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Relative & Absolute Dating C 1.5- PINPOINTING TIME DATING TECHNIQUES Dating techniques refer to methods scientists use to figure out the age of something, such as a rock or a fossil There are two ways of doing this: 1. 2. Relative dating Absolute dating RELATIVE DATING Recall, relative dating is a technique used for fossils or rocks when you already know some information about the history of fossils and rocks in the area It uses the relative position of the item (e.g. on top of fossil A but below fossil B) to make conclusions about its age (e.g. older than fossil A but below fossil B) ABSOLUTE DATING Relative dating is useful for coming up with an approximate date, but is not very exact Absolute dating involves analyzing the chemical content of the rock to come up with a more exact date. It employs the idea of radioactivity, originally discovered by Marie Curie in the early 20th century. Radioactivity studies the emission of energy from the nucleus of an atom as is becomes more stable. THE CLOCKS IN ROCKS Rocks, such as those examined by geologists, contain radioactive elements like rubidium and uranium. Over time, these parent isotopes decay, releasing energy and forming stable daughter isotopes. This breaking down of the parent occurs at a predictable rate, so by comparing the ratio of parent to daughter isotopes in a particular rock, it is possible to determine its age (in millions of years). Why is this useful information? THE CLOCKS IN ROCKS By finding the age of a rock, we can also, by default, age any fossils found in that rock. Radioactive dating is the process of finding the age of a mineral based on the rate of decay of the elements found in that mineral. It involves two steps: counting the number of daughter isotopes in the mineral using the known decay rate to calculate the length of time required to produce that number of daughters. THE CLOCKS IN ROCKS How is the composition of the rock analyzed? We’ll use zircon crystals, such as those formed during a volcanic eruption, as an example From the moment a zircon crystal forms, it is tough, dense, inert and nonmagnetic It resists weather, withstands extreme temperatures, and is incredibly stable From the moment a zircon crystal forms, unstable uranium (within the crystal) decreases, and the amount of lead increases When scientists study a zircon crystal, they measure the ratio of uranium to lead This is like a very precise stopwatch that can date events from a few hundred thousand years ago to several billion years ago It takes 4.468 billion years for half of the uranium to change to lead (called the half life) When a volcano erupts, gases originally dissolved in the magma form bubbles so quickly that they make the magma explode into crystals Common minerals found in volcanic ash are feldspar, quartz, mica and zircon Unlike the others, zircon is extremely durable It contains radioactive uranium, but no lead. After the volcanic ash falls down on an area, the ash could be washed away, or become trapped in a crack, or in a lake or swamp Over time, this ash is buried and becomes an identifiable layer in the sedimentary rock Zircon crystals buried in the ash is like a mineral stopwatch, ticking away as the uranium decays into lead. In the lab, geologists crush the ash sample and wash away the fine dust Zircon’s unique properties make it possible to separate it out it is dense, but not magnetic a magnet will separate out the magnetic minerals because of its density the zircon will settle at the bottom, while the rest will float After removing the zircon from the heavy liquid, scientists dissolve the crystals and separate the uranium and lead from the other elements in the zircon to read the stopwatch, the lead and uranium are put in a mass spectrometer, a machine that separates and counts individual atoms the ratio of uranium to lead atoms allows scientists to calculate how many years have passed since the volcano erupted RADIOACTIVE DATING CALCULATIONS PERFORMING RADIOACTIVE DATING CALCULATIONS The graph to the left is referred to as a decay curve. along the x-axis is time, given in millions of years (or number of half-lives elapsed) along the y-axis is the percentage of the parent isotope present in the sample PERFORMING RADIOACTIVE DATING CALCULATIONS notice that over time, the percentage of the parent decreases, and the daughter’s percentage increases correspondingly the total percentage in the ratio of parent to daughter is always 100% PERFORMING RADIOACTIVE DATING CALCULATIONS an element’s half life is the amount of time it takes for half the parent to decay into the daughter after one half life, the ratio is 50:50 after two half lives, the ratio is 25:75 after three half lives, the ratio is 12.5:87.5 and so on… STEPS TO CALCULATING THE AGE OF A MINERAL USING RADIOACTIVE DATING: Step one: Find the element on the “elements for radioactive dating” table, and determine if it is the parent or daughter nuclide STEPS TO CALCULATING THE AGE OF A MINERAL USING RADIOACTIVE DATING: Step two: Determine the percentage of remaining parent material in some cases, this will be given to you in other cases, you will be given the percentage of the decay (daughter nuclide), and the parent’s percentage can be found by subtracting the daughter’s percentage from 100% STEPS TO CALCULATING THE AGE OF A MINERAL USING RADIOACTIVE DATING: Step three: Using the decay curve, determine the number of half-lives this percentage corresponds to Step four: Multiply that number of half-lives by the “approximate half-life” listed on the “elements for radioactive dating” for that element EXAMPLE #1: A rock containing a fossil is found to contain 25% lead-206. What is the most likely age of the fossil? EXAMPLE (SOLUTION) Step one: Find the element on the “elements for radioactive dating” table, and determine if it is the parent or daughter nuclide lead-206 is the daughter nuclide EXAMPLE (SOLUTION) Step two: Determine the percentage of remaining parent material If the sample contains 25% daughter nuclide, it must contain 75% parent nuclide (100% - 25%) Step three: Using the decay curve, determine the number of half-lives this percentage corresponds to 75% 0.5 half-lives EXAMPLE (SOLUTION) Step four: Multiply that number of half-lives by the “approximate half-life” listed on the “elements for radioactive dating” for that element Age of the fossil = (half-life) x (# of half-lives) Age of the fossil = (4.47x109 years per half-life) x (0.5 half-lives) Age of the fossil = 2.2x109 years or 2.2 billion years EXAMPLE #2: A rock is analyzed and contains 10% carbon-14. What is the age of the rock? EXAMPLE (SOLUTION) 1. 2. 3. 4. carbon-14 is the parent nuclide the sample contains 10% carbon-14 (given in question) 10% corresponds to 3.5 half-lives the age of the fossil is… Age of the fossil = (half-life) x (# of half-lives) Age of the fossil = (5.73x103 yrs/hl)(3.5 hl) = 20 020 years = 2.0x104 years 10% 3.5 half-lives ASSIGNMENT Practice Problems 21-23 (pg 323) & 24 & 25 (pg 324) Radioactive M&M’s Worksheet