GEOL Lab 8

GEOL 1301 Lab 08 Geologic Time In this lab, you are going to review the concepts of relative and numerical geologic time and work on examples to establish the order of geologic events and to use radiometric dating. Please upload your completed worksheets on Canvas. Helpful Websites: http://en.wikipedia.org/wiki/Law_of_superposition http://en.wikipedia.org/wiki/Principle_of_original_horizontality http://en.wikipedia.org/wiki/Principle_of_cross-cutting_relationships http://en.wikipedia.org/wiki/Unconformity http://en.wikipedia.org/wiki/Radiometric_dating Key to rocks and symbols for the following activity: 1 Name:__________________________ For each of the following cross sections, determine the relative age sequence of the rocks and answer the additional questions. 1. Rock sequence from oldest to youngest: What type of unconformity is represented between layers E and C? Which principle is used to determine the time sequence of layers C, D, and A? 2 2. Rock sequence from oldest to youngest: What type of unconformity is represented between layers A and H (on the left side of the picture)? Is fault B older or younger than layers C and A? Which principle is used to determine this age relationship? 3 3. What is the age relationship between H and O, and how can you tell? What type of unconformity is represented between layers B+L+J and M? Is intrusion A older or younger than layers H, B, L, and J? Which principle do you apply to solve this? What type of unconformity is represented between layers M and G? Is fault F older or younger than intrusion A? Which principle do you apply to solve this? Rock sequence from oldest to youngest: 4 Name:__________________________ Numerical Geologic Time – Calculation A granite contains zircon crystals with 275 billion 235U atoms and 1925 billion 207Pb atoms. The half-life for 235U-to-207Pb decay is 704 million years. How old is the granite? Hint: In order to solve this problem, you have to calculate the total number of parent and daughter atoms (equaling the original number of parent atoms when the rock formed) and then figure out how many half-lives have gone by until the current number of parent atoms was reached (by repeatedly dividing the original number by 2). The final step is to multiply this number of half-lives with the years that correspond to one half-life. – List all the steps in your answer. 5