Printed from The Discovery of Hydrothermal Vents - 25th Anniversary
CD-ROM ©2002 Woods Hole Oceanographic Institution
EARLY CLUES: Evidence
"The Missing Heat"
Perhaps the most important clue for hydrothermal vents was not something
found, but something that wasnt found.
Here was the theory: The radioactive decay of rocks in Earths 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 Earths 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 surfacesuch
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 Earths crust to the seafloor. They found that the heat flow was
lower at the mid-ocean ridge crests than they had predicted. It didnt
make senseunless 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. Heres 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.
close window
|