Minerals Report - International Seabed Authority

volcaniclastic rocks (9) because of the younger age of the limestones and
therefore shorter time for crust growth coupled with the instability and mass
wasting of the limestone. Fe-Mn crusts are usually thin down to as much as
3000 m water depth on the submarine flanks of islands and atolls because of
the large amounts of debris that are shed down the flanks by gravity
processes (10). Reworked crust fragments occur as clasts in breccia, which is
one of the most common rock types on seamount flanks (11). Regional mean
crust thicknesses mostly fall between 5 and 40 mm. Only rarely are very thick
crusts (>80 mm) found, most being from the central Pacific, for which initial
growth may approach within 10-30 Ma the age of the Cretaceous substrate
rock. Clearly, while most Pacific seamounts are 65-95 Ma old, most crusts
collected on those seamounts represent less than 25 Ma of growth because of
reworking and episodic sediment cover. Thick crusts are rarely found in the
Atlantic and Indian Oceans, with the thickest (up to 125 mm) being recovered
from the New England seamount chain (NW Atlantic), and a 72 mm-thick
crust being recovered from a seamount in the Central Indian Basin (12).
The distribution of crusts on individual seamounts and ridges is
poorly known. Seamounts generally have either a rugged summit with
moderately thick to no sediment cover (0-150 m) or a flat summit (guyot) with
thick to no sediment cover (0-500 m) (Appendix 2). The outer summit margin
and the flanks may be terraced with shallowly dipping terraces headed by
steep slopes meters to tens of meters high. Talus piles commonly accumulate
at the base of the steep slopes and at the foot of the seamounts; thin sediment
layers may blanket the terraces alternately covering and exhuming Fe-Mn
crusts. Other seamount flanks may be uniformly steep up to 20˚, but most
seamount flanks average about 14˚ (13). The thickest crusts occur on summit
outer-rim terraces and on broad saddles on the summits. Estimates of
sediment cover on various seamounts range from 15% to 75%, and likely
averages about 50%. Crusts are commonly covered by a thin blanket of
sediments in the summit region and on flank terraces. It is not known how
much sediment can accumulate before crusts stop growing. Crusts have been
recovered from under as much as 2 m of sediment without apparent
dissolution (14). Based on coring results, Yamazaki (15) estimated that there
are 2-5 times more Fe-Mn crust deposits on seamounts than estimates based
on exposed crust outcrops because of their coverage by a thin blanket of
INTERNATIONAL SEABED AUTHORITY 197