Minerals Report - International Seabed Authority

Phosphorite and fresh basalt are strong, competent rocks and contrast
significantly with crusts, which are weak, light-weight, and porous (Table 4);
the other rock types, including altered basalt, may not contrast much in
physical properties with Fe-Mn crusts. In general, crusts are much more
porous (mean 60%) than most substrate rocks and have an extreme amount of
specific surface area, which averages about 300 m2 /g (Table 4), similar to the
surface area of silica gel. The surface area decreases by up to 20% when
measured one month after collection of the crust and up to 40% after two
months (45). This clearly shows that many physical properties that are
measured a long time after collection of the crusts may not closely
approximate in situ crust properties. The mean wet bulk density of crusts is
1.90 g/cm3 and the mean dry bulk density is 1.30 g/cm3 . The P-wave velocity
of crusts may be less or more than that of sedimentary substrate rocks, but is
generally less than that of basalt. This variable contrast will make it difficult
to develop sonic devices for measuring in situ crust thicknesses. The most
distinctive property of Fe-Mn crusts is their gamma radiation level, which
averages 475 net counts/min in contrast to sedimentary rock substrates (101)
and basalt substrates (146; Table 4). Gamma radiation may be a useful tool for
crust exploration under thin-sediment cover and for measuring crust
thicknesses in situ.
INTERNATIONAL SEABED AUTHORITY 204