Geologic Time Scale Basics

The history of the Earth covers a vast expanse of time, so scientists divide it into smaller sections that are associ-ated with particular events that have occurred in the past. The approximate time range of each time span isshown on the poster. The largest time span of the geologic time scale is the eon. It is an indefinitely long period oftime that contains at least two eras. Geologic time is divided into two eons. The more ancient eon is called thePrecambrian, and the more recent is the Phanerozoic. Each eon is subdivided into smaller spans called eras. ThePrecambrian eon is divided from most ancient into the Hadean era, Archean era, and Proterozoic era. See Figure 1.

Figure 1. Eras of the Precambrian Eon

Single-celled and simple multicelled organisms first developed during the Precambrian eon. There are many fos-sils from this time because the sea-dwelling creatures were trapped in sediments and preserved.

The Phanerozoic eon is subdivided into three eras – the Paleozoic era, Mesozoic era, and Cenozoic era. An era isoften divided into several smaller time spans called periods. For example, the Paleozoic era is divided into theCambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods.

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Exploring Geologic Time PosterIllustrated Teacher's Guide

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Background

Precambrian Eon

Proterozoic Era 2500 - 550 million years ago

Archaean Era 3800 - 2500 million years ago

Hadean Era 4600 - 3800 million years ago

Paleozoic Era

Permian Period 300 - 250 million years ago

Carboniferous Period 350 - 300 million years ago

Devonian Period 400 - 350 million years ago

Silurian Period 450 - 400 million years ago

Ordovician Period 500 - 450 million years ago

Cambrian Period 550 - 500 million years ago

Figure 2. Periods of the Paleozoic Era

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The Mesozoic Era contains the Triassic, Jurassic, and Cretaceous periods, as illustrated in Figure 3. It is oftencalled the Age of the Dinosaurs because of its famous inhabitants.

Figure 3. Periods of the Mesozoic Era

The two periods of the Cenozoic Era are the Tertiary and Quaternary. A period is divided into an even smallerunit called an epoch. The Tertiary period of the Cenozoic era is comprised of the Paleocene, Eocene, Oligocene,Miocene, and Pliocene. The Quaternary period of the Cenozoic era includes the Pleistocene and Holocene(Recent) epochs. See Figure 4.

Figure 4. Periods and Epochs of the Cenozoic Era

Mesozoic Era

Cretaceous Period 150 - 65 million years ago

Jurassic Period 200 - 150 million years ago

Triassic Period 250 - 200 million years ago

Cenozoic Era

Quaternary Period 2 million years ago - present

Recent (or Halocene) 0.01 million years ago - present

Pleistocene 2 - 0.01 million years ago

Tertiary Period 65 - 0.01 million years ago

Pliocene 5 - 2 million years ago

Miocene 25 - 5 million years ago

Oligocene 35 - 25 million years ago

Eocene 55 - 35 million years ago

Paleocene 65 - 55 million years ago

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Our Changing Earth

Scientists measure the breakdown of certain atoms in rocks to estimate theage of the Earth. Current measurements suggest that the Earth is approxi-mately 4600 million years old (4 600 000 000 or 4.6 billion years old). It hasundergone many changes throughout its long history. Some changes causedmass extinctions to occur. A mass extinction is the complete removal of manyspecies from the surface of the Earth at the same time. Fossils, the preservedremains of organisms from the past, and other evidence of mass extinctionsshow significant changes in the balance of life on Earth. Scientists have subdi-vided the geologic time scale to reflect these significant events throughoutEarth’s history. Although unfortunate for the organisms at the time, massextinctions allow us to identify transitions from one period to another.

The physical features of the Earth have also changed over time. Alfred Wegener(1880 – 1930) proposed the theory of continental drift to explain the distribu-tion of fossils, patterns and formation of mountain ranges, and how the shapes ofthe continents appeared as if they could fit together much like pieces of a jigsawpuzzle. He suggested that, at one time, the continents were joined together as asingle supercontinent called Pangaea. Over millions of years, Pangaea broke apart and the continents graduallymoved to their present locations. At the time,Wegener could not explain what caused the massive continents tomove. Scientists later developed the theory of plate tectonics to explain how and why continents move.The surfaceof the Earth, called the crust, is broken into several large chunks or plates. As currents of molten (magma) or partiallymolten rock beneath a section of crust move, they drag the floating plate along with them. Fossil evidence of tropicalplants in Antarctica and elsewhere suggest that as the plates moved, their climate dramatically changed. Organismsthat could not cope with these changes died and some were preserved as fossils.

Figure 5. A fossil of Archeopteryx

Figure 6. The break up of Pangaea and movement of the continents over time

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Worksheet # 1 – The Geologic Time ScaleAnswer the questions in the spaces provided.

1. Explain the relationships among eons, eras, epochs, and periods of the geologic time scale.

2. How did scientists account for fossils and other geological evidence as they developed the geologic timescale?

3. Match the description with the appropriate subdivision of the geologic time scale. Use each term once.

Description Term

_____ Hadean, Archaean, and Proterozoic A. Paleozoic

_____ Triassic, Jurassic, and Cretaceous B. Cenozoic

_____ Tertiary and Quaternary C. Precambrian

_____ Cambrian, Ordovician, Silurian, D. MesozoicDevonian, Carboniferous, and Permian

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COPYMASTER: Permission granted to make unlimited copies.Copy use confined to educational purposes within a single school building.Copyright © Neo/SCI.

Worksheet # 2 – Our Changing EarthAnswer the questions in the spaces provided.

1. What was Pangaea? What evidence supports the past existence of Pangaea?

2. Describe the theory of continental drift and identify who first proposed this theory.

3. How did the theory of plate tectonics improve on the theory of continental drift?

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Worksheet # 1 – The Geologic Time Scale(Expected Results)

Answer the questions in the spaces provided.

1. Explain the relationships among eons, eras, epochs and periods of the geologic time scale.

Eons are the largest spans of time in the geologic time scale. Eons are divided into smaller units callederas. Eras are subdivided into periods. Periods are subdivided into even smaller time spans called epochs.

2. How did scientists account for fossils and other geological evidence as they develop the geologic time scale?

Scientists associated the subdivisions of the geologic time scale with events that occurred in Earth’s past.Fossils and other geological evidence were used to identify significant changes in the Earth, which some-times caused mass extinctions.

3. Match the description with the appropriate subdivision of the geologic time scale. Use each term once.

Description Term

_____ Hadean, Archaean, and Proterozoic A. Paleozoic

_____ Triassic, Jurassic, and Cretaceous B. Cenozoic

_____ Tertiary and Quaternary C. Precambrian

_____ Cambrian, Ordovician, Silurian, D. MesozoicDevonian, Carboniferous, and Permian

C

D

B

A

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Worksheet # 2 – Our Changing Earth(Expected Results)

Answer the questions in the spaces provided.

1. What was Pangaea? What evidence supports the past existence of Pangaea?

Pangaea was a single super continent formed in the past when all the landmasses of the Earth were con-nected. The shapes of the continents appear to fit together like pieces of a jigsaw puzzle. The distributionof fossils of various types and patterns of mountain ranges suggest that the continents were once con-nected.

2. Describe the theory of continental drift and identify who first proposed this theory.

Alfred Wegener first proposed the theory of continental drift. This theory explained that the continentsfloated on a liquid core and at one time were connected to form the supercontinent called Pangaea.Pangaea then gradually broke apart and the continents drifted to their present locations.

3. How did the theory of plate tectonics improve on the theory of continental drift?

Both theories explained that the current location of continents is different from their locations in the past.The theory of continental drift included the idea that the continents floated on a liquid core. The theory ofplate tectonics further explained that the crust or surface of the Earth is broken into plates or chunks,which float on a liquid core. As currents in the hot liquid flow, they drag the plates. Thus the theory ofplate tectonics explained how huge landmasses could be moved.