Radiometric dating and geologic ages exercise 7. Radiometric dating problems

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Radiometric Dating and the Geological Time Scale

radiometric dating and geologic ages exercise 7

When a geologist collects a rock sample for radiometric age dating, or collects a fossil, there are independent constraints on the relative and numerical age of the resulting data. A critique of conventional geologic time scale should address the best and most consistent data available, and explain it with an alternative interpretation, because that is the data that actually matters to the current understanding of geologic time. Using these principles, it is possible to construct an interpretation of the sequence of events for any geological situation, even on other planets e. Stratigraphic position is an obvious one, but there are many others. After some initial and prolonged troubles over many years, the bed was eventually dated successfully by careful sample preparation that eliminated the detrital minerals. A few principles were recognized and specified later.

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Chapter 9 Geologic Time Flashcards

radiometric dating and geologic ages exercise 7

Geochronologists do not claim that radiometric dating is foolproof no scientific method is , but it does work reliably for most samples. A radiometric age for the Cretaceous-Tertiary boundary based on K-Ar, Rb-Sr, and U-Pb ages of bentonites from Alberta, Saskatchewan, and Montana. The geological time scale and the techniques used to define it are not circular. For example, the boundary between the Cretaceous and Tertiary periods is recognized on the basis of the extinction of a large number of organisms globally including ammonites, dinosaurs, and others , the first appearance of new types of organisms, the presence of geochemical anomalies notably iridium , and unusual types of minerals related to meteorite impact processes impact spherules and shocked quartz. The rock mass adjacent to the one containing the inclusion must have been there first in order to provide the fragment.

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Radiometric Dating and the Geological Time Scale

radiometric dating and geologic ages exercise 7

Dinosaurs were found after the first occurrence of land plants, insects, and amphibians. There is almost zero chance that the broad understanding of geological history e. Given the background above, the information used for a geologic time scale can be related like this: How relative dating of events and radiometric numeric dates are combined to produce a calibrated geological time scale. My thanks also to Brett Vickers for maintaining the talk. If the standard is of historically known age, such as would likely be used for testing Ar-Ar dating on sample of known age, then one would be more likely to get the correct age.

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1. EXERCISE 7: HISTORICAL GEOLOGY AND FOSSILS

radiometric dating and geologic ages exercise 7

The other dates are completely consistent with a lower boundary for the Campanian of 83±1 million years ago, as suggested by which Obradovich revises to 83. Because any newly-studied locality will have independent fossil, superpositional, or radiometric data that have not yet been incorporated into the global geological time scale, all data types serve as both an independent test of each other on a local scale , and of the global geological time scale itself. K-Ar and Ar-Ar can result in negative ages when atmospheric argon is considered. Conclusions Skeptics of conventional geology might think scientists would expect, or at least prefer, every date to be perfectly consistent with the current geological time scale, but realistically, this is not how science works. A profusion of terms is applied to the different concepts, and, confusingly to the uninitiated, to the names applied to subdivisions of them e. Spore-bearing land plants like ferns were always found before the occurrence of flowering plants.

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Geology Final Exam

radiometric dating and geologic ages exercise 7

The age of a particular sample, and a particular geological time scale, only represents the current understanding, and science is a process of refinement of that understanding. The first radiometric ages from the Judith River Formation Upper Cretaceous , Hill County, Montana. These flashcards will equip you with content that you need to succeed. The results are therefore highly consistent given the analytical uncertainties in any measurement. These are often characterised as the norm, rather than the exception. Despite this, the can be used to determine the sequence of deposition, folds, and faults based on their intersections -- if folds and faults deform or cut across the sedimentary layers and surfaces, then they obviously came after deposition of the sediments.

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1. EXERCISE 7: HISTORICAL GEOLOGY AND FOSSILS

radiometric dating and geologic ages exercise 7

Cambridge University Press: Cambridge, 131p. Uinkaret Plateau Ages of Rocks in Millions of Years K-Ar Rb-Sr Rb-Sr Isochron Pb-Pb Isochron 0. The isotopic make-up of original material is important, as is mixing of magma with surrounding material. A recent geological time scale, based on As another example, and present radiometric dates that bracket the ages of Late Cretaceous fossil occurrences i. This was true at a regional, and even a global scale. A challenge to this interpretation came in the form of Lord Kelvin's William Thomson's calculations of the heat flow from the Earth, and the implication this had for the age -- rather than hundreds of millions of years, the Earth could be as young as tens of million of years old. .

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PRACTICE EXERCISES ON RADIOMETRIC DATING

radiometric dating and geologic ages exercise 7

What percentage of this isotope should be found as daughter material in this rock? Etna basalt Sicily May 1964 0. The principle of superposition therefore has a clear implication for the relative age of a vertical succession of strata. Estimates of the age of the Earth again returned to the prior methods. The semantic difference exists to distinguish between the different but relatable types of observations and interpretation that go into them. In fact, the numbers that became available were significantly older than even some geologists were expecting -- rather than hundreds of millions of years, which was the minimum age expected, the Earth's history was clearly at least billions of years long.

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Geology Final Exam

radiometric dating and geologic ages exercise 7

Because of the chemistry of rocks, it was possible to calculate how much radioactive decay had occurred since an appropriate mineral had formed, and how much time had therefore expired, by looking at the ratio between the original radioactive isotope and its product, if the decay rate was known. Skeptics of radiometric dating procedures sometimes claim these techniques should not work reliably, or only infrequently, but clearly the results are similar: for intervals that should be about 70-80 million years old, radiometric dates do not yield for example 100 or 30 million years, let alone 1000 years, 100 000 years or 1 billion. The real question is what happens when conditions are ideal, versus when they are marginal, because ideal samples should give the most reliable dates. Various attempts were made as far back as the 1700s to scientifically estimate the age of the Earth, and, later, to use this to calibrate the relative time scale to numeric values refer to by Richard Harter and Chris Stassen. There is no way for a geologist to choose what numerical value a radiometric date will yield, or what position a fossil will be found at in a stratigraphic section.

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Geology Final Exam

radiometric dating and geologic ages exercise 7

With it factored in, the Earth could be vastly older. The same trend can be observed for other time periods. There are situations where it potentially fails -- for example, in cave deposits. So if these are real dates then you can hold a rock in your hand that won't form for hundreds of thousands or even millions of years yet. Biostratigraphy As geologists continued to reconstruct the Earth's geologic history in the 1700s and early 1800s, they quickly recognized that the distribution of fossils within this history was not random -- fossils occurred in a consistent order. By the 1830s, fossil succession had been studied to an increasing degree, such that the broad history of life on Earth was well understood, regardless of the debate over the names applied to portions of it, and where exactly to make the divisions. The methods work too well most of the time.

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