Explain how oceanic-floor rocks and sediments are evidence with sea-floor spreading.
look up Alfred Wegener on wikipedia
Sea floor spreading. In the 1960s, geologist Harry Hess proposed that the sea floor was moving outward from the midoceanic ridges. His theory of sea floor spreading maintained that new basaltic oceanic crust forms at a midoceanic ridge and is slowly pushed away on both sides toward the continents as more new crust is produced. (Measurements indicate that new crust moves away from a ridge at rates from 2 to 10 cm/year.) A midoceanic ridge is called a spreading axis or spreading center. Subduction is the process by which the oceanic crust is pushed against, and finally underneath, continental or oceanic crust. Subduction zones are often marked by overlying chains of volcanic islands called island arcs. Geologists believe sea floor spreading results from convection in the mantle and lower crust that brings hotter, less dense, and more plastic material up toward the surface; the colder, more dense rock and sediment, such as subducted crustal material, sinks toward the mantle (Figure 1 ). These convective forces tear the ocean crust apart at the midoceanic ridge, forming a rift valley marked by high-angle faults, basaltic lavas, and high heat flows. The Mid-Atlantic Ridge is one of the best studied midoceanic ridges. It separates North America from Europe. Its 10,000-foot-tall mountain peaks lie about a mile below the surface of the ocean.
