Title

Dedicated to the teaching of Earth Sciences

GEOLOGY RESOURCES FOR STUDENTS,
TEACHERS, AND RESEARCHERS

Prof. Sean Tvelia Suffolk County Community College

 

Developing the Theory

Interest in the theory of continental drift was revived after WWII. In 1947 the National Geographic Society commisioned Maurice Ewing to explore an odd rise in the Atlantic Ocean known as the Mid Atlantic Ridge. The ridge itself had been discovered in the 1850's by Mathew Maury who was attempting to create the first map of the ocean floor.

Based on his previous work, Ewing, like most others of his time believed that the ocean floor would be thick with sediment and would provide a rich fossil record of Earth's entire history. What Ewing found instead would surprise all. Beyond the continental shelf the ocean floor contained only recent sediment and unlike what was expected for the 3 billion years of accumulation, instead of being 12 miles thick, the sediment at its thickest was only 1000ft. Furthermore, as Ewing reached the ridge not only did the sediment get even thinner but dredges also began collecting volcanic rock. Later seismic data would eventually show that the sea floor was composed of basalt, an extrusive igneous rock, and was only a few miles thick.

In the early 1950s Ewing, with the help of Marie Tharp, a cartographer, mapped the seafloor using available echo profiles. To their's and the worlds surprise, the resulting mapped showed a large canyon that traversed the length of the ridge. This seemed almost impossible. Then, a few years later, a study of mid-atlantic earthquakes revealled that the earthquake epicenters followed the same valley that had been mapped by Ewing and Tharp. When Ewing heard this he began compilling data from all mid-ocean earthquakes and found that the resulting plot formed a line that seemed to wrap around the globe like the stiching on a baseball. Ewing discovered the existence of the worlds oceanic ridge system. The largest mountain range on earth stretching a total of 40,000 miles under water.

Ewing's discoveries lead him to believe that the oceans were widening but his conclusion did not fit well with the established belief that the Earth was actually contracting do to a loss of heat and so the idea of continental drift was still just that.

By this point the evidence of continental drift was overwhelming but still nobody could provide an acceptable mechanism, that is until Harry Hess.

Harry Hess was a professor from Princeton who had a new idea. He believed that the ocean floor acted as a conveyor belt and the continents simply travelled across the globe on top. However, unlike his predecessors Hess understood the implications of trying to convince an unbelieving world of this radical idea and so in one way Harry Hess becomes the Galileo of the geologic world. In 1960 Hess wrote, "The History of Ocean Basins," a paper in which he claimed that the oceans were not permanant. He states that the Mid-Ocean ridges are actually cracks in the Earth's crust where magma continually wells up and spreads outward. Like Galileo, Hess knew what kind of reception his idea would recieve so cleverly Hess begins his paper by warning his readers. He states, "birth of the oceans is a matter of conjecture," and that his paper is more of a "essay in geopoetry." Hess knew that was the only way to keep his readers reading.

His paper goes on to describe how new crust is created on either side of the ridges (causing the sea-floor to spread) at a rate of one-half inch per year. At this rate the Atlantic Ocean would have been created during the past 200 million years--just as Wegner suggested! He further explains that this would not cause the Earth to expand. Hess explains that as new crust is being formed at the ridges old ocean crust is being destroyed as it is pulled back into the Earth in deep ocean trenches which lie along the edges of continents called subduction zones. Hess went even further to say that it was convection within the mantle that caused this movement. Finally a mechanism.

Hess's idea seemed to answer all questions but without scientific data to back it up his paper was treated just as he suggested it be, as conjecture and fantasy.

Chapter Contents:

3.0: Continental Drift

3.1: Evidence of Continental

3.2: Developing the Theory

3.3: Paleomagnetism and Continental Drift

3.4: Plate Tectonics