Minerals Report - International Seabed Authority

are formed where one plate or a part of the earth's crust is
under-thrust or overridden by another plate. These zones are
usually marked by thrust faulting, folding, uplift, seismic and
volcanic activity. In some places small ocean basins and
spreading ridges lie adjacent to deep-ocean trenches and can
have an abyssal plain below a depth of 2,000 meters. Those
that border landmasses often contain thick accumulations of
sediment.
(6) Seamounts are formed on the oceanic crust in conjunction
with the spreading ridges, subduction zones, and mantle
plumes (hot spots). They form circular, conical or irregular
structures that may rise more than 1000 m above the seafloor.
Many Island States have developed on seamounts that have
subaerial summits, some with coraline overgrowths (i.e.
atols). These seamounts are developed by a combination of
geological processes involving mainly volcanism, but also
including tectonic uplift of the earth's crust.
2.2 Composition of oceanic and continental crust
In contrast with continental crust, oceanic crust is thin (5-10 km), of
relatively uniform composition and exposed at or near the surface over
much of the mid-ocean ridges. Sediments increase in thickness, on the
older parts of the ocean floor (reflecting greater accumulation time),
toward the ocean/continent boundary, continental rises and continental
margins. Throughout the majority of the ocean basins, away from
continental rises, sediments do not attain a thickness of more than a few
hundred meters (Figure 5).
The oceanic crust originates by partial melting of rocks in the
underlying mantle during seafloor spreading. Other rocks and minerals
produced by this process include polymetallic sulphides (PMS). In
addition, the ocean basins contain large concentrations of manganese
oxide nodules and crusts on the surface of the seafloor that are rich in
manganese, nickel, copper and cobalt (and other trace metals). The areas of
the sub-sea physiographic provinces are summarised in Figure 2.
Compared to the ocean floor, submerged continental margins and small
ocean basins contain the majority of potential sub-sea non-living
resources. This is both in terms of their variety and potential value. Most
important among these are the hydrocarbons, most of which probably
INTERNATIONAL SEABED AUTHORITY 677