Did Old Metamorphic Rocks Form in Just 10 Years?
A discovery in Norway may collapse a geological process by five or six orders of magnitude. A paper by Camacho et al. announced in Nature,1 yielded this comment by Simon Kelley (Open University, UK) in the same issue,2 “Conventional wisdom says that changes to crustal rocks pushed down deep when continents collide develop over millions of years. But it seems that some metamorphism may be caused by tectonic events lasting only a decade” (emphasis added in all quotes).
The gist of the story is that certain rocks called eclogites, long thought to have formed slowly over millions of years, might have formed rapidly instead, maybe in only ten. The authors of the paper deduced that they could not have remained at the temperatures assumed for very long without losing all their argon. Kelley explains why the mixtures in the rock suggest conflicting requirements for their formation:
The authors go on to estimate the temperature in the granulite lens during eclogite formation. Their conclusion – less than 400 °C – is a problem for the conventional interpretation of these rocks, given that a temperature of around 700 °C is required for the formation of the adjacent eclogites. Camacho et al. calculate that the total heating durations must have been around 18,000 years to explain the 40Ar-39Ar age profiles, but that individual fluid-flow events must have lasted just ten years to avoid significant heating of the granulite regions between the shear zones. This model evokes a radically different picture of the conditions during eclogite formation; but any alternative explanation would have to invoke a mechanism that explains why these phlogopites retained argon despite exceeding temperatures at which the gas would normally escape.
Kelley explains why the overturning of this classic case of a slow process points out an assumption that may need just as radical an overturn: “However, the very short timescales involved will make this idea controversial, as existing work on garnet seems to indicate processes operating on a million-year timescale; but also, perhaps, simply because we geologists are attuned to thinking in millions of years, whereas the features we observe may be just the aggregations of many shorter events.”
1Camacho et al., “Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids,” Nature 35, 1191-1196 (30 June 2005) | doi: 10.1038/nature03643.
2Simon Kelley, “Geophysics: Hot fluids and cold crusts,” Nature 435, 1171 (30 June 2005) | doi: 10.1038/4351171a.
Now there was a daring and honest admission: perhaps geologists are just in the habit of throwing around millions of years, when the features they observe could just as well be “aggregations of many shorter events.” Wow. Think about that. Here was a classic case of long ages from the Bergen Arcs in Norway that now must be reinterpreted. Neither Kelley or Camacho are claiming that this formation came into being recently, but it represents, nevertheless, a monumental shift in thinking about geological processes in general.
Dr. Terry Mortenson did his PhD thesis on the origin of old-earth thinking. He found that most scientists until the late 18th century believed the earth was young, and that the revisions upward to millions of years were due primarily to theological and philosophical attempts to discredit the early chapters of Genesis. Darwin, of course, later found all that extra time essential for his theory of evolution. Today, biologists and geologists don’t dare question the vast ages because Charlie needs the time: in fact, Darwin was aggravated to a pique when Lord Kelvin robbed him of the millions of years he required (see 02/02/2004 entry). Geologists found ways to steal those years back using radiometric dating methods, and have relaxed in complacency with their textbook geologic column, mumbling out those millions & billions nonchalantly, without much challenge (at least among the Darwin Party brethren). But what if (as many other dating methods suggest) things are really not that old? Follow the chain links on Dating Methods for examples. These articles in Nature, the most prestigious scientific journal in the world, should be a wake-up call for geologists not to take vast ages for granite (which, by the way, also shows evidence of rapid formation; see 12/07/2000 entry).