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The Dating Game data | hook | main | background & resources | student Background There are several methods available to the geologist for acquiring an absolute date. A suitable material must be available to date, however. The ice in ice cores is not a material that can provide a date radiometrically. In areas of high accumulation like Greenland and Siple Dome, Antarctica ice cores can provide a direct date by comparison. However, a volcanic ash layer in the ice core is a material that can provide an absolute date using the appropriate geochemical method of dating. Likewise, the carbon contained in any shells or plant material can be dated directly. There are several different dating techniques; two will be discussed briefly below. Radiometric Dating. The minerals that make up rocks may contain radioactive elements (usually in small quantities!!). These radioactive elements spontaneously decay, meaning that they release electrons or neutrons and protons and subsequently form a different element. The average rate of decay is constant. No chemical or physical process alters this decay rate (except substantial re-heating to reform the mineral). When an igneous rock is created, it has a set amount of a radioactive material and that material begins to decay. The dating comes from examining the rock and finding the proportions of the original element (parent element) and the newly formed element (daughter element). If the decay rate is known, the scientists can calculate the time when only parent elements and no daughter elements existed. Scientists speak in terms of half-lives. A half-life is when one half of the original parent element has decayed into the daughter element. In the M&M activity, the parent is represented by the "unchanged" and the daughter is represented by the "changed. "So, in the first half life, half of the parent element has decayed and half remains. At the end of the second half life, a quarter of the parent remains; at the end of the third half life, 1/8 of the parent remains, and so on. The instruments that measure the amount of parent material and daughter material are sensitive. However, there are limits to how small of a quantity they can measure. Usually the amount of parent material remaining after five half lives is starting to be too small to measure (1/32 of the original quantity of the parent element). Thus, depending on the rate of decay, some radioactive elements offer a long time span over which they can be measured, and others offer only a short time span because they decay so quickly. Some of the more important radioactive isotopes and their useful time measurements are listed below. Potassium/Argon and Carbon 14 (discussed below) are the most important for dating glacial events of the Cenozoic in Antarctica. The other methods have been used to date the age of the oldest rocks on earth (3.7 billion years). Because of the slow decay rate of the uranium group, they have not decayed sufficiently to be measurable in the most recent glacial sediment. Parent................Daughter.......Half-Life...........Useful Dating Range Uranium 238.......Lead 206.........4510 million......>5 million years Uranium 235.......Lead 207..........713 million......>60 million years Potassium 40......Argon 40.........1300 million......>50,000 years Carbon 14..........Nitrogen 14.......5,730 year........40,000 - 70,000 years Short-hand for Carbon 14 is 14C; Carbon-12 is written as 12C. Ash layers are helpful materials to find in the depositional environment, or in a sediment or ice core. Ash layers are formed when a volcano erupts, sending tiny parcticles into the atmosphere. The circulation of the atmosphere distributes this material over a wide region. Slowly, the ash settles back to the surface of the earth. Sometimes thick layers of ash accumulate (depending on the closeness of the volcano, the altitude the ash was injected into the atmosphere and the quantity of material produced by the eruption). The radioactive material in the ash layer can be dated directly. Ash flows also are a useful relative dating tool. The composition of volcanos differs (and varies with the stage of the eruption and age of the volcano). Thus, for any one eruption, the composition of the ash layer is the same. Researchers can identify the same volcanic event over the area of ash distribution. If the ash layer has been dated in one area the age is good for the ash layer where ever it is found. Lava flows also are found in Antarctica, sometimes in association with glacial sediment. The lava flow contains radioactive elements that can be used to directly date the creation of the flow. This, in turn, provides information about the age of the glacial deposit. Glacial deposits under the lava flows cannot be younger than the lava flow; material over the lava flow cannot be older than the flow. It usually is not useful to date radioactive elements from the minerals and rock fragments in sediment. Sediment, and sedimentary rock, is made of parcticles broken down from other rocks. These parcticles, therefore, formed at different places and different times, and will not reveal the age of the formation of the sediment. The 14C Dating Method. The Carbon 14 method is the most widely used method for dating sedimentary materials less than 70,000 years old. It is also a radiometric decay process, where the 14C (pronounced carbon-14) isotope decays into more stable nitrogen-14 and releases an electron in the process. Carbon-14 is incorporated with the other carbon isotopes (12C and 13C) by plants and animals. When the organism dies, it stops taking up 14C. The 14C begins to decay into 14N at a constant rate. The decay rate is rapid. The half-life of 14C is approximately 5730 years. Carbon-14 can be measured accurately to approximately 40,000 years. Recent developments of parcticularly sensitive instruments have allowed successful detection of the parent element in materials as old as 70,000 years. Much of the datable organic material occurs in the sedimentary environment (trees, bone, peat, calcium-carbonate shells of marine organisms). Some has been transported to a parcticular environment by depositional processes, and therefore is older than the deposits. Some of this material, however, grows in place, providing a valuable dating tool for the geologist and/or biologist. In Antarctica, subglacial till usually lacks fossils, so we date the microfossils in the sediment directly overlying the till using the 14C method. This assumes that little time passed between till deposition and the deposition of the capping sediment. Dating in this manner implies that the date acquired represents the youngest possible age of the till (the till could be older, but it is not younger than the overlying sediment). From the Glacier Web site: www.glacier.rice.edu Resources and Reference Materials Skinner, Brian J. and Stephen C. Porter, 1992. The Dynamic Earth An Introduction to Physical Geology. John Wiley and Sons, Inc: New York. Stroud, Sharon M. and Jeffrey C. Callister,1993. Earth at Hand A Collection of Arcticles from NSTA's Journals. National Science Teachers Association: Washington. Radiometric Dating and the Geological time Scale. http://www.talkorigins.org/faqs/dating.html#specific Radiometric Datinghttp://www.dc.peachnet.edu/~pgore/geology102/radio.htm Back to: TEA Activities Page |