Below you will find graphs that illustrate the decay of parent isotopes and the growth of daughter isotopes for four geologically important isotopic systems. The half-life of U-235 decaying to Pb-207 is 713 million years. Note that this half-life can be obtained from the graph at the point where the decay and growth curves cross. Determine the half-lives for the other three isotopes and enter your estimate into the text fields below each graph. Note the differences in scale between the various graphs. Click the Next button when you are ready to go continue.