Continental Drift & Seafloor Spreading Chapter 7: Plate Tectonics

Restless Continents In the early 1900’s, a German meteorologist named Alfred Wegener (VEG e nuhr) suggested there was a supercontinent, called Pangaea (meaning "all land"), that broke up millions of years ago, slowly moved to their current positions, and continue to move today.

Evidence for Continental Drift When Wegener first noticed the similarities in the shoreline of continents on both sides of the Atlantic Ocean, he then began fitting them together. He particularly liked the way in which South America and Africa fit. Wegener used documented cases of fossil organisms that had been found on different landmasses that could not have crossed the current oceans.

More Evidence… Matching mountain ranges on each side of the Atlantic Ocean: The Appalachian Mountains that disappear off the coast of Newfoundland match mountains in the British Isles and Scandinavia which are comparable in age and structure. Ancient climates: When the continents are put together to form Pangaea the remains of glacial materials fit together to form a pattern like the large ice sheets that cover our poles today.

Seafloor Spreading Harry Hess, professor of geology at Princeton and Navy captain during WWII, was able to conduct echo-sounding surveys in the Pacific while cruising from one battle to the next. He said that new ocean floor was being created at mid-ocean ridges and destroyed at subduction zones. He called his theory SEAFLOOR SPREADING! One of his biggest pieces of evidence was being able to calculate the age of the seafloor at the ridges and subduction zones. Hess died before he could prove it.

Magnetic Striping The volcanic rocks which make up the sea floor have magnetism because, as they cool, magnetic minerals within the rock align to the Earth's magnetic field. Scientists noticed that some rocks were aligned to the “positive” or north end of the Earth and some were aligned to the “negative” or south end. Also, the ages of the rocks at the ridge were much younger than those found at the subduction zones. A hypothesis was presented in 1963 by Fred Vine and Drummond Matthews to explain this pattern. They proposed that lava erupted at different times along the rift at the crest of the mid-ocean ridges preserved different magnetic properties.

This provided strong support for sea floor spreading. 1 2 Vine and Matthews proposed that lava erupted on the sea floor on both sides of the rift, solidified, and moved away before more lava was erupted. If the Earth's magnetic field had reversed (changed from one geographic pole to the other) between the two eruptions, the lava flows would preserve a set of parallel bands with different magnetic properties. This provided strong support for sea floor spreading. 3