Seafloor age is the most important geologic evidence of plate tectonics. Magnetic surveys of the ocean floor starting in the 1940s revealed symmetrical striped patterns of magnetizationmagnetization acquired by lava flows as they cool from the molten state. Soon afterward it was learned that the Earth's magnetic field undergoes occasional reversals. Then came the insight that the magnetic pattern is how these reversals are recorded by the fresh lava at mid-ocean ridges as they spread apart. The mid-ocean ridges are represented by the black lines with the youngest (red) rock flanking them (see the separate map of the mid-ocean ridges).
Click the image to see this map at full size. The map shows that Antarctica, Africa, South America and Australia have been spreading apart from each other for a long time. Running this motion backward in geologic time shows that they were once a single landmass. Fossil evidence from rocks on land had already suggested this story decades earlier, and the former supercontinent even had a name, Gondwanaland. Today we know that for a while Gondwanaland was part of a universal supercontinent, Pangea. On the other side of the globe, the Pacific Ocean is shrinking as old seafloor is subducted beneath the continents on its west, north and east sides. When Pangea existed, the corresponding ocean (named Panthalassa) was very large indeed.
This map is from a compilation published in 2008. Geoscientists continue to map the seafloor in detail as they refine our picture of the ancient Earth. Because there is no seafloor remaining that is older than 180 million years (except possibly a bit in the Mediterranean), we cannot use this map to go farther into the past than the Jurassic Period. But seafloor mapping is still helping us fill in details of geologic time since then. And the youngest seafloor ages very closely match the ongoing plate movements today that are measured in real time using the GPS system.
See related maps, such as age errors and sperading asymmetries, on the EarthByte site.