In 1958, Tasmanian geologist Samuel Warren Carey, published "The tectonic approach to continental drift" " as support theory for expansion of the global crust, from here developping the theory of tectonics of plates.
According to theory, the expansion is done by raising at the surface of some new rocks, along ocean's dorsals. Without postulating the existence of subduction zones and processed faults , the theory could not explain why there is not an unlimited growing of the earth.
Harry Hess, geologist at Princeton University, and retired Admiral Robert S. Dietz,scientist at the U.S. Coast and Geodetic Survey, have been particularly concerned about this thing. They have launched first, the term of expansion of the ocean floor, associating expansion with repeated inversions of the magnetic field of rock, found both sides of an ocean's dorsals, switches arranged i, symmetrical bands,parallel, with similar magnetisation.
Remarkable is that, since 1928, G.A.F. Molengraff from the Institute of Technologyof Delft (now University) launched a recognizable theory in the later ocean floor expansion theory, to explain the opening of Atlantic Ocean, but also the East African Rift.
Hess thought that if the oceanic crust is spread over ocean's dorsals, then it decreases in other part.
After Hess, Atlantic Ocean expands, while the Pacific Ocean is diminishing. Also Hess, published a paper,in 1962 year, where he has suggested an opposite theory to continental drift one, namely that ocean basin with that Continent that borders it, move on the same crustal unit or plate.
In 1967, W. Jason Morgan suggested that the surface of the Earth is composed of 12 rigid plates that move relative to one another, and two months later, Xavier Pichon published a complete model, based on 6 major plates, moving relatively.
Finally, in the late '60s, is taking shape theory of plate tectonics as a synthesis of theory of continental drift, from the first half of the twentieth century and the concept of expansion of ocean floor, developed over 60 years.
Plate tectonics theory argues that the lithosphere is composed of tectonic plates, seven major plates and many other smaller, which are floating on astenosphere and may include either continental crust, or oceanic crust, or both.
These plates are in a dependent moves one another, leading to three types of plate margins: convergent,when plate margins collide and / or a plate are immersing the other, melting slowly into the mantle, divergent, whenplates depart from each other, and processed when plates pass along the other one.
Dr. Harry Hess proposed the theory for sea floor spreading. I did a major college research paper on plate tectonics theory and had found only four layers for the earth. These being the inner core, outer core, mantle, and crust. However, the mantle and crust can be subdivided.
As shown on the chart in the link, the depths are as follows:
Lithosphere 0- 37 miles
Crust 0-22 miles
Upper Mantle 22-37 miles
Mantle 22-1790 miles
Asthenosphere 62-125 miles
Upper Mantle 22-410 miles
Lower Mantle 22-410 miles
Outer Crust 1790-3160 miles
Inner Crust 3160-3954 miles
There is a nice diagram on the link.