flow or violent jets. Subsequent expeditions somewhat farther north along the East
Pacific Rise found venting of dense black plumes loaded with minerals from tall
chimneys. Those were immediately termed black smokers. Quite similar biota
surrounded them.
(^) Nine years after the initial vent discovery, Rona et al. (1986) found black smokers
on the subtropical section of the Mid-Atlantic ridge, vents with substantially different
fauna. Vents have been explored now on “back-arc” subduction areas in the western
Pacific and on the central Indian Ocean spreading ridge. Hydrothermal vents are
located along much of the volcanically and tectonically active ridge systems of the
world’s oceans (Fig. 15.1). Different ridges spread at different rates, 0.1 to 17.0 cm yr
−1, depending upon the activity in the subtending magma chambers and their
proximity to the ridge surface. Spreading speed is determined from the ages of
magnetic reversals in the surrounding seafloor basalt (ages from comparisons to
potassium–argon-dated magnetic-field signatures of Earth’s polarity reversals in
stacked basalts on land). Fast spreading is associated with magma as shallow as 1 km
below the seafloor, slowest spreading with deeper and less-continuous sources of heat
and molten rock. Greater hydrothermal activity is generally associated with faster
spreading and closer proximity of water to actual magma. Vents along back-arc
trenches occur where continental crust is moving away from subducting oceanic plate,
magma filling between them. The plumbing of vents is not fully known, but upward
channels through cracked basalt overlie recent lava intrusions into the upper crust,
often at sites right in the axial valleys (“linear calderas”) at the centers of spreading
ridges. The upward flow is replenished with cooler seawater percolating through
subsurface fissures from the sides and eventually from the ocean above. Heating can
be sufficient to cause phase separation of brines and supercritical liquid–vapor
mixtures at subsurface levels. One vent system discovered in 2005, at 3000 m depth at
5°S on the Mid-Atlantic Ridge, emits steam at up to 464°C (Koschinsky et al. 2008).
In this supercritical phase, above 407°C, greater pressure does not condense water to
liquid. However, water emitted from most vents is at 300°C or less with no obvious
steam eruption (bubbles) above the seafloor. The hydrostatic pressure is usually great
enough to sustain the liquid phase as temperatures cool on approach to the surface of
the crust (Von Damm 1995).
Fig. 15.1 Locations of hydrothermal vents on volcanically active mid-ocean ridges
and subduction zones. Spreading centers are shown by parallel lines with offsets at
transverse faults. Subduction zones are shown by single lines with arrow heads
pointing in the direction of subduction on the side of the overriding plate. Sites
sampled for biology are located at the circles; sites at the Xs have been identified
from temperature only.
(^) (Map modified by Chris German after German & Von Damm, 2003, published with permission from Dr. German
and Elsevier.)