Fossils and The Law of Superposition

Fossils and The Law of Superposition Liz LaRosa 5th Grade Science http://www.middleschoolscience.com 2009 This PPT was created with the information from the FOSREC Activity “Who’s on First?” and “Fossil Inferences” by UEN.

Fossils and Superposition • What is a fossil? The trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock • What is a superposition? Younger rocks lie above older rocks if the layers have not been disturbed

Relative Dating and Index Fossils • What is relative dating? • Any method of determining whether an event or object is older or younger than other events or objects. • What is an index fossil? • A fossil that is found in the rock layers of only one geologic age and is used to establish the age of the rock layers. • Is found in rock layers around the world, ex Trilobites

Activity # 1 • On your desk, you have 8 large colored index cards with nonsense letters placed on them. • Your task is to determine what the correct sequence of the letters are. • You have two clues: • The card with the letters “C” and “T” 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

You have two clues:The card with the letters “C” and “T” is on the bottom, or the oldest layerLook for a card that has either a “T” or “C” written on it for the second layer Questions when you finish: What letter is the oldest? What letter is the youngest? What letter showed up the most? Which letters only showed up once? Which letters could be index fossils? How did you know which was older: “M” or “X”?

M D X O N B U A G C T This is one possible way to arrange the cards. Questions: What letter is the oldest? What letter is the youngest? What letter showed up the most? Which letters only showed up once? Which letters could be index fossils? How did you know which was older: “M” or “X”? MD DXO ON NB NBU UA AGC C T

Activity # 2 • Flip your eight index cards over • Arrange the index cards that represent layers of rock and fossils. Each card represents one layer of rock. • Clues: • The oldest layer has the letter “M” in it • Find a rock layer that has at least one of the fossils you found in the oldest rock layer • Extinction is forever - once an organism disappears from the sequence it cannot reappear later

Teacher Note: I replaced the letters with nonsense letters b/c spelling the word “organism” was too easy for my 5th graders

A N S WE R

To think about… • What problems did you run into when trying to arrange the fossils into the correct sequence? • Would this have been more difficult if you did not know which layer was the oldest to start the activity? • Which organism is the most complex of all the fossils and why?

Fossils of an organism can be found within multiple “layers” or periods in history if it lived for a long period of time. If a fossil that was only found in one layer, however, it would be considered an index fossil because it only lived for a short time.

Rock layers can be helpful in figuring out what order geologic events took place. • We will be looking at cross sections and using the Law of Superposition to figure out the correct order of layers from youngest to oldest.

Law of Superposition: Oldest sedimentary rock layers will be on the bottom, youngest on top. • Principle of Original Horizontality: When layers of sedimentary rock are first made, they are layered horizontally.

Rock Layers Can Change!  Rock layers can fold underground. Can you still figure out the order from oldest to youngest? Intrusions of igneous rock can cut through different layers, like A. The intrusion is always younger than the layers it cuts through (even if it comes from the bottom!).

A fault or crack could form in the middle of rock layers (B). If this happens, the fault is always younger than the layers it cuts through.

Task: Put the layers in order from oldest to youngest. • Hints: • Start at oldest first – look at the bottom layers • Igneous intrusions are younger than the layers they cut through • Fault lines are younger than the layers they cut through • If igneous and metamorphic are side by side, metamorphic was there first

Fossils and The Law of Superposition