Unlocking Time: How Radioactive Decay Changed Our Understanding of Earth's History

UCL Earth Sciences Digital Student Ambassador Ruby tracks how the discovery of radioactivity provided geologists with a powerful tool to unlock Earth's storied history.

On an overcast day in March 1896, French physicist Henri Becquerel made a serendipitous discovery that would transform our understanding of the natural world. He was investigating the relationship between phosphorescence and X-rays, recently discovered by Wilhelm Roentgen. Becquerel hypothesised that uranium salts, when exposed to sunlight, would emit penetrating radiation similar to X-rays. To test this, he wrapped a photographic plate in black paper and placed uranium salt crystals on top, leaving them in the sun. 

However, cloudy weather prevented him from conducting his experiment as planned. Instead, he stored the photographic plate and uranium salts in a dark drawer for several days. When he later developed the plate, he was astonished to find that it had been exposed, showing clear foggy imprints of the uranium crystals. This unexpected result revealed that uranium emitted its own invisible radiation, independent of any external light source. 

This accidental discovery laid the foundation for groundbreaking research by Marie and Pierre Curie, who discovered two new radioactive elements: polonium and radium. 

Radioactive Decay as a Clock 

The discovery of radioactivity provided geologists with a powerful tool to unlock Earth's history. The discovery soon revealed that radioactive elements decay at predictable rates, forming the basis for radiometric dating. Scientists realised they could use this natural process as a ‘clock’ to determine the ages of rocks and fossils. This method hinges on the concept of half-life, the time required for half of a radioactive isotope to decay into its daughter product. Each isotope has a unique half-life, ranging from a few to billions of years. 

By measuring the ratio of parent isotopes to their decay products in a sample, geologists can calculate how much time has passed since the rock formed. This realisation turned radioactivity into a precise geological clock that enables scientists to date ancient materials with remarkable accuracy. 

UCL's Research and Isotope Geology 

In my second year at UCL, I studied Isotope Geology as part of my MSci Environmental Geoscience course. This module gave me insight into how isotopic decay is applied to uncover Earth's history. We examined the physical and chemical processes that determine the isotopic composition of Earth materials, including nucleosynthesis, radioactivity and fractionation processes. We also learned about the mathematical foundations of radiometric dating and how scientists use this method to understand past natural events. 

A great example of isotope dating in action comes from recent UCL research. A study confirmed the discovery of a rare "Snowball Earth" event by dating Scottish and Irish rocks. This breakthrough, published in August 2024, provides critical evidence of one of the planet's most dramatic climate events, highlighting how radiometric dating can shed light on pivotal moments in Earth's past. 

The Power of Radiometric Dating in Geoscience 

By dating rock layers containing fossils, scientists have pieced together the timeline of evolution, including the rise and fall of different species. It has been instrumental in identifying and confirming the timing of mass extinctions, such as the event that led to the demise of the dinosaurs 66 million years ago. Volcanic ash layers, precisely dated using potassium-argon and uranium-lead methods, have allowed researchers to correlate extinction events with catastrophic occurrences like asteroid impacts and large-scale volcanic eruptions. 

Another major application of radiometric dating lies in the study of past climates. Ice cores and deep-sea sediments, analysed through isotopic dating, provide invaluable records of ancient climate changes. By examining fluctuations in isotopic compositions, scientists can reconstruct past atmospheric conditions, shedding light on natural climate cycles and helping predict future climate trends. 

With future Mars missions set to return samples to Earth, radiometric dating will continue to be an essential tool in unravelling the planet’s geological past and assessing its potential for ancient life. More than a century after Henri Becquerel’s discovery of radioactivity, its profound impact on science continues to grow, shaping our understanding of time, Earth and the universe itself.

Author: Ruby Tavola
Image Credit: iStock, George Clerk