Fossils and The Law of Superposition

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Superposition What is relative age dating? Determining whether an event or object is older or younger than other events or objects. What is the Law of Superposition? Younger rocks lie above older rocks if the layers have not been disturbed

Example of Superposition

Relative Age Dating with Index Fossils What is a fossil? The trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock What is an index fossil? A fossil that is used to establish the age of the rock layers. It is found in the rock layers of only one geologic age. It is found in rock layers around the world. ex Trilobites

Activity # 1 Obtain Set A materials: 8 index cards or slips of paper with letters on them. Each slip of paper represents a rock layer. Each letter represents a kind of fossil. Your task is to determine what the correct sequence of the letters are, from oldest to youngest.

Method: The card with the letters “T” and “C” is on the bottom, or the oldest layer. Look for a card that has either a “T” or “C” written on it for the second layer. Since this card has a common letter with the first card, it must go on top of the first card. This second layer is younger than the TC layer.

3. Sequence the remaining cards using the same process. Note – Sometimes organisms disappear or become extinct. Extinction is forever - once an organism disappears from the sequence it cannot reappear later

M D X O N B U A G C T Answer This is the sequence of life on the planet

Questions for Activity 1: Neatly list the layers in order, with the oldest on the bottom and the youngest at the top. Label as such on the side of your listing. Explain why “M” is the same age as “D” in the layer “DM”. Explain why “D” in rock layer “OXD” is older than the “D” in rock layer “DM”. Which letter(s) showed up the most? Which letter(s) showed up only once? What could have happened to the organisms represented by the letters that disappeared? Which letter(s) could be index fossils? Explain why you made this choice.

Activity # 2 Obtain Set B materials: 8 index cards or slips of paper with fossils on them. Each slip of paper represents a rock layer. Your task is to determine the correct sequence of the letters, from oldest to youngest. The oldest layer has the letter “M” in it. Use the same method that you used in Activity # 1.

S I N M O R G A

A N S WE R

Questions for Activity #2: Neatly list the layers in order, with the oldest on the bottom and the youngest at the top. Label as such on the side of your listing. Explain why a pelicypod and an ammonite are the same relative age. Which is older – a crinoid or a horn coral? Why? Which fossils could be used as index fossils? Explain why - how is an index fossil is different than just any fossil?

Questions for Activity 2: 5. What kind of rocks would you find fossils in? 6. Explain how this activity illustrates the Law of Superposition and Relative Age Dating? 7. Compare these fossil images to the real fossils provided to you. Describe.

What Did You Learn? How does the Law of Superposition help us determine the relative age of rock layers? How do fossils help us determine the relative age of rock layers? What two characteristics must a fossil have to be considered an index fossil?