Seafloor Spreading

Introduction:

The theory of continental drift was centered on evidences found on land.
1930's to early 1950's: Wegener's theory of continental drift received little attention.
After World War II, technological advances lead to two discoveries in the oceans that would build the theory of plate tectonics.

Systematic mapping of the ocean floor lead to the discovery of the Mid-Atlantic ridge (M-AR).
The M-AR is a submerged system of ridges and mountains.
M-AR stands 2500 to 3000 meters above the surrounding ocean floor.
M-AR runs in a sinuous curve down the middle of the Atlantic ocean from above Iceland to the tip of South America.
The M-AR is just part of a network of ridges encircling the Earth.
Mid-ocean ridges make-up over 20% of the Earth's surface.
In the early 1960's, Harry Hess of Princeton University proposed that the ridges were a surface feature caused by upwelling mantle materials.
At the ridges, the magma was extruded and displaced to the sides to make room for more upwelling magma.
The ocean floor actually becomes a giant conveyor belt
The oldest ends of the belt (the oldest magma /ocean bottom) being consumed at the deep-ocean trenches.
Two pieces of evidence supported Hess' theory:
- Radioactive dating of the rocks shows the age of the rock increases with distance from the ridge.
- The thickness of sediments increases the farther away you get from the middle of the ridge (more time for the sediments to accumulate).