Other Plate Tectonics Info:
Plate Tectonics Intro
Plate Tectonics Basics
Other Implications

More links to web sites that contain information about Plate Tectonics can be accessed through the Links Page of this web site.  The figures shown here were taken and/or modified from the USGS Plate Tectonics web pages.


Plate  Tectonics, History of the Theory:

Questions that you should be able to answer at the end of the Plate Tectonics: People, Evidence, Etc. lesson:

  1. What were the earliest ideas about how the continents moved, and when?
  2. What kinds of evidence were used to suggest that the continents had moved?
  3. What advances had to be made for this evidence to be gathered?
  4. Who were the people that made these suggestions?
  5. Who was Alfred Wegener and why was he so important?
  6. What was his evidence?
  7. What was Continental Drift?
  8. What is a Supercontinent?
  9. What were the names of the Supercontinents?
  10. What did Wegener suggest was happening with Continental Drift?
  11. Why were his suggestions largely disregarded?
  12. What hypotheses were suggested to explain his evidence?
  13. What was the evidence for Plate Tectonics?
  14. When was that evidence gathered?
  15. Who did the studies that led to Plate Tectonics?
  16. What is the difference between Continental Drift and Plate Tectonics?
  17. Why is Plate Tectonics so important in Geology?

For a cool website that shows what the surface of the earth has looked like in the geologic past, as well as a lot of other plate tectonic information, click here.

Early Suggestions about a changing Earth

 
Abraham Ortelius (1528-1598) - suggested in 1596 that the Americas were "torn away from Europe and Africa . . . by earthquakes and floods" and went on to say: "The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three [continents]."
Francis Bacon (1561-1620) - suggested Western Hemisphere once joined with Eurasia.

Why did it take until the 1500s for people to realize that the world's land masses had moved around?  In a word, technology.  We couldn't figure out that the continents had moved until we could figure out where they were, and what their shape was.  In this case, the technology necessary is the ability to navigate precisely, which led to good quality maps.  What technology was necessary?  Two devices come to mind - the Sextant, which enabled exact latitudes to be determined, and the Chronometer, which enable longitude to be determined.


A sextant is used to measure elevation of astronomical objects above the horizon, which gives you latitude.
 
This is the first chronometer made by John Harrison in 1741.  He also made the first successful chronometer, in 1761.
Note that this is AFTER the 1500s, which is one reason why maps from that time sometimes look significantly different from modern maps.

Technology will continue to be important in the development of Plate Tectonics as well.  Keep this in mind!

Other ways and people that showed the continents had moved


In 1858, geographer Antonio Snider-Pellegrini made these two maps showing his version of how the American and African continents may once have fit together, then later separated. Left: The formerly joined continents before (avant) their separation. Right: The continents after (aprés) the separation. (Reproductions of the original maps courtesy of University of California, Berkeley.) (from USGS website - click on images to go to it)
Eduard Suess - 1885-1909 - noted correspondence of geologic formations in S. Hemisphere landmasses, suggested once formed a single continent he called GONDWANALAND -
  • Although Antarctica and Australia are completely in the Southern Hemisphere, and South America is mostly in the Southern Hemisphere, Africa is mostly in the Northern Hemisphere, and India is completely in the Northern Hemisphere.
  • They are called "Southern Hemisphere landmasses" because that's where they came from, not because that's where they are now.
Note that this gives us 3 "supercontinents":


Wegener's genius is in that he used so MANY kinds of evidence, and he traveled the world collecting it.
What was Wegener's evidence?

Wegener's Evidence:
  • (Figure is from USGS website)
  • Glossopteris Flora
  • Mesosaurus
  • Cynognathus
  • Lystrosaurus
    • Present day distribution of coral reefs
    • Coral reefs need warm, shallow water.  They are almost all confined to the tropics
    • Fossil coral reefs, on the other hand, are found all over the world.
    • Why?

    There may be a number of reasons for this.

    The problem: Mechanism:
    Wegener envisioned continents plowing through oceanic crust kind of like how an icebreaker plows through ice, pushed by two forces.

    Result: belief in Stable Earth

    What alternatives to Wegener's mechanism were suggested?
    The data still had to be explained, even if his mechanism was rejected.  There were a number of ideas proposed:
    Other ideas explaining Wegener's data:

    What then led up to Plate Tectonic Theory?
    WWII - and improvements in technology during war time.

    One big problem was that during WWII, Allied convoys carrying food, arms, fuel, ammunition, soldiers, equipment from the US and Canada to Great Britain were being attacked and sunk by (among other things) German submarines (U-Boats).  The allies obviously wanted to stop this.

    Ways used to combat submarines:
    Harry Hess (1962) used echo-soundings to map the seafloor, and based on this suggested that molten rock (magma) oozes up from the Earth's interior along the mid-oceanic ridges, creating new seafloor that spreads away from the active ridge crest and, eventually, sinks into the deep oceanic trenches. (from a USGS website)  This was not quite Plate tectonics, but it started people thinking. new data:

  • These Marine Magnetic Anomalies (MMA) had a number of key characteristics:
  • Not only was the pattern of MMA the same along the Mid-Atlantic Ridge, but the same pattern is found in the Indian and Pacific Oceans.
  • Magnetic anomaly
                  Patterns from 3 oceans Top: South Atlantic magnetic anomaly pattern, made by the RV Vema.

    Middle: North Pacific magnetic anomaly pattern, made by the RV Vema.

    Bottom: Southern Indian Ocean magnetic anomaly pattern, made by the RV Eltanin.

    In all, "0 km" (near the right hand side of these patterns) is at the crest of the respective mid-ocean ridges.  Note that the anomaly pattern is symmetrical around the 0 km line.

    The same anomalies (numbered in the top pattern) are present, slightly compressed or expanded, in all of the patterns.  The black and white striped pattern represents positive (black) and negative (white) marine magnetic anomalies.

    dj
  • Paleomagnetism = study of Earth's magnetic field as recorded in rocks
  • Beginning as early as the 19th Century, researchers began measuring the magnetism recorded in rocks of the Earth.
  • It was discovered that, as they cooled, magnetic minerals in igneous rocks became magnetized in the same direction as the Earth's field.
  • Once the rocks drop below the "Curie Point" (usually 500-800oC), the magnetization is locked in.
  • Small magnetic mineral grains in sediments would also rotate during deposition to orient themselves in the same direction as the Earth's field.
  • Thus the Earth's magnetic field was recorded in igneous and sedimentary rocks.
  • A geologist going to a place where there were many thin lava flows (Iceland or Hawaii) could sample these rocks, bring them back to the lab, and measure the direction of magnetization.
  • The result would look something like this:
  • Fred Vine (l), Drummond Matthews (r)
    Result: Marine Magnetic Anomalies (MMA) - used by Fred Vine and Drummond Matthews (King's College, Cambridge) to infer Seafloor Spreading in 1963, based on MMA obtained during the 1950s and 60s.  (click on figure above to see USGS discussion of MMA's)



    Additional study in the years following has established that the changes in magnetic polarity:

    The MMA's are produced by sequential changes in the the earth's magnetic field, as seafloor spreading occurs:


    (from USGS Website - click on image to go to that site)

    Result (map view):


    Comparing the pattern of Marine Magnetic Anomalies to the pattern in the Geomagnetic Polarity Time Scale has allowed us to determine the age of the ocean floor over virtually the entire Earth:
    (Click on this image
    to see the entire Earth)

    This has also allowed us to map out probable spreading patterns over time.

    From this website:

    http://www.searchanddiscovery.net/documents/97019/index.htm

    From this website:

    https://sos.noaa.gov/catalog/datasets/plate-movement-200-million-years-ago-to-today/

    Click on the image to the left to see an animation.

    The net result is Divergent Boundaries.

    Earthquake information:

    • Hugo Benioff, 1954 - discovered the "Benioff Zone" -
    • American (of Russian heritage) seismologist.
    • Invented seismological instruments
    • Also invented electronic musical instruments!
    • Examined earthquakes near Kamchatka Peninsula (Russia)
  • These data from earthquakes (Convergent Boundary) and paleomagnetism (Divergent Boundary) allowed the discovery of what eventually became the Theory of Plate Tectonics in the middle 1960's.


    Other Plate Tectonics Info:
    Plate Tectonics Intro
    Plate Tectonics Basics
    Other Implications

    More links to web sites that contain information about Plate Tectonics can be accessed through the Links Page of this web site.  The figures shown here were taken and/or modified from the USGS Plate Tectonics web pages.