The concept of geologic time is often difficult for students to comprehend. To a thirteen-year-old, ten years is a long time. Millions or billions of years is just another long time, longer than ten years, but how much longer is meaningless. To give students a feeling for geologic time, it needs to be seen relative to a time they understand. For that purpose, a timeline can be constructed. This exercise is designed to show the entire history of the Earth and of Connecticut in a single timeline. A football field or some other long, straight space can be used to construct the time line.
Give the students the list of ages of geologic events. Assign each of them an event. Ask them to make signs on pieces of poster board for their event. Then glue sticks to the two sides of the boards so that the tops of the sticks are even with the tops of the signs. If you want the signs to last long enough for the rest of the school to see the timeline, cover the signs with plastic bags taped in place. Using books as references, they should then make a poster depicting what a landscape might have looked like at that time. The finished posters can be posted around the room in chronological order, to create a scenic time line.
Next, have three students measure the length of the field or other
space to be used for the timeline. If there is no long field, or
the weather is too bad, a long hallway inside the school can be used, but
some of the more recent events will be too close together to include all
of them. Calculate the average to use as the length. They can
calculate the location of the different events along the timeline, then
place their signs in the appropriate places.
The Earth is very old – 4,600,000,000 years old. That is 4.6 billion years. But a billion years is difficult to talk about and even more difficult to imagine. Because of the long time periods involved when talking about events in the far past, geologists talk in geologic time. Geologic time means a veeery long time. To try to picture what a billion years is like, or even a million years, you can make a timeline. Along a timeline you can see when events in the Earth’s geologic history happened in relation to other events.
Table 1. Important Dates in CT geologic history
MYA = million years ago
4600 MYA Beginning of Earth
3600 MYA Blue-green algae and stromatolites
2500 MYA Primitive aquatic plants
750 MYA Assembly of supercontinent, called "Rodinia"
630 MYA Rifting of Rodinia, begin to open Iapetus Ocean; proto-North
terrane in Connecticut is along shore with offshore reefs
670 MYA Oldest fossil evidence of marine worms and jellyfish
570 MYA Trilobites dominant
550 MYA Iapetus Ocean begins to close, island arc begins to form
offshore of Connecticut,
behind subduction zone
500 MYA First fish
435 MYA Early plants & animals on land
370 MYA First amphibians
360 MYA Sharks & large primitive trees
330 MYA Fern forests, abundant insects, first reptiles
250 MYA Pangea complete. Some of Iapetos ocean floor and island
arc added to Connecticut between
proto-North America and Avalonia. Largest known mass extinction (90% of all species disappear).
240 MYA Coniferous trees abundant
228 MYA First dinosaurs (about the size of cats)
205 MYA First birds & mammals, abundant dinosaurs
200 MYA Basalts begin erupting in central Connecticut as Pangea
is breaking up and Atlantic
Ocean will soon begin to form. Large mass extinction (50-60% of all species).
140 MYA Flowering plants
65 MYA Mass extinction. Comet or meteor collides with the Earth, and most animals on land and sea weighing more that 3-kg perish. Or the extinctions may have been caused by large volcanic eruptions, which darkened the skies for years, cooling the climate suddenly. End of dinosaurs.
4.5 MYA First humans appear on Earth
26,000 years ago Last glacier begins to cover Connecticut
15,000 years ago Glacier completely gone from Connecticut
1. To see how long 1 billion seconds is, figure out how many years equal
1 billion seconds.
Begin by calculating the number of seconds in a year = _____________. Then divide 1
billion seconds by that number. The number of years = _________________.
2. Make a sign with the age of a geologic event. At the top of the sign put the number of years ago the event happened, such as 240 Million Years Ago. Below that, write the event – Coniferous Trees Abundant.
3. Using classroom books as references, make a poster of what the scenery might have looked like at the time of your event. Print the time on the bottom of the poster, as you printed it on your sign. These can then be placed around the classroom walls, in time order, as a pictorial timeline.
4. Three students from the class should measure the length of the space where a timeline will be constructed. The length is _______ meters.
5. Multiply the length in meters by the number of centimeters in a meter. The length in centimeters is _______ centimeters.
6. To determine the number of centimeters needed to represent 1 million years, divide the length of the timeline in centimeters by the age of the Earth (in millions of years) = ___________ cm/million years.
7. Age of your event = ________ million years. How many centimeters on the timeline is this?
___________ cm or _________ meters
8. Measure the distance along the timeline representing your event, starting from the present end. Place your sign there.
9. Divide the number centimeters representing 1 million years by 1 million
to find out what part of a centimeter represents 1 year = _______ cm.
Now multiply that number by your age. Your age is represented by _____
cm. In relation to the history of the Earth, the number of
years since you were born is too small to represent on the timeline.
Geologic history is veeeery long!