The early geologists built a detailed relative history of the Earth, much of which holds true today, but they had no idea of how long that history was in years. ‘Modern breakthroughs and revolutions’ explains that the ability to measure Earth time transformed geology. With the discovery of radioactivity in 1896, radiometric dating of rocks became a possibility. This numerical dating, when combined with the relative dating provided by fossils, provided a means to calibrate the long and eventful history of our planet. The study of ocean floor geology and the birth of the revolutionary plate tectonics concept explained the patterns of mountain ranges, volcanism, and earthquakes.