Relative & Absolute Dating

 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. Relative dating 2. Absolute 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)

 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 20 th century.  Radioactivity studies the emission of energy from the nucleus of an atom as is becomes more stable.

 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?

 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.

 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 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

 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

 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%

 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…

 Step one: Find the element on the “elements for radioactive dating” table, and determine if it is the parent or daughter nuclide

 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%

 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

 A rock containing a fossil is found to contain 25% lead-206. What is the most likely age of the fossil?

 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

 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

 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 = (4.47x10 9 years per half-life) x (0.5 half-lives)  Age of the fossil = 2.2x10 9 years or 2.2 billion years

 A rock is analyzed and contains 10% carbon-14. What is the age of the rock?

1. carbon-14 is the parent nuclide 2. the sample contains 10% carbon-14 (given in question) 3. 10% corresponds to 3.5 half-lives 4. the age of the fossil is… Age of the fossil = (half-life) x (# of half-lives) Age of the fossil = (5.73x10 3 yrs/hl)(3.5 hl) = years = 2.0x10 4 years 10% 3.5 half-lives

 Practice Problems (pg 323) & 24 & 25 (pg 324)  Radioactive M&M’s Worksheet