Key concepts Absolute Dating The problem: By the mid 19th century it was obvious that Earth was much older than years, but how old? This problem attracted the attention of capable scholars but ultimately depended on serendipitous discoveries. Initially, three lines of evidence were pursued: Hutton attempted to estimate age based on the application of observed rates of sedimentation to the known thickness of the sedimentary rock column, achieving an approximation of 36 million years. This invoked three assumptions: Constant rates of sedimentation over time Thickness of newly deposited sediments similar to that of resulting sedimentary rocks There are no gaps or missing intervals in the rock record. In fact, each of these is a source of concern. The big problem is with the last assumption.
Fossils – What is a Fossil? A Fossils are the remains and traces of ancient organisms. A cluster of fossil ammonites, an extinct cephalopod. The convention is that a fossil must predate recorded human history. While there is no defined date, typically something must be older than 10, years to be considered a fossil. The oldest fossils in the fossil record date from 3.
The oldest rocks on Earth, found in western Greenland, have been dated by four independent radiometric dating methods at billion years. Rocks billion years in age have been found in southern Africa, western Australia, and the Great Lakes region of North America.
So, how do we know how old a fossil is? There are two main methods determining a fossils age, relative dating and absolute dating. Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages. Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it. Relative Dating The majority of the time fossils are dated using relative dating techniques.
Using relative dating the fossil is compared to something for which an age is already known. For example if you have a fossil trilobite and it was found in the Wheeler Formation. The Wheeler Formation has been previously dated to approximately million year old, so we know the trilobite is also about million years old. Scientists can use certain types of fossils referred to as index fossils to assist in relative dating via correlation.
Index fossils are fossils that are known to only occur within a very specific age range. Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites work best as index fossils.
There are two primary ways of dating used by geologists: Relative Dating Consider this cross section, based on an area near Port Waikato. The units on the key in the left are not shown in the correct order. Can you work them out?
Potassium–argon dating, abbreviated K–Ar dating, is a radiometric dating method used in geochronology and is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar). Potassium is a common element found in many materials, such as micas, clay minerals, tephra, and evaporites.
Problems in Dating Sedimentary Rocks Sidebar 3 Stratified sedimentary rocks and their included fossils confront us with important concepts relating to the history of the earth. Are sedimentary rocks and fossils millions of years old? Radioisotope dating of sedimentary rocks might be supposed to provide the answer. However, the radioisotope dates that have been obtained by standard geochronological practice have been recognized to be doubtful because: For example, radiogenic parent 40K is often uncommon in sedimentary rocks, or the radiogenic daughter 40Ar is lost from open mineral structures.
Thus, carbonate rocks, evaporates, iron minerals, and phosphorites are usually classified as unsuitable for analysis of age. Minerals from three types of sedimentary rock could be potentially datable:
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Radiometric dating! Radioactive elements were incorporated into the Earth when the Solar System formed. All rocks and minerals contain tiny amounts of these radioactive elements.
There are a few ways to go about that. The first is the most indirect. Ti is especially useful because it serves as a thermometer in zircon – when zircon grows in the presence of certain other minerals rutile and quartz in particular , the Ti content of zircon is a direct function of the temperature that the zircon grows at. So – if you want to date minerals that don’t have U, Th, etc.
Sometimes these radioactive-element-bearing minerals will also be zoned, and you can see how these minor and trace elements evolve with time and perhaps get a sense of timing for a larger window of the metamorphic path. Another more direct way of doing this is to look for inclusions of datable minerals like zircon, rutile, monazite, etc. Recalling the principle of included fragments that you may have learned early on – an inclusion in a metamorphic rock is similar, it must have formed before or at the same time as the mineral enclosing it.
There are lots of caveats with this, but it’s a viable if still indirect method. Finally, lots of metamorphic minerals do have radioactive elements in them and can be dated directly.
Mod Problems in Dating Sedimentary Rocks
Example[ edit ] For example, consider the case of an igneous rock such as a granite that contains several major Sr-bearing minerals including plagioclase feldspar , K-feldspar , hornblende , biotite , and muscovite. Rubidium substitutes for potassium within the lattice of minerals at a rate proportional to its concentration within the melt. The ideal scenario according to Bowen’s reaction series would see a granite melt begin crystallizing a cumulate assemblage of plagioclase and hornblende i.
This then causes orthoclase and biotite, both K rich minerals into which Rb can substitute, to precipitate. The resulting Rb-Sr ratios and Rb and Sr abundances of both the whole rocks and their component minerals will be markedly different. This, thus, allows a different rate of radiogenic Sr to evolve in the separate rocks and their component minerals as time progresses.
Dating – Rubidium–strontium method: The radioactive decay of rubidium (87Rb) to strontium (87Sr) was the first widely used dating system that utilized the isochron method. Rubidium is a relatively abundant trace element in Earth’s crust and can be found in many common rock-forming minerals in which it substitutes for the major element potassium.
Radioactive decay[ edit ] Example of a radioactive decay chain from lead Pb to lead Pb. The final decay product, lead Pb , is stable and can no longer undergo spontaneous radioactive decay. All ordinary matter is made up of combinations of chemical elements , each with its own atomic number , indicating the number of protons in the atomic nucleus. Additionally, elements may exist in different isotopes , with each isotope of an element differing in the number of neutrons in the nucleus.
A particular isotope of a particular element is called a nuclide. Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will undergo radioactive decay and spontaneously transform into a different nuclide.
Carbon, Radiometric Dating
Dating rocks and minerals The radiation causes charge to remain within the grains in structurally unstable “electron traps”. Another possibility is into two or more nuclides. Nuclear Methods of Dating.
Radioactive dating Radioactive dating is a method of dating rocks and minerals using radioactive isotopes. This method is useful for igneous and metamorphic rocks, which cannot be dated by the stratigraphic correlation method used for sedimentary rocks.
Dinosaurs disappeared about 65 million years ago. That corn cob found in an ancient Native American fire pit is 1, years old. How do scientists actually know these ages? Geologic age dating—assigning an age to materials—is an entire discipline of its own. In a way this field, called geochronology, is some of the purest detective work earth scientists do. There are two basic approaches: Here is an easy-to understand analogy for your students: