Oceanic crust is continuously being created at mid-ocean ridges. As plates diverge at these ridges, magma rises into the upper mantle and crust. As it moves away from the ridge, the lithosphere becomes cooler and denser, and sediment gradually bui... read more
New ocean floor rock comes up from the mantle at mid-ocean ridges. As the ridge widens, new rock flows up to fill the “gap”. For an obvious example, see the Mid-Atlantic Ridge. read more
These form regoliths known as"drop stones" on the ocean floor, and are indicative of cold conditions. Alluvium may also be from volcanic activity, turbidite, or other weathering events. Rocks exposed on the ocean floor, that are not regoliths, can indicate the direction of magnetic north when that rock formed. read more
New rock is formed by magma at the mid-ocean ridges, and the ocean floor spreads out from this point. When the magma cools to form rock, its magnetic polarity is aligned with the then-current positions of the magnetic poles of the Earth. read more