Hydrothermal Resources At Mid-Oceanic Ridge Axes

The axes of mid-oceanic ridges are the locus of seafloor spreading. Molten material continually is emplaced along the ridge axis. The most promising hydrothermal resource at the ridge axis are natural high temperature vents and hot-dry rock at depths greater than a kilometer beneath the seafloor. The ridge axis can be located by analysis of magnetic anomalies and high-precision bathymetry with relatively little effort. Bottom photography is useful in refining the position of the ridge axis and for locating high temperature vents. Water escapes through isolated vent clusters at rates of up to around 100 kg/sec with temperatures of about 350 C. Limited sampling indicates that the maximum power from a single vent cluster is 200 Mwatt but that power of a few 10's of Mwatts is more common. The average heat loss from such vents is less than 10 Mwatt per kilometer of ridge (for a full spreading rate of 60 mm/yr), which is equivalent to one vent cluster every 10 km of ridge axis. Obvious problems in utilizing this resource include the low demand for power at the middle of the ocean and the difficulties in running a heat engine at the bottom of the sea. A less obvious problem is that hydrothermal vents are short lived and may die after a few years. The conductive heat losses from active vents are so low that the only benefits in drilling a vent cluster would be more efficient collection of the water and perhaps some extension of the life of the vent field from clogging. Leakage from natural vent clusters apparently is self-sealed by precipitation of minerals dissolved in the hydrothermal water. Much larger amounts of heat are potentially available from hot-dry rock. The magma chamber of the ridge axis is a potential resource which can be located by multichannel seismic reflection. The top of the chamber is 2 to 3 km deep and about 4 km wide. Moderate temperature hot-dry rock is present everywhere near the axis.