How do we date the Earth's oldest rocks using the chemistry of ancient volcanoes?

Nature · Published 3 October 2026 Swipe more like this →

In short: Rubidium-strontium dating is a powerful geological clock that tracks the radioactive decay of rubidium-87 into strontium-87. By measuring these isotopes in rock samples, scientists can reconstruct the age of igneous formations, provided the rock has remained a closed system since its initial crystallization from a molten state.

The rubidium-strontium method was the first to widely employ the isochron technique, a clever mathematical approach to dating. Rubidium is a trace element that frequently substitutes for potassium in common rock-forming minerals. As liquid rock cools, minerals precipitate at different times, creating varying rubidium-to-strontium ratios across the sample. Because these minerals initially share the same strontium isotopic signature, their subsequent evolution allows geologists to plot them on an isochron diagram—a straight line where the slope directly reveals the time elapsed since the rock solidified.

For this dating to be valid, the rock must meet three strict conditions: all samples must have started with the same initial strontium ratio, they must have remained closed to chemical exchange, and they must share the same age. If the data points do not form a perfect straight line, it suggests the system was disturbed by weathering, leaching, or reheating events. Volcanic rocks are particularly susceptible to these errors due to their fine-grained structure, while meteorites, preserved in the deep freeze of space, often provide the most reliable samples.

Beyond dating individual rocks, this method helps us understand the history of the Earth's crust. Because rubidium is concentrated in continental crust, the strontium-87 ratio in continental rocks is significantly higher than in the mantle-derived lavas found at oceanic ridges. By analyzing these ratios, scientists can distinguish between different magma sources and even estimate when continental crust first formed. This isotopic signature acts as a geochemical fingerprint, allowing researchers to trace the evolution of the planet's surface over billions of years.

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