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#35-1146 Laminated
Geologic Time Scale Basics
The history of the Earth covers a vast expanse of time, so scientists divide it into smaller sections that are associated with particular events that have occurred in the past. The approximate time range of each time span is shown on the poster. The largest time span of the geologic time scale is the eon . It is an indefinitely long period of time that contains at least two eras. Geologic time is divided into two eons. The more ancient eon is called the
Precambrian, and the more recent is the Phanerozoic . Each eon is subdivided into smaller spans called eras . The
Precambrian eon is divided from most ancient into the Hadean era, Archean era, and Proterozoic era. See Figure 1.
Proterozoic Era
Archaean Era
Hadean Era
2500 - 550 million years ago
3800 - 2500 million years ago
4600 - 3800 million years ago
Figure 1. Eras of the Precambrian Eon
Single-celled and simple multicelled organisms first developed during the Precambrian eon. There are many fossils 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 is often divided into several smaller time spans called periods . For example, the Paleozoic era is divided into the
Cambrian , Ordovician , Silurian , Devonian , Carboniferous , and Permian periods.
Permian Period 300 - 250 million years ago
Carboniferous Period 350 - 300 million years ago
Devonian Period
Silurian Period
400 - 350 million years ago
450 - 400 million years ago
Ordovician Period
Cambrian Period
500 - 450 million years ago
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 often called the Age of the Dinosaurs because of its famous inhabitants.
Cretaceous Period 150 - 65 million years ago
Jurassic Period 200 - 150 million years ago
Triassic Period 250 - 200 million years ago
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 smaller unit 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.
Quaternary Period 2 million years ago - present
Recent (or Halocene) 0.01 million years ago - present
Pleistocene
Tertiary Period
2 - 0.01 million years ago
65 - 0.01 million years ago
Pliocene
Miocene
Oligocene
Eocene
Paleocene
5 - 2 million years ago
25 - 5 million years ago
35 - 25 million years ago
55 - 35 million years ago
65 - 55 million years ago
Figure 4. Periods and Epochs of the Cenozoic Era
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Our Changing Earth
Scientists measure the breakdown of certain atoms in rocks to estimate the age of the Earth. Current measurements suggest that the Earth is approximately 4600 million years old (4 600 000 000 or 4.6 billion years old). It has undergone many changes throughout its long history. Some changes caused mass extinctions to occur. A mass extinction is the complete removal of many species from the surface of the Earth at the same time.
Fossils , the preserved remains of organisms from the past, and other evidence of mass extinctions show significant changes in the balance of life on Earth. Scientists have subdivided the geologic time scale to reflect these significant events throughout
Earth’s history. Although unfortunate for the organisms at the time, mass extinctions 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 distribution of fossils, patterns and formation of mountain ranges, and how the shapes of the continents appeared as if they could fit together much like pieces of a jigsaw puzzle. He suggested that, at one time, the continents were joined together as a
Figure 5. A fossil of Archeopteryx single supercontinent called Pangaea . Over millions of years, Pangaea broke apart and the continents gradually moved to their present locations. At the time,Wegener could not explain what caused the massive continents to move. Scientists later developed the theory of plate tectonics to explain how and why continents move.The surface of the Earth, called the crust , is broken into several large chunks or plates . As currents of molten (magma) or partially molten rock beneath a section of crust move, they drag the floating plate along with them. Fossil evidence of tropical plants in Antarctica and elsewhere suggest that as the plates moved, their climate dramatically changed. Organisms that could not cope with these changes died and some were preserved as fossils.
Figure 6. The break up of Pangaea and movement of the continents over time

Answer 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 time scale?
3. Match the description with the appropriate subdivision of the geologic time scale. Use each term once.
Description
_____ Hadean, Archaean, and Proterozoic
Term
A. Paleozoic
_____ Triassic, Jurassic, and Cretaceous
_____ Tertiary and Quaternary
B. Cenozoic
C. Precambrian
_____ Cambrian, Ordovician, Silurian,
Devonian, Carboniferous, and Permian
D. Mesozoic
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Answer 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?
COPYMASTER:Permission granted to make unlimited copies.Copy use confined to educational purposes within a single school building.Copyright © Neo/SCI.

(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 called eras. 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 sometimes caused mass extinctions.
3. Match the description with the appropriate subdivision of the geologic time scale. Use each term once.
Description Term
A. Paleozoic
B. Cenozoic
C. Precambrian
Devonian, Carboniferous, and Permian
D. Mesozoic
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(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 connected. The shapes of the continents appear to fit together like pieces of a jigsaw puzzle. The distribution of fossils of various types and patterns of mountain ranges suggest that the continents were once connected.
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 continents floated 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 of plate 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 of plate tectonics explained how huge landmasses could be moved.