Geologic Time Scale

Geologic Time Scale

Middle School

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Learning Objectives

• Define the geologic time scale and its purpose in studying Earth's history.

• Explain how the time scale is organized into eons, eras, periods, and epochs.

• Describe evidence like fossils, rock layers, and radioactivity used to develop the time scale.

• Identify major divisions of geologic time and the key events that separate them.

Key Vocabulary

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Geologic Time Scale

A system that divides Earth's history into named blocks of time based on fossil evidence and major events.

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Eon

The largest division of geologic time, which is further subdivided into eras, periods, and epochs.

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Era

A major division of geologic time that is a subdivision of an eon and contains multiple periods.

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Period

A division of an era, often named for the geographic region where its rocks were first studied.

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Epoch

A subdivision of a period, representing a smaller unit of time within the geologic time scale.

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Radiometric Dating

The scientific method used to determine the exact age of rocks, providing absolute dates for the time scale.

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What is the Geologic Time Scale?

• Geologists divide Earth's 4.5-billion-year history into smaller, named periods of time.

• This organization helps reference specific events and developments throughout Earth’s past.

• These divisions mark major events in the history of life on Earth.

• Events are identified using fossil evidence found in the rock record.

Multiple Choice

What is the primary basis for the divisions in the geologic time scale?

• The names of different European mountains

• The discovery of radioactivity

• The regular passage of a million years

• Major events in life history found in the fossil record

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Building the Time Scale: Relative and Absolute Dating

Relative Dating

• ​The first time scale was built by correlating rock layers from different locations around the world.

• ​​Geologists used Steno's laws, which state that older rocks are found below younger rocks in layers.

• ​This method places events in the correct order, creating a relative history of Earth over time.

Absolute Dating

• ​The discovery of radioactivity in the late 1800s introduced a way to find exact dates.

• ​​This process allows scientists to determine the exact age in years of certain types of rocks.

• ​It helps assign absolute dates to time scale divisions, like the start of the Jurassic period.

Multiple Choice

How did the discovery of radioactivity change the geologic time scale?

• It only confirmed that older rocks are at the bottom.

• It was used to discover the principles of relative dating.

• It helped name the periods after European locations.

• It allowed scientists to assign exact ages in years to the time scale divisions.

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How Geologic Time Is Organized

• Geologic time is sorted hierarchically, with the youngest ages displayed at the top.

• The largest units of time are Eons, which are then split into Eras.

• Eras are divided into Periods, and Periods can be broken into smaller Epochs.

• Remember the order with this mnemonic: Every Elephant Plays Energetically.

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Multiple Choice

Which of the following correctly lists the divisions of the geologic time scale from the largest unit to the smallest unit?

• Era, Eon, Period, Epoch

• Period, Era, Eon, Epoch

• Epoch, Period, Era, Eon

• Eon, Era, Period, Epoch

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Major Divisions of Geologic Time

• Most of Earth's history is in the Pre-Archean, Archean, and Proterozoic Eons.

• Our most recent eon, the Phanerozoic, is divided into three major eras.

• The Paleozoic Era ('Old Life') spanned from 541 to 252 million years ago.

• The Mesozoic Era ('Middle Life') is known as the age of the dinosaurs.

• This era includes the Triassic, Jurassic, and Cretaceous periods (252 to 66 million years ago).

• Our current Cenozoic Era ('New Life') began about 66 million years ago.

Multiple Choice

The Triassic, Jurassic, and Cretaceous periods all belong to which era?

• Proterozoic Eon

• Mesozoic Era

• Paleozoic Era

• Cenozoic Era

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Common Misconceptions

Misconception	Correction
Geologic time units are all equal in length.	Durations vary as they are defined by major geological or biological events.
Dinosaurs and early humans coexisted.	Dinosaurs became extinct millions of years before the first modern humans appeared.
An epoch is the longest span of geologic time.	An eon is the longest time span; an epoch is much shorter.

Multiple Choice

Based on how the geologic time scale is constructed, what is the primary difference between how the 'Jurassic Period' and 'Pleistocene Epoch' are defined?

• The Jurassic is part of the Cenozoic era, while the Pleistocene is part of the Mesozoic.

• The Jurassic is a longer and larger-scale division of time than the smaller, more recent Pleistocene epoch.

• The Pleistocene is named after a place in Europe, while the Jurassic is not.

• The Jurassic is defined by a major life event, while the Pleistocene is a fixed length of time.

Multiple Choice

How did Steno's laws and the discovery of radioactivity work together to create the modern geologic time scale?

• Steno's laws established the relative order of events, and radioactivity provided the absolute ages for those events.

• Radioactivity was used to order the rock layers, and Steno's laws provided the exact dates.

• Radioactivity proved Steno's laws were incorrect, forcing a new time scale to be made.

• Steno's laws were used to name the time periods, and radioactivity was used to find the fossils.

Multiple Choice

A geologist discovers a fossil in a rock layer directly above a layer dated to 240 million years ago and below a layer dated to 205 million years ago. Based on the provided information, what is the most likely period this fossil is from?

• Permian

• Paleogene

• Triassic

• Cambrian

Multiple Choice

The end of the Cretaceous Period marked a major extinction event. What larger division in the geologic time scale also concluded at this exact same point in time?

• The Phanerozoic Eon

• The Paleozoic Era

• The Mesozoic Era

• The Cenozoic Era

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Summary

• The geologic time scale divides Earth's history based on major life events.

• It began as a relative scale, but now uses radioactivity for absolute dating.

• Time is organized into eons, eras, periods, and epochs.

• We live in the Cenozoic Era, which followed the Mesozoic and Paleozoic eras.

Poll

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

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