Fossils and The Law of Superposition

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

– Superposition – Oldest Rocks on the Bottom. According to the principle of superpostion, in undisturbed layers of rock, the oldest rocks are on the bottom and the rocks become progressively younger toward the top.

Rock Layers Sediment forms sedimentary rocks by building up in sedimentary rock layers. The first layer to form is on the bottom. The oldest layer is on the bottom. Sometimes forces in the Earth can turn layers over. When layers have been turned upside down, it’s necessary to use other clues in the rock layers to determine their original positions and relative ages.

Rock Ages The relative age of something is its age in comparison to the age of other things. Geologists determine the relative ages of rocks and other structures by examining their places in a sequence. Relative age determination doesn’t tell you anything about the age of the rock layers in actual years.

Other Clues Help If rock layers haven’t been faulted or turned upside down, it is easy to determine relative age.. In cases where rock layers have been disturbed you might have to look for fossils and other clues to date the rocks.

Other Clues Help What other clues can you use to find the relative age of rocks? Fossils because they are specific to an era and once a species is extinct they will never return.

Other Clues Help If you find a fossil in the top layer that’s older than a fossil in the lower layer, you can hypothesize that the layers have been turned upside down by folding during mountain building.

Example Number the rock layers below from 1-5, from the oldest relative age to the youngest relative age *Bonus: Who came up with Principle of Superposition? Avicenna

Who’s on First? Relative Age Activity

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: 1.The card with the letters “C” and “T” is on the bottom, or the oldest layer 2.Look for a card that has either a “T” or “C” written on it for the second layer

C T AGC UA NBU NB ON DXO MD This is one possible way to arrange the cards. Questions: 1.What letter is the oldest? 2.What letter is the youngest? 3.What letter showed up the most? 4.Which letters only showed up once? 5.Which letters could be index fossils? 6.How did you know which was older: “M” or “X”?

Activity # 2 Flip your eight index cards over Arrange the index cards that represent layers of rock and fossils Clues: 1.The oldest layer has the letter “M” in it 2.Find a rock layer that has at least one of the fossils you found in the oldest rock layer 3.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 5 th graders

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?