6. Geologic Timelines

For students, a few days can feel like a very long time. Thus, my students have a hard time conceptualizing the difference between 1 thousand, 1 million and 1 billion years. In this activity, students develop a sense of just how long the geologic time scale really is by creating a to-scale geologic timeline. This lesson begins with students guessing how long ago different events happened – when the Earth was formed, when life began, when dinosaurs roamed, and when humans first appeared. Then students redraw the periods and eras of the Phanerozoic Eon to scale using adding machine tape (1 million years = 1 millimeter). Then a teacher created scroll containing the other eons: Proterozoic, Archean and Hadean is unrolled to give students a visual sense of just how long Earth history really is. Finally, there are some analogies for students to contemplate, such as when different events would have occurred if Earth history were condensed into a calendar year or into a cross country trip.

Can read information from a Geologic Time Scale
Recognize that many changes in biodiversity have occurred since life evolved on Earth
Can describe the major forms of life in each eon and in each era of the Phanerozoic Eon
Develop a sense of the vastness of geologic time

Geologic time
Geologic time scale
Phanerozoic Eon
Proterozoic Eon
Archean Eon
Hadean Eon
Paleozoic Era
Mesozoic Era
Cenozoic Era

Attachment Size
geologic_time_handout.doc 49 KB
6geologic_timelines_v2.doc 68.5 KB

6. Geologic Timelines - Logistics

10 minutes guess where events occurred and discussion
30 minutes create timescales
5 minutes add Pre-Cambrian eons to the timeline
5 minutes discuss analogies



  • Copies of a geologic time scale (download the attachment on the bottom of the summary page, see the sources, or create your own)
  • Adding machine paper, at least 60 cm per student and an additional 5 meters for the class
  • Meter sticks or rulers, enough for 1 per student
  • Optional: Colored pencils
  • Optional: Calendar or map of the United States


6. Geologic Timelines - Background

Teacher Background
The major goal in this activity is for students to gain a sense of scale and a general feel for the vastness of geologic time. An understanding of the major biodiversity changes in the various eons and eras is secondary. The timelines students create use a 1 million years to 1 millimeter scale, resulting in a Phanerozoic eon that is 54 cm long and a geologic time scale that is 4.6 meters long. Students create only the more interesting Phanerozoic timelines while the full timeline is unrolled for dramatic effect near the end of the period.

The geologic time scale is a tool used by geologists to break up the history of Earth (all 4.6 billion years of it) into chunks that are more manageable. These divisions are determined by the major changes in biodiversity (and therefore the appearances of some fossils and the disappearance of others) that occurred throughout time. As new information is discovered, the geologic time scale is refined to reflect these new discoveries. The dates given in the handout are derived from a 2004 revision of the time scale endorsed by the International Commission on Stratigraphy. Several changes such as the phasing out of the Tertiary/Quaternary periods in favor of a Paleogene/Neogene division is included here.

The largest divisions are eons that define the most major developments in Earth’s history. The most ancient eon, the Hadean (from 4.6 – 3.8 billion years ago), finds the Earth as it coalesces and cools into a more stable planet. The first life appears in the form of ancient bacteria during the second eon, the Archean (from 3.8 – 2.5 billion years ago). Gradually, more complex eukaryotic life including algae and the first multicelluar organisms evolve during the third eon, the Proterozoic (from 2.5 – 0.54 billion years ago). These early photosynthetic organisms produced oxygen that accumulated in the Earth’s atmosphere. Finally, there is the Phanerozoic eon (from 542 million years ago to the present) encompassing the evolution of most complex life on Earth.

Eons are divided into eras that are in the range of hundreds of millions of years long. The major eras of the Pharnerozoic include the Paleozoic (from 542 – 251 million years ago), the Mesozoic (from 251 – 65 million years ago) and the Cenozoic (from 65 million years ago to the present). These can broadly be described as the “Age of Fish and Amphibians”, the “Age of the Dinosaurs”, and the “Age of Mammals”. Each era ended with a major extinction event. Following eras, the divisions of time are called: periods, epochs, and ages.

Student Prerequisites

Familiarity with the idea of relative and absolute time. Familiarity with the organization of the geologic time scale into eons, eras and periods is helpful.

6. Geologic Timelines - Getting Ready

Getting Ready

  1. Make photocopies of the Geologic Time Scale.
  2. Cut adding tape into 54 cm long strips, enough for one per student with a few extras for mess-ups.
  3. Cut one strip of adding tape 4.6 meters long to show all 4 eons. On this strip, measure off the first 54 cm to represent the Phanerozoic eon, the next 2 meters to represent the Paleozoic eon, the next 1.3 meters to represent the Archean eon, and the last 80 cm represents the Hadean eon. Label each section accordingly. Roll the strip up again so that the most recent events are on the outside of the roll (start rolling from the Hadean eon end) and secure with a rubber band or paper clip.
  4. On the board, draw a line across its entire length. Label the left end “The Earth and solar system form”. Label the right end “Today”. Below this write “Draw an arrow on the timeline where you think dinosaurs roamed the Earth. Draw a dot on the timeline where you think humans first appeared.”
  5. Measure the line across the board and approximate where the Mesozoic era would be (65-250 mya = dinosaurs) and where the Neogene period began (23 mya = first hominids). Don’t label these places, just remember where they are approximately for later in the class.

6. Geologic Timelines - Lesson Plan

Lesson Plan

  1. As students trickle into class, direct them to the front of the room and have them add an arrow or dot to the timeline on the board. (If this format is too chaotic for your classroom situation, add labels and tick marks every 200 million years or so and have students write down their numerical guesses on a sheet of paper in their seats.)
  2. When all students have made a guess, briefly discuss their hypotheses. In particular, ask them about their reasoning behind their guesses.
  3. Distribute the geologic time scale handouts. Give students a moment to see if they can determine how many years ago the Earth and solar system formed (4,570 million or 4.57 billion years ago). Allow the student that first discovers this information to explain how s/he figured out the answer. Point out how the geologic time scale has the most recent events on the top and the most ancient events on the bottom, just like rock layers.
  4. Ask students to find out when dinosaurs roamed the Earth. This time, ask them to tell you not only how many million years ago but also during what eon, era, and periods dinosaurs lived. Again, allow the student who discover this information to share how s/he read the answer. Point out how the eon, era and period can be determined by the first 3 columns in the table.
  5. Ask students to find out when the first humans appeared. Is this a long time ago compared to the entire history of Earth or a short time?
  6. Give students a brief overview of the 4 major eons and the types of life that were present in each. Refer to the geologic time scale frequently to reinforce how to read information from the table.
  7. Ask students which of these eons was the longest? Which was the shortest? How do you know? Point out that the timescale is not broken into even chunks of time. That some eons are longer than others, just as some eras are longer than others and some periods are longer than others. Thus, while the geologic time scale is good for finding information, it’s not an accurate way to visually see how long one chunk is compared to another.
  8. Tell students that they will now create a timeline representing the most recent eon, the Phanerozoic. On the board write “1 millimeter = 1 million years; 1 centimeter = 10 million years”. Show students a strip of adding machine paper and hold a meter stick up next to it. Model for them how to keep the meter stick in the same place, with one end of the paper aligned with the 0 mark, and how to mark off boundaries between the eras and periods and label each mark with the number of millions of years ago that mark represents.
  9. Give students their own adding machine tape and a meter stick. Remind them with your own set up how to mark off the boundaries between the periods using the meter stick and their time scales. Once they have the general idea, wander around the classroom helping individuals that are having difficulty.
  10. Once the time boundaries are marked off, have students identify each period and write in the name of the period and one of the major biological events that happened in that period.
  11. If you have time, have students color their time scales. I had my students color each of the eras a different color to visually link the periods in the same era together. Alternatively, you could color each period a different color.
  12. When students are done or 10 minutes before the end of class, have a student that is finished hold up their timeline. Tell students that you created a timeline that includes all of Earth history, from when the Earth and solar system formed until today. Match the “today” end of your rolled up timeline with the “today” end of your student’s timeline. Unroll yours behind or below your student’s. When you reach the end of the Paleozoic, have a student come up to hold that section and briefly review the major events in that eon. Continue unrolling and stop again at the end of the Archaen eon to have a student hold that section and to review the major events. Finish in the same fashion with the Hadean eon.
  13. With the remaining time, give students one or two analogies as “food for thought” to depart class with.

Geologic time as a calendar year:

  • Imagine that the geologic time scale is contained in 1 calendar year (each second is around 146 years). January 1 is 4.6 billion years ago.
  • The oldest known rocks are formed in early March
  • The first forms of life (bacteria and algae) are preserved as fossils in late March
  • The first multi-celled creatures (seaweed) appear on September 3
  • Phanerozoic Eon (most recent eon) begins on November 11
  • Reptiles appear on December 5
  • The first mammals appear December 14
  • Dinosaurs go extinct December 26
  • The first hominids (human-like ancestors) appear at 5 PM on December 31 (7 hours before midnight)
  • The first modern humans (Homo sapiens) appear at 11:48 PM on December 31 (12 minutes to midnight)
  • The last glacier receded at 11:58:45 p.m. on December 31 (1 minute and 15 seconds before midnight)
  • Written history begins at 11:59:30 on December 31 (30 seconds before midnight)
  • Columbus lands in the Americas 3 seconds before midnight
  • You were born 1/10th of a second before midnight

Geologic time as the distance from Los Angeles to New York City:

  • The distance from LA to NYC is approximately 4500 kilometers (km). Therefore, each kilometer represents one million years of Earth history. Look at a map of the United States. Start the geologic time journey in LA.
  • Precambrian would last until Pittsburgh
  • Paleozoic would be entirely in Pennsylvania
  • Mesozoic would get us to New Jersey, only 66 km from NYC.
  • The most recent Ice Age ended about 10,000 years ago. 10,000 years = 1/100 of a million years = 10 meters.
  • The past 2000 years of history would be represented by a sidewalk (2 meters)
  • A human lifespan would be half of the width of a curb (100 millimeters)


6. Geologic Timelines - Assessment


  1. This is a good time for a short quiz on the geologic time scale. Have students use their geologic time scales to find information. For example:
    • The __________________________ is an outline of the major events in the earth's history. The largest grouping of time is a __________________. The next largest grouping of time is a __________________.
    • Describe the forms of life that were around 190 million years ago.
    • The first mammals appeared during the _________________ period in the __________________ era and ___________________eon.
  2. Watch the movie “Ice Age” and critique the scientific inaccuracies in it using your geologic time scale.

Going Further

  1. The Kentucky Geological Survey provides a fantastic list of ideas for how to modify this activity to create timeline showing the geologic time scale down a hallway, using calendars, across a school gym, using toilet paper, etc.
  2. Study fossils. Learn about how fossils form (see Fossil Adventure lesson). Go on a archaeological dig. Go to a museum with fossils. Study the Mesozoic era and the rise and fall of dinosaurs (kids love dinosaurs).

6. Geologic Timelines - Sources and Standards

The geologic time scale used in this lesson was based on information from the International Commission on Stratigraphy and on Wikipedia.

The lesson itself was inspired by several other similar lessons including one by Judy Scotchmoor called “What Came First?” and a series of lessons by the Kentucky Geological Survey called It’s About Time.

If you don’t like the style of the Geologic Time Scale provided here, try these:

  • From the UC Museum of Paleontology. Definitely check out their geologic time machine where you can explore each period, era and eon in greater detail.
  • From the Geological Society of America. This version has the greatest level of detail I have seen.
  • From the British Geological Society.
  • From the Association of Professional Geoscientists of Ontario.
  • From geology.com.

For more information on the geologic time scale, see:

  • The USGS provides a great, short overview of geologic time.
  • So does the Kentucky Geological Survey.

Grade 7
Earth and Life History (Earth Sciences)
Evidence from rocks allows us to understand the evolution of life on Earth. As a basis for understanding this concept:
a.    Students know Earth processes today are similar to those that occurred in the past and slow geologic processes have large cumulative effects over long periods of time.
b.    Students know the history of life on Earth has been disrupted by major catastrophic events, such as major volcanic eruptions or the impacts of asteroids.
d.    Students know that evidence from geologic layers and radioactive dating indicates Earth is approximately 4.6 billion years old and that life on this planet has existed for more than 3 billion years.
e.    Students know fossils provide evidence of how life and environmental conditions have changed.
g.    Students know how to explain significant developments and extinctions of plant and animal life on the geologic time scale.