The best-known radiometric dating method involves the isotope carbon-14, with a half life of 5,730 years. Wait another 4.468 billion years and only about 250,000 atoms of uranium will remain (Fig. The rest of the uranium will have decayed to 500,000 atoms of other elements, ultimately to stable (i.e., nonradioactive) atoms of lead-206. If you have a collection of one million atoms of a radioactive isotope, half of them will decay over a span of time called the “half life.” Uranium-238, for example, has a half life of 4.468 billion years, so if you start with a million atoms and come back in 4.468 billion years, you’ll find only about 500,000 atoms of uranium-238 remaining. These isotopes are unstable, so they gradually break apart or “decay.” Radiometric dating works because radioactive elements decay in predictable fashion, like the regular ticking of a clock. Trace amounts of isotopes of radioactive elements, including carbon-14, uranium-238, and dozens of others, are all around us-in rocks, in water, and in the air (Table 1). This remarkable technique, which depends on measurements of the distinctive properties of radioactive materials, is called radioisotope geochronology, or simply “radiometric dating.” The physical process of radioactive decay has provided Earth scientists, anthropologists, and evolutionary biologists with their most important method for determining the absolute age of rocks and other materials (Dalrymple 1991 Dickin 2005). Exacting satellite measurements over the past two decades reveal an average spreading rate of 2.5 centimeters per year, so the approximate age of the Atlantic is easily estimated as: New crust forms along the Ridge, as Europe and Africa move away from the Americas. These continents were once joined into the supercontinent Pangaea the Atlantic Ocean formed when Pangaea split down the middle and formed a divergent boundary, now marked by the Mid-Atlantic Ridge (Fig. The near perfect fit of the East Coast of South America with the West Coast of Africa provided key evidence for plate tectonics, the idea that Earth’s crust is broken into about a dozen thin, brittle plates that shift positions in response to convection in Earth’s deep interior.
You can do a similar calculation to date the Atlantic Ocean, which is about 3,700 kilometers wide and grows wider every year. The other islands that string out to the northwest, each with now-dormant volcanoes, are progressively older (and a new island, dubbed Loihi, is already forming as volcanoes erupt on the ocean floor southeast of the big island). This is a rough estimate, to be sure, but it jives well with other methods that date the big island of Hawaii as about a million years old.