Rock formation and deformation - Fundamentals of the Physical Environment - page 357

Figure 12.16 General representations of evaporite rocks in
marine and/or terrestrial environments.
as material of varying saturation and competence is squeezed, slumps or intrudes other
layers. In addition to these largely mechanical effects, dewatering through high pore
pressures and dehydration through surface evaporation precipitates dissolved minerals,
especially carbonates, silicates and iron compounds. These cement the grains and give
cohesive strength to the developing rock mass.
THE ROCK CYCLE (4) OCEAN ENVIRONMENT
It is reckoned that the entire global ocean is cycled through oceanic crust every few
million years. This process is driven by the same thermal convection responsible for the
mid-ocean ridges and is known as hydrothermal circulation . It alters the condition of
sea-floor rocks and is capable of generating new minerals. Oceans are also a major route
for recycling crustal lithosphere by reprocessing terrigenous sediments via subduction or
accretion, albeit on far longer time scales. Together these processes form a major part of
the rock cycle and influence the quality (geochemistry and turbidity), performance and
biochemical processes of ocean water.
HYDROTHERMAL CIRCULATION AND METASOMATISM
Ocean water comes into contact with the sea bed but also circulates to depths of several
kilometres in oceanic crust, penetrating the basalt and gabbro layers and maybe also
reaching upper-asthenosphere peridotite. It gains access to the lithosphere via faults and
fractures generated by mid-ocean ridge rifting, post-formational cooling contraction and
subsidence. This process occurs over a very wide area, perhaps over 30 per cent of the
ocean floor, driven by mantle convection. Typical heat fluxes exceed 50 mW m −2 in
'new' crust up to 50 Ma old and 200-250 mW m −2 at the ridges. This draws water in over
a wide area and pumps it in concentrated hydrothermal plumes through axial vents.
Although the process is known simply as hydrothermal circulation , it is clear that water-