There is a lot of overlap between oceanography and geology. Scientists now believe that the oceans formed shortly after the earth itself and have always been just about as deep and just about as salty as they are today. They have not become more salty over the eons, Byerly said, because they are constantly drying up and re-forming elsewhere, leaving behind great deposits of salt. Scientists used to believe that the ocean basins were ancient and the continents were relatively young. Now we know just the opposite is true.
Ocean basins are the most geologically active places on earth.
The average age of the Atlantic, for instance, is between 75 and 150 million years. Although the earth itself is estimated to be 4.5 billion years old, no rocks have survived from that early date because geological forces have recycled them. Life probably began in the warm, shallow waters of even earlier oceans but did not survive. The earth was struck by large meteors for perhaps the first 700 million years of its history. Many of these would have been big enough to evaporate all the oceans, sterilizing life.