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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.These haloes were considered to be the result of damage to the crystal structure of the host minerals caused by high energy alpha particles.In numerous papers published in scientific journals in the 1970s and 1980s, Gentry built the case that the different alpha decay energies of various naturally occurring radioactive isotopes resulted in distinctly different halo diameters.Granite is a well-known type of plutonic igneous rock, but there are many others as well.

Igneous rocks form from molten material, and are further subdivided into two main categories, the volcanic rocks which form from lava extruded at or near the surface; and plutonic rocks which form from magma, deep within the crust.

The boxes on the next few pages present a brief tutorial in rocks, minerals, and radioactivity.

Certain minerals, such as zircon and monazite, which form as common trace constituents in igneous rocks, have crystal structures which can accommodate varying amounts of the naturally occurring radioactive elements, uranium and thorium.

In metamorphic rocks, new minerals form that are more stable at higher temperatures and pressures.

Sometimes the minerals segregate into distinct bands.

Both types of igneous rocks comprise a mixture of different minerals.