How do scientists determine the relative age of a fossil?
Answers
The age of fossils is determined through relative dating, and objective dating.
Fossils themselves usually can’t be directly dated, because most of them have little or no of the organic material left, and dating by Carbon 14 is only useful up to around 35000–50000 years. Fossils older than that must be dated by the rock layer in which they are found.
Fortunately, scientists have been studying rock strata for over 300 years, and have worked out very precisely the order in which the rock layers have been laid down. Very roughly, rocks are ranked in eras from oldest to youngest:
PreCambrian, Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene. Throughout the world, rocks are always found in this order, unless there’s been some obvious disturbance. This allows scientists to place a fossil found within a particular rock layer within these particular age brackets.
The only problem with this, is that knowing which layer is older than the next doesn’t give us an objective date for the rock layer. We can only know, from the above, that a cretaceous fossil is younger than a Jurassic one.
What allows scientists to put an objective date is radiometric dating. Sedimentary rock, which the vast majority of fossils are encased in, can’t be dated directly, but igneous rock can be. By measuring the amount of radioactive decay in particular elements since the igneous rock solidified, one can work out a fairly precise date for the rock. If you find a igneous layer dated at 2.4 million years old on top of a sedimentary layer you can be sure the sedimentary rock was laid down before 2.4 million years ago. A second igneous layer below dated to 3.2 million years, tells you that the sediment was laid down between 3.2 and 2.4 million years ago. A fossil within that layer then must be older than 2.4 million years, and younger than 3.2 million years.
The most commonly used elements for radiometric dating of rocks are Argon, Uranium, Potassium, and Rubidium because they have fairly long half lifes. Carbon isn’t used for dating rocks because it’s half life is too short.
There are various methods by which the scientists determine the age of the fossil.
● one being the determination of era of fossil through the idea of layers of rocks in which it is found.
● Other is by the use of radioactivity. In this method there is continuous bombarding of atoms in minerals in earth surface, electrons get excited and get trapped in crystal structures. Then through various radiometric methods, scientists determine the age of the minerals which in turn is used to determine the age of the rocks and then of the fossils.
● There is another method called as Carbon-14 method. This method is used to tell the date of fossils by its carbon. The percentage amount of carbon present helps in determining the age of the fossil.