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Permeable convection above magma bodies—discussion
Tectonophysics,
353
95 (1983) 353-357
Elsevier Science Publishers
B.V.. Amsterdam
- Printed
in The Netherlands
Discussion PERMEABLE
JEAN
CONVECTION
ABOVE MAGMA BODIES-DISCUSSION
GOGUEL
Bureau de Recherches Gbologiques et Mini&es, 103 rue de Lille, Paris 75007 (France) (Received
May 17, 1982; accepted
November
29, 1982)
ABSTRACT
Goguel,
J., 1983. Permeable
Thermal limited
two phases
by the amount
phases convection
convection
permeable of water
above magma
convection available
bodies-discussion.
in the cooled
layer,
or by the rate of surface
at depth must take into account
Tectonophysics,
at the top of a former cooling,
the actual properties
96: 353-354. lava lake, is
not by permeability.
Two
of steam.
The paper by H.C. Hardee (1982) gives very interesting temperature profiles, measured on two lava lakes. But it seems clear that the heat flow in the almost isothermal, thick, pervious layer (30 m and over), in which two-phase convection occurs, is not limitated by the permeability, but perhaps by the amount of water available (340 cm a year, for q = 243 W/m2), from rain, or more likely also from the condensation of vapor in the top layer, cooled by air, wind, or radiation. Any increase in the mass-flow of vapor would only increase the pressure gradient for the up-flowing
steam,
and
at the same
time,
increase
the thermal
gradient,
assuming the steam to be everywhere saturated, i.e., in equilibrium with liquid water. The down-flow of liquid water, which occupies only a very small part of the pores volume, is no problem, as long as this water is not completely vaporised. Equation 15, in Hardee’s
paper, is not correct, because
it does not take into account
the large
difference between the specific volumes of steam and water. The temperature profiles in the lava lakes have been measured in open holes and show no thermal gradient; but it is not sure that they are representative of the actual temperature profiles in the rock itself, because expansion of the vapor to the atmospheric pressure in the open holes involves cooling, down to the boiling temperature. In the pervious rock, there may be a small thermal gradient, linked with the pressure gradient which moves the vapor upward, but this thermal gradient cannot be measured in an open hole. We can only assume that these gradients are small. These interesting measurements, at the surface of lava lakes, cannot be used directly to describe two-phase permeable convection over a deep-seated magmatic 0040-1951/83/$03.00
0 1983 Elsevier Science Publishers
B.V.
354
body. It is necessary
to take into account,
properties
and water
of steam
published
paper (Goguel,
equations
7- 10 in Hardee’s
or temperature,
This has been done
1982) using the same assumptions
the actual
in a recently
as those expressed
by
paper.
The result of these computations permeability,
for any pressure
in equilibrium.
is that the apparent
is very large in the two-phases
domain,
conductivity,
increasing
for a given
up to the critical
temperature. Thus, one may expect a much smaller thermal gradient, when where temperature and fluid pressure reach vaporisation conditions; above a matic body, this may happen at a depth less then 2700 m (at larger depths, pressure is likely to be larger than 225 bars), if temperature reaches 374°C. If conditions, allowing two-phase permeable convection, are reached somewhere, are likely to extend upward; transportation of heat by this process becomes so
and magfluid these they easy,
that a steady-state model is inadequate; heating of the rock must be taken into account. Anyhow, we may expect a temperature profile, with a much smaller gradient all over the space of equilibrium conditions between the two phases of water, than outside of this interval. The basic assumption of local, one-dimension convection, that is, upward flow of vapor and downward flow of water at the same place, may not remain valid. We cannot exclude apparition of convective cells, with predominance of upward flow of vapor in the hot cells, and downward flow of cold water in others cells. In such a case, the temperature profiles in these two sorts of cells shall soon become very different. This is the case in geothermal fields, such as Wairakei, New Zealand (Grindley, 1965). REFERENCES
Hardee,
H.C., 1982. Permeable
Goguel,
J., 1982. The behaviour
Grindley,
G.W.,
New Zealand.
PERMEABLE
convection of vapour
1965. The geology,
above magma dominated
structure
bodies. Tectonophysics,
reservoirs.
and exploitation
Geothermics, of the Wairakei
84: 179-195. 1I
(I): 3- 13.
geothermal
field, Taupo,
N.Z. Geol. Surv., Bull., 75: 109.
CONVECTION
ABOVE
MAGMA
BODIES-
REPLY
H.C. HARDEE Geothermal Research, Division 9743, Sandia National Laboratories, (Received
October
12, 1982; accepted
November
Albuquerque,
NM 87185 (U.S.A.)
29, 1982)
The lava lake calculations by Hardee (1982) require the presence of water. This water may well have reached the lava lake in the form of rainfall or condensation. In
353
95 (1983) 353-357
Elsevier Science Publishers
B.V.. Amsterdam
- Printed
in The Netherlands
Discussion PERMEABLE
JEAN
CONVECTION
ABOVE MAGMA BODIES-DISCUSSION
GOGUEL
Bureau de Recherches Gbologiques et Mini&es, 103 rue de Lille, Paris 75007 (France) (Received
May 17, 1982; accepted
November
29, 1982)
ABSTRACT
Goguel,
J., 1983. Permeable
Thermal limited
two phases
by the amount
phases convection
convection
permeable of water
above magma
convection available
bodies-discussion.
in the cooled
layer,
or by the rate of surface
at depth must take into account
Tectonophysics,
at the top of a former cooling,
the actual properties
96: 353-354. lava lake, is
not by permeability.
Two
of steam.
The paper by H.C. Hardee (1982) gives very interesting temperature profiles, measured on two lava lakes. But it seems clear that the heat flow in the almost isothermal, thick, pervious layer (30 m and over), in which two-phase convection occurs, is not limitated by the permeability, but perhaps by the amount of water available (340 cm a year, for q = 243 W/m2), from rain, or more likely also from the condensation of vapor in the top layer, cooled by air, wind, or radiation. Any increase in the mass-flow of vapor would only increase the pressure gradient for the up-flowing
steam,
and
at the same
time,
increase
the thermal
gradient,
assuming the steam to be everywhere saturated, i.e., in equilibrium with liquid water. The down-flow of liquid water, which occupies only a very small part of the pores volume, is no problem, as long as this water is not completely vaporised. Equation 15, in Hardee’s
paper, is not correct, because
it does not take into account
the large
difference between the specific volumes of steam and water. The temperature profiles in the lava lakes have been measured in open holes and show no thermal gradient; but it is not sure that they are representative of the actual temperature profiles in the rock itself, because expansion of the vapor to the atmospheric pressure in the open holes involves cooling, down to the boiling temperature. In the pervious rock, there may be a small thermal gradient, linked with the pressure gradient which moves the vapor upward, but this thermal gradient cannot be measured in an open hole. We can only assume that these gradients are small. These interesting measurements, at the surface of lava lakes, cannot be used directly to describe two-phase permeable convection over a deep-seated magmatic 0040-1951/83/$03.00
0 1983 Elsevier Science Publishers
B.V.
354
body. It is necessary
to take into account,
properties
and water
of steam
published
paper (Goguel,
equations
7- 10 in Hardee’s
or temperature,
This has been done
1982) using the same assumptions
the actual
in a recently
as those expressed
by
paper.
The result of these computations permeability,
for any pressure
in equilibrium.
is that the apparent
is very large in the two-phases
domain,
conductivity,
increasing
for a given
up to the critical
temperature. Thus, one may expect a much smaller thermal gradient, when where temperature and fluid pressure reach vaporisation conditions; above a matic body, this may happen at a depth less then 2700 m (at larger depths, pressure is likely to be larger than 225 bars), if temperature reaches 374°C. If conditions, allowing two-phase permeable convection, are reached somewhere, are likely to extend upward; transportation of heat by this process becomes so
and magfluid these they easy,
that a steady-state model is inadequate; heating of the rock must be taken into account. Anyhow, we may expect a temperature profile, with a much smaller gradient all over the space of equilibrium conditions between the two phases of water, than outside of this interval. The basic assumption of local, one-dimension convection, that is, upward flow of vapor and downward flow of water at the same place, may not remain valid. We cannot exclude apparition of convective cells, with predominance of upward flow of vapor in the hot cells, and downward flow of cold water in others cells. In such a case, the temperature profiles in these two sorts of cells shall soon become very different. This is the case in geothermal fields, such as Wairakei, New Zealand (Grindley, 1965). REFERENCES
Hardee,
H.C., 1982. Permeable
Goguel,
J., 1982. The behaviour
Grindley,
G.W.,
New Zealand.
PERMEABLE
convection of vapour
1965. The geology,
above magma dominated
structure
bodies. Tectonophysics,
reservoirs.
and exploitation
Geothermics, of the Wairakei
84: 179-195. 1I
(I): 3- 13.
geothermal
field, Taupo,
N.Z. Geol. Surv., Bull., 75: 109.
CONVECTION
ABOVE
MAGMA
BODIES-
REPLY
H.C. HARDEE Geothermal Research, Division 9743, Sandia National Laboratories, (Received
October
12, 1982; accepted
November
Albuquerque,
NM 87185 (U.S.A.)
29, 1982)
The lava lake calculations by Hardee (1982) require the presence of water. This water may well have reached the lava lake in the form of rainfall or condensation. In