Click to enlarge » Scientists predicted that the flow of heat through the seafloor should decrease farther away from the ridge. Although this is true, the measured values of heat flow near the ridge (dashed line) were lower than the predicted values (solid line). Could hydrothermal circulation explain the difference? (Illustration by E. Paul Oberlander, WHOI)

The Importance: Heat from the Earth

One of the early clues for seafloor hydrothermal activity was the difference between the heat generated in the Earth’s interior and escaping through the Earth’s crust to the seafloor, and the heat flow that scientists had predicted should be taking place. This difference suggested that some other unknown process was going on to remove that heat at the mid-ocean ridges.

The process responsible for the ‘missing’ heat turned out to be hydrothermal circulation, a process driven by “convection.” Cold seawater circulates through the ocean crust, is heated by molten or hot solid rocks below, and is transformed into a hydrothermal fluid as it interacts with crustal rocks. The hot fluid rises and is discharged at the seafloor, thereby removing heat from the Earth’s crust.

When scientists looked at the plot of measured and predicted heat lost from the ocean crust, they noticed that the curves merge when the ocean crust is about 65 million years old. This told them that hydrothermal circulation must continue in the ocean crust for tens of millions of years as it moves away from the Mid-Ocean Ridge where it is created. Scientists were also able to calculate that the amount of heat escaping from the crust by hydrothermal circulation is about 30% of the total heat lost from all ocean crust. This is the difference between the measured and predicted curves. So how much heat is lost at the Mid-Ocean Ridge itself? The answer is about 30% of all the heat removed through hydrothermal circulation in ocean crust.