Determining the Age of Rocks We’ve already taken a look at types of fossils and how they form, but how do we know how old they are? We can determine the age of a fossil by finding the age of the rock it was found in. There are two types of dating, one gives an exact number, and the other a comparison. In these notes you will learn how a rock layers can help a geologist determine the age of fossils and rock. _______________________________ – age compared with ages of other rocks. _______________________________ – number of years since the rock formed. _______________________________ – in horizontal sedimentary rock layers the oldest layer is at the bottom; each higher layer is younger than the layer below it. Discuss: How did the Snicker’s activity demonstrate the law of superposition? The _________________________________________ means that layers of sediment are generally deposited in a __________________________ position. Discuss: What would cause the layers to not be horizontal after time? Write down at least 2 reasons. _______________________ – lava that hardens (igneous rocks) on the surface. _______________________ – magma that cools and hardens into a mass of igneous rocks beneath the surface. _______________________ – break in Earth’s crust which is always younger than the rock it cuts through. _______________________– gap in the geological record where some rock layers have been lost because of erosion. Draw a sketch of the picture and put the Layers in order, youngest to oldest: Draw a sketch of the picture and determine which is older, the fault or the intrusion? Explain why. Draw a sketch of the picture and put the layers in order, oldest to youngest: Absolute Dating Discuss: How is absolute dating different from relative dating? Draw a sketch of an atom and label the protons, neutrons, and electrons. Basic structure of an atom Orbiting the nucleus are electrons, which are negative electrical charges. Atomic number is the number of protons in the atom’s nucleus. Mass number is the number of protons plus the number of neutrons in an atom’s nucleus. Geologist use _________________________________ to determine the ___________________ ages of rocks. ___________________________ – all the atoms of a particular type of matter are the same. ___________________________ – process by which one unstable element breaks down into another element that is stable. ___________________________ – time required for half of the unstable element to decay. Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs. Radiometric dating is the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes.