Age of the Ocean Floor
National Geophysical Data Center
The seafloor has been widely mapped by shipborne magnetometers, allowing us to delineate zones of normal and reversed magnetic polarity. As the oceanic plates grow, like paper coming from a fax machine, the magnetic zones form distinct stripes on the map. This map is colored according to the magnetic chron, calibrated against absolute age in million years before present (Ma).
Very little ocean floor is older than 125 million years, as shown by the blue areas. Almost all of it has been subducted, carried down into the mantle and recycled. So while this map is great for studying plate motions in the recent geologic past, for times before the Cretaceous we must rely more and more on paleomagnetic studies on the continents.