Posted by: paulgarner | April 18, 2009

Learning to live with uncertainty

Being a young-age creationist while you’re an undergraduate studying geology is a challenging experience. I remember it well. Many of the things I was being taught posed genuine questions for the biblical view of earth history, especially its short timescale, and often I didn’t have good answers.

During that time, I had to learn some important lessons: to hold on to God and his Word even in the face of perplexity, to be honest about the data and its challenges without being overwhelmed by it, and to seek new ways of understanding the world that were consistent with both the scientific data and biblical revelation.

Those lessons stood me in good stead. (In fact they’re helpful in the life of faith generally, not just in thinking about science). I needed to realize that it was okay to have unanswered questions. In fact, science is so much more fun because of all those unanswered questions! Of course, I’m also really interested to see how much light has been shed in the last couple of decades upon some of the things that bothered me as an undergraduate.

One of the problems I recall being particularly exercised about was the time needed for the formation of granite plutons. I was familiar with the conventional picture of how granites formed. From the generation of the magmas by partial melting of the lower crust, to their gradual ascent through the crust as balloon-shaped diapirs, their piecemeal assembly into batholiths with deep roots, their slow cooling dominated by conduction, and their eventual unroofing by erosion, they must have taken hundreds of thousands to millions of years to form. I just couldn’t see how it could be done within the time constraints imposed by biblical history.

Then came the granite revolution (Petford et al. 2000; Snelling 2008). Geologists began to call into question almost every aspect of this old view of granite formation. Granite magmas were no longer thought to ascend slowly through the crust as diapirs, but within days by dyke injection; the timescale for the filling of plutons was reduced to centuries or even months; batholiths were found to be mostly tabular structures, without deep roots, thus reducing the time needed for their cooling; a greater role for convection also served to lower estimated cooling times; phreatic stripping and other processes suggested that unroofing could occur rapidly. Even the generation of the magmas by partial melting was now thought to take only years to decades.

Furthermore, research on the formation of isolated polonium radiohalos within biotite flakes in granite plutons began to suggest even tighter constraints on their cooling history. The hydrothermal fluid transport model proposed by Snelling (2005) indicates that very short-lived polonium isotopes were separated from their parent uranium source and transported short distances by hydrothermal fluids where they became concentrated into new radiocentres. However, the requirement for an abundant supply of polonium and the fact that halos could only be preserved once the temperature of the host mineral had fallen below 150°C (their annealing temperature), implied a startlingly short timescale of formation of only hours to days. Incidentally, predictions based on this hydrothermal fluid transport model have recently been confirmed by data collected from a nested suite of granite plutons in Yosemite National Park (Snelling and Gates 2009).

The point is that the problem of granite formation, which so troubled me as an undergraduate, no longer seems quite so insurmountable. Of course, that hasn’t happened in every case. Other problems haven’t been resolved yet. Some even seem greater today than they did back then. But the revolution in thinking about granites gives me encouragement to think that answers to these other problems might be forthcoming one day too. I may or may not be around to see them, but that’s okay. In the meantime, I’ve learned to live with the uncertainty.

References

Petford N., Cruden A. R., McCaffrey K. J. W. and Vigneresse J.-L. 2000. Granite magma formation, transport and emplacement in the Earth’s crust. Nature 408:669-673.

Snelling A. A. 2005. Radiohalos in granites: evidence for accelerated nuclear decay. In: Vardiman L., Snelling A. A. and Chaffin E. F. (editors). Radioisotopes and the Age of the Earth: Results of a Young-Earth Creationist Research Initiative, Institute for Creation Research, El Cajon, California and Creation Research Society, Chino Valley, Arizona, pp.101-207.

Snelling A. A. 2008. Catastrophic granite formation: rapid melting of source rocks, and rapid magma intrusion and cooling. Answers Research Journal 1:11-25.

Snelling A. A. and Gates D. 2009. Implications of polonium radiohalos in nested plutons of the Tuolumne Intrusive Suite, Yosemite, California. Answers Research Journal 2:53-77.

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Responses

  1. There are still insurmountable problems in young-Earth attempts to explain the formation of batholiths and other igneous intrusions. Most of these fit into the category of “too many events, too little time.” Here are a few:

    –Even with advection (cooling by groundwater circulation), the amount of time that it would take to crystallize and cool a large batholith is in the tens of thousands of years.

    –Large batholiths, such as the Sierra Nevada, are composite features. There is clear field evidence that individual plutons crystallized before the next intrusion event. The Sierra Nevada Batholith is composed of hundreds of individual plutons.

    –A number of batholiths intrude into Cenozoic sedimentary rocks. This means that these would have had to been post-flood events (according to your view that Cenozoic sediments are post-flood), which compresses the time scale to the extreme.

    –It looks like the articles about cooling and crystallization rates referred to by Snelling mostly refer to studies done on small plutons. In Snelling’s model, one would have to extrapolate these to much larger plutons in order to be valid.

    –Many batholiths are composed of diorite and granodiorite, not granite. These crystallize at higher temperatures, and will therefore take longer to cool than the granite addressed by Snelling.

    –The Cenozoic sedimentary rocks around the Sierra Nevada batholith contain fragments of the batholith. This means that the Sierra Nevada batholith had completely cooled and been uncovered by the time these sediments were deposited.

    –Many batholiths have been faulted prior to deposition of overlying and adjoining Cenozoic sediments. This means that the batholiths would have had to have been completely crystallized by the end of the flood.

    A good overview of the many problems related to igneous and metamorphic rocks and the flood is found in Chapters 11 and 13 of “The Bible, Rocks and Time” by Young and Stearley. The authors are both geologists who hold to the inerrancy of Scriptures and an old Earth.

    • You’re right to say that the most sophisticated models of cooling times have been applied only to small plutons and need to be extended to batholith-size bodies. However, Snelling and Woodmorappe (1998) undertook a simpler analysis to show that timescales of about 3,000 years for the cooling of an 11 km wide, 16.5 km thick batholith buried 20 km below the surface were not unreasonable. With regards to the Sierra Nevada batholith, it’s noteworthy that it is, in part at least, composed of intrusions with sheet-like geometries, which probably diminishes cooling times considerably. Neither is it necessarily the case that plutons had crystallized completely before the next intrusive event occurred or even before they were being unroofed by erosion. Some evidence, such as the presence of satellite intrusions and pegmatites, suggests that the centres of such bodies may still have been hot.

      On the other hand, I think you’ve actually underplayed the stratigraphic constraints on cooling times that exist with some plutons. The Sierra Nevada batholith was being unroofed in the Cenozoic; in my reckoning that’s during post-flood time. But other plutons were emplaced, uplifted and eroded within the year of the flood itself. One of the best examples is the Shap Granite, a stock of the Lake District batholith of northwest England. According to radiometric dates, this granite was intruded in the early to mid Devonian, yet clasts of the granite are found in a nearby basal conglomerate of early Carboniferous age. However, Snelling (2008) has documented an abundance of polonium radiohalos in the Shap Granite, consistent with the hydrothermal fluid transport model for Po radiohalo formation and with catastrophically rapid granite formation. In fact, the evidence suggests that the granite formed within 6-10 days and its Po radiohalos within hours to days once the granite had cooled below 150oC. Hydraulic fracturing of the host rocks overlying the granite probably facilitated its subsequent rapid unroofing.

      By the way, I have a copy of Young and Stearley’s book right here on my “to read” pile. I understand that it updates and expands upon the arguments in Davis Young’s earlier books, which I read several years ago, so I shall look forward to seeing what the authors have to say.

      References

      Snelling A. A. And Woodmorappe J. 1998. The cooling of thick igneous bodies on a young earth, in: Walsh R. E. (editor), Proceedings of the Fourth International Conference on Creationism, Creation Science Fellowship, Pittsburgh, pp.527-545.

      Snelling A. A. 2008. Radiohalos in the Shap Granite, Lake District, England: evidence that removes objections to Flood geology, in: Snelling A. A. (editor), Proceedings of the Sixth International Conference on Creationism, Creation Science Fellowship, Pittsburgh and Institute for Creation Research, Dallas, pp.389-405.

  2. This is way above my head, being a non-geologist, so I’m mostly asking for some clarification of both the questions and the answers.

    First, what the heck is a pluton?

    Point: “There is clear field evidence that individual plutons crystallized before the next intrusion event.”

    Counter: “Neither is it necessarily the case that plutons had crystallized completely before the next intrusive event occurred or even before they were being unroofed by erosion. Some evidence, such as the presence of satellite intrusions and pegmatites, suggests that the centres of such bodies may still have been hot.”

    (If I understand those statements correctly:)
    While maybe not all the plutons (whatever they are) were cooled before the next event, if at least some of them had clearly cooled first, wouldn’t that still cause problems? Taking 3000 years to cool (at fastest estimate) before the next event comes along starts causing some troubles.

    Could they cool part way in just a couple hundred years and then have a new event happen? Does that work, or do all the coolings and events have to take place within a couple years total?

    How short of a time-frame do we have to work with for the things to form? If we’re talking the first year or two after the flood, that sounds tough to have hundreds of different layers laid down, lifted, eroded, etc. But, if we’re talking about all this taking place in 1000 years after the flood, that might be different.

    • Okay, first things first: a pluton is an igneous rock body that crystallised from a magma at depth. Batholiths are larger bodies, usually composed of many individual plutons.

      In the construction of a batholith, while it’s not necessary for each individual pluton to have cooled before the next is emplaced, that may have occurred in at least some instances. In such instances, the estimated time for each pluton to cool is critical because cross-cutting relationships show that many batholiths were emplaced during the single year of the global flood (assuming that the Palaeozoic and Mesozoic sediments are flood-deposited). Obviously timescales of even years or decades would be problematic.

      But here’s where Andrew Snelling’s work on radiohalos comes in, because his data suggests that these granite plutons were actually cooled within only days to weeks. See for example
      this recent paper
      (to which I referred in my original post) on the implications of polonium radiohalos in nested plutons of the Tuolumne Intrusive Suite, part of the enormous Sierra Nevada Batholith of central California.

      Controversial? Yes. But those who don’t like it need to explain the radiohalo data some other way – and that hasn’t been done yet.


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