M6 Hydrothermal Circulation

Topic

Hydrothermal circulation is the movement of a fluid—predominantly water—through the Earth's crust, driven by pressure and temperature gradients. At mid-ocean ridges, cold seawater penetrates the fractured basaltic crust and heats up as it approaches the underlying magma chamber, reaching temperatures of up to 400°C; it dissolves metals and sulfur from the rock it traverses and rises back up, now enriched with metals, emerging from hydrothermal vents that spew "black smoke" or "white smoke," depending on the minerals that precipitate upon contact with the cold seawater.

On land, geothermal systems—such as those in Iceland, Yellowstone Park, or New Zealand—similarly mobilize water of meteoric origin that is heated from below by a nearby magmatic intrusion.

The fundamental physics underlying this hydrothermal circulation combines the law describing flow in porous media with the equation describing heat transport: the fluid moves in response to driving hydraulic pressure gradients, but this movement carries heat along with it; this transported heat, in turn, modifies the rock's temperature field, thereby altering the fluid's own density and viscosity. This coupling between fluid movement and heat transport gives rise to true hydrothermal convection when the parameter measuring the ease with which a fluid begins to convect exceeds a well-defined critical value—close to 39—within a uniform porous layer.

The expected behavior of this hydrothermal convection is the formation of complete circulation cells: a hot, upward flow concentrated in zones of high permeability, such as faults and fractures, and a diffuse, downward flow occupying the cooler zones of the rock. The maximum temperature a hydrothermal fluid can reach is limited by water's critical point—approximately 374°C and 22.1 MPa of pressure—beyond which water ceases to behave as a liquid or gas distinguishable from one another.

At the slowest-spreading mid-ocean ridges, the heat driving hydrothermal circulation does not originate from a magma chamber but directly from the exhumed mantle itself, through the serpentinization of its minerals; this closes the link between the pressure and temperature gradients mentioned at the start of this description and the various heat sources capable of sustaining full-scale hydrothermal circulation.

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