Way to Tell Age of Earth Dates Back 100 Years
By Walter Witschey
Accurately dating the age of the Earth is commonplace today. Take a few rocks, analyze them on fancy machines, and presto, out pops the Earth’s age.
But these techniques are not new. A century ago, B.B. Boltwood outlined the basics.
Boltwood got his doctorate from Yale University in 1897. A decade later, he was back as professor of physics. In 1907, he published a list of geologic ages – time spans for ancient rock formations.
His dates have been revised since, but the U.S. Geological Service says Boltwood was right saying the age of the oldest rocks would be "hundreds to thousands of millions of years old," that is hundreds of millions to billions of years old.
Using elements that are radioactive (such as Carbon-14 or Potassium-40) and measurements of residual radioactivity or radio decay by-products, scientist can tell the age of rocks and fossils.
One must know that Potassium-40, to pick one example, radioactively decays, producing Argon-40 as a by-product. Out of every 1,000 atoms of Potassium-40, 500 will decay in 1 1/4 billion years.
By counting the ratio of atoms of Potassium-40 to Argon-40 in a rock sample, we can tell how old it is.
From Boltwood’s observations in 1907, it took 40 years until W.F. Libby not only discovered the decay properties of Carbon-14, but also realized their implication for dating human activity around the planet. For that, he won a Nobel Prize.
In the past 60 years, accurately dating prehistoric burials, wooden huts, and ancient boats has resulted in a detailed story of humans on the planet. The story is especially rich and detailed in the past 40,000 years.
During this exciting time, our human ancestors settled down, adopted religious practices, domesticated food plants and animals, and emerged as city dwellers with complex societies, cell phones, and iPods.
Today, many geology and archaeology projects rely heavily on radiometric dating, based on the century-old insights of Boltwood. They tell us the "when" of our human past.
Virginia’s science Standards of Learning, especially ES-10, cover using rocks and fossils to study history, and the evolution of life and Earth.
Walter R.T. Witschey is professor of anthropology and science education at Longwood University.