Bellwork: Tues. Feb. 13, 2018 Radiometric 1. _______________ or Radioactive dating is the use of half-lives to determine the age of a sample. 2. _______________ dating places rocks in sequence in which they were formed. 3. ________-_______length of time required for half of the radioactive atoms in a sample to decay. Radiometric Relative Half life 1

Notes: back of Radiometric Notes Bracketing the fossils Fossils are usually found in sedimentary rock—not igneous rock. Sedimentary rocks can be dated using C14 (radioactive carbon dating), but because carbon decays relatively quickly, this only works for rocks younger than about 50-60 thousand years. http://evolution.berkeley.edu/evolibrary/article/si de_o_0/radiodating_01

To date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil. Scientists date igneous rock using elements that are slow to decay, like uranium and potassium.

Bracketing: dating fossils by “sandwiching” igneous rock layers to determine youngest & oldest ages within the sedimentary rock layer.

Radioactive parent element decays into a stable daughter element Half-Life paper cutting. Radioactive parent element decays into a stable daughter element 1 2 3 4 5 6 7 8 9 10 1/2 1/4 1/8 1/16 1/32 1/64 1/128 1/256 1/512 1/1024 100% 50% 25% 12.5% 6.25% 3.125% 1.56% 0.781% 0.39% 0.195% 0.0975% 0:1 1:1 3:1 7:1 15:1 31:1 63:1 127:1 255:1 511:1 1023:1

Small-Scale Investigation: Radioactive Decay Radioactive elements decay at a constant, measurable rate. The time it takes for half of any given amount of an original element to change into a new isotope or element is called a half-life. You can demonstrate the principle of radioactive decay with a simple model. Materials: clock or watch with a second hand; sheet of notebook paper, about 28 x 22 cm; scissors Procedure 1. Record the time. 2. Wait 20 seconds, then carefully cut the sheet of paper in half. Select one piece, and set the other piece aside. 3. Wait 20 seconds, then cut the selected piece of paper in half. Select one piece, and set the other piece aside. 4. Repeat Step 3 until nine 20-second intervals have elapsed.

Analysis and Conclusions 1. In terms of radioactive decay, what does the whole piece of paper used in this investigation represent? 2. What do the pieces of paper that you set aside in each step represent? ____________________________ 3. What is the half-life of your “element”? (in seconds)_____ 4. How much of your paper “element” was left (in %) after the first three intervals? _______ after six intervals? _________ after nine intervals?________ 5. What two factors in your model must remain constant for your model to be accurate? Explain your answer.

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Bellwork: Tues. Feb. 21, 2017 Use the figure to answer questions 1 through 4 1. The half-life of this parent isotope is _____ 2. Percent of parent isotope remaining after 2 million years will be _______ 3. By the 3rd half-life, how muchof the parent isotope remains?________ 4. How many half-lives are represented in the graph? ___ 9

Bellwork Friday: Feb. 19, 2010 Layers A & C are igneous rock & have been radiometrically dated. Layer B is sedimentary & older than 60,000 years…so Approximately how old is layer B? 2. What is the technique used to date layer B (think “rock sandwich or [ ]”)? If radiometric dating was not available what conclusion could you still make about the age of layers A, B, and C (think about the other type of dating rocks)? Look at your Geologic Time Scale: 4. What era and period is layer B from? 5. What major evolutionary event involving plants occurred in this period? 10

Draw this in your notes: A 320 m.y.a. Igneous B ? Sedimentary C 408 m.y.a. Rock Layers