The submarine hydrothermal activity on and near the Galápagos Rift has been explored with the aid of the deep submersible Alvin. Analyses of water samples from hydrothermal vents reveal that hydrothermal activity provides significant or dominant sources and sinks for several components of seawater; studies of conductive and convective heat transfer suggest that two-thirds of the heat lost from new oceanic lithosphere at the Galápagos Rift in the first million years may be vented from thermal springs, predominantly along the axial ridge within the rift valley. The vent areas are populated by animal communities. They appear to utilize chemosynthesis by sulfur-oxidizing bacteria to derive their entire energy supply from reactions between the seawater and the rocks at high temperatures, rather than photosynthesis.

We report an extensive suite of geothermal measurements in the deepest borehole yet drilled into the oceanic crust, hole 504B of the Deep Sea Drilling Project. Located in 6.2‐m.y.‐old crust of the Costa Rica Rift, hole 504B was cored during legs 69 and 70 in late 1979 and leg 83 in late 1981, to a total depth of 1350 m beneath the seafloor, through 274.5 m of sediment and 1075.5 m of basalt. During the three drilling legs, downhole temperatures were logged 11 times, and the thermal conductivities of 239 sediment and basalt samples were measured. The results indicate a dominantly conductive mode of heat transfer through the complete section, at 190±10 mW/m2. This is consistent with the predicted plate heat transfer and the hypothesis that the thick sediment cover acts as a seal against hydrothermal circulation of seawater to basement. For over 2 years after this sediment seal was penetrated, borehole temperatures were nearly isothermal to about 350–370 m, indicating that ocean bottom water was flowing down the hole into the upper ∼100 m of basement. This downhole flow was driven by the underpressure of the basement pore fluids, which is of indefinite, but possibly hydrothermal, origin (Anderson and Zoback, 1982). The flow rate decreased from 6000–7000 1/h in late 1979 to about 1500 1/h 2 years later; altogether over 50×106 kg of seawater has been drawn into the basement. We estimate a permeability of ≳6×10−14 m2 for the reservoir in the upper ∼100 m of basement. This zone seems to correspond to a layer of high apparent porosity (Becker et al., 1982), which has been tentatively identified as a thin layer 2A (Anderson et al., 1982a).

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A total of 194 new measurements of heat flow through the eastern Pacific Ocean floor are presented. They range in value from essentially zero to 8.04×10−6 cal/cm2 sec. The crest of the east Pacific rise is systematically associated with high values, a strip 200 to 300 km wide at the crest having an average heat flow of about 3×10−6 cal/cm2 sec. Within this strip, the highest values occur in two narrower zones which appear to be approximately parallel and symmetrically oriented to the crest. The source of the high heat flow in each of these zones is probably a region of unusually high temperature a few tens of kilometers wide located about 10 km beneath the ocean floor. One‐fourth of all the measurements gave low heat‐flow values (≤0.80×10−6 cal/cm2 sec). Two approximately equidimensional regions near the equator and to each side of the rise show generally low heat‐flow values. These regions are 2 to 4×10−6 km2 in area, and their average heat flows are about half the normal oceanic value of 1.2 to 1.4×10−6 cal/cm2 sec. In other areas, many of the isolated low heat‐flow values are correlated with flat topography which suggests an effect of the local environment; these values are presumably not representative of the regional geothermal heat flow. The systematically distributed variations suggest an origin associated with large‐scale thermal convection in the mantle.

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