It is estimated that the world's first oceans formed about four billion years ago as a result of the cooling of the primitive earth. It is thought that volcanic eruptions discharged huge amounts of hot water vapor and other gases that cooled to form liquid water on the rugged surface. The volume of the oceans grew as volcanoes continued to emit gases from the molten rocks below. The chemical composition of the ocean water became salty as sodium, calcium, and magnesium were freed by chemical weathering and swept into the ocean by erosional processes. Chlorine and other elements were contributed from other volcanic gases.
Oceans cover about 70 percent of the earth's surface. The geology of the sea floor was largely unknown until the last half of the twentieth century, when the rapid advance of new technologies allowed geologists to study the sea floor in great detail.
Mapping the topography of the sea floor is accomplished using an echo sounder. This procedure calculates the depth to the ocean floor using the time it takes a sound wave sent from a ship to bounce off the bottom and return. A seismic profiler is similar to an echo sounder but uses lower frequencies that penetrate farther and result in more detailed profiles that show underlying structures such as faults and buried topography.
Other sampling techniques include using a rock dredge to collect rock samples from the bottom and a corer to retrieve a column of sediment in a weighted steel pipe. Huge derricks allow drilling directly into the ocean floor to recover solid cylindrical cores of ocean bedrock. Submersibles such as the HMS Challenger have taken geologists down to the ocean floor faults and allowed them to directly observe the geologic features, including active hydrothermal vents precipitating mineral deposits.