# Introduction to Radioisotope Geochronology/Part Eight - Examples of Application
## Introduction

Radio-isotopic dating is the determination of ages of geologic materials through the comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates.

## Age of the Earth
### Brief History of Determining the Age of the Earth

In regards to the age of the Earth, the first accurate estimate that is still broadly accepted today was proposed by Claire Patterson in 1953. Patterson's dissertation project was focused on using radiogenic lead (i.e. lead that was produced by the radioactive decay of uranium) to calculate the age of meteorites. It was assumed that meteorites and asteroids represented the left-over materials from the formation of the Solar System and had remained relatively undisturbed and therefore by measuring the lead isotopes of a meteorite on could assume the age of the Earth to be equivalent. Patterson found that the age of five meteorites from Canyon Diablo in Arizona, Nuevo Laredo in Mexico, and Henbury in Northern Australia was 4.55±0.07 billion years old. Amazingly, the current estimate for the age of the Earth is still within uncertainty of Patterson's first calculation at 4.54 ± 0.05 billion years (4.54 × 10$^9$ years ± 1%).

## Stratigraphic Applications

The Cretaceous–Tertiary extinction event, which occurred approximately 65.5 (Ma), was a large-scale mass extinction of animal and plant species in a geologically short period of time. Widely known as the K–T extinction event, it is associated with a geological signature known as the K–T boundary, usually a thin band of sedimentation found in various parts of the world. K is the traditional abbreviation for the Cretaceous Period derived from the German name Kreidezeit, and T is the abbreviation for the Tertiary Period. The event marks the end of the Mesozoic Era and the beginning of the Cenozoic Era. With "Tertiary" being discouraged as a formal time or rock unit by the International Commission on Stratigraphy, the K–T event is now called the Cretaceous–Paleogene (or K–Pg) extinction event by many researchers.

## Quaternary Geochronology

See Stirling and Mortensen (2009) for a recent review of the U-Th chronometer applied to fossil coral reefs.

## References

* ^ Uranium-series dating of fossil coral reef: Extending the sea-level record beyond the last glacial cycle. Earth and Planetary Science Letters, 284 (2009) 3269-283
* ^ Yuri Amelin, et al. Science 297, 1678 (2002)
* ↑ Patterson, C. (1956). "Age of meteorites and the earth". Geochimica et Cosmochimica Acta. 10: 230–237. doi:10.1016/0016-7037(56)90036-9.
* ↑ "Age of the Earth". U.S. Geological Survey. 1997. Archived from the original on 23 December 2005. Retrieved 2006-01-10. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
* ↑ Dalrymple, G. Brent (2001). "The age of the Earth in the twentieth century: a problem (mostly) solved". Special Publications, Geological Society of London. 190 (1): 205–221. Bibcode:2001GSLSP.190..205D. doi:10.1144/GSL.SP.2001.190.01.14.
* ↑ Fortey, R (1999). Life: A Natural History of the First Four Billion Years of Life on Earth. Vintage. pp. 238–260. ISBN 9780375702617.
* ↑ Gradstein, F. A Geologic Time Scale 2004. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)