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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.

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Presentation on theme: "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."— Presentation transcript:

1 Relative and Absolute Dating

2 What is Dating? When geologists date rocks, they are determining how long ago they formed. Two ways to do this: Relative Dating Absolute Dating

3 Relative Dating Determining how old something is compared to something else Use words like older or younger instead of exact numbers

4 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)

5 Order these layers from oldest to youngest Youngest layer Oldest layer

6 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

7 Order the events from oldest to youngest Fault Gray Layer Multicolored layer Tan Layer OLDEST LAYER/EVENT YOUNGEST LAYER/EVENT

8 Absolute Dating Determining how old something is Use numbers (in millions of years, mya) Only works for Igneous Rocks

9 How Absolute Dating Works When magma/lava cools, radioactive elements are incorporated into the minerals Examples: -- Potassium Uranium 235

10 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.

11 What can we learn from this? T-Rex Fossil Geologist Basalt, dated to 80 mya Basalt flow, dated to 100 mya Sandstone 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

12 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


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