Geoscience Reference
In-Depth Information
rived from the continents, tells us to expect it, but at the margins. To paraphrase
the cartographers of old: “Here Be Surprises.”
Scientists measuring the amount of heat escaping from the ocean basins found it
to be highest right over the rifts. Earthquake epicenters, instead of being scattered
randomly, tended to lie under the rifts. Geologists thought that the oceans had ex-
isted for hundreds of millions of years, if not always, as Bailey Willis would have
had it. During all that time, a thick layer of sediment ought to have accumulated
on the ocean floor. Yet drilling cores brought up only a fraction of the expected
amount. Either the ocean basins are much younger than geologists had thought or
some process had swept their floors clear of sediment, as though the master sushi
chef had tidied up before turning off the lights.
The raft of new information from the ocean basins, but especially the Heezen-
Tharp map, deeply impressed geologists. Those of a certain age may never forget
the first time they saw the map. It was impossible not to recognize that something
horrific had happened to the Atlantic Ocean floor, something beyond the ken of
continent-bound geologists. The permanent, immobile continents had begun to
teeter on their supposedly rock-solid foundations.
Fossil Compass Needles
Another line of research provided rigorous quantitative evidence that bore on con-
tinental drift. It came from the fossil magnetism of rocks. Since antiquity, people
have known that a sliver of lodestone (the ironbearing mineral magnetite) points
toward the North (Magnetic) Pole, providing mariners with a life-saving tool.
Scientists later learned that when magnetic materials cool from high temperatures,
