Measuring Deep Time: How Scientists Unraveled Earth's 4.5-Billion-Year History
Introduction There is a particular difficulty in thinking about geological time. A human life is measured in decades. Written history reaches back only a few thousand years. Even the oldest monuments built by civilizations occupy almost no space at all when placed against the history of the planet. Yet the rocks beneath our feet preserve evidence of events that began hundreds of millions, and in some cases billions, of years ago. Earth is about 4.54 billion years old . Today, that number appears in textbooks, geological maps, scientific papers, and laboratory reports with almost casual familiarity. But the number was not always known, and it was not obtained from a single experiment. Scientists had to discover a way of measuring time on a scale for which human experience offered no useful reference. There was no geological clock large enough to watch for billions of years. There was no written record from Earth's earliest history. Even the oldest rocks preserved at the ...
