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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.

This left branch is called a discontinuous reaction series because each mineral has a different crystalline structure.

Recall that olivine is composed of a single tetrahedra and that the other minerals in this sequence are composed of single chains, double chains, and sheet structures, respectively.

The mineral makeup of an igneous rock is ultimately determined by the chemical composition of the magma from which it crystallized.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.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.

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