Earth Space Science: 06 Geologic Time: 06.01 Geologic Time Scale

Geologic Time Scale Activity


Understanding geologic time is critical to relating events in Earth’s history to those of today. Scientists use several forms of the geologic time scale to classify the age of objects and to become familiar with other events occurring simultaneously. Choose one of the options listed in the procedures below to complete this assignment.


If you would like to create your geologic time scale by hand, you will need the materials listed.

Note: There are alternative procedures that do not require these materials.

Adding machine paper, or pieces of paper taped together to form a continuous line
ruler or meter stick
colored pencils, crayons, or markers
digital camera, mobile device, or laptop camera to take a photograph of your time scale


Option 1 Procedures with materials

Cut a piece of adding–machine tape approximately 4.6 meters long or attach pieces of paper in a continuous line using tape so it is approximately 4.6 meters long.
Tape the ends of your paper onto your table so it is easier to measure and label.
Label your paper “Geologic Time Scale” at the top left.
At the top right of your paper, include the following scale:
Starting from the left side of the paper, measure 5 centimeters to the right and draw a vertical line. Label the line “today”.
Plot each era, period, and important event on the paper using the scale provided and Table 1 below.
When you have completed your geologic time scale model, take an image of it and place it into this document for your instructor to view.

Option 2: Procedures without materials

There are many different types of models. A timeline is one way to represent the geologic time scale, but there are other ways as well.

Below Table 1, you have been provided with the geological time scale. The time scale is an approximation and is not to scale.
You will need to complete the scale by recording the significant biological and geological events. You will need to identify at least two biological and two geological events for each period. You may need to perform additional research to gather your information to complete the time scale.
After you have completed the time scale answer the Reflection Questions.

Reflection Questions

Do not answer until you have completed the time scale located on the page following Table 1.

How long (in centimeters and years) was the Precambrian period compared to the rest of the scale?
How many years and “centimeters” of time separated the dinosaurs and humans on Earth?
How many years and centimeters of time passed before life appeared on Earth?
For how many years of geologic time have humans existed?
Did dinosaurs exist at the same time as mammals? Explain your answer using the geologic time scale.
Table 1

Complete the total time column for the table below to assist with answering the reflection questions. The first one has been completed for you.

bya= billion years ago
mya= million years ago
1cm= 10 million years


Geologic Time Scale

Scientists estimate that Earth is 4.6 billion years old. They document significant events in Earth’s geologic past using the geologic time scale which is based on geologic evidence, careful observations, and the correlation of events relative to each other. The scale is subdivided into discrete time frames, based on the times at which the events occurred.

The following activity lists several significant events in Earth’s history. Use the timeline provided to predict when each event listed occurred on the geologic time scale.

Geologic Timeline — Text Version

The front of the card:

Think about what point on the geologic time scale the following events happened?
Timeline: 4.6-3.8 billion years ago, 3.5-3.0 billion years ago, 3.0-2.5 billion years ago, 146-100 million years ago. Events: An atmosphere with oxygen first formed on Earth. The first flowering plants on land appear. The first microscopic life forms appear. The first rocks are formed from volcanoes.

Show the back of the card:

Show Quiz

Record your predictions in your science notebook. By the time you reach the end of this lesson, you will have learned when each of the events in this activity happened, and you will have an opportunity to check your answers.

As you proceed through the lesson, you will learn the basic organization of the geologic time scale and key events that have shaped our understanding of the past.

How do scientists organize the major events of Earth’s history?
At the end of this lesson, you will be able to:
examine the differences in time between each era and period on the geologic time scale
describe the reasons for the organization of the scale
create a scale model of the geologic time scale
What is the Geologic Time Scale?
When speaking about Earth history, scientists refer to the past as the geologic past. The geologic past covers such a long period of time that it can seem impossible to grasp. To help organize the events of the geologic past, scientists represent large segments of time as periods on a geologic time scale.

Time PeriodsGeologic Time Scale

The geologic time scale is a model that represents segments of time in Earth’s past. Geologic time does not use such small lengths of time as seconds, minutes, months, or even years; it uses much longer time periods.

There are multiple ways to show the geologic time scale. Several examples are shown in the images below.

The first representation. See text version.
The second representation. See text version.
The third representation. See text version.
The third representation. See text version.
The third representation. See text version.
Select the images above to enlarge.

Although the geologic time scale can be graphically represented in different ways, scientists generally agree upon which significant events to include in the scale. You can get an idea of the lengths of time and sequence of events by compressing geologic time into a standard calendar year.

Use the calendar below to see how the geologic time scale would appear if it is compressed into a single year. A calendar year is divided into months, weeks, and days. Geologic time is measured in eons, which are divided into eras, periods, and epochs.

Geologic Time — Text Version
The front of the card:

January 1 represents the beginning of Earth, 4.6 billion years ago. December 31, at midnight, represents the present day. Try to determine where you think each event belongs on the time scale. Select the image to check your work.

January 1st at midnight, March 29th at 6:43 a.m., November 8th at 4:35 p.m., November 19th at 7:40 p.m., November 28th at 9:36 a.m., December 2nd at 3:45 a.m., December 13th at 8:37 p.m., December 19th at 4:01 p.m., December 26th at 7:12 p.m., December 31st at 11:50 p.m.

The oldest humanlike ancestors appear.
The oldest marine worms and jellyfish appear.
The first birds appear.
The first bacteria (according to fossil evidence) appear.
The first dinosaurs appear.
The first land animals (according to fossil evidence, millipedes) appear.
The first amphibians appear.
Earth begins.
The first fish appear.
Dinosaurs become extinct.

Show the back of the card:
Scientists developed the geologic time scale based on available data. One of the problems in developing the scale is the incompleteness of the fossil record. On Earth today, there are an estimated 10 million to 13 million species of plants, animals, insects, bacteria, and other living things. The vast majority of Earth’s species are extinct, however, and extinct organisms often do not show up in the fossil record.

Significant EventsPractice

We do not know precisely how many species have existed across all of the geologic time. Scientists estimate that modern living species account for perhaps one percent of all the species that ever lived. Many species became extinct during very short time periods. Some of these mark significant boundaries of the geologic time scale. These boundaries help us represent the vast periods of the geologic time scale in manageable segments.

The scale below shows the Hadean, the Archean, and the Proterozoic eons, followed by the current Phanerozoic eon, which is subdivided into eras, and further divided into periods. Explore each Eon, Era, and period in the geologic time scale.

Instructions: Select the step groups to read more. Then, select individual steps to read more. Numbers represent millions of years ago. * Note geologic time scales can be represented in order from most recent to past or from past to most recent.

Text Version for Geologic Past Interactive
Earth’s geologic past can be divided into eons, eras, and periods, as described below:

Precambrian: The earliest time in Earth’s history, the Precambrian, is the least understood. The Precambrian began approximately 4.6 billion years ago. It is by far the longest segment of Earth’s geologic past. The Precambrian consists of three eons: Hadean, Archaean, and Proterozoic.

Hadean: 4.6 to 3.8 billion years ago
The Hadean marks the very beginning of Earth history. Earth was hot and molten, later cooling to form a crust. Comets and asteroids constantly bombarded the Earth at this time.
Life during the Hadean: No life on Earth was present during the Hadean.
Geology of the Hadean: Volcanoes and molten rock were common. Any rocks that may have formed in this time would later have been destroyed.
Other events during the Hadean: The sun and all the planets in the solar system were forming at the same time.
Archaean: 3.8 to 2.5 billion years ago
Life during the Archaean: Bacteria have been found dating back to 3.5 billion years ago. This is the first time that life appeared on Earth, according to the fossil record.
Geology of the Archaean: Rocks began to cool enough for continents to form during the Archaean.
Other events during the Archaean: The atmosphere was very different than the atmosphere today. Photosynthetic organisms were present, which may have led to the formation of the modern atmosphere.
Proterozoic: 2.5 billion to 543 million years ago
Life on Earth began to flourish, though the forms of life were relatively simple compared to the complex organisms of today.
Life during the Proterozoic: The first organisms more complex than bacteria formed during the Proterozoic.
Geology of the Proterozoic: The first crustal plates, which would later lead to plate tectonics, began to take shape during the Proterozoic.
Other events during the Proterozoic: Photosynthetic organisms caused a buildup of oxygen in the primitive atmosphere.

Paleozoic Era: The Paleozoic Era had an astonishing diversity of life. Sometimes known as the “Age of Fish,” the Paleozoic was marked by numerous types of aquatic organisms. Many of these organisms had hard parts, such as shells and teeth, that are preserved in the fossil record. Many others, however, did not have hard parts to be preserved. Nevertheless, we know these organisms existed because they left behind other traces of their existence, such as imprints and burrows.

Cambrian: 543 to 490 million years ago
Life on Earth exploded into myriad new life forms.
Life during the Cambrian: Sponges, trilobites, worms, and other primitive sea creatures began to flourish in the newly formed oceans.
Geology of the Cambrian: Most of the landmasses present on Earth were concentrated in the Southern Hemisphere.
Other events during the Cambrian: The diversity of life that began in the Cambrian has been called the “Cambrian Explosion.”
Ordovician: 490 to 443 million years ago
Life continues to flourish during the Ordovician. Landmasses moved within regions in the Southern Hemisphere.
Life during the Ordovician: Early vertebrates, or animals with a spine, appeared in the world’s oceans.
Geology of the Ordovician: Much of the land on Earth was concentrated on one continent, known as Gondwana.
Other events during the Ordovician: At the end of the Ordovician, a mass extinction wiped out a large percentage of the species of plants and animals on Earth.
Silurian: 443 to 417 million years ago
The continued movement of landmasses on Earth contributed to changes in climate, and therefore to life on Earth.
Life during the Silurian: Coral reefs became widespread during the Silurian. The first true fish became common during the Silurian.
Geology of the Silurian: Many of the mountain chains common on Earth today started forming during the Silurian.
Other events during the Silurian: A warmer climate caused the melting of glaciers, and therefore a significant rise in sea level.
Devonian: 417 to 354 million years ago
Amphibians and fish dominated Devonian times.
Life during the Devonian: The first evidence of true plants, including ferns, occurred during the Devonian. Amphibians first appeared in the fossil record during this time as well.
Geology of the Devonian: North America and Europe were a part of a single landmass in the Northern Hemisphere. South America, Africa, and Antarctica were joined in another landmass in the Southern Hemisphere.
Other events during the Devonian: A variety of other life forms, including insects, first appeared during the Devonian.
Carboniferous: 354 to 290 million years ago
This time period is often broken down into two distinct parts: the Mississippian and the Pennsylvanian subperiods.
Life during the Carboniferous: Animals begin laying shelled eggs. The hard shells provided the young organisms greater protection, allowing them to develop in more complex ways and leading to a greater diversity of life on land.
Geology of the Carboniferous: The Appalachian Mountains formed as collisions of landmasses caused a “wrinkling” of Earth’s crust.
Other events during the Carboniferous: Much of the coal present today formed during the Carboniferous.
Permian: 290 to 245 million years ago
Life on Earth continued to flourish throughout the Permian, but the period soon led the way to the end of the Paleozoic Era.
Life during the Permian: Pine trees and other types of conifers appeared during the Permian.
Geology of the Permian: A supercontinent, known as Pangaea, formed. During this time, almost all of Earth’s landmasses were joined.
Other events during the Permian: The largest mass extinction in Earth’s geologic history happened at the end of the Permian.

Mesozoic Era: During the Mesozoic Era, some of the largest animals that ever lived roamed the planet. It may be fun to watch movies and read science fiction in which humans and dinosaurs existed together; however, tens of millions of years separated the end of the dinosaurs and the beginnings of humanlike life.

Triassic: 245 to 206 million years ago

As the world’s landmasses continued to shift, life on Earth slowly rebounded from the mass extinctions of the Permian period.
Life during the Triassic: More plant species began to flourish during the Triassic. Dinosaur-like animals became dominant on land.
Geology of the Triassic: The supercontinent Pangaea began to break apart during the Triassic, leading to a diversification of life on different continental regions.
Other events during the Triassic: Mammals, though small at the time, first appeared during the Triassic.
Jurassic: 206 to 144 million years ago
Known as the “Age of the Dinosaurs” and popularized by science-fiction movies, the Jurassic contained some of the largest life forms ever to exist on Earth.
Life during the Jurassic: The largest dinosaurs appear during the Jurassic. The land and the oceans were filled with life.
Geology of the Jurassic: The continents began to move toward their present-day positions during the Jurassic. The Atlantic Ocean began to open at this time.
Other events during the Jurassic: Much of the oil today used as fuel began to form during the Jurassic period.
Cretaceous: 144 to 65 million years ago
The last period of the Mesozoic era was when many species of dinosaurs became extinct, and mammals began to be more common on Earth.
Life during the Cretaceous: Flowering plants, more modern mammals, and many types of birds evolved during the Cretaceous.
Geology of the Cretaceous: The continents continue to move toward their present-day positions.
Other events during the Cretaceous: Global climate began to cool, leading to seasonal changes in weather and in vegetation present on land.

Cenozoic Era: The Cenozoic is the era you live in today, though the plants and animals have significantly changed over time. Often known as the “Age of Mammals,” the Tertiary and Quaternary periods were a time of transition and change for global climate and for life on Earth.

Tertiary: 65 to 1.8 million years ago

Life during the Tertiary: Horses, which were initially very small compared to their modern equivalents, and other grass-grazing animals became common. Many other mammals were very large, including several species of rodents.
Geology of the Tertiary: The Himalayan Mountains formed when India collided with Southern Asia. In the Western Hemisphere, Earth’s moving plates also formed the Rocky Mountains in North America and the Andes in South America.
Other events during the Tertiary: Though the climate was initially warm, changing climate patterns caused regions to cool significantly by the end of the Tertiary period.
Quaternary: 1.8 million years ago to today
Life during the Quaternary: Saber-toothed tigers, early humanlike ancestors, and woolly mammoths were among the common creatures in North America.
Geology of the Quaternary: Massive sheets of ice covered much of the northern and southern regions of Earth. The Quaternary contains what is now known as the last Ice Age. Most of the ice retreated toward the poles about 11,000 years ago.
Other events during the Quaternary: The continents arrived at their recognizable, present-day positions at the beginning of the Quaternary.

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