Geoscience Reference
In-Depth Information
one-quarter of the global heat flux (Table 7.3). This cooling affects the plate thick-
ness (Section 7.5) because the top few kilometres of plate are 'instantly' cooled.
In effect, the cooling reduces the mean age of the oceanic crust before subduc-
tion. As the oceanic plate moves away from the ridge and cools, the hydrothermal
systems slowly lose their vigour. Thus hydrothermal circulation is the cooling
radiator of the front of the Earth's engine, like the splendid radiator on a Rolls-
Royce, but it is much more than just a radiator.
Hot water is less dense than cold water. Any water that enters the porous, rubbly
flank of a volcano heats up and rises, to be replaced by cold water from rain or
from the sea. At mid-ocean ridges this process is intense, and water penetrates
very deeply into the new oceanic crust. The system is so vigorous that huge, well-
organized circulations become established, with cold water entering, warming as
it penetrates to depths of perhaps several kilometres and then rising. The rising
water may pass through well-defined channels to debouch back into the ocean
at temperatures that are nearly critical, often approaching 400
◦
Catthe ambient
pressures of the ocean floor. These hydrothermal fluids are generally very acid,
with a pH as low as 2. Enormous volumes of water pass through these systems,
so great that in total a volume equal to the entire volume of the oceans passes
through the ridge hydrothermal systems in about ten million years. The spacing
between hydrothermal vent fields along the mid-ocean ridges is dependent upon
the spreading rate (and hence the magmatic budget): it is predicted to be a linear
function of the spreading rate. Data from the East Pacific Rise fit this prediction
with frequent vent fields, but spacing on the slow-spreading Atlantic ridges is
much more variable: 30-130 km on the Mid-Atlantic Ridge (11-38
◦
N), but there
is only one vent field along 600 km of the Reykjanes Ridge. Hydrothermal vent
fields on the very-slow-spreading Southwest Indian Ridge are likely to be rare.
Off the ridge axis, hydrothermal circulation continues to take place as the
young lithosphere cools. Here the circulation is probably confined to the upper-
most permeable crust. Water temperatures and circulation rates are low for off-
axis systems, but this circulation continues until the volcanic surface is sealed
by sediments and the fractures become clogged. This average
sealing age
for the
oceanic lithosphere is 65
10 Ma. Passive off-axis hydrothermal circulation is
not dramatic but may overall account for over 80% of the heat loss by hydrother-
mal circulation. Overall, the volume of water circulating through 1-65-Ma-old
oceanic crust may be 10
2
-10
3
±
times that circulating through young (
<
1 Ma)
oceanic crust.
There are immediate chemical consequences of hydrothermal circulation. As
sea water passes through rock, it exchanges cations such as sodium and calcium,
dissolving some, moving or precipitating others. This changes the chemistry
and the mineralogy of the rock, altering and metamorphosing it, depending on
the depth and extent of hydration. Slightly away from the axis, the new basalt
is already a much rearranged metamorphic rock, typically with its fine-grained
minerals and with any glassy groundmass partly or substantially changed. Water
