Earth Science: Geologic Time Scale: Eons, Eras, and Periods Worksheet

Question 1

The geologic time scale is organized from largest time unit to smaller time unit. Put these units in order from largest to smallest: period, eon, era.

Accepted Answer

The correct order from largest to smallest is eon, era, period. An eon contains eras, and an era contains periods.

Question 2

Earth is about 4.6 billion years old. The Phanerozoic Eon began about 541 million years ago. Which lasted longer, the time before the Phanerozoic Eon or the Phanerozoic Eon itself?

Accepted Answer

The time before the Phanerozoic Eon lasted longer. It lasted for more than 4 billion years, while the Phanerozoic Eon has lasted about 541 million years.

Question 3

Name the three eras of the Phanerozoic Eon in order from oldest to youngest: Cenozoic, Paleozoic, Mesozoic.

Accepted Answer

The three eras of the Phanerozoic Eon in order from oldest to youngest are Paleozoic, Mesozoic, and Cenozoic.

Question 4

A fossil is found in rock layers from the Jurassic Period. Which era does the Jurassic Period belong to?

Accepted Answer

The Jurassic Period belongs to the Mesozoic Era. The Mesozoic Era includes the Triassic, Jurassic, and Cretaceous Periods.

Question 5

The Paleozoic Era includes several periods, such as the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. Which of these periods came first?

Accepted Answer

The Cambrian Period came first. It is the oldest period listed from the Paleozoic Era.

Question 6

Look at this sequence: Triassic, Jurassic, Cretaceous. What do these three time units have in common?

Accepted Answer

The Triassic, Jurassic, and Cretaceous are all periods in the Mesozoic Era. They are also the periods during which dinosaurs lived.

Question 7

The Cenozoic Era includes the Paleogene, Neogene, and Quaternary Periods. Which period are we living in now?

Accepted Answer

We are living in the Quaternary Period. It is the most recent period of the Cenozoic Era.

Question 8

Explain why scientists divide Earth's history into eons, eras, and periods instead of using one long list of years.

Accepted Answer

Scientists divide Earth's history into eons, eras, and periods to make the huge amount of time easier to study and describe. These divisions also help connect major events, fossils, and rock layers to specific parts of Earth's history.

Question 9

A student says, "The Mesozoic Era is older than the Paleozoic Era because dinosaurs lived a long time ago." Is the student correct? Explain.

Accepted Answer

The student is not correct. The Paleozoic Era is older than the Mesozoic Era, even though the Mesozoic Era also happened a very long time ago.

Question 10

The end of the Permian Period marks a major mass extinction. Which era ended at the same time as the Permian Period?

Accepted Answer

The Paleozoic Era ended at the same time as the Permian Period. The Permian was the last period of the Paleozoic Era.

Question 11

Classify each item as an eon, era, or period: Phanerozoic, Mesozoic, Cretaceous.

Accepted Answer

Phanerozoic is an eon, Mesozoic is an era, and Cretaceous is a period. These units go from larger to smaller parts of geologic time.

Question 12

A rock layer contains many fossils of early marine animals from the Cambrian Period. Which eon and era does this rock layer belong to?

Accepted Answer

The rock layer belongs to the Phanerozoic Eon and the Paleozoic Era. The Cambrian Period is the first period of the Paleozoic Era.

Question 13

Use this time information: The Paleozoic Era lasted from about 541 million to 252 million years ago. About how long did the Paleozoic Era last?

Accepted Answer

The Paleozoic Era lasted about 289 million years. This is found by subtracting 252 million from 541 million.

Question 14

Put these periods in order from oldest to youngest: Cretaceous, Triassic, Jurassic.

Accepted Answer

The correct order from oldest to youngest is Triassic, Jurassic, Cretaceous. These are the three periods of the Mesozoic Era in order.

Question 15

A simplified geologic time scale shows the Hadean, Archean, Proterozoic, and Phanerozoic Eons. Which eon is the most recent, and why?

Accepted Answer

The Phanerozoic Eon is the most recent eon. It includes the present day and contains the Paleozoic, Mesozoic, and Cenozoic Eras.

Earth Science: Geologic Time Scale: Eons, Eras, and Periods

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