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By measuring the parent isotope radioactive and the daughter isotope radiogenic in a system for example, a rock , we can tell how long the system has been closed in our example, when the rock formed. The process of radiogenic dating is usually done using some sort of mass spectrometer. A mass spectrometer is an instrument that separates atoms based on their mass. Because geochronologists want to measure isotopes with different masses, a mass spectrometer works really well for dating things.
I do think that radiometric dating is an accurate way to date the earth, although I am a geochronologist so I have my biases. Most estimates of the age of the earth come from dating meteorites that have fallen to Earth because we think that they formed in our solar nebula very close to the time that the earth formed. The fact that the age we calculate is reproducible for these different systems is significant. We have also obtained a very similar age by measuring Pb isotopes in materials from earth.
I should mention that the decay constants basically a value that indicates how fast a certain radioactive isotope will decay for some of these isotope systems were calculated by assuming that the age of the earth is 4. The decay constants for most of these systems have been confirmed in other ways, adding strength to our argument for the age of the earth. Radiometric dating depends on the chemistry and ratios of different elements.
It works like this:. Take, for example, zircon, which is a mineral; its chemical formula is ZiSiO 4 , so there is one zirconium Zi for one silicon Si for four oxygen O. One of the elements that can stand in chemically for zircon is uranium. Uranium eventually decays into lead, and lead does not normally occur in zircon, except as the radioactive decay product of uranium. Therefore, by measuring the ratio of lead to uranium in a crystal of zircon, you can tell how much uranium there originally was in the crystal, which, combined with knowing the radioactive half-life of uranium, tells you how old the crystal is.
Obviously, if the substance you are measuring is contaminated, then all you know is the age since contamination, or worse, you don't know anything, because the contamination might be in the opposite direction - suppose, for example, you're looking at radio carbon carbon 14, which is produced in the atmosphere by cosmic rays, and which decays into nitrogen. Since you are exposed to the atmosphere and contain carbon, if you get oils from your skin onto an archeological artifact, then attempting to date it using radio carbon will fail because you are measuring the age of the oils on your skin, not the age of the artifact.
This is why crystals are good for radiometric dating: The oldest crystals on Earth that were formed on Earth are zircon crystals, and are approximately 4. Asteroids in the solar system have been clocked at 4.
We assume that the Earth is probably as old as the asteroids, because we believe the solar system to have formed from a collapsing nebula, and that the Earth, being geologically active, has simply destroyed any older zircon crystals that would be its true age, but we can't really be certain.
The building blocks that the Earth is made of, the asteroids are 4. Based on astronomical models of how stars work, we also believe the Sun to be about 4. Radiometric dating is a widely accepted technique that measures the rate of decay of naturally occurring elements that have been incorporated into rocks and fossils. Every element is defined by the particular number of protons, neutrons, and electrons that make up it's atoms.
Sometimes, the number of neutrons within the atom is off. These atoms, with an odd number of neutrons, are called isotopes. Because they do not have the ideal number of neutrons, the isotopes are unstable and over time they will convert into more stable atoms.
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Scientists can measure the ratio of the parent isotopes compared to the converted isotopes. The rate of isotope decay is very consistent, and is not effected by environmental changes like heat, temperature, and pressure. This makes radiometric dating quite reliable.
However, there are some factors that must be accounted for. For example, sometimes it is possible for a small amount of new "parent" isotopes to be incorporated into the object, skewing the ratio. This is understood and can be corrected for. Carbon is the most commonly used isotope for dating organic material plants, animals. Plants and animals continually take in carbon during their lifespan. From the s onwards, geologists noted how fossils became more complex through time.
The oldest rocks contained no fossils, then came simple sea creatures, then more complex ones like fishes, then came life on land, then reptiles, then mammals, and finally humans. Since , paleontologists, or fossil experts, have searched the world for fossils. In the past years they have not found any fossils that Darwin would not have expected. Darwin and his contemporaries could never have imagined the improvements in resolution of stratigraphy that have come since , nor guessed what fossils were to be found in the southern continents, nor predicted the huge increase in the number of amateur and professional paleontologists worldwide.
All these labors have not led to a single unexpected finding such as a human fossil from the time of the dinosaurs, or a Jurassic dinosaur in the same rocks as Silurian trilobites. Paleontologists now apply sophisticated mathematical techniques to assess the relative quality of particular fossil successions, as well as the entire fossil record. These demonstrate that, of course, we do not know everything and clearly never will , but we know enough.
Today, innovative techniques provide further confirmation and understanding of the history of life. Biologists actually have at their disposal several independent ways of looking at the history of life - not only from the order of fossils in the rocks, but also through phylogenetic trees.
Phylogenetic trees are the family trees of particular groups of plants or animals, showing how all the species relate to each other. Phylogenetic trees are drawn up mathematically, using lists of morphological external form or molecular gene sequence characters.
More Bad News for Radiometric Dating
Modern phylogenetic trees have no input from stratigraphy, so they can be used in a broad way to make comparisons between tree shape and stratigraphy. The majority of test cases show good agreement, so the fossil record tells the same story as the molecules enclosed in living organisms. Dating in geology may be relative or absolute. Relative dating is done by observing fossils, as described above, and recording which fossil is younger, which is older. The discovery of means for absolute dating in the early s was a huge advance. The methods are all based on radioactive decay:. The first radiometric dates, generated about , showed that the Earth was hundreds of millions, or billions, of years old.
Accuracy of Fossils and Dating Methods
Since then, geologists have made many tens of thousands of radiometric age determinations, and they have refined the earlier estimates. Age estimates can be cross-tested by using different isotope pairs. Results from different techniques, often measured in rival labs, continually confirm each other. Every few years, new geologic time scales are published, providing the latest dates for major time lines.
Older dates may change by a few million years up and down, but younger dates are stable. For example, it has been known since the s that the famous Cretaceous-Tertiary boundary, the line marking the end of the dinosaurs, was 65 million years old. Repeated recalibrations and retests, using ever more sophisticated techniques and equipment, cannot shift that date.
It is accurate to within a few thousand years. The fossil record is fundamental to an understanding of evolution. Fossils document the order of appearance of groups and they tell us about some of the amazing plants and animals that died out long ago. Fossils can also show us how major crises, such as mass extinctions, happened, and how life recovered after them. If the fossils, or the dating of the fossils, could be shown to be inaccurate, all such information would have to be rejected as unsafe.
Radiometric Dating: Methods, Uses & the Significance of Half-Life
Geologists and paleontologists are highly self-critical, and they have worried for decades about these issues. Repeated, and tough, regimes of testing have confirmed the broad accuracy of the fossils and their dating, so we can read the history of life from the rocks with confidence. Educators have permission to reprint articles for classroom use; other users, please contact editor actionbioscience.