Absolute Dating

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What is absolute dating?

Absolute dating is used by geologists to determine the actual age of a material. It can be achieved through the use of historical records and through the analysis of biological and geological patterns. Although development of radiometric methods led to the first breakthroughs in establishing an absolute time scale, other absolute methods have limited applications. Chief among these are dendochronology, varve analysis, hydration dating, and TL dating.

Dendochronology
This method of dating is based on the number, width, and density of annual growth rings of long-lived trees. A master tree-ring index has been constructed in the southwestern United States for the Douglas fir and bristlecone pine. Dendochronologists can use this index to date accurately events and climatic conditions of the past 3000-4000 years.

Varve Analysis
This technique was developed by Swedish scientists in the early 20th century. A varve is a sedimentary bed, or sequence of beds, deposited in a body of still water within a yearís time. Counting and correlation of varves have been used to measure the age of Pleistocene glacial deposits.

Obsidian Hydration Dating
This method is used to calculate ages in years by determining the thickness of rims (hydration rinds) produced by water vapor slowly diffusing into freshly chipped surfaces on artifacts made of obsidian or recent volcanic glass. This method can be applied to glasses 200 to 200,000 years old.

Thermo Luminescence (TL) Dating
This method is based on the phenomenon of natural ionizing radiation inducing free electrons in a mineral that can be trapped in defects of the mineralís crystal lattice structure. When trapped electrons escape as TL and are heated to a temperature below incandescence, the TL of a mineral such as quartz, can be recorded. Assuming a constant natural radiation level, the last drainage of the trapped electrons can be dated back to several hundred thousand years.

Radiometric Dating
Radiometric methods measure the time elapsed since the particular radiometric clock was reset. There are lots of ways to estimate ages, and geologists knew the earth was old a long time ago. Radiometric dating actually allows the measurement of absolute ages, so it is deadly to the argument that the earth cannot be more than 10,000 years old.



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