Strontium has four naturally occurring isotopes: ⁸⁸Sr; ⁸⁷Sr; ⁸⁶Sr; and ⁸⁴Sr. Of these, ⁸⁷Sr is perhaps the most important because it is formed by the natural radioactive decay of the rubidium isotope ⁸⁷Rb; this decay provides the basis for one of the most important geochronological methods. At its simplest, the initial strontium ratio (common strontium) of a rock is the ratio between the radioactively produced isotope ⁸⁷Sr and the ‘ordinary’, non-radiogenic isotope ⁸⁶Sr at the time when the rock crystallized. In a hypothetical rock containing no rubidium this ratio would remain unchanged for ever, but most rocks contain some rubidium; thus radioactive decay constantly increases the amount of ⁸⁷Sr, and the ratio of ⁸⁷Sr to ⁸⁶Sr constantly increases, at a rate proportional to the amount of rubidium in the rock. The initial strontium ratio of a rock is determined by measuring the present-day ⁸⁷Sr:⁸⁶Sr ratio in several of its constituent minerals. At the time when it first crystallized each mineral in the rock would have had the same ⁸⁷Sr:⁸⁶Sr ratio, but each mineral contains a different amount of rubidium, so after any given time its ⁸⁷Sr:⁸⁶Sr ratio will have increased away from the initial value by an amount exactly determined by the relative proportions of rubidium and strontium in it. Therefore, both the initial strontium ratio and the age of the rock can easily be found by plotting the measured present-day ratios of the constituent minerals on an isochron diagram. In petrology, initial strontium ratios are important because they provide information that otherwise would not be available on the chemical composition and ages of the source regions of igneous rocks. For example, an igneous rock that has a very young radiometric age but a very high initial strontium ratio must have been derived from a source rich in ⁸⁷Sr. Such a source must have been rich in rubidium and old enough for the ⁸⁷Sr to have accumulated by the radioactive decay of ⁸⁷Rb. Young granites in continental collision zones (e.g. the Alps) have extremely high (up to 0.8) initial strontium ratios and formed by the melting of old crustal gneisses. Granites formed by melting of the same rocks at different times can be distinguished immediately by their initial strontium ratios since the source rocks would have accumulated different amounts of ⁸⁷Sr when melting occurred. Granites formed in island arcs are derived from young mantle materials and have strikingly lower (0.704–0.706) initial strontium ratios. Similar arguments can be applied to the source regions of basalts in the mantle, though the differences between present and initial ⁸⁷Sr:⁸⁶Sr ratios are very much smaller because the mantle is poor in rubidium; typical ocean ridge basalts have initial strontium ratios close to 0.703. At the time of its formation, 4.6 billion years BP, the bulk Earth ratio is thought to have been 0.699. See also isochron.