4-3 Radioactive Dating

Essential Question:

How do Scientist study Earth’s past?

What is Radioactive Decay?

What is Radioactive Dating?

How do Scientist Determine Earth’s Age?

Earth’s History - Course 2

My Planet Diary pg. 152Marie CurieIn 1896, French scientists named Marie and Pierre Curie heard about experiments that had been done by another scientist, Henri Becquerel.

Becquerel had shown that by placing some uranium salt on a photographic plate, covered with black paper, the plate would be affected as if light had fallen on it. The effect is produced by special rays which are emitted by the uranium salt…. My determinations showed that the emission of the rays is an atomic property is an atomic property of the uranium.

Marie Curie later described what happened: The property that Becquerel and the Curies discovered was called radioactivity. Today, radioactivity is used for many purposes—including finding the age of rocks!

1.What did Marie and Pierre Curie discover about radioactivity?

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2. What does the discovery of radioactivity tell you about how scientists work together? _________________________________________________

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What is Radioactive Decay? Pg. 153

Some elements decay, or break down, over time, releasing particles and energy. This process is called radioactive decay, and the elements are considered to be radioactive.

During radioactive decay, the atoms of one element break down to form atoms of another element.

The half-life of a radioactive element

is the time it takes for half of the

radioactive atoms to decay.

The rate of decay of each

radioactive element never changes.

Radioactive Dating

Half-LifeThe half-life of a radioactive element is the amount of time it takes for half of the radioactive atoms to decay. What pattern do you see in the graph?

Assess Your Understanding pg. 153

I get it! Now I know that radioactive decay occurs when

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What is Radioactive Dating? Pg. 154Radioactive elements in igneous rocks decay over time into other elements. This slowly changes the composition of the rock. Geologists use radioactive dating to determine the absolute ages of rocks.

In radioactive dating, scientists first determine the amount of a radioactive element in a rock. Then they compare that amount with the amount of the stable element into which the radioactive element decays.

They calculate the age of the rock using this information and the half-life of the element.

A rock contains 25% of the potassium-40 it started with. How old is the rock?Step 1: Determine how many half-lives have passed.

-After one half-life, 50% of the potassium would remain. After two half- lives, 25% of the potassium would remain. So two half-lives would have passed.Step 2: Find the half-life of potassium-40.

-The half-life of potassium-40 is 1.3 billion years.Step 3: Multiply the half-life by the number of half-lives that have passed.

-1.3 billion years/half-life x 2 half lives = 2.6 billion years, so the rock is about 2.6 billion years old.

1.Calculate: A rock fromThe moon contains 12.5 % ofThe potassium-40 it beganWith. How old is the rock?Hint 12.5%=1/8

2. Calculate: A fossil contains1/16 of the carbon-14 it beganWith. How old is the fossil?

Radioactive Dating pg. 154

Potassium-Argon Dating and Carbon-14 DatingSome elements used by scientists in radioactive dating include potassium-40 and carbon-14.Potassium-40 has a long half-life, which is useful in dating the most ancient rocks. All plants and animals contain some carbon-14, which decays after the organism dies. Scientists measure the amounts of carbon-14 to determine the age of a rock sample.

Real World Inquiry using Carbon-14 Dating

Scientists have dated these skeletons to 5,000-6,000 years ago. But they do not use radioactive dating to find the age of stone artifacts made by people. Why not?

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Assess Your Understanding pg. 155

a. Identify Scientists use the method of (radioactive dating / relative dating) to find the absolute age of a rock.

b. Apply Concepts The half-life of thorium-232 is 14 billion years. A rock with 25% of its throrium-232 remaining is __________years old.

c. Challenge A scientist finds stone tools in the ruins of an ancient house. The house also has ashes in a fireplace. How could the scientist estimate the age of the stone tools? __________________________________________________________________________________________________________

I get it! Now I know that radioactive dating is done by _____________

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How Do Scientists Determine Earth’s Age? Pg. 156

To determine Earth’s age, scientists use radioactive dating and evidence from rocks on Earth and the moon.

Using radioactive dating, Earth rocks are estimated to be between 3.8 and 4.28 billion years old.

However, Earth rocks have changed over time due to the processes of plate tectonics and erosion.

Moon rocks, fortunately, have not changed over time.

According to one theory, scientists infer that the moon formed when another object collided with Earth.

Since the moon came from Earth, the moon is about the same age as Earth.

Radioactive dating estimates moon rocks to be 4.6 billion years old. Scientists estimate that Earth is about the same age.

Radioactive Dating – pg. 157

Formation of the MoonAccording to one theory, a planet-sized object struck Earth. Material from the object and material from Earth were knocked into orbit around Earth. Gravity pulled the material together to form the moon.

How do moon rocks help scientists estimate the age of Earth?

Assess Your Understanding pg. 157

I get it! Now I know that scientists determine Earth’s age by

_________________________________________________________Rock DatingThis rock is from the Jack Hills in Australia.Tiny zircon crystals in this rock have survivedFrom some of the earliest rocks that formedon Earth. The crystals preserved minerals that were trapped inside when the crystals formed. By examining these minerals, cientists can determine the conditions under which the crystal formed.

Why would only tiny crystals from the earliest rocks have survived?_________________________________________________________