Radiometric dating paper
The following quote from The Earth: An Introduction to Physical Geology by Tarbuck & Lutgens, pp.55-57, (1987), gives us an idea of the tremendous complexity of the processes that occur when magma solidifies.Lava erupting earlier would come from the top of the magma chamber, and lava erupting later would come from lower down.A number of processes could cause the parent substance to be depleted at the top of the magma chamber, or the daughter product to be enriched, both of which would cause the lava erupting earlier to appear very old according to radiometric dating, and lava erupting later to appear younger.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.This left branch is called a discontinuous reaction series because each mineral has a different crystalline structure.
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.
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.
Evidence of this type led them to look into the possibility that a single magma might produce rocks of varying mineral content. Bowen discovered that as magma cools in the laboratory, certain minerals crystallize first.