Scientists explain: this is how ancient material is dated



How can we find out how old an excavated object, a dinosaur skeleton or our planet is? And is it possible to get an answer as to when modern humans appeared? Archaeologists, geologists, palaeontologists and geneticists have several different methods at their disposal for dating ancient material. We present a selection of them here.

Other methods for older fossils

The carbon-14 method does not work on fossils or rocks older than 50,000 years. In such cases, there are other radiometric methods to investigate the radioactive decay in other elements.

“If you want to know how old the earth is, for example, you can use something called the uranium-lead method. There are different types of uranium that decay into different types of lead. The two that are being examined have half-lives of about 4.5 billion years and 700 million years. With those methods, they were able to determine the age of the earth at 4.5 billion years in the 1950s,” says Lougheed.

The radioactive decay in these substances occurs more slowly than in carbon-14 and the precision will therefore not be as high either. If uranium-lead is used, the margin of error is a million years here or there compared to carbon-14, where the margin of error can be as low as 30 years.

In addition to analysing radioactive decay, geologists also use so-called magnetic dating when they want to find out how long ago a certain volcano erupted, for example. Then they study how magnetic minerals in the lava are oriented. The Earth’s magnetic field varies, namely, and the magnetic poles even switch places with each other from time to time, meaning the magnetic north pole becomes the south pole and vice versa. This results in magnetic minerals being fixed in what was a north-south direction when the lava solidified. How and when the Earth's magnetic field varied is fairly well researched.

Bryan Lougheed puts a sample into an magnetometer

Using a magnetometer, geologist Bryan Lougheed examines the direction of various magnetic minerals in a rock sample. As the Earth’s magnetic field has shifted over billions of years, the method works for determining age. Photo: Tobias Sterner/BILDBYRÅN

Just as archaeologists use stratigraphies, geologists use stratigraphic sequences for relative dating that can sometimes stretch back billions of years. Rocks formed from sediments, for example limestone or clay, accumulate in different layers that can be dated partly by magnetic dating but also by radiometric dating of embedded fossils. In this way, chronological sequences have been pieced together concerning very long periods of time.

Of all the life that has existed on earth, only an extremely small proportion has left traces. Therefore, fossils do not provide a comprehensive picture of when today’s species arose. However, those answers can be found in our genetic material.

Read the full article here: Scientists explain: this is how ancient material dated

News from the Department of Earth Sciences

Last modified: 2022-09-30