The Shape and Age of the Ocean Floor
The expectation of geologists until the mid-1900's was that the rocks of the ocean floor should be very old. This expectation was based on two arguments. First, one would expect there to be little to no erosion of the rocks on the ocean floor -- it is already at low potential energy -- so that the rocks that formed as the Earth formed should still be preserved beneath the ocean. Now you might expect that the ocean floor would be covered with thick layers of sedimentary rocks eroded from the continents, but beneath the sediments, there should be some of the oldest rocks of the Earth. The second reason is that geologists viewed the ocean as a kind of giant valley. If you look at the Grand Canyon, as you go deeper into the canyon, the rocks get older and older. While this was the expectation, the reality was that no one had ever found really old rocks from the ocean floor. Dredging and drilling operations have never recovered rocks more than 150 million years old. Given that the Earth is about 4.5 billion years old this would seem to present a problem. Using SONAR to map the ocean floor in detail radically changed geology's view of the oceans and the earth itself.
1. Explain the reasoning that led geologists to expect to find very old rocks in the oceanic crust.2. On a map of the Pacific Ocean locate the mid-ocean ridge and the position of trenches.
3. Draw simple cross sections illustrating the principles of superposition and crosscutting relationships.
4. Describe the distribution of ages of the oceanic crust in the Atlantic Ocean.
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