““When rocks are liquid or nearly liquid, what geologists call plastic, they contain a certain amount of rubidium and a certain amount of strontium. The mixture is measurable by diligent radiochemists. When that molten rock spews out of a volcano, say, it solidifies. By looking at the ratios of certain elements frozen in with rubidium and strontium, radiochemists and geochemists can determine how long ago the melt, as it’s called, turned solid. In the case of rubidium and strontium, we can count on precisely half of the rubidium-87 to transmute to strontium-87 (now with 49 protons) in 48.8 billion years.””
