Geoscience Reference
In-Depth Information
they collapse in a pile of debris. In this way, considerable deposits of potentially valuable
sulphide minerals can build up. Where the water is more acidic but slightly cooler, more
zinc sulphide is dissolved, creating a white smoker. These are slower growing and gener-
ally cooler and thus better habitats for some of the amazing life forms that cluster around
such hydrothermal vents. Life down here is based entirely on chemical energy, not sunlight.
Primitive bacteria flourish in the hot and often acidic conditions. Blind shrimps, crabs, and
giant clams feed off them, and giant tube worms containing symbiotic bacteria filter nutri-
ents from the water. It has been suggested that life on Earth first began in such places, so
they are causing much excitement among researchers.
Wealth from the sea
One of the surprise discoveries on the
Challenger
voyages in the 1870s was the return, in
dredge samples from the deep ocean floor, of strange black nodules. These nodules are es-
pecially rich in manganese and iron oxides and hydroxides, together with potentially valu-
able metals such as copper, nickel, and cobalt. Known as manganese nodules, they are now
known to pepper large areas of the deep ocean floor. Exactly how they form is uncertain,
but it seems to be in a slow chemical process with the metals derived from sea water and
possibly the underlying sediments. The nodules often grow in concentric onion-like layers
around a small, solid nucleus, perhaps a chip of basalt, a bead of clay, or a shark's tooth. Es-
timates of their age suggest that they are very slow-growing, perhaps adding only a couple
of millimetres in a million years. In the 1970s there were various proposals to mine them
with scoops or suction, but so far this has not happened due to technological, political, eco-
logical, and economic hurdles.
Pushes, pulls, and plumes
It does not seem as if sea floor spreading is the result of the ocean floor pushing apart
from the mid-ocean ridge system. For most of their length, the ridges do not have substan-
tial mantle plumes of hot material rising beneath them. It seems more as if they are being
pulled apart, with new material rising to fill the gap. Beneath the ridge there is no thick,
hard lithosphere, just a few kilometres of ocean crust. As mantle material rises under the
ridge, the pressure drops and so does the melting point of some of the minerals. This leads
to partial melting of as much as 20 or 25% of the material, producing basalt lava. The rate
of magma formation is just right to produce ocean crust of a fairly uniform thickness of 7
kilometres.
A notable exception is Iceland, where a mantle plume and a mid-ocean ridge coincide.
Here, far more basalt is erupted and the crust is around 25 kilometres thick, so that Iceland
rises above the Atlantic. The history of that mantle plume can be traced in thickened basalt
