Half-Life and Radioactive Decay

3.10 Radioactive dating and
Half-Life

Lesson Objectives

Learners can:

• identify the concept of radioactive decay and the role of unstable isotopes in the dating of rocks and fossils

• explain how various radioactive isotopes are used in different dating methods

• evaluate the strengths and limitations of radioactive dating in Earth Science research

Vocabulary: carbon-14, radioactive decay, uranium-lead dating

Prerequisite Vocabulary: half-life, protons, neutrons, radioactive isotope

Curriculum Kit Materials: Earth Science Notebook

Class Resources

• Class Genially with vocabulary​

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Notebook

Page 47-48 in the Earth Science Notebook
Unit 3 Notebook

        

Isotopes

Isotope- Atoms of an element have the same number of protons but different neutrons so have differing mass

• Element type is set by the number of protons

​Amount in nature 98.93% 1.07% 0.0000000001%

​Radioactive!

Radioactive Decay

Radioactive Decay- Certain elements are unstable. They break apart and release radiation

• Radiation is made from high-energy particles or waves

• They can be dangerous in high amounts

• Low levels of it exist everywhere

​Most radioactive material doesn't glow in real life

​Uranium metal

Radiation

All elements have varieties of the atoms that are radioactive
You get a small dose of radiation by existing called the background

• Your body is designed to handle it

• Potassium has a higher-than-average number of radioactive atoms so eating a banana gives you a radiation dose

  • Banana .000000098 sieverts. Lethal dose 4 sieverts

• Living in a stone building gives .00000007 sieverts dose yearly due to trace amounts of uranium in the stone

• Normal USA background dose is .00000062 sieverts

​THIS IS ALL FINE AND NORMAL

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Radioactive Decay

Parent Isotope- In radioactive decay it is the original element that is breaking apart

• In geology it is mainly Uranium or Thorium

​Uranium-92

​Thorium-90

Radioactive Decay

Daughter Isotope- In radioactive decay it is the element that is being formed

• Always less radioactive than the parent isotope

• Eventually will keep decaying until lead is formed as that is stable and will not decay any more

​Lead

Half-Life

Half-Life- The rate at which elements break apart

• Half-life tells you when you will have half of a sample left

• It is unchanging for a specific element

Some Half Lives

• Thorium-232 14 billion years

• Uranium-238 4.46 billion years

• Carbon-14 5,730 years

• Cobolt-58 70.8 days

• Radium-213 2.74 seconds

Radiometric Dating

Radiometric Dating- Using the ratio of parent-to-daughter isotopes and their half-life the absolute age of a material

U²³⁵ has a half-life of 5.27 years. If we find a sample of rock with 1/4 of its U²³⁵ left then we know it is 1,401 million years old or that it has undergone 2 half-lives

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Types of Radiometric Dating

Carbon-14 dating
Used for dating things that were once living
Half-life- 5,730 years
Can be used to date things up to 55,000 years old
Primarily used by archeologists

Ötzi the iceman is a mummy preserved in a glacier near the modern Austria/Italy Border. Thanks to carbon dating we know he lived around 5,350 years ago

Types of Radiometric Dating

Uranium dating
Used for dating rocks
Half-life- 710 million to 4.47 billion
It can be used to date things 1 million- 4.5 billion years old
As there are two isotopes of uranium U²³⁵ and U²³⁸ the dating of both can be used to check for errors

​The first eruption of the Yellowstone supervolcano was 2.1 million years ago

Types of Radiometric Dating

Uranium dating
Uranium dating is normally done on crystals called zircons because they are very tough and won't allow any material into or out of them

Most zircons used in dating are much smaller than this one
Gilgit, Pakistan

Types of Radiometric Dating

There are other forms of radiometric dating using potassium-40, rubidium-87 and samarium-147. They can be used for looking at different time ranges and to check each other

Limitations

Radiometric dating cannot be used on sedimentary rock as it is made of bits of tons of other rocks

• Generally, they will look for inclusions of igneous rocks to do the dating on

Limitations

Radiometric dating cannot be used on metamorphic rock as when it undergoes heat and pressure material can be added or removed

• Can be used to tell when it became metamorphic rock

​Gneiss pronounced nice

Poll

How was class today?

• Too Hard

• Just Right

• Too Easy

Song of the Day

"Radioactive" by Jonny Berliner