GEO 1001
Continental Drift and Sea Floor Spreading
Reading: p. 531-538


 Early Ideas about Continents, Ocean Basins, and Mountains

Continental Drift: The Story on Land
 
In 1912 the German meteorologist Alfred Wegener suggested that there was once a supercontinent which he named Pangaea (meaning "all lands").  He proposed that Pangaea began to break up by continental drift  during Late Carboniferous time, about 300 million years ago. (Fig. 20.1)

The theory of continental drift was supported by various types of geologic evidence including:

(1) continental "fit" across the Atlantic
(2) matching geologic features on the margins of continents
(3) fossil evidence
(4) record of paleozoic glaciation.


Figures from Volcano World

Despite the geological evidence, many scientists remained unconvinced because no one could explain how continents of granite could "plow" through the ocean basins.
 
 

From Volcano World
CLICK HERE FOR MORE INFO ON CONTINENTAL DRIFT AND ALFRED WEGENER

 Paleomagnetism and Apparent Polar Wander

For example, when Europe and North America were previously joined, the paleomagnetic field preserved in their rocks would indicate a single pole location until they drifted apart (B). The sequence of rocks on each continent would then show the pole taking a different path to its present position (A)
 
 
 
 
 
 
 
 
 
 
 

 

The break-up of Pangaea and the opening of the Atlantic Ocean
(From This Dynamic Earth-USGS)


SEA-FLOOR SPREADING: THE STORY AT SEA
Renewed interest in oceanographic research in the 1950's led to extensive mapping of the world's ocean basins and to the discovery of the world wide oceanic ridge system. In 1962, Harry Hess postulated that the ocean floors are spreading apart due to convection currents in the mantle.
According to this theory, magma from the interior of the earth is injected at mid-ocean ridges resulting in new oceanic crust spreading symmetrically away from mid-ocean ridges.


 
 
Deep Sea Drilling: In 1968, a large effort was made to learn about the nature of the ocean floor by collecting sea floor samples from long (up to two kilometers) drill cores.  A drilling ship was specially constructed (the Glomar Challenger), which enabled scientists to collect samples of sea floor sediments and rocks.