by Tas Walker

One objection to the earth being only 6,000 years old as described in the Bible is the claim that the coarse-grained texture of granites shows they cooled slowly over millions of years. Sceptics allege that if the magma had cooled quickly, the texture would have been a fine-grained lava-like rock or even a volcanic glass. However, this objection is based on the false idea that grain size depends solely on cooling rate. Granitic dykes and pegmatites are well-known examples of granitic rocks that formed when cooling was more rapid than the cooling of a pluton. Also, large fine-grained rhyolite bodies show that coarse-grained texture does not necessarily form when cooling is slow within them. Crystal size depends on many factors, including nucleation rate, viscosity, original composition of the melt, pressure variation and the amount of volatiles. In fact, fluid inclusions in crystals within granites and the lack of evidence of crystal accumulation in the granite plutons indicate that crystallization occurred quickly. Another evidence of rapid crystallization is the presence of radiohalos of short-lived isotopes of polonium in biotite and muscovite crystals in granite. Rather than being a problem for rapid cooling, the crystals point to rapid crystallization. The Tatoosh pluton in Washington State is an example of a shallow granite pluton that crystallized quickly after rapid reduction in vapour pressure due to a sudden loss of volatiles. This is exactly the opposite of the conventional ideas on which the sceptic’s claims are made.

The granite cooling objection

One of the many objections to the earth being only 6,000 years old as described in the Bible is the idea that large bodies of magma (molten rock) need millions of years to accumulate and cool.1–4 Today huge volumes of once-molten rock, called plutons, are exposed at the surface of the earth, having been unroofed by erosion of the overlying material (Figures 1 and 2). Most plutons intruded into the upper crust are of granitic composition. That is, they are comprised mainly of the light-coloured minerals, the feldspars and quartz—a composition often referred to as ‘felsic’. Although dark minerals such as biotite and hornblende are usually present, giving the rock a speckled appearance, they are never abundant.

It is important to remember that such objections to the young-age for the earth are not based on observations, but on assumptions about the past. The behaviour of magma deep in the earth in the past is deduced from evidence observed at the surface of the earth in the present—evidence such as field relationships, laboratory experiments, thermodynamic modelling, and isotopic measurements. All conclusions about magma behaviour depend directly on how the researcher interprets his observations, and his interpretations are constrained by what he already believes about the past.

Such beliefs (or assumptions) cannot be checked or proven because no-one has ever monitored the cooling of a pluton as conventionally described. First, the body of magma is inaccessible to direct observation because it accumulates deep (supposedly) inside the earth. Next, the process supposedly occurs over a time frame of tens of thousands to millions of years, far too slowly for changes to be measured. And then, the events occurred in the past, millions of years ago it is claimed, but no-one was present to observe them happening….

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