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Usually those who present this as a problem for a short chronology have based their inferred rates of cooling (and thus, the perceived problem) on the rates of conduction of heat by solid rock. Rock is, in general, a very good insulator, and thus greatly retards the outward conduction of heat. When the figures for time of cooling calculated using the assumption of only conductive cooling are applied to any significant batholith (large buried intrusion of molten rock or magma), the calculations indicate hundreds of thousands or millions of years would have been required to bring a large molten mass of rock to ambient temperature. There are two aspects to this problem that we will consider. The first concerns the question of whether a batolith cools only by conduction. The second concerns the issue of whether many large masses of granitic rocks were ever molten. We will consider these points in order.

Batholith cooling

Underlying the calculations using only conductive heat loss is the assumption that the batholith is a closed system; that the only way heat can be removed from it is by conduction--molecule to molecule transfer of heat. The alternative method of heat loss involves an open system in which the transfer of heat to circulating fluids, generally water, could quickly deplete the batholith of heat, not only from the surface, but from deep within, through circulation in cracks and crevices. Probably the chief reason for using the simplifying assumption of conductive heat loss only is that it greatly simplifies (or maybe, makes possible) the calculation. Trying to factor in the contribution of convection when neither the surface area exposed to the fluids nor the volumes and flow rates of fluids involved is known, involves assumptions of high order.

However, it is very clear that cooling rates can be perturbed by several orders of magnitude if the system were open. For example, having water circulating in the environs of an intrusion (convection) can result in orders of magnitude faster cooling than with static conduction-only cooling. The calculations are difficult, but unless the efforts are made, we will never know the answers.

The assumption of closed systems (i.e. conduction only) is probably never entirely warranted. There is plenty of field evidence to support the idea of openness (convection). Faulting, jointing and fracturing in granitic plutons are common. Even apparently solid granites have no shortage of channels for water movement. One of the reasons the Hot Dry Rock energy generating project failed (in Cornwall, UK) is that the injected water leaked away too quickly, and the research team could not confine the steam. There were just too many routes for water to escape.

The "convective cooling" scenario is not intended as a panacea to permit short timescales, but where it is applicable, it can dramatically shorten timescales. It may not always be applicable: if geology is to be a science, theory must be sensitive to the data. This is, ultimately, the reason for challenging the "conduction-cooling" model. Even with convective cooling the heat problem remains a challenge to explain within the framework of naturalistic science.

Is convection a valid model?

Consider the modern Oceanic Ridge systems, the one instance where we do have good data about water circulation in and around plutons. These systems are buried several kilometres down, but with enormous water convection cells cooling them. The amount of water moving through the oceanic crust is truly staggering. Conductive heat loss is negligible compared with convective loss.

Edmond et al (1982) suggest that "a volume of water equivalent to the whole ocean must circulate through the high temperature intrusion zone in the ridge axis ... every 8-10 Myr".

Macdonald et al (1980) estimate that a single vent provides a hydrothermal heat loss of (6 2) x 107 cal/s. This is compared with the conductive heat loss over a 60 square kilometre area of 0.23 x 107 cal/s. The heat flows are so high that the authors estimate the vents are active for less than 10 years. This study "emphasises the importance of hydrothermal activity in the global heat budget".

Cann and Stiens (1982) argue that the heat source for black smokers must be magma chambers - nothing else will satisfy the modelling constraints. Large sulphide deposits require the crystallisation of large volumes of magma. If larger deposits have to be formed, more black smokers can be invoked and there can also be replenishment of the magma chamber. Thus, three black smokers could produce a 1 million ton ore deposit in 320 years, solidifying 7 cubic kilometres of magma in this time. One black smoker is said to have a mass flow rate of 160 kg/s.

Even though these timescales are geologically short, I am of the opinion that they are unrealistically long. Compared with some of the Cyprus sulphide deposits, the present day deposition of sulphides are quite minuscule. There are no adequate modern analogues for producing 1 million ton deposits. These deposits are begging for catastrophist mechanisms and models.

Anderson et al (1979) say "By carefully measuring the nonlinearity of temperature profiles, we have calculated both the conductive and convective components of the total heat flow at the sea floor. Significant convective heat transfer is occurring through the crustal and sedimentary layers even at the oldest (55 x 10^6 years) experimental site".

For a popular overview, see Macdonald and Luyendyk (1981).

Thus, convective cooling is the predominant mechanism for heat loss on the present ocean floor, and this mechanism accounts for an estimated 30 times as much heat loss as conduction from the sources in question. If we apply even this correction to the heat loss problems of fossil batholiths, the time problem is greatly altered in the direction of a short time scale.

Granitization

A number of granitic batholiths have indications of granitization (modification of a host rock to granite by solid state diffusion) apparent to the careful observer. For example the granite upon which the University of California Riverside campus sits is well exposed behind the campus and displays well defined sedimentary stratification. A few tens of miles further inland near Palm Springs is a remarkable sandstone which grades laterally into a perfectly good granite. If granitization is the explanation for the origin of a significant fraction of plutons (igneous bodies of presumed deep origin), this may impact the time problem with respect to cooling of batholiths, since cooling is no longer an issue in solid state diffusion. The problem is not resolved at present, but the lines are well drawn. For example, a popular Physical Geology textbook (Leet & Judson), states that Plutonists believe 85% of the granites are magmatic in origin and 15% are due to granitization. The Granitists believe that 85% of the granites are metasomatic in origin, 15% are magmatic.

Although metasomatism (modification of a host rock by mineral-charged fluids), which was popular as an explanation of granites in the 1940's and 50's, has largely fallen into disrepute, some competent geologists still consider it the best explanation for such deposits. One of the greatest draws for this explanation and for granitization as well, aside from the field relationships was that it offered a solution to the "room problem", which remains one of the great enigmas in geology: Large magma bodies moving upwards through the crust of the earth have the problem of "What creates the space into which they move?" Certainly the prevalent paradigm today is magmatic, but this leaves the "room problem" as a festering sore. If granitization is the correct explanation for many plutonic bodies, this solves at once the time problem for short chronologists, and the room problem for everyone.

 References

Anderson, R.N., Hobart, M.A. and Langseth, M.G. 1979. Geothermal convection through oceanic crust and sediments in the Indian Ocean. Science, 204(25 May), 828-832.

Cann, J.R. and Stiens, M.R. 1982. Black smokers fuelled by freezing magma. Nature, 298(8 July), 147-149.

Edmond, J.M., Von Damm, K.L., McDuff, R.E. and Measures, C.I. 1982. Chemistry of hot springs on the East Pacific Rise and their effluent dispersal. Nature, 297(20 May), 187-.

Macdonald, K.C., Becker, K., Spiess, F.N. and Ballard, R.D. 1980. Hydrothermal heat flux of the "Black Smoker" vents on the East Pacific Rise. Earth and Planetary Science Letters, 48, 1-7.

Macdonald, K.C. and Luyendyk, B.P. 1981. The crest of the East Pacific Rise. Scientific American, 244(May), 86-99.

Parmentier, E.M. and Schedl, A. 1981. Thermal aureoles of igneous intrusions: some possible indications of hydrothermal convective cooling. Journal of Geology, 89, 1-22.

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Ó 2010 Arthur V. Chadwick, Ph.D.