area’s permeability thereby facilitating the circulation of seawater and hydrothermal fluids around and/or between the solid rocks.
Missing Heat
Before the discovery of the ocean’s submarine geysers and other formations, now
called ‘‘hydrothermal sites’’, scientists suspected the existence of hydrothermalism
at oceanic spreading centers. Mid-Oceanic ridges where fresh lava is extruded are
also sites of heat production and heat loss as the lithospheric plates move away
from their point of origin at the ridge axes. We can expect to observe that the crust
cools and contracts as it slips away. When the crust was about 60–80 million years
old after its creation, it had already lost much of its original heat and thermal
convection in the lithosphere was much lower. Subsequent phenomena of metamorphism, sediment filling and alteration sealed much of the original rocks’
porosity and closed some of the fractures of the older oceanic crust during the
lithosphere’s aging process.
In the 1960s, when scientists calculated the amount of magma formed at ridge
axes and determined how long it would take for the crust to cool, the amount of
heat produced at any given time was found to be lower than what was expected.
Measurements of the ocean crust’s thermal regime was conducted on the sea floor
using special thermal probes that had the shape of a large needle and which were
pushed into the sediment by a coring tube. Probing the crust’s thermal temperature
was conducted at various distances perpendicular to the accreting ridge axes.
These temperature probes revealed that the ridges had a heat flow deficit, which
was not what was expected according to the theories based upon conductive
cooling models (Parker and Oldenburg 1973; Sclater and Francheteau 1970).
When the crust is heated from below, it will cool due to thermal conductivity
through the overlying sediment. During this process of cooling, no material is
transported. Therefore, it would be expected that the crust would be colder at a
distance from the ridge axis. In fact, the anomaly noted was that the amount of heat
lost due to its conduction in sediment was found to decrease towards a ridge axis
instead of increase, as would be expected. Thus the heat deficit observed between
the theoretical values of a predicted model on conductive cooling and the reality of
the temperatures measured on site led scientists to conclude that significant
quantities of heat must have been lost by a non-conductive mechanism beneath the
ridge crests, and this was interpreted as being due to the circulation of seawater as
a heat exchange agent.
It was therefore speculated that seawater circulation was a factor which directly
affected the cooling of crust formed on the Mid-Oceanic Ridges (Williams and
Von Herzen 1974). The theory postulated that, beneath the ridge axis, cold sea
water percolating down through the various porosities of the oceanic crust would
descend until it reached the vicinity of hot magmatic upwelling zones, at which
point, when the seawater had been reheated after having cooled its immediate
146
6 Hydrothermal Activity and Metalliferous Deposits
Missing Heat
Before the discovery of the ocean’s submarine geysers and other formations, now
called ‘‘hydrothermal sites’’, scientists suspected the existence of hydrothermalism
at oceanic spreading centers. Mid-Oceanic ridges where fresh lava is extruded are
also sites of heat production and heat loss as the lithospheric plates move away
from their point of origin at the ridge axes. We can expect to observe that the crust
cools and contracts as it slips away. When the crust was about 60–80 million years
old after its creation, it had already lost much of its original heat and thermal
convection in the lithosphere was much lower. Subsequent phenomena of metamorphism, sediment filling and alteration sealed much of the original rocks’
porosity and closed some of the fractures of the older oceanic crust during the
lithosphere’s aging process.
In the 1960s, when scientists calculated the amount of magma formed at ridge
axes and determined how long it would take for the crust to cool, the amount of
heat produced at any given time was found to be lower than what was expected.
Measurements of the ocean crust’s thermal regime was conducted on the sea floor
using special thermal probes that had the shape of a large needle and which were
pushed into the sediment by a coring tube. Probing the crust’s thermal temperature
was conducted at various distances perpendicular to the accreting ridge axes.
These temperature probes revealed that the ridges had a heat flow deficit, which
was not what was expected according to the theories based upon conductive
cooling models (Parker and Oldenburg 1973; Sclater and Francheteau 1970).
When the crust is heated from below, it will cool due to thermal conductivity
through the overlying sediment. During this process of cooling, no material is
transported. Therefore, it would be expected that the crust would be colder at a
distance from the ridge axis. In fact, the anomaly noted was that the amount of heat
lost due to its conduction in sediment was found to decrease towards a ridge axis
instead of increase, as would be expected. Thus the heat deficit observed between
the theoretical values of a predicted model on conductive cooling and the reality of
the temperatures measured on site led scientists to conclude that significant
quantities of heat must have been lost by a non-conductive mechanism beneath the
ridge crests, and this was interpreted as being due to the circulation of seawater as
a heat exchange agent.
It was therefore speculated that seawater circulation was a factor which directly
affected the cooling of crust formed on the Mid-Oceanic Ridges (Williams and
Von Herzen 1974). The theory postulated that, beneath the ridge axis, cold sea
water percolating down through the various porosities of the oceanic crust would
descend until it reached the vicinity of hot magmatic upwelling zones, at which
point, when the seawater had been reheated after having cooled its immediate
146
6 Hydrothermal Activity and Metalliferous Deposits