Minerals Report - International Seabed Authority

times greater than atmospheric pressure, the reacting seawater reaches
temperatures up to 350-400°C without boiling. At these temperatures the
altered fluids do become extremely buoyant, however, with densities only ~2/3
that of the down welling seawater, so that they rise rapidly back to the
ocean/seafloor interface as hydrothermal fluids. The movement of the fluid
through the rock is such that, whilst the original downward flow proceeded
by gradual percolation over a wide area, the consequent upflow is often much
more rapid and tends to be focussed into natural channels emerging at "vents"
on the seafloor.
Beneath the seafloor, the reactions between seawater and fresh basalt
remove the dissolved Mg2+ and SO4 2- ions that are typically abundant in
seawater and lead to precipitation of a number of sulphate and clay minerals.
As the water seeps lower into the crust and the temperature rises, metals,
silica and sulphide are all leached from the rock to replace the original Mg2+ and SO4 2- ions. The hot and by now metal-rich and sulphide-bearing fluids
then ascend rapidly through the ocean crust to the seafloor. When they begin
to mix with the ambient cold, alkaline, well-oxygenated deep ocean waters;
the result is instantaneous precipitation of a cloud of tiny metal-rich sulphide
and oxide mineral grains. These rise within the ascending columns of hot
water giving the impression of smoke. Precipitation around the mouths of the
vents over time builds chimneys through which the smoke pours giving rise
to the term "black smokers"; hot water gushes out of these tall chimney-like
sulphide spires at temperatures of ~350°C and at velocities of 1-5 m/sec. Upon
eruption, this hydrothermal fluid continues to rise several hundred metres
above the seabed, mixing with ordinary seawater all the time, in a buoyant
turbulent plume (Figure 1).
INTERNATIONAL SEABED AUTHORITY 378