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)

Early Clues: The "missing heat"

Perhaps the most important clue for hydrothermal vents was not something found, but something that wasn’t found.

Here was the theory: The radioactive decay of rocks in Earth’s deeper layers produces heat, which rises toward the surface and escapes. This process, called “conduction,” is similar to what happens when you put a cold spoon in your coffee. The tip of the spoon in the coffee heats up first, and then the heat is gradually conducted toward the other end of the spoon. This is going on all over Earth’s surface.

It was natural to expect that heat flowing through the seafloor would be highest in places where hot mantle material is rising to the surface—such as the mid-ocean ridges. And it would make sense that the heat flow would diminish as you moved farther from the crests of the ridges.

But scientists were surprised in the 1960s when they began to build “heat-flow” probes that they thrust into the seafloor to measure the heat flow from the Earth’s crust to the seafloor. They found that the heat flow was lower at the Mid-Ocean Ridge crests than they had predicted. It didn’t make sense—unless some other unknown process was going on to remove the heat at the ridge crests.

Clive Lister of the University of Washington was among the first scientists to propose a solution to the mystery. He said that hydrothermal vents could account for the “missing heat.” Here’s how it works: Seawater circulating in porous oceanic crust is heated by molten rocks below. The heated fluid rises and is discharged at the seafloor. Excess heat is carried off with the fluids and dispersed into the ocean.

In this way, vents literally “ventilate” the mid-ocean ridges in a process called “convection.” The same process occurs in a pot of boiling water. Heated water rises to the top of the pot and discharges heat to the air above it. The cooler, and now denser, water then sinks to the bottom of the pot to be heated again.

So, by the early 1970s, scientists had predicted that there might be hydrothermal vents at mid-ocean ridges. But until 1977, no human had ever seen hydrothermal vents on the seafloor, nor could they prove they really existed.

[ Go to the next section: A Scientific Revolution ]