Geologic Time and Earth History Part 2 – Absolute Age

Uniformitarianism A principle that states that geologic processes that have been occurring today have been occurring since Earth formed. Key to the science of geology and studying Earth’s past!

Essential Questions How can the amount of parent material in a rock sample be determined by using half-lives? How does the amount of parent material in a sample change as time increases?

Absolute Dating Radioactive isotope content can be used to identify the amount of time passed since the rock formed. Rocks that can be dated like this are mostly igneous, but sometimes metamorphic. Sedimentary rocks cannot be dated this way. The process is also known as radiometric analysis.

Radioactive Decay Method that is used to determine the absolute age of a rock or fossil. When an atom breaks down or decays over time until it is stable.

Half-Life Half-life: The time required for half of a sample of a radioactive isotope to break down by radioactive decay to form a daughter isotope. Ranges from seconds to billions of years depending on the material.

Half-Life For example, carbon-14 takes 5730 years to break down. So, if I have 100g of carbon-14, after 5730 years I will only have 50g. How much will be left after another 5730 years? How much time would have passed? Answer: You would have 25g of carbon-14 11,460 years of total time would have passed.

Using Half-Life Parent: unstable radioactive isotope Daughter: isotope resulting from the decay of the parent. # of Half Lives Parent Isotopes Daughter Isotopes 100% 0% 1 50% 2 25% 75% 3 12.5% 87.5% 4 6.25% 93.75%

Parent and Daughter material in terms of half-life By comparing the percentage of an original element (parent) to the percentage of the decay element (daughter), the age of rock can be calculated. The ratio of the two atom types is a direct function of its age because when a rock was formed, it had all parent atoms and no daughters.

Calculating Half-Life How old would a fossil be if it had only 1/8 of its original carbon-14 content remaining? Remember carbon-14 takes 5730 years to divide in half. Create a table! Time, Amount of parent/daughter isotopes, and # of half-lives

Problem #1 Answer: 6.25 grams The half-life of radon-222 is 3.8 days. How much of a 100g sample is left after 15.2 days? Answer: 6.25 grams Time Amount 0 Days 100 grams 3.8 50 grams 7.6 25 grams 11.4 12.5 grams 15.2 6.25 grams

Problem #2 Answer: 8.75 grams Carbon-14 has a half-life of 5730 years. If a sample contains 70g originally, how much is left after 17,190 years? Time Amount 0 years 70 grams 5730 years 35 grams 11,460 years 17.5 grams 17,190 years 8.75 grams Answer: 8.75 grams

Problem #3 Answer: 15.625 grams How much of a 500g sample of potassium-42 is left after 62 hours? The half-life of potassium-42 is 12.4 hours. Time Amount 0 hours 500 grams 12.4 hours 250 grams 24.8 hours 125 grams 37.2 hours 62.5 grams 49.6 hours 31.25 grams 62 hours 15.625 grams Answer: 15.625 grams