Plate Tectonics I

Tectonics is the large-scale movement of a planetary body's solid material through time. On Earth, this is dominated by the global process of plate tectonics We know that the mantle convects, bringing heat toward Earth's surface by physically overturning. Plate tectonics are how this action is reflected in surface features. It's recognition is one of the great breakthroughs in geology, so we approach it historically in this presentation.


HMS Challenger from Wikipedia
The first practical exploration of the ocean basins occurred in 1872 when the British government sponsored the first interdisciplinary research expedition to expore the world's oceans - the four-year voyage of the H. M. S. Challenger. The deep oceans defied expectations:
Clearly the geology of the oceans was unlike that of the continents. What's up with that?

Continental Drift - the beginnings of an answer:


Harding Ice Sheet, AK
The German meteorologist and geophysicist Alfred Wegener (1880-1930) performed field work in Greenland, covered by a continental ice sheet. There, he had ample opportunity to observe the behavior of glaciers. He observed that ice, when greatly compressed, flowed plasticly, allowing the ice sheet to glide slowly across the underlying rock, and apparently began wondering if rock did the same thing on a larger scale. He noticed the following patterns:

"Continental Drift:" To explain this, Wegener proposed the hypothesis of "continental drift:" i.e. that the location of continents was not fixed, and that they had "drifted" across the globe. Specifically, Wegener thought that the continental crust slid over the oceanic crust like glacial ice sliding over bedrock.


Pangaea from the Christopher Scotese Paleomap Project


Problem: While Wegener was a genius at making observations and recognizing patterns, he was not able to provide a theory to explain and predict the movements of continents, i.e. to say how it happened. Of course, no one who favored stable continents could begin to explain either why the patterns Wegener saw existed or where mountains came from, but that didn't really matter. The Geological profession didn't like amateurs claiming to solve puzzles that had defied them for 40 years. In 1926, Wegner proposed as motivating forces:

In 1928, Harold Jeffries published an analysis demonstrating that neither of these forces could begin to propel continents around. Continental drift was relegated to the idiot fringe. No US textbook mentioned it until 1960. In 1930 Wegener died in a freak storm while doing field work in Greenland.


Diagram of sea-floor spreading as put forward by Arthur Holmes

Geologists didn't entirely forget Wegener's evidence for moving continents. In the southern hemisphere (where much of the best evidence originated) his ideas never really went out of style. In 1944, the prescient Arthur Holmes one of the key developers of the methods of radiometric dating, proposed seafloor spreading - that slow convection currents in the mantle carried the overlying crust with them, and caused the formation of new seafloor. Continental crust would be carried passively along on top of the oceanic crust. Elegant speculation that lacked only evidence. (Holmes' ilustration at right.)

Paleomagnetism:

After WWII, several lines of evidence from the study of the intrinsic magnetic fields of igneous rocks came together to support Holmes' notion.



Key concepts and vocabulary:

  • "Continental drift"
  • Alfred Wegener
  • Wegener's evidence
  • Pangaea (also spelled "Pangea")
  • Harold Jeffries
  • Seafloor spreading
  • Arthur Holmes (again)
  • Paleomagnetism
  • Polar wander
  • Geomagnetic reversals
  • Geographic geomagnetic reversal pattern in sea floor