modern plate tectonic theory now affirms that oceanic crustal material eventually plunges back into the earth and melts in a process known as subduction. After surface rocks melt during subduction, they form a new supply of molten magma. Eventually, magma from other locations deep in the earth wells up at mid-oceanic ridges to form new igneous rocks, in a process known as seafloor spreading. It follows that any oceanic sediments deposited atop the oceanic igneous crust will have a limited “life span” on the surface of the earth. Eventually, these sedimentary rocks collide with the continental
...more
