Page 145 - Global Tectonics

OCEAN RIDGES 131

coming into contact with near-molten material are of magmatic activity at these points, producing higher
expected to be short-lived, but the relatively gentle cir- temperatures at shallow depths (Michael et al., 2003).
culation of cool water, driven by heat conducted from Further evidence that hydrothermal circulation
below, should persist for some time. However, as the occurs comes from the presence of metalliferous depos-
oceanic crust moves away from the ridge crest, and its at ridge crests. The metals are those known to be
subsides, it is blanketed by impermeable sediments, and hydrothermally mobile, and must have been leached
the pores and cracks within it become clogged with from the oceanic crust by the ingress of seawater which
minerals deposited from the circulating water. Ulti- permitted their extraction in a hot, acidic, sulfi de-rich
mately heat fl ux through it is by conduction alone and solution (Rona, 1984). On coming into contact with

hence normal heat flow measurements are obtained. cold seawater on or just below the sea floor the solu-

This “sealing age” of oceanic crust would appear to be tions precipitate base metal sulfide deposits. The pres-
approximately 60 Ma. ence of such deposits is corroborated by studies of

Detailed heat flow surveys on the Galapagos Rift ophiolites (Section 13.2.2).
revealed that the pattern of large-scale zoning and the
wide range of individual values are consistent with
hydrothermal circulation (Williams et al., 1974). Small-
scale variations are believed to arise from variations in 6.6 SEISMIC
the near-surface permeability, while larger-scale varia-
tions are due to major convection patterns which exist EVIDENCE FOR AN
in a permeable layer several kilometers thick which is
influenced by topography, local venting, and recharge

at basement outcrops. The penetration of this convec- AXIAL MAGMA
tion is not known, but it is possible that it is crust-wide.
It is thought that hydrothermal circulation of seawater CHAMBER
in the crust beneath ocean ridges transports about 25%
of the global heat loss, and is clearly a major factor in
the Earth’s thermal budget (Section 2.13). Models for the formation of oceanic lithosphere nor-
The prediction of hydrothermal circulation on mid- mally require a magma chamber beneath the ridge axis
ocean ridges, to explain the heat fl ow values observed, from which magma erupts and intrudes to form the lava

was dramatically confirmed by detailed investigations flows and dikes of layer 2. Solidification of magma

at and near the sea floor at ridge crests, most notably within the chamber is thought to lead to the formation

by submersibles. Numerous hydrothermal vent fi elds of most of oceanic layer 3 (Section 6.10). Evidence for

have been discovered on both the East Pacific Rise and the presence of such a magma chamber has been sought
the Mid-Atlantic Ridge, many of them revealed by the from detailed seismic surveys at ridge crests employing
associated exotic and previously unknown forms of life refraction, refl ection, and tomographic techniques.

that survive without oxygen or light. The physical and On the fast-spreading East Pacific Rise many of the

chemical properties of the venting fluids and the surveys have been carried out in the area north of
remarkable microbial and macrofaunal communities the Siquieros Fracture Zone between 8° and 13°N. The
associated with these vents, have been reviewed by area centered on the ridge crest at 9°30′N has been
Kelly et al. (2002). The temperature of the venting fl uids particularly intensively studied (e.g. Herron et al., 1980;
can, exceptionally, be as high as 400°C. The chemistry Detrick et al., 1987; Vera et al., 1990). More recently
of the hydrothermal springs on the East Pacifi c Rise and additional experiments have been carried out at 14°15′S,
Mid-Atlantic Ridge is remarkably similar, in spite of the on one of the fastest spreading sections of the ridge
great difference in spreading rates, and suggests that (Detrick et al., 1993a; Kent et al., 1994). All of these
they have equilibrated with a greenschist assemblage of studies have revealed a region of low seismic velocities
minerals (Campbell et al., 1988). Surprisingly perhaps, in the lower crust, 4–8 km wide, and evidence for the
because of the cooler environment at the ridge crest, top of a magma chamber at varying depths, but typi-
there are high levels of hydrothermal activity at certain cally 1–2 km below the sea floor. There is some indica-

locations on the very slow- and ultraslow-spreading tion that the depth to the magma chamber is
Gakkel Ridge. This appears to result from the focusing systematically less at 14°S compared to 9°N on the East