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Plutonic rocks on the other hand cool very slowly, on the order of a million years or more for some deeply buried and insulated magmas.The mineral grains in these rocks can grow very large and are readily distinguished in hand samples.Note that the halo has the highest intensity of discoloration near the source, gradually fading with distance in the host mineral to a "fuzzy" edge.
The zone of damage is roughly spherical around a central mineral inclusion or radioactive source.
When burial pressures and temperatures get too great, the rocks melt completely, becoming new igneous rocks.
To fully understand Gentry's hypothesis a basic background in geology, mineralogy, and radiation physics is helpful.
The crystals develop an interlocking texture with some of the trace minerals becoming completely surrounded by later forming crystals.
Volcanic rocks, because they are able to cool and crystalize rapidly, have a very fine-grained texture; the individual mineral grains are too small to see easily with the naked eye.