Changes

Extensive exploration and mapping of the seafloor after World War II led to the discovery of a deep rift running down the center of the Mid-Atlantic Ridge. In the early 1960s Harry Hess of Princeton University and Robert Dietz of the University of California suggested that the seafloor separates along the rifts in mid-0ceanic ridges and that new seafloor forms by upwelling of hot mantle materials in these cracks, followed by lateral spreading. By 1967 separate lithospheric plates had been identified, which explained phenomena such as high levels of volcanic and earthquake activity that take place between the plates. By the end of the 1960s the theory of plate tectonics proved to be a unifying concept that pulled together diverse theories  and explained a the large body of observations in the field. <ref>Press, Frank and Siever, Raymond.1998."Understanding Earth", 2nd Ed. New York: W. H. Freeman and Company</ref>

Extensive exploration and mapping of the seafloor after World War II led to the discovery of a deep rift running down the center of the Mid-Atlantic Ridge. In the early 1960s Harry Hess of Princeton University and Robert Dietz of the University of California suggested that the seafloor separates along the rifts in mid-0ceanic ridges and that new seafloor forms by upwelling of hot mantle materials in these cracks, followed by lateral spreading. By 1967 separate lithospheric plates had been identified, which explained phenomena such as high levels of volcanic and earthquake activity that take place between the plates. By the end of the 1960s the theory of plate tectonics proved to be a unifying concept that pulled together diverse theories  and explained a the large body of observations in the field. <ref>Press, Frank and Siever, Raymond.1998."Understanding Earth", 2nd Ed. New York: W. H. Freeman and Company</ref>

Twelve or so plates slide over a partially molten lithosphere, and the continents, embedded in some of the plates, are carried by them. Where plates collide, mountains such as the Himalayas or the Rocky Mountains are formed. In some places, called subduction zones one plate sinks beneath another plate and returns to the molten regions of the athenosphere. Along mid-oceanic ridges magma emerges to  expand the plate and create new sea floor. [10][11]

Twelve or so plates slide over a partially molten lithosphere, and the continents, embedded in some of the plates, are carried by them. Where plates collide, mountains such as the Himalayas or the Rocky Mountains are formed. In some places, called subduction zones one plate sinks beneath another plate and returns to the molten regions of the athenosphere. Along mid-oceanic ridges magma emerges to  expand the plate and create new sea floor.

==How Old is the Earth==

==How Old is the Earth==