GO IN DEPTH

Fossils and Geologic Time

 
Fossils and Geologic Time Photo Credit: Clipart.com

Purpose

To help students understand the development of the geologic time scale. Also, to introduce students to the major time periods in earth's history, as well as to the role fossils play in helping us understand this history.


Context

This lesson is based on an online booklet that provides an introduction to the study of earth's history, published by the USGS. Using careful analogies and written historical records, the authors help students understand the development of the geologic time scale, including how this depended on gathering evidence and making comparisons. The major time periods in earth's history are introduced, as well as are fossils and the role they play in helping us understand this history.

Students will likely have been introduced to the geologic time scale in earlier grades, so this lesson is structured loosely to enable you to adapt the content to match the levels and needs of your particular students. Prerequisite knowledge for this lesson includes the idea that: "Sediments of sand and smaller particles (sometimes containing the remains of organisms) are gradually buried and are cemented together by dissolved minerals to form solid rock again." (Benchmarks for Science Literacy, p. 73.)

Concepts covered in this lesson, including geologic history, age dating, plate tectonics, timelines, and fossils are prerequisite concepts for understanding the theory of evolution, which is another topic taught at this grade level.

One chapter called Fossil Succession, found in the online booklet Fossils, Rocks, and Time, is particularly useful in helping students understand that the basic idea of biological evolution is that the earth's present-day species developed from earlier, distinctively different species. (Benchmarks for Science Literacy, p. 125.) Specifically, it helps students understand that the kinds of animals and plants found as fossils change over time. It discusses how Darwin's theory gave scientific meaning to the observed succession of once-living species in the record of earth's history preserved in the rocks.


Planning Ahead

This lesson is based on information included in Fossils, Rocks, and Time, part of the United States Geological Survey website.

Preview the information at this site and decide whether it would be best to print the text ahead of time, or allow students to read it online. Another option would be for you to order print materials directly from the USGS.


Motivation

Students will likely have been introduced to the geologic time scale in earlier grades, so take this opportunity to review basic concepts and find out the extent to which they are familiar with this topic. To get a feel for students' understanding of the age of the earth, ask the following question:

  • If you wanted to have a birthday party for the earth, how many candles would you put on the cake? (Accept all answers, but ask students to give reasons for their answers.)

Development

This section of the lesson is based on information found in the online booklet, Fossils, Rocks, and Time, by Lucy E. Edwards and John Pojeta, Jr. The content in this booklet could be presented in several ways, depending on the background, interests, and needs of your students. Students could read the entire text in one chunk, or you could break it up and ask guiding questions along the way.

For example, you could present the content like this:

Part I:
Have students read the Introduction; Putting Events in Order; The Relative Time Scale; and Rocks and Layers. Then ask the following questions:

  • Explain the difference between "relative time" and "numerical time."
  • Explain the overarching structure of the geologic time scale (i.e., the placement of eons, eras, periods, and epochs).
  • What is a paleontologist?
  • Why do the authors say that the layers of rocks are the pages in earth's history book?
  • What are sedimentary rocks?
  • What is the Law of Superposition and why is it critical to our interpretation of earth's history?
  • What is the Law of Original Horizontality and what does it help us understand about sedimentary rocks that are no longer horizontal?

Part II:
Have students read Fossils and Rocks; Fossil Succession; and The Numeric Time Scale. Again, ask students the following questions:

  • What types of fossils do most paleontologists study?
  • What is the Law of Fossil Succession?
  • What would happen if we started at the present and worked backwards to examine older and older layers of rock?
  • How does Darwin's theory of evolution explain what is seen in the fossil record?
  • Why are scientific theories continually being corrected and improved?
  • What are index fossils?
  • How did the discovery of radioactivity help scientists calculate the age of a rock?

Most of the vocabulary in the booklet is clearly explained and used to enhance understanding of basic concepts. If you'd like to emphasize some or all of the vocabulary, you could make a list of the terms found in the text ahead of time and ask students to record definitions as they read, building their own glossary.


Assessment

Revisit the question asked in the Motivation and assess the extent to which students' understanding has moved forward:

  • If you wanted to have a birthday party for the earth, how many candles would you put on the cake?

To help summarize students' understanding of the reading, have students expand upon their answers to the fourth question in Part I above, citing specific evidence from the online text:

  • Why do the authors say that the layers of rocks are the pages in earth's history book?

Extensions

Students could complete the Geologic Time Activity, on the Exploring the Environment website, in which they compare geologic time to the length of a football field, as well as take an in-depth look at the geologic time periods.



Students could access The Geology Wing, an online exhibit on the Museum of Paleontology website, and take a close look at the time periods making up the geologic time scale.



Students could complete the Sequencing Time activity in which they create a personal time line in an effort to better understand the geologic time line. This is most appropriate for students with minimal understanding of the geologic time scale.


Did you find this resource helpful?

Lesson Details

Grades Themes Project 2061 Benchmarks
AAAS Thinkfinity