Geologic Time Scale

Angelica saw an example of the geologic time scale in her book while studying the history of Earth. She is interested in learning more about the geologic time scale, including its purpose. What is the purpose of the geologic time scale?

• to organize the eras and periods of major events of fossils

• to organize the kinds of plants and animals that lived during those times on Earth

• to organize the kinds of rocks in order for people to study the life forms and classify fossils

• to organize Earth's history in a record of geologic events and show the evolution of life forms

Answer explanation

The purpose of the geologic time scale is to organize Earth’s history in a record of geologic events and to show the evolution of life forms in a timeline. The geologic time scale does use eras and periods for organization, but it includes more than fossils. The scale also organizes plants and animals that lived then, but it includes fossils and rocks that are organized. Rocks are organized on the time scale, but it is not only to study fossils and life forms.

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NGSS.MS-ESS1-4

Robert looked at a timeline showing the geologic time scale. He was surprised at the amount of information included in it and wondered what scientists used to create it. What evidence did scientists use to develop the geologic time scale? Choose the three that apply.

• collected rocks

• unconformities

• radioactive dating

• eras and periods

• worldwide rock layers and index fossils

Answer explanation

Scientists collected evidence from unconformities to determine when rocks were laid down in layers, and they compared rocks and index fossils worldwide to organize the scale. Radiometric dating later provided more evidence of the order of the timeline. While rocks were studied, collecting rocks was not a critical part for determining ages of rocks unless they could be compared with rocks from all over Earth. Eras and periods are part of the geologic time scale, not evidence found to make it.

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NGSS.MS-ESS1-4

Ana studied this diagram of the geologic timeline to complete her homework assignment. Ana became curious about the different divisions that formed the sections of an era. What is the name for the groups into which eras are divided?

• sections

• periods

• divisions

• Precambrian

Answer explanation

The correct answer is “periods.” This is the name for eras when they are divided into smaller sections of time. The periods are divisions, but they are not called that. Precambrian is an era, not a period. 

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NGSS.MS-ESS1-4

Ana studied this diagram of the geologic timeline to complete her homework assignment. She then began looking at fossils and categorizing them into the different eras of the geologic timeline. A fossil of a fish called Xiphactinus lived around 75 million years ago. During which part of the Mesozoic Era did it live?

• Triassic

• Jurassic

• Cretaceous

Answer explanation

The correct answer is “Cretaceous.” Because the fossil was determined to live around 75 million years ago, it fits into the Cretaceous period in the Mesozoic era. The Cretaceous period was 79 million years ago, so the fossil is in that time period. The Triassic period was 51 million years ago, so “75” doesn’t fit in that period. The Jurassic period was 56 million years ago, so “75” doesn’t fit in that period, either.

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NGSS.MS-ESS1-4

NGSS.MS-LS4-1

Nikki was putting together a presentation about the geologic time scale to share in class. What is the purpose of the geologic time scale?

• to organize and sequence events by eras and periods in the different divisions of time and to serve as a timeline

• to show the evolution of life forms and to model how those life forms behaved and what they ate

• to organize and sequence events in the history of Earth and to serve as a model for a vast amount of time covered

• to determine the relative ages of rocks and when and how they formed to create a timeline of their development

Answer explanation

The correct answer is C. Our understanding about the geologic time scale changes as the technology used to analyze rocks and fossils advances. That’s why geologists refine the geologic time scale. Physical evidence supports changes to the geological time scale. For example, if the evidence shows that humans lived long ago in a different time from what was previously identified, then the geologic time scale will change. However, discoveries do not indicate that changes to the geologic time scale needs to change. Discoveries could further support that the organism or rock feature was already properly placed on the geologic time scale, so A and B are incorrect. A recent appearance of newly formed rocks is recent geologic history, and would not cause a change to the geologic time scale, so D is incorrect.

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NGSS.MS-ESS1-4

Daniel recently read a news article about geologists updating the geologic time scale. This happens when new physical evidence is discovered. Which of the following would provide physical evidence that could change the divisions on the geologic timeline?

• the discovery of ancient ruins buried deep in ocean floor sediments

• the discovery of fossilized microorganisms in ocean floor sediments

• the discovery of evidence of humans who lived long ago

• the discovery of evidence of different kinds of rocks that recently appeared

Answer explanation

The correct answer is B. The discovery of microorganisms in the ocean sediment would provide evidence that scientists’ previous understanding of the geologic timescale might not be completely accurate. Therefore, scientists might respond to this evidence by changing the divisions of the geologic time scale.

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NGSS.MS-ESS1-4