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An important consideration in the exploration and exploitation of potential crust
resources is the contrast in physical properties between crusts and substrate rocks. Those
comparisons are complicated by the fact that crusts grow on a wide variety of substrates
including breccia, basalt, phosphorite, limestone, hyaloclastite, and mudstone in that
order of abundance. 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 m 2/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/cm 3 and the mean dry
bulk density is 1.30 g/cm 3. 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.
Table 4. Physical properties of Fe-Mn crusts and substrate rocks, from Hein et al. 37
Physical Properties n
Fe-Mn
Crusts
n
Sed. rock
substrate
International Seabed Authority 51
n
Basalt
substrate
Porosity (volume %)
Range 13 52-66 8 18-47 2 15-37 38
Mean 13 61 8 39 2 26 38
- 41-74 - 7-69 - 7-67 39
- 55 - 35 - - 40
Wet bulk density (g/cm 3 )
Range 13 1.83-2.04 8 2.04-2.57 2 2.22-2.62 38
Mean 13 1.92 8 2.18 2 2.42 38
Range 18 1.90-2.44 7 1.59-2.68 23 2.06-2.66 41
Mean 18 2.00 7 1.90 23 2.34 41
Dry bulk density (g/cm 3 )
Range 13 1.18-1.48 8 1.56-2.38 2 1.84-2.46 38
Mean 13 1.29 8 1.78 2 2.15 38
- 1.31 - - - - 40
Ref.