MiddleSchoolPortal/Geologic Time: Eons, Eras, and Epochs

From NSDLWiki

<keywords content="middle school geology teacher content knowledge billion million earth space science scientific notation numerical literacy standards inquiry lessons methods conceptions" />

<metadescription content="This free, standards-based, online publication, developed for middle school science teachers, explores the eons, eras and epochs of geologic time by linking to and describing inquiry-based lessons and activities" />

Geologic Time: Eons, Eras and Epochs - Introduction

How old is the universe? How old is our sun? How old is the earth? When did life first appear on the earth? How long have Homo sapiens inhabited the earth? The answers to most of these questions differ by billions of years. However, to many middle school students, these answers boil down to variation on a single theme: really old, a long time ago, and for a long time. Ask your colleague teaching history: She or he can tell you students have difficulty conceptualizing and contrasting the different historical eras, such as the medieval period in Europe and the colonial period in America. Thus, it is no surprise that geologic time is difficult for students of all ages to conceptualize.

The Earth and Space Science content standards, grades 5-8, of the National Science Education Standards include: Structure of the earth system; Earth's history; and Earth in the solar system; none of which can be mastered without good conceptual understanding of geologic time. In order to gain that understanding, students need numerical literacy, including both comprehension of scientific notation and an accurate concept of the difference between a million and a billion. Additionally, knowledge of the nature of science in general, and geologic science in particular, is needed.



Not a geology major or mathematician? No worries! This publication contains resources designed to do three things. The first is to complement teacher content knowledge and its relationship to the nature of geologic science. Geology is not a laboratory-based science lending itself to traditional notions of controlled experiments; rather it is a historical science requiring different methods. Second, we supply teachers with hands-on/minds-on lessons to develop student understanding, and third, we provide career-oriented resources to expose students to scientists whose work involves concepts in geologic time.

Background Information for Teachers

Understanding geologic time is a sub-concept of earth science. The resources in this section are thus either a subdivision of a larger earth science resource or the larger resource within which geologic time is addressed. Parts of some of these resources are appropriate for students, while others are more appropriate for teachers or more advanced learners only. There are links to texts, graphics, interactive presentations, and online courses for teachers.

The first resource is the NSDL Strand Map Service. These maps illustrate connections between concepts and across grade levels. Use them to contextualize and sequence your instruction. Geologic Time is captured in at least two different areas. One is under the Common Themes of science--Scale. The other is under the Physical Setting--Changes in the Earth's Surface which includes rates of change. An image of the middle grades (6-8) only part of the Changes in the Earth's Surface map appears below. Clicking on a concept within the maps will show NSDL resources relevant to the concept, as well as information about related AAAS Project 2061 Benchmarks and National Science Education Standards. Move the pink box in the lower right hand corner of the page to see the grades 6-8 learning goals.

Changes in the Earth's Surface

View individual map Printable view of map

The Big Ideas From the many facts, theories, and questions found in earth and space science, scientists and educators have identified a certain small set as the "big ideas" that organize the intellectual domain. This project synthesized these ideas into a compact and defensible list. The list of 25 concepts is divided into seven topic areas, including the Nature of Earth Science, which contains items 21-25.

Geologic Time Scale This information about geologic time and the geologic time scale defines the terms relative time (chronostratic) and absolute time (chronometric). Relative time can be thought of as the subdivisions of rock found in the earth’s stratigraphy and absolute time as the measurements taken of rock types to determine the actual time that has expired. Absolute time measurements can be used to calibrate the relative time scale, producing an integrated geologic or geochronologic time scale. The site contains a multicolor time scale and references for more information. The text is appropriate as teacher background knowledge; the graphic is appropriate for students assuming some discussion and explanation accompany it.

Once in a Million Years: Teaching Geologic Time This article outlines pedagogical approaches to teaching geologic time and describes common student preconceptions and misconceptions. Several activities will assist students in conceptual change.

Geologic Time: Online Edition An online edition of a general interest publication from the U. S. Geological Survey gives an overview of the concepts associated with the age of the earth. Section headers are: Geologic Time, Relative Time Scale, Major Divisions of Geologic Time, Index Fossils, Radiometric Time Scale, and Age of the Earth.

Fossils, Rocks, and Time This online book, published by the U. S. Geological Survey, discusses the use of fossils in determining the age of rocks. The book covers how to place events in correct temporal order, a description of the geologic time scale, the use of fossils to indicate rock ages, the law of fossil succession, index fossils, and radioactive dating.

A Formative Assessment of Geologic Time for High School Earth Science Students Earth science courses typically include the concept of geological time. The authors of this study attempt to move past traditional assessment practices and develop a formative assessment of students’ understanding of the construction of the geologic time scale and how it is interpreted. Through this approach, students are challenged to conceptualize the geologic time scale by comparing it to a student-produced time scale for an older adult’s life. This formative assessment allows the teacher to alter instruction based on students’ feedback.

Lessons and Activities

As in the Background Information section, activities specific to the geologic time scale are often embedded in larger earth science units. Thus, parts of some of these resources will overlap with concepts not apparently tied to the geologic time scale directly. However, if we keep in mind the vastness and abstract quality of geologic time, it is not hard to see how a grasp of geologic time facilitates understanding all other earth science concepts as well as evolutionary theory. Recall the article Once in a Million Years from the Background Information section. We will not repeat the resource here, but it also contains activity ideas and links to more resources.

Geologic Time: The History of Earth This interactive tutorial provides students with an overview of earth’s history and its relation to geologic time. Topics include the age of the earth, the use of time lines, and the concepts of relative and actual age. Once these topics have been covered, their applications to rocks and fossils are explained through the concepts of superposition (oldest rocks on the bottom), the use of fossils to determine relative age, and the use of radiometric dating to determine absolute age. There is also an interactive geologic time scale where students can find descriptions of what the earth was like by clicking on the eons, eras, or periods.

Who's on First? A Relative Dating Activity In this activity, students are introduced to sequencing and geologic time through relative dating techniques. Students begin by categorizing cards of nonsense words, then move on to cards with pictures of fossils. Once students begin to grasp relative dating, they can extend their knowledge of geologic time by exploring radiometric dating and developing a time line of earth’s history. There is a teacher’s guide to this activity with background information and templates to use for teaching about relative dating. There are also objectives, materials, procedures, and questions.

A Geological Time Spiral A graphic representation of the past 4.5 billion years is in PDF and downloadable. See USGS Fact Sheet 2007-3015 for ages of geologic time periods. Ages in the spiral have been rounded from the age estimates in the fact sheet.

Sequencing Time In this activity, students gain an understanding of relative and numerical time by placing events in sequence and assigning relative times to the events. This will familiarize students with the methods used by scientists to develop the geologic time scale. The activity contains objectives, materials, procedure, and extensions.

Evolution Animation This interactive site can be investigated by students independently as enrichment to their study of geologic time. The length of the time line helps reinforce the idea of the immense age of the universe. Flash animation provides a tour of the history of the universe, the solar system, and earth. Moving the slider allows viewers to progress from the Big Bang, almost 14 billion years ago, to the beginnings of life on earth in the Proterozoic era, through the age of the dinosaurs, and finally to the time of Homo sapiens. When the slider stops moving, animations and text appear, highlighting important events. Other animations accompany the time scale and show the movements of the continents, the advance and retreat of the polar ice caps, and changes in the oxygen content of the atmosphere. A French translation is available.

Date a Rock! This lesson shows students that age dating of rocks involves counting atoms and comparing the counts. Students use simulated rock samples, which show a highly magnified selection of 128 atoms. Each sample has a different proportion of the atoms of two different elements: a parent radioisotope and its daughter product. By counting the parent radioactive atoms and knowing the half-life of those atoms, students can figure the number of half-lives since the sample solidified, and therefore the age of the sample.

Scientists: Their Science and Geologic Time

Here we highlight some career fields associated with geologic time: geology, paleontology, astronomy, and cosmology. However, these resources do more than just list what each career requires in terms of education. Geology, for example, is presented in an ask-an-expert format, astronomy considers the experience of early female astronomers, and cosmology is discussed in a biographical sketch of Edwin Hubble, all connecting to the History and Nature of Science Content Standard.

USGS and Science Education In its science education resources, the U.S. Geological Survey (USGS) provides information about geology-related careers. Click on Careers in Science to find a link to Scientists in Action! The latter contains several links, including Geologists in the Park with photographs of college geology students and recent graduates, accompanied by a short narrative of their work and quotes from the persons featured.

Frequently Asked Questions: What is Paleontology? This site asks and answers questions about paleontology, fossils, and dinosaurs. Paleontology questions are: What is paleontology? How does paleontology differ from anthropology and archaeology? What are the practical uses of paleontology? How do paleontologists know how old their fossils are? What training is necessary to become a paleontologist? What organizations exist for paleontologists?

Looking at the Sky Through a Glass Ceiling: Women in Astronomy This article looks at the involvement and acceptance of women in astronomy, including three significant female astronomers from the 1900s and three from the present day. Statistics are also provided concerning contemporary women's participation in astronomy. The three astronomers featured from the 1900s are Henrietta Swan Leavitt, Cecilia Payne-Gaposchkin, and Jocelyn Bell Burnell. Each woman's most notable contribution to astronomy is described. The three featured contemporary astronomers participate in the Hubble Space Telescope project. They each supplied paragraph-long answers to the question: What is it like to be a woman in astronomy today? For five of the six featured astronomers, a link connects to a fuller description of the astronomer's work.

Edwin Hubble: The Great Synthesizer: Revealing the Breadth and Birth of the Universe This biographical article explains why Edwin Hubble is the appropriate namesake for the Hubble Space Telescope. The article recounts how Hubble made two extraordinary discoveries in the early 1900s that revolutionized science's understanding of the universe. Hubble first discovered that our galaxy, the Milky Way, is not the whole of the universe. He then discerned that the universe is expanding. This expansion is now known as Hubble's law, and the discovery paved the way for the development of the Big Bang theory and modern cosmology.

Latest Science News from the New York Times

NYT > Paleontology

News about Paleontology, including commentary and archival articles published in The New York Times.

SMARTR: Virtual Learning Experiences for Students

Visit our student site SMARTR to find related science-focused virtual learning experiences for your students! The SMARTR learning experiences were designed both for and by middle school aged students. Students from around the country participated in every stage of SMARTR’s development and each of the learning experiences includes multimedia content including videos, simulations, games and virtual activities.


The FunWorks Visit the FunWorks STEM career website to learn more about a variety of science-related careers (click on the Science link at the bottom of the home page).

National Science Education Standards

The references, lessons and activities provided in this publication align with the following content standards from the National Science Education Standards.

Science as Inquiry: Content Standard A

As a result of their activities in grades 5-8, all students should develop

Abilities necessary to do scientific inquiry

  • Develop descriptions, explanations, predictions, and models using evidence. Students should base their explanation on what they observed, and as they develop cognitive skills, they should be able to differentiate explanation from description--providing causes for effects and establishing relationships based on evidence and logical argument. This standard requires a subject matter knowledge base so the students can effectively conduct investigations, because developing explanations establishes connections between the content of science and the contexts within which students develop new knowledge.
  • Think critically and logically to make the relationships between evidence and explanations. Thinking critically about evidence includes deciding what evidence should be used and accounting for anomalous data. Specifically, students should be able to review data from a simple experiment, summarize the data, and form a logical argument about the cause-and-effect relationships in the experiment. Students should begin to state some explanations in terms of the relationship between two or more variables.
  • Recognize and analyze alternative explanations and predictions. Students should develop the ability to listen to and respect the explanations proposed by other students. They should remain open to and acknowledge different ideas and explanations, be able to accept the skepticism of others, and consider alternative explanations.
  • Use mathematics in all aspects of scientific inquiry. Mathematics is essential to asking and answering questions about the natural world. Mathematics can be used to ask questions; to gather, organize, and present data; and to structure convincing explanations.

Understandings about scientific inquiry

  • Different kinds of questions suggest different kinds of scientific investigations. Some investigations involve observing and describing objects, organisms, or events; some involve collecting specimens; some involve experiments; some involve seeking more information; some involve discovery of new objects and phenomena; and some involve making models.
  • Current scientific knowledge and understanding guide scientific investigations. Different scientific domains employ different methods, core theories, and standards to advance scientific knowledge and understanding.
  • Mathematics is important in all aspects of scientific inquiry.
  • Technology used to gather data enhances accuracy and allows scientists to analyze and quantify results of investigations.

Scientific explanations emphasize evidence, have logically consistent arguments, and use scientific principles, models, and theories. The scientific community accepts and uses such explanations until displaced by better scientific ones. When such displacement occurs, science advances.

  • Scientific investigations sometimes result in new ideas and phenomena for study, generate new methods or procedures for an investigation, or develop new technologies to improve the collection of data. All of these results can lead to new investigations.

Life Science: Content Standard C

As a result of their activities in grades 5-8, all students should develop understanding of

Diversity and Adaptations of Organisms

  • Biological evolution accounts for the diversity of species developed through gradual processes over many generations. Species acquire many of their unique characteristics through biological adaptation, which involves the selection of naturally occurring variations in populations. Biological adaptations include changes in structures, behaviors, or physiology that enhance survival and reproductive success in a particular environment.
  • Biological evolution accounts for the diversity of species developed through gradual processes over many generations. Species acquire many of their unique characteristics through biological adaptation, which involves the selection of naturally occurring variations in populations. Biological adaptations include changes in structures, behaviors, or physiology that enhance survival and reproductive success in a particular environment.

Earth and Space Science: Content Standard D

As a result of their activities in grades 5-8, all students should develop an understanding of

Science as a human endeavor

  • Women and men of various social and ethnic backgrounds--and with diverse interests, talents, qualities, and motivations--engage in the activities of science, engineering, and related fields such as the health professions. Some scientists work in teams, and some work alone, but all communicate extensively with others

Nature of science

  • Scientists formulate and test their explanations of nature using observation, experiments, and theoretical and mathematical models. Although all scientific ideas are tentative and subject to change and improvement in principle, for most major ideas in science, there is much experimental and observational confirmation. Those ideas are not likely to change greatly in the future. Scientists do and have changed their ideas about nature when they encounter new experimental evidence that does not match their existing explanations.

History of science

  • Many individuals have contributed to the traditions of science. Studying some of these individuals provides further understanding of scientific inquiry, science as a human endeavor, the nature of science, and the relationships between science and society.
  • Tracing the history of science can show how difficult it was for scientific innovators to break through the accepted ideas of their time to reach the conclusions that we currently take for granted.

Read the entire National Science Education Standards online for free or register to download the free PDF. The content standards are found in Chapter 6.

Author and Copyright

Mary LeFever is a resource specialist for the Middle School Portal 2: Math & Science Pathways project, a doctoral candidate in science education at Ohio State University, and presently teaches introductory biology at a Columbus, Ohio local high school. She has taught middle school and high school science and is an adjunct instructor of biology and natural sciences at Columbus State Community College.

Please email any comments to

Connect with colleagues at our social network for middle school math and science teachers at

Copyright September 2007 - The Ohio State University. Last updated August 22, 2010. This material is based upon work supported by the National Science Foundation under Grant No. 0424671 and since September 1, 2009 Grant No. 0840824. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.