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Such a large variety of igneous rocks exists that it is logical to assume an equally large variety of magmas must also exist.

However, geologists have found that various eruptive stages of the same volcano often extrude lavas exhibiting somewhat different mineral compositions, particularly if an extensive period of time separated the eruptions.

Most scientists today believe that life has existed on the earth for billions of years.

This belief in long ages for the earth and the existence of life is derived largely from radiometric dating.

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.

Oftentimes the rate of cooling occurs rapidly enough to prohibit the complete transformation of calcium-rich feldspar into sodium-rich feldspar.

In these instances, the feldspar crystals will have calcium-rich interiors surrounded by zones that are progressively richer in sodium.

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This calls the whole radiometric dating scheme into serious question.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.Although these minerals crystallize in the order shown, this sequence is not a true reaction series.Ordinarily, these reactions are not complete so that various amounts of each of these minerals may exist at any given time.The right branch of the reaction series is a continuum in which the earliest formed calcium-rich feldspar crystals react with the sodium ions contained in the melt to become progressively more sodium rich.