Problems with radiometric dating techniques are greatly overstated by most young-Earth advocates. They often cite isolated instances of implausible dates, but these are generally caused by obsolete dating methods, contamination that a good scientist would detect (and which does not affect the large majority of dates), or by attempting to date materials that are younger than the dating method's margin of error (using radiometric methods to date recent Hawaiian lavas or living sea creatures, for example, is akin to using an unmarked yard-long stick to measure the thickness of a human hair). Most modern radiometric dating uses the isochron method, which measures several different samples (and sometimes different decay paths) and correlates them. The isochron method basically cross-checks itself constantly, and results that do not represent real ages will fail the isochron tests. Consequently, the isochron method does not require any assumption about initial amounts of parent and daughter elements (a common young-Earth objection to radiometric dating). An important consideration with the isochron method, however, is to know what it is that you're dating. Plotting several samples on an isochron will tell you how long ago the samples were separated from one another. For samples from the same lava flow, this will give you the age of the lava flow. But for samples from different flows, there can be a residual isochron giving the age of the melting event in the Earth's mantle from which the flows were derived. This is exactly what ICR geologist Steve Austin measured in the Grand Canyon. He claims that Rb/Sr isochron dating of lavas at the top of the Grand Canyon gives ages older than Grand Canyon basement rocks (ICR Impact #224,http://www.icr.org/pubs/imp/imp-224.htm). However, since Austin's samples came from several different flows, he could not have been dating the age of a single flow, rather he was dating the magma chamber beneath the Grand Canyon, from which all of the flows came. Although Austin claims that his "anomalous ages" cast doubt on radiometric dating in general, the fact is that geochemists often use the same method Austin used to date melting events much earlier than the formation of the flows themselves. For a more detailed discussion of ICR's "Grand Canyon Dating Project," see Stassen (1999),http://www.talkorigins.org/faqs/icr-science.html. Another common objection to radiometric dating is that addition or removal of parent or daughter elements may have occurred. However, in most cases this would leave tell-tale chemical clues that scientists could detect (and indeed this frequently happens), and furthermore such contamination could not possibly account for all of the world's radiometric measurements, which are in good agreement. Finally, radioactive decay rates are known to be constant under all relevant physical conditions. The fact is that, although radiometric dating is imperfect like any other science, there is tremendous overall agreement among radiometric ages, as well as with stratigraphic (relative) ages, giving very strong circumstantial evidence for the reliability of radiometric dating methods. For a general overview of radiometric dating, see Radiometric Dating: A Christian Perspective by Christian geologist Roger Wiens (http://asa.calvin.edu/ASA/resources/Wiens.html).