HEAT FLOW
The first successful instrument for measuring the flow of heat through the ocean floor was laboriously devised at
One of the disadvantages of the original Bullard probe was the necessity of taking a separate sediment core for thermal conductivity measurements immediately after the probe lowering, which not only required considerable extra time but also could occur some distance from the probe measurement because of ship drift. Scripps and its frequent competitor in creativity, Lamont Geological Observatory, naturally devised slightly different techniques for solving this problem. The Scripps approach was to attach a conductivity needle to a slider on the probe, which measured the conductivity at an average depth of 50 cm below the surface of the sediment. Lamont developed a heat probe attached to the piston corer.
The streamlined instrument devised by Charles Corry, Carl Dubois, and Victor Vacquier in 1968 has proved to be a convenient device at sea, even in weather scarcely comfortable for its handlers: “with the ship rolling up to 35° and in wind speeds up to 60 knots.”[17] The heat probe appears to be more susceptible to breakage or bending on deck than on the sea floor.
The results from the first measurements of heat flow through the sea floor were among several surprises gathered on the first major Scripps expedition, Midpac, in 1950: the temperature gradient was very similar to that measured on land, whereas it had been expected to be considerably less. “The only adequate source of heat that has been suggested is radioactivity within the earth,” noted Bullard,[18]
Other surprises were in store, as scattered measurements of heat flow were taken on various expeditions in Atlantic and Pacific. From Capricorn Expedition in 1952 Maxwell and Revelle reported an unusually high value on the broad topographic feature first known as the Albatross Plateau (part of the East Pacific Rise), which was not recognized until later as having been taken on a ridge. About the same time, Bullard, then at the National Physical Laboratory in England, found a high value in the Atlantic Ocean. As measurements continued, a pattern began to emerge. Heat flow on the crests of the East Pacific Rise and the Mid-Atlantic Ridge, and in the Gulf of California, was generally higher than elsewhere. Geologists were examining these regions closely and by every means at their disposal, as these anomalous regions appeared to be the foci of intensive sea-floor activity. As theories developed around the concept of sea-floor spreading, the measurements of heat flow fitted into the emerging picture: high heat flow indicated areas of more intensive crustal activity.
The development of a convenient shipboard device for measuring the flow of heat resulted in the gathering of more measurements at sea than on land. In 1967 Von Herzen (by then at Woods Hole Oceanographic Institution) and Vacquier estimated that only 11 percent of the 1,300 measurements to that time had been taken ashore. In an ingenious use of the shipboard technique, these researchers carried a temperature probe to Africa and obtained “land” heat-flow measurements aboard a Fisheries Research Unit vessel in Lake Malawi in the east African rift zone. Later Vacquier and John Sclater used a temperature probe in Lake Titicaca in South America. The heat-flow enthusiasts