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These long time periods are computed by measuring the ratio of daughter to parent substance in a rock and inferring an age based on this ratio.
This age is computed under the assumption that the parent substance (say, uranium) gradually decays to the daughter substance (say, lead), so the higher the ratio of lead to uranium, the older the rock must be.
Lava (properly called magma before it erupts) fills large underground chambers called magma chambers.
Most people are not aware of the many processes that take place in lava before it erupts and as it solidifies, processes that can have a tremendous influence on daughter to parent ratios.
During the last stage of crystallization, after most of the magma has solidified, the remaining melt will form the minerals quartz, muscovite mica, and potassium feldspar.
Further, the silicon content of the melt becomes enriched toward the latter stages of crystallization.Such processes can cause the daughter product to be enriched relative to the parent, which would make the rock look older, or cause the parent to be enriched relative to the daughter, which would make the rock look younger.This calls the whole radiometric dating scheme into serious question.Bowen also demonstrated that if a mineral remained in the melt after it had crystallized, it would react with the remaining melt and produce the next mineral in the sequence shown in Figure 3.6.For this reason, this arrangement of minerals became known as Bowen's reaction series.