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.

Seafloor Spreading

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).

Iron-rich minerals occur in the lava flows found in the ocean ridges.
As the lava cools to below about 580°C (the Curie point), the individual mineral grains become magnetized in the direction of the Earth's magnetic field.
The rocks record the magnetic fields that existed when they cooled.
Preserving the direction of the ancient magnetic field is called paleomagnetism.
Examining cooled lava from various places on Earth showed the Earth's magnetic field periodically reverses direction.
The Earth's magnetic north pole would be at the geographic south pole.
These reversals happen every few 100,000 years to several million years apart.
Sensitive instruments showed that the rocks on either side of the Mid-Atlantic ridge showed a pattern of 'normal' and 'reversed' magnetism.
In 1963, Vine and Matthews suggested that the patterns which ran || (parallel) to the ridge were a recording of the changes of the Earth's magnetic field made as lava was extruded from the ridge and displaced by later lava flow.

The mammal-like dinosaur: Cyanognathus

The extinct reptile: Lystrosaurus

Magnetic stripes of the mid-ocean ridges:

Earth's Tectonic Plates: