Age of the Earth Periods 2,4,6

Age of the Earth Periods 2,4,6

How old is the Earth? Scientists know our planet is about 4.5 billion years old. But how did they find out Earth's age? 

First Estimate Was Far OffIn the 1800s, scientists tried to find the age of the Earth. One of these scientists was Lord Kelvin. He studied physics and math. Lord Kelvin knew that Earth was very hot when it first formed. He thought about how long it would take to cool down. He said that the Earth was between 20 million years old and 400 million years old. 

Lord Kelvin's estimate was wrong. That is too young for the Earth. But the way he tried to answer the question was right. He used the scientific method.

From Millions To Billions Of Years OldScientists also tried to compare layers of rock to get the age of the Earth. This is called relative dating. They compare one layer to another layer. This can show which layers are older or younger. But it cannot give an actual age. However, it did tell the scientists that the Earth was billions of years old. 

Advances in science continued. In the early 1900s, scientists found elements that break down into other elements. They are called radioactive elements. The time it takes for half of the element to break down is called its half-life. 

Scientists use a certain process to tell the age of a rock. They find radioactive elements in the rocks. They look at how much of the element is left. They look at how much of the new element has formed. They use the half-life to tell how much time has passed. This is called radiometric dating.

Looking Beyond Earth.One problem with radiometric dating is the the rock cycle. During the rock cycle, rocks change forms. Old rocks are destroyed as they slide back into the Earth. New rocks form when melted rock cools and becomes solid. 

The first rocks that formed on the Earth are no longer here. This makes it hard to find Earth's age. The oldest rocks that have been found are about 3.8 billion years old. Some tiny minerals are 4.2 billion years old.

Scientists have looked at rocks from the moon and from meteorites. These rocks have not been changed by the rock cycle. Scientists used radiometric dating on those rocks. Scientists have gathered a lot of data from the rocks on Earth and beyond. They have used the data to estimate that Earth is 4.5 billion years old.

Multiple Choice

What is one MAIN idea of the article?

• (A) Scientists have tried different ways to find out how old Earth is.

• (B) Lord Kelvin thought that Earth was at least 20 million years old.

• (C) Radiometric dating does not always work to find Earth's age.

• (D) The oldest rocks on Earth are 3.8 billion years old.

Multiple Choice

Read the following paragraph from the section “Looking Beyond Earth." Scientists have looked at rocks from the moon and from meteorites. These rocks have not been changed by the rock cycle. Scientists used radiometric dating on those rocks. Scientists have gathered a lot of data from the rocks on Earth and beyond. They've used the data to estimate that Earth is 4.5 billion years old.

What is the MAIN idea of this paragraph?

• (A) Radiometric dating is the process of using a half-life to tell time.

• (B) Radiometric dating does not work very well during the rock cycle.

• (C) Scientists use rocks from outside Earth to help estimate Earth's age.

• (D) Scientists think that Earth is more than 4.2 billion years old.

Multiple Choice

If readers are looking for information on HOW scientists figured out that Earth is 4.5 billion years old, which section should they read?

• (A) introduction [paragraph 1]

• (B) "First Estimate Was Far Off"

• (C) "From Millions To Billions Of Years Old"

• (D) "Looking Beyond Earth"

Multiple Choice

What does the section “First Estimate Was Far Off” show the reader?

• (A) why scientists tried to estimate Earth's age

• (B) how many people have estimated Earth's age

• (C) what the estimate of Earth's age is today

• (D) who made an estimate of Earth's age