Problems with dating methods
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Phylogenetic trees are drawn up mathematically, using lists of morphological external form or molecular gene sequence characters.
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. Since then, geologists have made many tens of thousands of radiometric age determinations, and they have refined the earlier estimates.
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Age estimates can be cross-tested by using different isotope pairs. Results from different techniques, often measured in rival labs, continually confirm each other.
As potassium decays, it turns into argon. This is how geologists explain away the old isochron at the top of the Grand Canyon. A pioneering investigation into the crystallization of magma was carried out by N. Once the rocks melt, a plume of molten material begins to rise in the crust. It is very likely that some of this melt will be squeezed from the mixture into the cracks which develop in the surrounding rock. At the current rate of decline it could take just 1, years to disappear, with increasing effects upon the electronic systems of satellites and spacecraft.
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.
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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. 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.
Currently, he is studying certain basal dinosaurs from the Late Triassic and the quality of different segments of the fossil record. He holds the Chair in Vertebrate Paleontology at the University of Bristol, UK, in addition to chairing the Masters program in paleobiology at the university. Your one-stop source for information on evolution. Michael Benton wrote another article, Evidence of Evolutionary Transitions , for this website which explains how fossils support the stages of evolutionary history. Data bases and software for studying the quality of the fossil record.
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Michael Benton has written over 30 books on dinosaurs and paleobiology. Two suggested readings are provided — the first for adults, the second for children:. An online directory of dinosaur exhibits fro around the world.
Many natural history museums and universities worldwide offer public participation programs in dinosaur events, such as fossil hunting or fossil cataloguing. No experience needed in most cases! The list is too long to mention here, so a couple of examples are provided to get you going on your search for programs in your area:. These methods — some of which are still used today — provide only an approximate spot within a previously established sequence: Think of it as ordering rather than dating. One of the first and most basic scientific dating methods is also one of the easiest to understand.
Everything Worth Knowing About ... Scientific Dating Methods
Paleontologists still commonly use biostratigraphy to date fossils, often in combination with paleomagnetism and tephrochronology. A submethod within biostratigraphy is faunal association: Sometimes researchers can determine a rough age for a fossil based on established ages of other fauna from the same layer — especially microfauna, which evolve faster, creating shorter spans in the fossil record for each species.
The polarity is recorded by the orientation of magnetic crystals in specific kinds of rock, and researchers have established a timeline of normal and reversed periods of polarity. Paleomagnetism is often used as a rough check of results from another dating method. Within hours or days of a volcanic eruption, tephra — fragments of rock and other material hurled into the atmosphere by the event — is deposited in a single layer with a unique geochemical fingerprint.
Researchers can first apply an absolute dating method to the layer. They then use that absolute date to establish a relative age for fossils and artifacts in relation to that layer. Anything below the Taupo tephra is earlier than ; anything above it is later. Generally speaking, the more complex a poem or piece of pottery is, the more advanced it is and the later it falls in the chronology. Egyptologists, for example, created a relative chronology of pre-pharaonic Egypt based on increasing complexity in ceramics found at burial sites. Sometimes called carbon dating, this method works on organic material.
Both plants and animals exchange carbon with their environment until they die. Afterward, the amount of the radioactive isotope carbon in their remains decreases.
The Age of the Earth
Measuring carbon in bones or a piece of wood provides an accurate date, but only within a limited range. It would be like having a watch that told you day and night. Also called single crystal argon or argon-argon Ar-Ar dating, this method is a refinement of an older approach known as potassium-argon K-Ar dating, which is still sometimes used. Both methods date rock instead of organic material.
As potassium decays, it turns into argon. But unlike radiocarbon dating, the older the sample, the more accurate the dating — researchers typically use these methods on finds at least , years old. While K-Ar dating requires destroying large samples to measure potassium and argon levels separately, Ar-Ar dating can analyze both at once with a single, smaller sample.
The uranium-thorium method is often helpful for dating finds in the 40, to ,year-old range, too old for radiocarbon but too young for K-Ar or Ar-Ar. Silicate rocks, like quartz, are particularly good at trapping electrons.
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Researchers who work with prehistoric tools made from flint — a hardened form of quartz — often use thermoluminescence TL to tell them not the age of the rock, but of the tool. After shaping flint, toolmakers typically dropped the rocks into a fire.