location of earthquakes, volcanoes, and midocean ridges, and the distribution of fossils, rock types, and ancient climatic zones provide evidence for plate tectonics.

         2. the solid Earth is layered with cold, brittle lithosphere; hot, convecting mantle; and dense, metallic core.

         3. lithospheric plates that are the size of continents and oceans move at rates of centimeters per year in response to movements in the mantle.

         4. how to explain major features of California geology in terms of plate tectonics (including mountains, faults, and volcanoes.

 Back to Top

II. Goals of this module -

• To learn the evidence for the theory of plate tectonics.

• To learn about the plate structure of the Earth’s lithosphere and how the plates move with respect to one another.

• To learn how movement of the plates effects the surface of the Earth.

• To learn the definitions of and proper spelling for scientific terms that are used commonly in newspapers, magazines, and books, and on television and radio programs to discuss plate tectonics.

 Back to Top

III. Continental drift -

• Continental drift is a forerunner hypothesis to the present theory of plate tectonics.

• Evidence for the hypothesis was presented in 1915 by Alfred Wegener.
  • Shape of the continents - If the continents are viewed as pieces of a puzzle, they fit together nicely.

  • Fossils - The same fern and reptile fossils are found in South America, Africa, and Australia, continents which today are separated by oceans.

    

    Source for Diagram: http://xray.geol.uni-erlangen.de/html/teaching/plate/pla_tec.html

  • Continuous mountain belts - Mountain ranges in North and South America are separated by the Atlantic Ocean from their respective continuations in Europe and Africa.

  • Ancient glacial period - Glacial grooves under 300,000,000-year- old glacial deposits in South America, Africa, Australia, and India indicate that the glaciers flowed from where today there is ocean. Problem is solved if all the continents are put together with the glaciated area centered over the South Pole.

Source for Diagram: http://xray.geol.uni-erlangen.de/html/teaching/plate/pla_tec.html

Source for Diagram: http://xray.geol.uni-erlangen.de/html/teaching/plate/pla_tec.html

 Back to Top

IV. Reasons why this early hypothesis was rejected -

• No viable driving force to move the continents was proposed.

• No suggestion was made as to how the rigid continental crust could move through the equally rigid oceanic crust.

Source for Diagram: http://xray.geol.uni-erlangen.de/html/teaching/plate/pla_tec.html

 Back to Top

V. Discovery of paleomagnetism -

• When iron-rich minerals melt, they lose all their magnetic properties.

• When magma cools and forms igneous rock, the newly formed iron-rich minerals in the igneous rock align themselves with the earth’s magnetic field (each one like a little compass).

• These rocks can be dated and used to tell the position of the earth’s magnetic pole in times past.

• It has been discovered by looking at rocks of different ages, that the north and south magnetic poles of the earth have switched positions every 1,000,000 years or so (normal polarity becomes reversed polarity).

 Back to Top

VI. New evidence from paleomagnetism that led to plate tectonics -

• Studies of very old rocks showed that ancient positions for the north magnetic pole were in the middle of the Pacific Ocean and that the north magnetic pole positions for North America and Europe were in different locations.

• The only way to resolve this problem and allow for only one north magnetic pole would be to move the continents.

• Magnetic studies across the midocean ridges showed that there were mirror-image magnetic zones or stripes in the rocks on the bottom of the ocean and that these zones were parallel to the ridges and were of alternating strong and weak magnetism.

  

  Source for Diagram: http://xray.geol.uni-erlangen.de/html/teaching/plate/pla_tec.html

• An explanation for this phenomenon is that the weakly magnetic zones represent rocks of reversed polarity.

• Because the rocks in the first weakly magnetic zone (reversed polarity) away from the midocean ridge could not have formed under present conditions of normal polarity, they must have formed during the most recent period of reversed polarity and, therefore, must be older than the rocks along the midocean ridge.

  

  Source for Diagram: http://xray.geol.uni-erlangen.de/html/teaching/plate/pla_tec.html

  

  Source for Diagram: http://xray.geol.uni-erlangen.de/html/teaching/plate/pla_tec.html

  

  Source for Diagram: http://xray.geol.uni-erlangen.de/html/teaching/plate/pla_tec.html

• Hence, ocean crust is being created at the midocean ridges and the sea floor is spreading outward away from the midocean ridges through time.

 Back to Top

VII. Hypothesis of plate tectonics is proposed -

• The rigid continental crust is not moving through an equally rigid oceanic crust. Instead, the two types of crust are attached and the two types together are floating on the mobile asthenosphere and are moving away from the midocean ridges.

  

  Source for Diagram: http://geology.er.usgs.gov/eastern/tectonic.html

• There are seven major plates in the world and several smaller ones.

  

  Source for Diagram: http://geology.er.usgs.gov/eastern/plates.html

• Plate boundaries consist of three types -

  

  Source for Diagram: http://pubs.usgs.gov/publications/text/Vigil.html

  • Divergent (midocean ridges or spreading centers) - Places where plates are moving away from each other. At divergent boundaries the plates are pulling apart and new oceanic crustal rocks are created in the gap by upwelling of magma from the mantle.

    

    Source for Diagram: http://xray.geol.uni-erlangen.de/html/teaching/plate/pla_tec.html

  • Convergent (ocean trenches) - Places where plates are moving toward each other. At convergent boundaries the subducting plate melts as it descends into the mantle and the rising magma forms chains of volcanoes and batholithic intrusions parallel to the subduction zones. Convergent boundaries are faults.

    

    Source for Diagram: http://pubs.usgs.gov/publications/text/understanding.html#anchor4665685

    

    Source for Diagram: http://pubs.usgs.gov/publications/text/understanding.html#anchor4665685

    

    Source for Diagram: http://pubs.usgs.gov/publications/text/understanding.html#anchor4665685

  • Transform (like the San Andreas fault) - Places where plates move laterally past one another. At transform boundaries the plates move laterally past one another between two offset divergent plate boundaries. Like convergent plate boundaries, transform boundaries are faults. The plates move past one another only between the offset divergent plate boundaries. Beyond the area of the offset, the plates move the same direction and at the same speed.

Source for Diagram: http://pubs.usgs.gov/publications/text/understanding.html#anchor4665685

 Back to Top

VIII. Testing the hypothesis -

• Six tests can be made to check the validity of the plate-tectonic hypothesis.
  • As distance from the midocean ridge increases, thickness of sediment on the ocean bottom should increase;

  • As distance from the midocean ridge increases, age of sediment at the bottom of the pile should increase;

  • As distance from the midocean ridge increases, age of volcanic rocks below the sediment should increase;

  • As distance from the midocean ridge increases, volcanic rocks should alternate from normal to reversed in magnetic polarity.

  • Earthquakes should be most abundant in places where plates are subducting or scraping past one another and there should be a systematic relationship between the focus of the earthquake and its depth.

  • Chains of volcanic islands, if they represent the passage of the crust over a hot spot in the mantle, should be old at one end and progressively decrease in age along the chain.

• All the above six tests have been completed and the results have supported the hypothesis, so the concept of plate tectonics has now advanced to the status of a theory.

• Plate velocity - Study of spreading rates at midocean ridges show that the plates move at an average rate of about 2 inches/year.

 Back to Top

IX. Implications of the theory of plate tectonics -

• The Atlantic Ocean is enlarging slowly and the Pacific Ocean is shrinking.

• The oldest rocks in the world are on the continents because continents are too light to get subducted and have been floating around on the surface of the earth ever since the first one was formed.

• Rocks in the ocean basins are constantly being consumed at the subduction zones and created at the midocean ridges; hence, they never have a chance to get very old. The oldest rocks in the ocean are no older than 200,000,000 years old.

Source for Diagram: http://gdcinfo.agg.nrcan.gc.ca/toc.html?/app/app3eng.html

 Back to Top

X. Driving force behind plate tectonics -

• A new spreading ridge is created when a plume of hot mantle material rises toward the earth's surface. The reason for this rise is presently unknown, but may be due to adjustments in the mantle following a giant meteorite impact.

• The rising mantle plume lifts the lithosphere above it and cracks it, commonly in a three-limbed fracture pattern. Once the lithosphere has been raised and fractured, gravity begins to pull the mobile plates down the slope and away from the fractured area, thus widening the fractures and starting the spread of the new plates away from the fractured ridge.

• At some relatively long distance away from the fractures, the lithosphere cracks again, so that subduction can start and the lithospheric plates can continue to slide down the slope at the base of the lithosphere under the influence of gravity. As subduction and spreading continue, two of the original three fractures are extended outward across the surface of the earth.

 Back to Top

XI. Possible Essay Questions -

• Describe the four lines of evidence that Wegener used to support his theory of continental drift.

• Describe and illustrate the three types of plate boundaries.

• Describe the six tests that were done to check the validity of the hypothesis of plate tectonics.

 Back to Top

XII. Practice Questions

 Back to Index