As of Sunday, July 14, 2013
Science sometimes advances by giant leaps, but more often moves in small steps by gradually gathering new evidence.
The calculated age of the Earth is a good example, having shifted from a few thousand to 4.5 billion years old over the last two centuries.
Aristotle thought the Earth was eternal. The ancient Chinese believed it was created and destroyed every 23 billion years, while the Hindus accepted a similar cycle of 4.32 billion years. Different literal interpretations of the Old Testament and several Middle Eastern groups placed Earth’s creation anywhere from 4,000 to 8,000 years ago.
In the late 1700s, European scientists began to question the widely accepted young age of the Earth. They realized a few thousand years simply couldn’t account for the geologic features they were observing. Mountain-building, erosion and sedimentation were seen as slow steady processes requiring enormous amounts of time to occur, perhaps even millions of years.
Exposed rock strata, including volcanic flows and sedimentary deposits, occurred in recognizable layers, with the most recent on top. Many layers contained fossils that always became more primitive with each deeper layer. Similar patterns were being described on every continent around the world.
These findings led to the first “relative” geologic time scale, in which Earth’s history could be diagramed back into the remote past. Specific ages and dates couldn’t be determined given the technology of the time, but rocks, fossils and geologic events could now be reliably arranged from youngest to oldest.
Another century would pass before the discovery of radioactivity, and decades more before ancient rocks and minerals could be accurately dated. In the meantime, other direct measurements began to confirm the Earth was older than a few thousand years. The best known of these involve trees and snow, and are still being studied today.
Counting tree rings, or dendrochronology, has measured time back nearly 11,000 years in parts of Europe. Tree rings record not just each year’s weather, but bug infestations, fires and other effects in any particular region. The pattern of rings over a period of years becomes distinctive, like a barcode at the store, and can be matched and overlapped back through generations of trees and logs.
Similarly, ice sheets and glaciers build up layers every year that can be directly counted. Patterns of snow accumulation, deposited dust and occasional volcanic ash left a record of passing time that goes back 65,000 years in Greenland and Antarctica. The oldest ice was laid down over 100,000 years ago, although the deepest layers are so distorted by time and pressure that newer scientific methods must be used to date them.
But modern science and technology were needed to increase the Earth’s age to billions of years. Most of these methods involve some form of radiometric dating, which depends on the steady decay of naturally occurring radioactive elements called isotopes.
Small amounts of different radioactive isotopes are trapped in rocks and minerals at the time they form. Each isotope decays at its own predictable rate, or half-life, into a new and different daughter isotope. By measuring the amount of original isotope remaining in a rock plus the amount of daughter isotope, scientists can calculate both the original total amount of isotope and the time elapsed since the rock’s formation. Other effects of radioactive decay, such as fission tracking, can be measured separately,
Over 40 different methods have been used to determine the age of our planet and solar system. Rocks as old as 3.8 billion years have been found in Greenland, and zircon crystals over 4 billion years old in Australia. Dozens of meteorites along with the oldest moon rocks consistently date to between 4.4 and 4.6 billion years of age.
Science can’t put the Earth’s exact birthdate on our calendars yet, and probably never will. But the ongoing accumulation of evidence from different sources and methods continues to narrow down and confirm its ancient age. In science, progress and learning are more often the goal than simply reaching a final answer.
Lifelong Oregonian Fred Schubert, a The Dalles biologist, has a lifelong interest in general science and science writing. Feel free to submit any comments on this article or suggestions for new topics to firstname.lastname@example.org.