TECTONICS/Hydrothermal Activity 365 (see Geysers and Hot Springs) It is the nature of the heat source that generally determines whether hydrothermal activity occurs at high (>150 C) or low temperature The circulation system consists of a recharge zone through which fluids enter the crust, a region in which the fluid takes up heat from its surroundings, and a discharge zone, through which the heated hydrothermal fluid emerges at the surface as a hot spring or hydrothermal vent Although fluid may sometimes recirculate several times before exiting the system, it is often convenient to describe circulation in terms of a simple single-pass circulation model Figure shows cartoons of single-pass models envisioned for high-temperature terrestrial and submarine systems and a low-temperature warm spring system In addition, all hydrothermal activity exhibits temporal variability, and chemical reactions between the circulating fluid and rock are often important Heat Geothermal gradient Conductive heat flux, H, is related to the geothermal gradient by H ¼ l dT/dz, where l is the thermal conductivity For rocks, l ranges from approximately 1.8 to W (m C) 1, with most igneous and metamorphic rocks falling into a narrower range between 2.0 and 2.5 W (m C) In older, stable continental cratons, the geothermal gradient may be as low as 10 C km 1, whereas in active volcanic regions it may be more than 100 C km A typical geothermal gradient of %25 C km gives a conductive heat flux of %60 mW m In terrestrial low-temperature hydrothermal activity, fluids driven by a topographic head circulate to a depth of $1–3 km in the crust where they are heated by the geothermal gradient The fluids emerge through faults at the surface as warm or hot springs with temperatures ranging from a few tens of degrees above ambient to the local surface boiling temperature (Figure 3C) Such springs are found worldwide in areas of both normal and elevated heat flow Low-temperature hydrothermal circulation in oceanic crust occurs from ridge axes to a lithospheric age of $60 My This circulation is partially controlled seafloor topography in combination with the geothermal gradient, with discharge occurring at highs and recharge occurring at topographic lows Type and thickness of sediment cover also influences this circulation More than 90% of all hydrothermal heat loss from the seafloor occurs at low temperature This circulation impacts geochemical cycles as the equivalent of an ocean volume approximately evens 106 years Magmatic heat High-temperature hydrothermal activity (typically classified as > 150 C) is associated with active volcanism In these settings, shallow magmatic intrusions provide the heat source Part of this heat comes from the latent heat of crystallization and part of the heat is derived from the cooling pluton Thermal buoyancy differences between the colder and hotter parts of the system drive convective fluid motions As volcanism is associated with ocean ridges, hot spots, and island arc systems (fore-arc, arc, and back-arc settings) at subduction zones, it is not surprising that essentially all high-temperature hydrothermal activity occurs in these regions (Figure 1) In terrestrial settings, boiling hot springs and geysers provide the surface expressions high-temperature hydrothermal activity Reservoir temperatures of these systems typically lie between 200 and 350 C In oceanic settings vigorous high-temperature hydrothermal activity is exhibited as ‘‘black smoker’’ venting at temperatures between 300 and 400 C (Figure 4) Lower temperature ‘‘white smokers’’ with temperatures $150–200 C are also common Because of the high pressure ($250 bars) at the seafloor, these hightemperature vents lie below the boiling temperature As discussed later, however, boiling and phase separation appear to occur in the subsurface of both terrestrial and submarine high-temperature hydrothermal systems Chemical heat It has long been recognized that hydration of peridotite is an exothermic reaction that produces heat, that alters the chemistry of the rocks and hydrating solutions involved, and that expands the volume of the rocks ($40%) It is only now emerging how widespread this process called the ‘‘serpentinization reaction’’ may be beneath ocean basins and possibly continents The reaction involves peridotite, the characteristic ultramafic rock type of the Earth’s upper mantle, and either seawater or meteoric water Serpentinization is commonly observed in ultramafic rocks recovered from the seafloor and in slices of ancient oceanic mantle exposed on land as ophiolites This reaction yields distinctive chemical solutions characterized by high alkalinity, high ratios of Ca to Mn and other metals, and abiogenic permeability and the formation of seafloor event plumes Journal of Geophysical Research 105: 8341 8354 (B) Analogous cartoon representing single pass flow in a high temperature terrestrial system Adapted from White (1973) Characteristics of geothermal resources In: Kruger P and Otte C (eds.) Geothermal Energy Stanford, CA: Stanford University Press (C) Cartoon of a terrestrial low temperature warm spring system Reproduced from Lowell (1992) Hydrothermal systems In: Encyclopedia of Earth System Science, vol 2, pp 547 557 San Diego, CA: Academic Press