Date a Rock Lab

Objective: Students will know that radioisotope half-lives provide a reliable measure for age-dating rocks.

Background:

How old is the earth?

Scientists have collected a lot of evidence, especially geological data which shows that the Earth is approximately 4.6 billion years old.

Geological Timeline:

 

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How do we measure the age of the earth?

One way geologists find the age of the Earth is to measure the age of rocks in the Earth’s crust. They do this by calculating amounts of radioisotopes in rock samples.

What are radioisotopes?

Rocks are made of atoms, some of which give off radiation and decay over time. These atoms are called radioisotopes and when they decay they change to form new atoms. Soon all the radioisotopes in a rock will become the atoms of the new element. Radioisotopes decay at different rates. Scientists know that, in a sample of radioisotopes, half of them will decay in a certain amount of time called the half-life. Half-lives for radioisotopes range in length from a few seconds to billions of years. Radioisotopes keep going through half-lives until the entire rock is made up of the new atoms.

 

Radioisotope

Half-life

Used for

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

How do scientists use these radioisotope clocks to measure the ages of rocks?

Scientists count the atoms in the rocks using sophisticated techniques. They compare the number of radioisotopes in the rock to the number of new atoms to see how many half-lives those radioisotopes have gone through. By knowing the half-life of the radioisotope they can then calculate the age of the rock.

Vocabulary:

Radioisotope –

Half-life –

Procedure:

You are going to be breaking up into pairs. Each pair is going to receive a rock sample with extra large atoms and radioisotopes (beans and corn kernels). The radioisotope you will measuring is Beanium which decays into an atom of Cornium. We know that is takes about 100 million years for half of the atoms in a sample of Beanium to decay into Cornium atoms. Your group’s job is to figure out how old your sample is.

Sample

Sample

Number of Beanium Atoms

Number of Cornium Atoms

Half-lives

Age (Millions of Years)

A

J

 

 

 

 

B

K

 

 

 

 

C

L

 

 

 

 

D

M

 

 

 

 

E

N

 

 

 

 

F

O

 

 

 

 

G

P

 

 

 

 

H

Q

 

 

 

 

I

R