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How old are fossils?

The oldest known fossils are the remains of unicellular organisms dating back, about 3.4 billion years. Although most fossils are a lot younger, when talking about the age of fossils, we often tend to speak about millions, tens of millions or even hundreds of million years ago. But how do we know this? And how can you find out, how old a particular fossil is?

A first insight is gained from the relative dating of rock layers. Over the centuries, geologists performed a lot of research on the different "strata" (stratigraphy). If layers are not disturbed by folding and/or mountain orogenesis, One can always assume that the younger layer are lying over the older layer (relative age). These layers can be read like the pages of a book. Each layer has its own characteristics and fossils, and by examining various places on Earth with different  layers, geologists can make a global classification.  At this point we know which pages of the geological book comes first, and which pages are coming after. In 1830, Charles Leyll the famous British geologist, already proposed some principles that are still valid today, only barring some exceptions. His main principles were stated as followed:

The layering of this coastal cliff is almost horizontal.

Additionally, one can also learn a lot from the fossils found in different layers. When studying paleontology it soon becomes clear, that there are certain patterns to discover when we take a closer look at fossils: In old layers for instance you will never find any fossils belonging to mammals. In young layers on the other hand, you won’t find any trilobites. The very foundation of this phenomenon, is the evolutionary development of life.

Not only  can one determine which deposits are older or younger than others, but by studying the fossils in these very same deposits in different parts of the world, we can connect layers from similar age to one another. An important role is fulfilled by so-called “guide” fossils. These fossils are the remains of organisms who: (1) had a worldwide abundance occurrence, and a large distribution, (2) are easily recognizable, and (3) only lived for a limited period in time. If you find such a fossil in a certain layer in Europe, and the same fossil in a layer in Africa, one can assume that these layers are approximately about the same age. These fossils are important "benchmarks" to correlate layers of the same age with each other.

 
A similar form of 'benchmarks' is provided by regional or global catastrophic events such as major volcanic eruptions or meteorite impacts. A well-known example is the meteorite impact, which forms the boundary between the Cretaceous and Paleogene. In this deposit a thin layer of iridium-rich material is deposited globally.

But in order to answer the question how old these layers are precisely, it’s necessary to give an absolute indication of time. In historical times, an indication of time, used to be estimated by observing the speed of recent geological processes, and extrapolating them. Obviously, this method had a very large error margin. A genuine revolution in dating geological time, came from nuclear physics. Radiometric dating allowed us to date rocks and minerals inside deposits, based on the decay of radioactive elements, or more precisely, counting the amount of radioactive isotopes.  For certain deposits, the results are very accurate.

In a natural way, atoms occur in various forms (with different amounts of atomic particles) we called them isotopes. Some isotopes are unstable and decay into other forms. In this process, radioactivity is released. The importance of this effect lies within the fact that this decay is always done at a constant speed, which is often expressed by the term 'half-life'. This is the time required to half the amount of isotopes present in a sample by counting the amount of decay products. The half-life time is different for each type of radioactive isotope.

For some radioactive (unstable) isotopes, it is known in what amounts they appeared in comparison to stable isotopes, when the rock was formed. By varying the ratio of these two types of isotopes in a rock that has to be examined, we may calculate accurately how long ago the rock was formed.

A highly simplified example: When the solidification of a lava flow occurs, it is known that there is no 'Lead-207' present in the lava, but the unstable isotope 'Uranium-235'. When this isotope starts to decay,  Lead-207 is formed . This decay of Uranium-235 occurs with a half-life of 700 million years. This means that half of the available Uranium-235 will be converted into Lead-207, after approximately 700 million years. When rock samples from the lava flow are examined in a laboratory, the  Lead-207/Uranium-235 ratio = 3/4. Based on the half-life, we can now calculate the approximate age: the lava is 3/4 x 700 = 525 million years old.

There are several isotopes which can be used for dating rocks. Summed up below, is a summary of the different radiometric methods. On the negative side, not all stones or geological formations, are suitable for radiometric dating sedimentary deposits are very difficult to date, because of the lack of any radioactive elements. Radiometric dating is therefore applied mainly to volcanic rocks. Several successive volcanic eruptions for example, can create layers in a geological profile, each layer containing valuable radiometric data. The dating of two independent volcanic layers provides an identification of the possible age of any of the intervening sediments between the 2 volcanic layers.

Isotope

Decay product 

Applicable on

Carbon-14

Nitrogen-14

Fossils/ Archeological artefacts between 1000 to 65.000 years old.

Uranium-235

Lead-207

Volcanic rock older than 110.000 years.

Potassium-40

Argon-40

Volcanic rock older than 100.000 years.

Uranium-238

Lead-206

Volcanic rock older than 10 million years.

Rubidium-87

Strontium-87

Volcanic & sedimentary rocks older than 10 million years.


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