Minerals Report - International Seabed Authority

earthquakes produced by motions between plates and mostly lie beneath the
oceans (Figure 2). In particular, divergent boundaries where plates are
separating are manifested as a submerged volcanic mountain range that
extends more-or-less continuously through all the ocean basins of the world.
This submerged volcanic mountain range is the largest geographic feature on
Earth, but is hidden from view because the ocean mostly covers it. Molten
rocks or magma buoyantly up well from the Earth's interior beneath the
submerged volcanic mountain range, cools, congeals, and accretes to either
side of the submerged volcanic mountain range, and forms two diverging
conveyor belts of new lithosphere in the process of seafloor spreading at a rate
of inches centimetres (inches) per year. These globe-encircling divergent plate
boundaries transfer heat and materials from the Earth's interior to the
lithosphere.
The ocean basins are leaky as containers of the oceans because the
seafloor is penetrated by fractures. Cold, dense, heavy seawater flows
kilometres downward through the fractures and is assimilated into the rocks
of the Earth's interior over much of the ocean basin. However, where the
cold, heavy seawater encounters the magma up welling beneath the
submerged volcanic mountain range at divergent plate boundaries, the
seawater is heated, expands, and the lighter fluid buoyantly rises. As the
heated seawater rises, it dissolves various elements that are present in low
concentrations in the rocks through which it flows, particularly metals (1).
The metals concentrated in the up welling solutions link with sulphur from
seawater and from the rocks to precipitate as metallic sulphide deposits
beneath and on the seafloor. The remaining high-temperature (up to 400
degrees Celsius; 750 degrees Fahrenheit), metal-rich solutions discharge at the
seafloor and almost instantaneously precipitate the metals to form the cloud
of black metallic mineral particles known as "black smokers" that may
buoyantly continue to rise up to hundreds of meters into the overlying water
column.
The divergent plate boundary at the globe-encircling submerged
volcanic mountain range constitutes a global system of exchange of heat and
materials from the Earth's interior to the lithosphere and into the ocean in
amounts that impact the ocean environment and contribute greenhouse gases
(carbon dioxide and methane) to global change. Hydrothermal mineral
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