Rock Cycle - Past Life (6)
Pre Lab 

  • Understanding how rocks record geologic time.
  • Exploring the importance of fossils.
  • evolution
  • fossil
  • geologic time
  • paleontology
  • stratigraphy
Student discuss geologic time.

Ammonites are not found in the Cenozoic


Students are familiar with dinosaurs and other fossils, but the idea that certain organisms lived at certain times is very difficult for them to comprehend. The oldest unmistakable fossils are about 3.8 billion years old. However, multicellular organisms did not appear in the fossil record until approximately 650 million years ago. Animals with hard body parts (skeletons) did not appear until about 580 million years ago.

Geologic time is often very difficult to understand. Most students have trouble understanding the impact of 10 years, not to mention the 4.5 billion years that have passed since the formation of the Earth. It is difficult to date exactly how old the Earth is because no one was there to record the event. There are many lines of evidence for the age of the Earth, within your students' lifetimes this number will probably change slightly as new technology helps us to better date the past.

An accurate sense of geologic time and Earth history was very difficult for scientists to construct. It was not until the mid 1700's that James Hutton, who developed some of the modern principles of stratigraphy, realized that the history of the Earth was of unimaginable length of time, and that the "present was the key to the past." Hutton tried to develop methods that used present rates of sediment deposition to interpolate how much time had passed since the deposition of a particular rock layer.

Organisms have evolved over the course of the Earth’s evolution. Since paleontologists have recorded and study these changes, geologists can use the type of fossils in a rock to tell estimate its age relative to other rocks. For example, the dinosaurs lived and became extinct long before human ancestors evolved. Therefore, a rock containing dinosaur fossils would be much older than one containing hominid remains. This type of geologic time telling is called relative geologic time; it gives the order of events, fossils, and rocks from oldest to youngest, without reference to numerical ages.

Relative time is based on several principles of stratigraphy (the arrangement of sedimentary rock layers. These are the principles of Original Horizontality, Superposition, and Faunal Succession. The principle of Original Horizontality, first proposed by William Smith in the late 1700's, states that sedimentary layers formed in a horizontal position. If mountain building took place and the layers were tilted or moved, this event must have occurred after the layers were laid down.

 The Principle of Superposition states that in a sequence of sedimentary rock layers, the bottom layers are older than the top layers. The bottom layers were deposited first. The principle of Faunal Succession is similar to Superposition; it states that the oldest fossils in a series sedimentary rock layers will be in the lowest layer. Progressively younger fossils occur in higher layers.

So time is determined by fossils that are in different layers (strata).   If you look at the horizontal component, you are looking at one time period and understand the paleoenvironment.  In this activity students are asked to look at a dinosaur gravesite and try to identify the type od dinosaur.

  1. llustrate geologic time using the analogy of a 24 hour clock representing the 4.5 billion years of earth history. Draw the clock (shown below) on your blackboard. The creation of the major types of animals was not until 20:00, and humans were not around until 30 seconds before 24:00! Humans have only been on this Earth for a very short period. Dinosaurs were on this planet for much longer than we have been
  2. Emphasize that the history of geologic time was very difficult for scientists to construct. Explain how relative time is based on stratigraphic principles. You may wish to use the diagram below to help illustrate these principles.

  3. The activity looks at a dinsosaur gravesite and have students looking at what bones can be identified. The  dinosaur is a Stegosaurus. 


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