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General: Many rocks contain very small amounts of radioactive elements which decay at some rate to stable elements. Extremely small content of Isotope requires very sensitive and precise laboratory techniques. When molten rock escapes to the surface, it is assumed that radioactive elements are "fresh" and no decay products are present. If daughter elements were present at T0, the apparent age would be greater. Below the granite basement of the crust of the earth is the mantle. The curst is solid and temperatures increase with depth. The mantle temperatures are so high that the mantle would be molten except for the created pressure - Seismic evidence indicates that the mantle is solid. When a fissure occurs in the curst and the surrounding pressure drops, the local rock along with its minerals melts and issues forth with the molten magma. This would mean that the ratios of parent to daughter compounds would be a mixture of the magma, the molten portions of the mantle and any crustal rock melted on the way to expulsion. If the flow is thru an area rich in isotopes, the new igneous rock will be quite different from other flows with lesser or different mixtures. Thus, no reliable dating would be possible at all. Also, concordant ages in a given specimen or area would be no assurance of reliability of the age, but would be confirmation of ratios carried over from the host rock. Age is measured by determining the content of daughter element and using a know rate of decay to calculate the elapsed time from eruption. All these radio nucleotides have a very long half-lives and therefore the methods are only for very long ages. This produces a very large gap between radiocarbon dating (oldest dates 25,000 years) and these other methods like K-AR (youngest date 43,000,000 years). Daughter elements such as 40AR are volatile and mobile even in dense rock like granite. Argon may migrate from lower rocks to higher ones in the strata giving lower ones the appearance of advanced age Changes in content of elements is know to occur by natural events such as water leaching. e.g.  an iron meteorite lost 80% of its potassium by distilled water running over it in just 4½ hours. Variance between laboratories is know to be very high. Variance within a single rock occurs with different samples