Geoscience Reference
In-Depth Information
trying to manoeuvre a large bucket, hanging on 5 km of wire, over a scarp slope,
which you can see only on the ship's echo sounder, and then attempting to collect
rocks from the debris at the scarp base. Dredging is also not particularly represen-
tative: a sample might not be typical. A much more precise way of sampling the
seabed is to use a submersible, a minisubmarine capable of descending to great
depths; there the geologist can sample the exact outcrop and rock type wanted.
Submersibles (both manned and remotely operated) have enabled scientists to
make detailed studies of small areas of the seabed, particularly the axial zones
of the mid-ocean ridges. Such operations are, however, extremely expensive and
still sample only the surface. Sampling the rock outcropping at the seabed does
not tell us what rocks make up the lower oceanic crust (even if the fault scarps
are such that deeper rocks could be exposed at their bases). However, it does
enable us to make informed guesses; for instance, since seabed samples include
basalts, gabbros, serpentinites and recent sediments, one would not guess at a
deep crust made up of granite (it took scientists a while to realize that granite
samples dredged from the seabed were not representative of the ocean crust but
had been dropped by icebergs, these samples being ice-rafted granite). Another
waytoobtain samples is to drill. Drilling into the oceanic crust is an expensive
and difficult operation compared with drilling on land. Not only is the rock hard
and frequently fractured, but also there are many kilometres of sea water between
the drilling ship and the top of the drillhole (Fig. 9.3). Drilling into the oceanic
crust is an international cooperative venture overseen by the Joint Oceanographic
Institutions for Deep Earth Sampling (JOIDES). Drilling, which started in 1968
as the Deep Sea Drilling Project (DSDP), in 1985 entered a new phase as the
Ocean Drilling Program (ODP) and then in 2003 became the Integrated Ocean
Drilling Program (IODP), has tremendously advanced our knowledge of the geo-
logical and geophysical structure of the uppermost crust. While 1.5 km of the
lower crust has been drilled on Atlantis Bank (a 5-km-high bank adjacent to the
Atlantis II transform fault on the ultra-slowly spreading Southwest Indian Ridge),
drilling into the upper mantle remains a project for the future.
Most of the seabed is sedimented. Figure 9.4 shows the sediment thickness
over the western North Atlantic. Thicknesses gradually increase from zero at the
active ridge axis to 0.5-1 km in the ocean basins. The pelagic sedimentation rate
over mid-ocean ridges is very low, millimetres per thousand years (m per Ma). The
greatest accumulation of sediments occurs beneath the continental margins; over
10 km of sediment is not uncommon, but thicknesses rarely reach 15 km. These
sediment accumulations, a consequence of the rifting apart of the old continent,
are discussed in Section 10.3.For detail of shallow sediments on continental
margins see Figs. 4.43 and 4.44.
Researchers began widespread oceanic seismic-refraction experiments in the
1950s and by the 1960s had made enough determinations of crustal seismic veloc-
ities over the oceans for it to be apparent that, unlike the continents, the oceanic
crustal structure varied little. Table 9.2 gives the standard seismic structure of
