Minerals Report - International Seabed Authority

The solid earth's surface consists of two physiographic provinces
(Figure 2): the ocean basins with a mean depth of 5,000 meters, and the
continents that rise to a mean height of about 5800 meters above the ocean
floor. The oceans extend over the margins of the continental masses for
distances ranging from a few tens of kilometres to more than 1,300 km.
The boundary between the continents and ocean basins is everywhere
submarine, generally ranging in depth from 2,000 to 4,000 m. The
physiographic contrast between the continents and the ocean basins
reflects fundamental geologic differences between them. Continental crust
(being richer in silica and the alkalis and poorer in iron and magnesia) is
less dense, and averages about 35 km in thickness, compared with the
approximately 5 km-thick oceanic crust. Many physiographic features of
the ocean basins are related to volcanism, crustal stretching and
subsidence, and seafloor spreading that brings basaltic igneous rock to the
surface along mid-ocean ridges and carries new crust away from the mid
oceanic ridge at the rate of up to 25 cm per year.
2.1 Physiographic features of the seafloor.
The deep seafloor is dominated by the following features:
(1) Continental margins that contain the continental shelf (down
to a depth of about 300 m), the continental slope (with typical
gradients of 1:20) and the continental rise (typically down to
4000 metres below sea-level). These features were formed
during rifting of larger continental blocks and the subsequent
development of oceanic crust by seafloor spreading (Figure
4). The continental margins have a history of subsidence and
are, therefore, mainly covered in thick sedimentary
sequences. These sequences may approach 20 km in thickness
and are typically composed of clastic material (sand and
mud), evaporites (salt deposits), carbonates, and pelagic
clays. In areas where volcanic activity accompanied initial
continental rifting, many continental margins also comprise
voluminous extrusive layers (flood basalt) that occur over
wide regions of many thousand square kilometres. In places,
the initial continental rifting may have involved several failed
episodes. During these, continental fragments may have been
partially separated from the main land-mass by the formation
of deep sedimentary basins underlain by stretched and
thinned continental crust and volcanic material. These
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