Relative and Absolute Dating Question of the Day Question: What are the 3 types of rocks and what is the rock cycle? Answer: … … … Relative and Absolute Dating What is Dating? When geologists date rocks, they are determining how long ago they formed. Two ways to do this: Relative Dating Absolute Dating Relative Dating Determining how old something is compared to something else Use words like “older” or “younger” instead of exact numbers Rules of Dating two rules for dating rocks: 1. Stuff on the bottom is oldest and the stuff on the top is youngest This is called the Law of Superposition This only works for sedimentary rocks (sometimes lava flows) Order these layers from oldest to youngest Youngest layer Oldest layer Rules of Dating two rules for dating rocks: 2. If a fault or another rock cuts through a layer it happened after (and is younger than) the layer This is called the Law of Cross-Cutting Relations Order the events from oldest to youngest YOUNGEST LAYER/EVENT Fault Gray Layer Multicolored layer Spotted Layer OLDEST LAYER/EVENT Absolute Dating Determining how old something is Use numbers (in millions of years, mya) Only works for Igneous Rocks How Absolute Dating Works When magma/lava cools, radioactive elements are incorporated into the minerals Examples: -- Potassium 40 -- Uranium 235 How Absolute Dating Works These elements begin to decay at a known rate starting when the rock cools We can measure how much of the element is left Tells us how much time has passed since the rock formed. What can we learn from this? Geologist Basalt, dated to 80 mya T-Rex Fossil Sandstone Basalt flow, dated to 100 mya So when did this T-Rex live? Between 100 and 80 mya This may not seem very accurate, but compared to the 4,500 million years the earth has been around it gives us a lot more information than we had before What can we learn from this? Absolute Dating Helps us determine the age of the earth Helps us determine when specific events in the history of the earth happened (ex. Extinction of the dinosaurs) Relative Dating Can help us estimate the time span between major earthquakes, storms, tsunamis etc Can help us determine the order that life forms developed on earth Lab: What’s Up?