But even if it is true that older radiometric dates are found lower down in the geologic column, which is open to question, this can potentially be explained by processes occurring in magma chambers which cause the lava erupting earlier to appear older than the lava erupting later.
Lava erupting earlier would come from the top of the magma chamber, and lava erupting later would come from lower down.
For example, at the stage when about 50 percent of the magma has solidified, the melt will be greatly depleted in iron, magnesium, and calcium, because these elements are found in the earliest formed minerals.
But at the same time, it will be enriched in the elements contained in the later forming minerals, namely sodium and potassium.
The plot of the ratios will then produce a straight line below the Concordia curve.
Most scientists today believe that life has existed on the earth for billions of years.
This commonly occurs if the system has been heated or otherwise disturbed, causing a loss of some of the lead daughter atoms.
The following quote from The Earth: An Introduction to Physical Geology by Tarbuck & Lutgens, pp.
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.
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.
The mineral makeup of an igneous rock is ultimately determined by the chemical composition of the magma from which it crystallized.
Such a large variety of igneous rocks exists that it is logical to assume an equally large variety of magmas must also exist.
Radiometric dating is largely done on rock that has formed from solidified lava.