Transcript ES 8.2 2006
Section 8.2 Determining Absolute Age 1 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. Absolute Age The numeric age of an object or event, often stated in years before the present. Absolute Dating Methods A variety are used, either by observing geologic processes or by measuring the chemical composition of a rock. Rates of Erosion By measuring the rate at which a stream erodes its bed, we can estimate the age of the stream. Practical only for features that formed within the past 10- Example Niagara Falls has been eroding at an average rate of 1.3 m per year for the past 9,900 years. For Older Surface Features The method is less accurate because rates of erosion may vary greatly over millions of years. Rates of Deposition Geologists can estimate the average rates of deposition for common sedimentary rocks (about 30 cm/1000 years). It Provides Only An Estimate Because Any given sedimentary layer may not have been deposited at the average rate. The rate of deposition may change over time. Varve Count Sediments can show definite annual layers that consist of a light-colored band of coarse particles and a dark band of fine particles. We can count these like tree rings. 10 Generally Form In Glacial Lakes Large light-colored sediments deposited quickly in the summer. Small particles and organic matter settle out in the winter, forming a dark layer. One light and dark layer thus equals one year. Radiometric Dating A method of determining the absolute age of an object by using radioactive decay. Isotopes Atoms of the same element with different numbers of neutrons. Radioactive Isotopes Atoms that break down and emit particles and energy. Since the decay rate is known we can use radioactive materials as natural clocks by comparing the ratios of two isotopes. Parent Isotope The original, radioactive isotope. Daughter Isotope The result of radioactive decay; the newly formed isotope. Using The Known Decay Rate Scientists compare the proportions of the parent and daughter isotopes to determine the absolute age of the rock. An Example Think of an hourglass filled with sand. The sand on top is the parent material and the sand falling to the bottom is the daughter product. As the parent material decreases the daughter product increases in proportionate amounts. 18 Half-Life The time it takes half the mass of a given amount of a parent isotope to decay into its daughter isotopes. The rate is assumed to be constant for each substance and is not effected by changes in temperature, pressure, or other environmental factors. This Only Works If The sample has not gained or lost either parent or daughter isotopes through leaking or contamination. Radioactive Isotopes (Table 1, p. 195) The amount of time that has passed since a rock has formed determines which radioactive element is used to date a rock. Time Is The Key Too little: Not enough daughter isotope to measure. Too much: Not enough parent isotope to measure. So the estimated age of the rock Carbon Dating Younger rocks may be dated by the remains of organic material found within the rock. Also known as radiocarbon dating, it is used for material less than 70,000 years old. 14C Is Created In the Atmosphere It then becomes part of carbon dioxide, which all organisms use as a carbon source. The ratio of 14C to normal 12C is a known constant, and is the same in all organisms. When The Organism Dies The ratio between the two isotopes begins to change because 14C is radioactive, with a half-life of 5,730 years. Thus the amount of 14C loss tells us how long ago the organism was Assignment Due EOP Thursday: Directed Reading 8.2 Key Terms Due EOP Friday: Licorice Lab Due BOP Monday: “Shaky” Chapter 20