Gentry has spent most of his professional life studying the nature of very small discoloration features in mica and other minerals, and concluded that they are proof of a young Earth.
Geologists classify rocks into three main categories - sedimentary, igneous, and metamorphic - based on the way in which they form.
s the creation/evolution debate continues, there has been an increasing sophistication of certain Creationist arguments and publications.
It can be an especially difficult challenge when the Creationist author has professional credentials and has published in mainstream scientific journals.
True granites are relative latecomers on the geologic scene as they required a number of recycles of crustal material to differentiate and concentrate potassium.
In an earlier edition of , Lorence Collins (March/April, 1999) provided a thorough overview of the origin and nature of granitic rocks.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.Discoloration haloes in younger rocks tend to be smaller and less intense than in older rocks, indicating that the zone of crystal damage increases with time.From these observations early attempts were made to use the dimensions of haloes as an age dating technique.Figure 1 shows a typical discoloration halo around a radioactive mineral inclusion in the mineral pyroxene.The zone of damage is roughly spherical around a central mineral inclusion or radioactive source.When these minerals occur as inclusions in certain other minerals, most notably the mica family, they are often seen to develop discoloration, or "pleochroic" haloes.The haloes are caused by radiation damage to the host mineral's crystalline structure.One such individual is Robert Gentry, who holds a Master's degree in Physics (and an honorary doctorate from the fundamentalist Columbia Union College).For over thirteen years he held a research associate's position at the Oak Ridge National Laboratory where he was part of a team which investigated ways to immobilize nuclear waste.