Geologic Time Scale

Geologic Time Scale

Middle School

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Learning Objectives

• Explain the geologic time scale and the evidence used to develop it.

• Describe the major divisions of geologic time, like eras and periods.

• Analyze how fossil and rock evidence helps organize the Earth's long history.

• Explain why scientists update the geologic time scale with new evidence.

Key Vocabulary

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Geologic Time Scale

A record of geologic events and the evolution of life as shown in rock and fossil records.

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Fossil

The preserved remains or traces of an organism that lived in the past, found in Earth's rocks.

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Unconformity

A gap in the geologic record where rock layers were lost due to erosion or non-deposition.

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Era

One of three long units of geologic time between the Precambrian and the present, like the Paleozoic.

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Period

A subdivision of an era in the geologic time scale, defined by specific geological or biological events.

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What Is the Geologic Time Scale?

• The geologic time scale organizes Earth's 4.6-billion-year history.

• It was created using relative dating with rock layers and index fossils.

• An unconformity, a gap in the rock record, is key evidence.

• Absolute dating was later used to assign specific ages to divisions.

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Multiple Choice

What is the main difference between relative dating and absolute dating?

• Relative dating assigns specific ages to rocks, while absolute dating places them in chronological order.

• Relative dating places rock layers in order, while absolute dating assigns specific ages to them.

• Relative dating is used to identify unconformities, while absolute dating is used to find index fossils.

• Relative dating is a more precise method than absolute dating.

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The Divisions of Geologic Time

• Divisions are based on major changes in life forms found in fossils.

• A species' disappearance often marks the beginning of a new time unit.

• Precambrian Time covers about 88% of the Earth’s entire history.

• Time is divided into major units called Eras, then into Periods.

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Multiple Choice

What is the primary basis that scientists use to separate geologic time into different divisions like Eras and Periods?

• Major changes in the types of life forms found in the fossil record.

• The shifting of the Earth's continents into new positions.

• Equal intervals of time, each lasting 100 million years.

• Changes in the types of rocks found in the Earth's crust.

A Closer Look at the Eras

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Paleozoic Era

• ​This era saw a dramatic increase in the complexity of life forms.

• ​​The supercontinent of Pangaea was formed during this time.

• ​Life was dominated by marine invertebrates, early fish, and sprawling forests.

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Mesozoic Era

• ​This era is famously known as the ‘Age of Dinosaurs’.

• ​​The supercontinent Pangaea began to break apart during this era.

• ​It includes the Triassic, Jurassic, and the Cretaceous periods.

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Cenozoic Era

• ​Mammals became the dominant land animals after the dinosaurs went extinct.

• ​​The continents shifted into their present-day positions during this time.

• ​This is our current era, and it is known as the ‘Age of Mammals’.

Multiple Choice

During which era did the supercontinent Pangaea form and life's complexity dramatically increase?

• Paleozoic Era

• Mesozoic Era

• Cenozoic Era

• Precambrian Time

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An Ever-Refining Timeline

• The geologic time scale is a dynamic model, not a fixed chart.

• Scientists refine it based on new evidence and better technology.

• For example, the Quaternary period’s start date was recently changed.

• It was moved from 1.8 to 2.6 million years ago.

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Multiple Choice

What does the fact that scientists updated the start date of the Quaternary period demonstrate about the geologic time scale?

• It is a dynamic model that is revised as new discoveries are made.

• It is a fixed and unchanging chart of Earth's history.

• The original dates set by scientists were significant errors.

• The timeline is simplified over time to make it easier to understand.

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Common Misconceptions about the Geologic Time Scale

Misconception	Correction
Geologic time divisions are equal in length.	Divisions are based on major events, not equal lengths of time.
Humans and dinosaurs lived at the same time.	Dinosaurs died out 66 million years before modern humans appeared.
The geologic time scale is a finished, unchanging chart.	It is a dynamic model that is updated with new discoveries.

Multiple Choice

How did the development of absolute dating improve upon the information provided by relative dating?

• It provided specific ages for time divisions, making the scale more precise.

• It allowed scientists to place rock layers in chronological order for the first time.

• It helped identify the locations of unconformities in the rock record.

• It was the first method to use fossils to organize Earth's history.

Multiple Choice

A geologist finds a rock layer with a wide variety of complex fossils, such as trilobites, directly above a layer with very few, simple fossils. This transition most likely represents the boundary between which two major time divisions?

• The Precambrian Time and the Paleozoic Era

• The Paleozoic Era and the Mesozoic Era

• The Mesozoic Era and the Cenozoic Era

• The Cenozoic Era and the present day

Multiple Choice

If a major extinction event, previously unknown, was discovered to have happened 50 million years ago, what would be the most likely impact on the geologic time scale?

• Scientists might propose a new period or boundary to mark this significant event.

• Scientists would ignore the discovery to keep the current time scale stable.

• The entire geologic time scale would have to be discarded as inaccurate.

• The dates of all other divisions would be shifted to accommodate the new event.

Multiple Choice

What is the main reason that dinosaurs on different continents began to evolve into different species after the breakup of Pangaea?

• The change in climate was too sudden for them to survive.

• The oceans created a physical barrier that isolated populations from each other.

• Volcanic eruptions wiped out all the dinosaurs on some continents.

• The dinosaurs were too old to adapt to new environment.

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Summary

• The Geologic Time Scale organizes Earth's history using rock and fossil evidence.

• It uses relative dating to order rock layers and absolute dating for specific ages.

• History is divided into units like eras, marked by major changes in life.

• The time scale is a dynamic model that scientists constantly refine with new evidence.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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• 4