EARTH AND SPACE SCIENCE Chapter 8 The Rock Record 8.2 Determining Absolute Age
8.2 Determining Absolute Age Objectives • Summarize the limitations of using the rates of erosion and deposition to determine the absolute age of rock formations. • Describe the formation of varves. • Explain how the process of radioactive decay can be used to determine the absolute age of rocks.
Introduction • When judging the relative age of a rock unit, one is only comparing it as older or younger than other rock units. • To further understand Earth’s history, it is necessary to determine the absolute age of a rock formation. • Absolute age is the numeric age of an object or event, often stated in years before present, as established by an absolute-dating process, such as radiometric dating.
Absolute Dating Methods • A variety of methods are used to arrive at an absolute age. • Some methods may involve geologic processes that can be observed and measured over time. • Other methods may involve examining the chemical composition of certain materials in rocks.
Absolute Dating Methods • Rates of erosion can be used to estimate the age of a stream. • Determination by measuring erosion rates is only practical for studying geologic features that have formed within the past 10,000 to 20,000 years. • Rates of erosion can vary greatly over millions of years. • Average rates of deposition for sedimentary rocks can also be calculated.
Absolute Dating Methods • One can, in general, expect about 30 cm of common sedimentary rock to be deposited over 1,000 years. • A particular sedimentary layer, however, may not have been deposited at an average rate. • A flood, for example, can deposit several meters of sediment in one day. • Rate of deposition may slow down considerably under certain circumstances as well. • Because of variability in rates of erosion and deposition, measurement of these processes is a poor way to determine absolute age.
Absolute Dating Methods • A varve is a banded layer of sand and silt that is deposited annually in a lake, especially near ice sheets or glaciers, and that can be used to determine absolute age (in context). • Varves appear as light colored bands of course sands and darker bands of finer silt particles. • During summer, when snow and ice melt rapidly, a rush of water can carry large amounts of sediment into a lake. • Most of the course particles settle quickly to form a layer on the bottom of the lake. • As winter approaches and the water freezes, clay particles that are still suspended in the water then settle slowly to form an fine overlying layer.
Radiometric Dating • Isotopes are atoms of the same element that have different numbers of neutrons. • Rocks generally contain small amounts of radioactive isotopes. • Isotopes undergo radioactive decay, releasing particles and/or energy, in order to reach a more stable state. • Scientists can use the radioactive decay process to determine the age of certain rocks because the rate of radioactive decay for a particular isotope is constant and cannot be changed.
Radiometric Dating • Scientists can measure the amount of radioactive parent isotope (original radioactive isotope) and compare that to the amount of newly formed stable daughter product. • The relative proportion of radioactive parent isotope to stable daughter product can then be used to determine the age of the object. • Half-life is the time required for half of a sample of radioactive isotope to break down into the stable daughter product. • For example, if you begin with 100 grams of radioactive parent isotope, after one half-life, you will have 50 grams of radioactive parent isotope and 50 grams of stable daughter product.
Radiometric Dating • After the next half-life, you will have 25 grams of radioactive parent isotope and 75 grams of stable daughter product. • After yet another half-life, you will be left with 12.5 grams of radioactive parent isotope and 87.5 grams of stable daughter product (and so on). • Comparing parent or daughter isotopes only works when the sample has not lost or gained radioactive parent isotopes or daughter product. • If too little or too much time has passed, an accurate reading may not be possible with the chosen isotopes to measure.
Radiometric Dating • Also, if the isotope you are measuring wasn’t in the sample to begin with, accurate results are impossible. • In order to date an inorganic rock, you would not use carbon-14 dating – why? • If you were dating a dinosaur bone, you would not use carbon-14 dating – why? • If you were dating a woolly mammoth, you would not use potassium/argon dating – why?
Carbon Dating • Materials containing organic remains less than 70,000 years old can be dated using carbon-14. • When using radiocarbon dating, one looks for the small percentage of carbon-14 that was bonded with oxygen to form carbon dioxide that was taken in by plants and incorporated into the organic structure of plants (and then animals as they ate the plants). • Carbon-14 has a 5,730 year half-life and undergoes beta decay to become nitrogen-14.
References • Erosion and Deposition -http://wwwbrr.cr.usgs.gov/projects/Burned_Watersheds • Varve - http://www.runet.edu/~rugs/saga.html • Radioactive Decay - http://spurr.pls.uni.edu/ctst/inspiration.html • Half-life - http://fig.cox.miami.edu/~cmallery/150/chemistry/C14dating.htm