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6 Infobase Publishing
6 Infobase Publishing
Figure 3.2 Scientists created a geologic time scale by studying rock layers to divide the Earth's history into different time periods.
rock layers from different areas of the planet, they slowly built up a geologic time scale for the planet. This chart neatly divided Earth history into different time segments called eons, eras, periods, and epochs.
Having a relative time scale to work with was very helpful, but what geologists really needed was a way of telling absolute time. An absolute age date tells you how old something is in terms of years. With an absolute time scale, geologists could tell exactly how old Earth was. It would also allow them to calculate how fast different changes took place.
By the end of the 1800s, several scientists had used some rather ingenious ways to try and calculate the absolute age of Earth. For example, in 1897, British physicist William Thomson (who would later be known as Lord Kelvin) used the cooling of Earth to come up with an age of about 40 million years for the planet. He got this number by calculating how long it would take for Earth to reach its current temperature if it started in a completely molten state. In 1899, another British scientist named John Joly came up with an age of about 90 million years. He based this figure on the concentration of salt in the ocean. By measuring the average salt content of streams, Joly estimated how long it would take to build up the current amount of salt in seawater. Neither of these estimates was even close to the correct number. This would change, though, when an American scientist named Bertram Boltwood used radioactive decay to finally get an accurate age for Earth.
BERTRAM B0LTW00D INVENTS "ATOMIC TIME"
Once Ernest Rutherford discovered the basics of how radioactive decay worked, scientists suddenly had a new tool at their disposal. Bertram Borden Boltwood was born in Amherst, Massachusetts, in 1870. At the age of 19, he entered Yale University and graduated with honors in only 3 years with a degree in chemistry. Boltwood spent the next two years in Germany at the University of Munich where he studied rare earth minerals, including many that contained the element uranium. Returning to Yale, he received his doctorate degree in chemistry in 1897.
After graduation, Boltwood teamed up with a classmate to run his own lab, and it was at this time that he first learned about the discoveries of Becquerel and the Curies. It was after reading several papers on radioactivity written by Ernest Rutherford that Boltwood began his own research into this new form of energy. He quickly became recognized as one of the leading authorities on radioactivity in the United States. In the process, he also became friends with Ernest Rutherford, with whom he frequently corresponded. Rutherford thought that it should be possible to calculate how old a rock was by measuring the amount of radioactive decay taking place in it. Boltwood was intrigued by the idea and began working on the problem.
In studying the radioactive decay of the element uranium, Boltwood found that the stable end product was always a form of the element lead. He figured that if a rock contained uranium, then the older it was, the more lead should be present. By comparing the amount of lead to the amount of uranium in the sample, he should be able to calculate the age of the rock. All he would need to know is the rate at which the uranium turned to lead. This number—called the half-life—would be something he could measure in the lab.
In 1907, Boltwood began to work with several different rock samples in his lab. His calculations astounded him: One sample was dated at over 500 million years old, while a second one came in at over 2 billion years. While the work of Hutton and Lyell led many geologists to conclude that Earth had to be old, these numbers were older than anyone had ever imagined up to that time.
Boltwood continued to work on his measurements and refined the age dating process. His later tests showed that the results were accurate. Earth was not a few hundred million years old—it was billions of years old. Boltwood had given geologists the exact tool they were looking for—an accurate geologic clock that was built right into the rocks! Using Boltwood's techniques, geologists have determined that our Earth is about 4.6 billion years old. Given this much time, it's easy to see how the slow steady changes that were proposed by Hutton and Lyell could easily reshape the entire planet.
How Does Radioactive age dating work?
The idea behind radioactive age dating is really quite simple. When igneous rocks crystallize, some of their minerals contain radioactive elements. The radioactive elements that a new rock starts with are called parent elements. Soon after the rock hardens, the parent elements begin to change, or decay, into different daughter elements. The rate at which a parent element changes into a daughter element is called the decay rate. Decay rates are different for different elements. Heating, cooling, or squeezing a rock does not change the decay rate of a particular element.
Decay rates for different elements are measured in a period of time called a half-life. The half-life is defined as the amount of time it takes for half of a parent element to change into a daughter element. For example, saying that the half-life of an element is 100 years means that after 100 years, half of the total amount of the element will have changed into the daughter element and half will remain as the parent. Using this information, the age of a rock can be figured by working backward. By measuring the relative amounts (the ratio) of parent element to daughter element found in a rock sample, scientists can calculate how many half-lives have gone by. Multiplying the number of half-lives by the decay rate produces a date. For example, if you measure the same number of daughter atoms as parent atoms, then you know that one-half of the parent atoms have decayed to the daughter. This tells you that the sample is one half-life, or 100 years, old. If there are three times as many daughter atoms as parent atoms, then the rock is two half-lives, or 200 years, old (after one halflife, you have half of the original atoms; after two half-lives, you have one-quarter of the original parent atoms).
While radioactive age dating is a great tool, it does not work with all rock types, only those where the minerals have
crystallized or re-crystallized from scratch. That means that igneous or metamorphic rocks work well for determining age, but sedimentary rocks give unreliable dates.
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