(^) The vents are not only of biological interest. Seawater dissolves copious amounts of
practically every element soluble in extremely hot water as it percolates through the
hot basaltic rocks of the spreading centers. For many of these elements, the vents have
been shown to be the main input term for their budget in seawater. Their
concentrations in seawater are what they are because of the input rates at the vents.
Since this sort of budgetary chemistry is complex; since we have only known about
the vents for 25-odd years; and since the vents can be visited only very occasionally,
the consequences of this observation are still being studied. Many of the metallic
species dissolved in vent discharge are not soluble at ordinary seafloor temperatures.
Thus, when the vent plume enters the cooler surrounding water, the metals precipitate
as salts, most often sulfides (e.g. iron pyrite) and sulfates, near the vent mouth. In
many cases, particularly for the vents richest in metallic solutes, “black smokers”, this
produces depositional chimneys around the discharge points. These chimneys are rich
in exploitable minerals, and interest in them as ores has been considerable, although
no deep-sea mining schemes have actually been started.
(^) Black smokers with circulation driven by hot magma are the dominant form of
hydrothermal vents along the ridge axes at spreading centers. However, in 2001,
Deborah Kelley and others (2005) discovered some tall, white chimneys in the
Atlantic, constituted mostly of limestone. These are not in the axis caldera, but well
up on the basalt walls to the side. What the discoverers called the Lost City field is in
fact on a huge undersea mountain, the Atlantic Massif (30°07′N, 42°07′W), which is
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