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Fluid pathways
diagram of fluid pathways in a seafloor hydrothermal system

Hydrothermal systems have three basic parts. The recharge zone is where seawater enters the permeable volcanic rocks of the seafloor, percolates down toward the heat source, and begins to undergo low temperature chemical reactions. The high temperature reaction zone is where the fluid passes near the magmatic heat source and reacts vigorously with the surrounding rock in a very hot, very corrosive environment.

The upflow zone is the path to the exit, where heated, buoyant hydrothermal fluids ascend rapidly to the seafloor vents. The reaction zone is located just above the molten (or partially molten) rock in the volcanic system. In cases like Axial Seamount and also many mid-ocean ridge segments, magma resides at 1-4 km depth within the oceanic crust, and the long-term location of the reaction zone is just above the magma, at the base of the dikes which form most of the upper crust. Fluids reach temperatures of up to 450°C, and pick up a heavy load of dissolved metals from reaction with the surrounding rock.
In the upflow zone, hot, buoyant hydrothermal fluids race to the seafloor. In some cases the fluid path is not well focused, especially after a disruption to the system such as an eruption. The fluids may mix with seawater before they reach the seafloor, cooling down and spreading out to produce diffuse venting. In other cases, the fluids take a direct path to the vent with little or no mixing and exit the seafloor at temperatures as high as 300-400°C.
The magma reservoir acts as a stable long-lived heat source for hydrothermal circulation, and indeed the presence of long-lived hydrothermal activity seems to reflect the presence of a substantial magma body at depth. Individual lava flows and dikes are small in volume by comparison, and are cooled within a few months to a few years after an eruption. Nevertheless, they support or augment wide-spread but short-lived diffuse venting. On the 1998 lava flow at Axial, a majority of the diffuse vents located in 1998 had already shut down by the summer of 2000. Those that remain vigorous, including the Marker 33/Cloud Vent area, are located near the 1998 eruptive fissure. In contrast, the 1998 eruption served to rejuvenate the preexisting, long-lived high temperature vents at ASHES and CASM. These focused vent sites are both located near the caldera fault system of Axial, with CASM actually at the intersection of caldera wall and the north rift zone.
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Related Virtual Sites:
ASHES | CASM | Marker 33

Other NeMO-related concepts:
Mid-ocean ridges | seafloor spreading | seamounts & hot spots | calderas | Axial volcano
Hydrothermal vents | fluid paths | focused vents | diffuse vents | sulfide | anhydrite
Animal Gallery | chemosynthesis | biological colonization of new lava

Lava morphology | sheets | pillows | lava contacts | skylights | pillars | the 1998 flow
NeMO at Axial | the 1998 eruption | the rumbleometer story | lava flow animation

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