PELAGITES (MANGANESE NODULES)
Some portions of the deepest seafloors of the world
have common to abundant, dark-colored, manganese-rich
nodules. These are called pelagites.
They were first discovered in the 1860s in the Kara Sea. They are
composed of various Mn, Fe, Cu, Ni, and Co minerals. Some materials
commonly identified in these nodules include todorokite ((Mn,Ca,Mg)Mn3O7áH2O),
birnessite (~Na4Mn14O27á9H2O),
amorphous hydrous Fe-hydroxyoxides, goethite, detrital volcanic
aluminosilicates, zeolites, etc. Pelagites are surprisingly lightweight
for their size, and do not have the look & feel of Ònormal rocksÓ.
Manganese nodules grow extremely slowly, estimated at
~5 mm per million years. Cross-sections show irregular concentric
layering. They appear to form intermittently, rather than by continuous
accretion of layers.
Manganese nodule formation is not well
understood. Unsolved problems include: 1) explaining how they stay at the
seafloor surface, even where sedimentation rates exceed nodule growth rates;
and 2) larger nodules appear to grow more quickly. More than one growth
mechanism may be involved in the genesis of manganese nodules around the world.
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Pelagite
(deep seafloor manganese nodule) (3.9 cm across) from an unrecorded locality in
the Pacific Ocean.
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Pelagite
(deep seafloor manganese nodule) - broken cross section surface of specimen shown
at top, 8-9 mm thick. Irregularly concentric banding is visible,
apparently nucleated around an orangish-brown rock fragment.
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Pelagite
(deep seafloor manganese nodule) (4.1 cm across). This specimen is
derived from the Clarion-Clipperton Fracture Zone in the subtropical eastern
Pacific Ocean. It was recovered from the seafloor at ~4500 to ~5500
meters water depth, at about 13¼ North latitude & 127¼ West
longitude. It was collected in the mid- or late-1970s by the Kennecot
Consortium.