GEOL388
6-10-08

Rocks and Plate Tectonics:


John Merck

Here is a map of the major islands of the Galápagos: :

Memorize the English and Spanish names of the major islands and be ready to identify them on a map. Don't be a baby.

Typically, visitors use the Spanish name. In cases where this is not the practice, we have boldfaced the usual name.

Most volcanically active islands have only one volcano however Isabela has six. Be prepared to identify them on a map.

  • Ecuador
  • Wolf
  • Darwin
  • Alcedo
  • Sierra Negra
  • Cerro Azul
  • That's nice. How did the islands get there? To answer this, we must indulge in a brief geological diversion.

    Rocks:

    What is a rock? A purely descriptive definition is that a rock is - A naturally occurring aggregate of minerals and other solid material. - Usually, there are several minerals in the aggregate, though some rocks may have only one. The other materials may include natural glasses, organic material, or fossils.

    Geologists usually think of rocks in a second important way, however. Please memorize this and recite it like a mantra:

    * A rock is a record of the environment in which it formed. *

    Rock types:

    Today we recognize three basic types:

  • Igneous: Rocks that form from the cooling and solidification of magma. Igneous rocks are generally have interlocking crystals that show no preferred orientation. There are two types:

  • Metamorphic: Rocks that form from the recrystallization of preexisting rocks under extreme heat and/or pressure. E.g.: Phyllite.

  • Ortega Quartzite in Picuris Mts., NM

  • Sedimentary: Rocks that form from transported fragments of preexisting rocks. E.g.:

    The Rock Cycle: Consider the three basic rock types and how they form: The beginnings of an answer came from an improbably source. The German meteorologist 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. Note: Wegener thought that the continental crust slid over the oceanic crust like glacial ice sliding over bedrock. 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: At the 1928 meeting of the American Association of Petroleum Geologists, Harold Jeffries presented an analysis demonstrating that neither of these forces could begin to propel continents around. Continental drift was relegated to the idiot fringe (among Northern Hemisphere Geologists, at least). No US textbook mentioned it until 1960. In 1930 Wegener died in a freak storm while doing field work in Greenland.

    New evidence and reconsideration

    Paleomagnetism After WWII, several lines of evidence from the study of the intrinsic magnetic fields of igneous rocks began to come together.

    Harry Hess puts the pieces together: Hess was an igneous rock geologist (a "hard-rock" man) who had participated in sea floor geologic surveys in the 1950s and early 60s. At the beginning of the sixties, he finally put the pieces together: Wegener had been right to say that the continents moved, but for the wrong reasons. Hess's view is the foundation of the theory of Plate Tectonics, which has become the unifying theory of modern Geology. Between 1960 and 1970, the academic community was won over to it. Here is its essence: Plate tectonics' explanatory power:

    The Galapagos Islands are the visible manifestation of an oceanic hot spot.

    Indeed, being near the spreading center between the Cocos and Nazca plates (which has, itself, drifted to the north over the hot spot) their history is somewhat like that of Iceland. The Galapagos hot spot is responsible for both the Cocos and Carnegie Ridges.

    BUT HEY! Don't accuse me of not teaching the alternative!