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Page 1: Absolute Ages of Rock Mr. Perez. Important vocabulary Isotopes Radioactive decay Half-life Radiometric dating Uniformitarianism

Absolute Ages of RockMr. Perez

Page 2: Absolute Ages of Rock Mr. Perez. Important vocabulary Isotopes Radioactive decay Half-life Radiometric dating Uniformitarianism

Important vocabulary

• Isotopes• Radioactive decay• Half-life• Radiometric dating• Uniformitarianism

Page 3: Absolute Ages of Rock Mr. Perez. Important vocabulary Isotopes Radioactive decay Half-life Radiometric dating Uniformitarianism

Absolute Ages• Absolute age is the age, in years, of a rock or other object.• Examples:

• How old are you?• How old is your house?• How old is your book bag?• How old is your pet?• How old is your sibling?

Page 4: Absolute Ages of Rock Mr. Perez. Important vocabulary Isotopes Radioactive decay Half-life Radiometric dating Uniformitarianism

Radioactive Decay• Atoms are made up of:• Nucleus- central region

• Includes the protons (+) and neutrons (0)

• Electrons- cloud surrounding center (-)

• The number of protons(+) determines the identity of the element

• The number of neutrons (0) determines the form of the element (aka isotope)

Page 5: Absolute Ages of Rock Mr. Perez. Important vocabulary Isotopes Radioactive decay Half-life Radiometric dating Uniformitarianism

Radioactive Decay• Some isotopes are unstable and break down into other

isotopes and particles• We call the above process radioactive decay

Page 6: Absolute Ages of Rock Mr. Perez. Important vocabulary Isotopes Radioactive decay Half-life Radiometric dating Uniformitarianism

Radioactive Decay- Alpha and Beta Decay

Beta Decay• Neutron breaks

down into proton and electron

• Electron leaves atoms a beta particle

Alpha Decay• Two protons

and two neutrons are given off

• Leave as alpha particle

Both types of decay form a new isotope

Page 7: Absolute Ages of Rock Mr. Perez. Important vocabulary Isotopes Radioactive decay Half-life Radiometric dating Uniformitarianism

Radiometric Decay- Half Life• Each parent isotope decays (breaks down) into its daughter

isotope at a certain rate• Based on this decay rate, it takes a certain period of time for

one half of the parent isotope to decay to its daughter product• The half-life of an isotope is the time it takes for half of the

atoms in the isotope to decay.• Example- It takes carbon-14 5,730 years for half of its atoms to

turn into nitrogen-14. Then, it takes another 5,730 years for half of the remaining carbon-14 atoms to turn into nitrogen-14, and so on…

THINK OF EXPONENTS! 23 = 8, 32 = 9, 44 = 256

Page 8: Absolute Ages of Rock Mr. Perez. Important vocabulary Isotopes Radioactive decay Half-life Radiometric dating Uniformitarianism

Radioactive Decay- Half-life• Fill in the following table using this information:• Mr. Perez has 100g of carbon-14. After 5,730 years, how many

grams of carbon-14 will still be carbon-14? After 2? After 3?...

Year Grams of carbon-14

0 100g

1

2

3

4

Page 9: Absolute Ages of Rock Mr. Perez. Important vocabulary Isotopes Radioactive decay Half-life Radiometric dating Uniformitarianism

Radiometric Dating

Page 10: Absolute Ages of Rock Mr. Perez. Important vocabulary Isotopes Radioactive decay Half-life Radiometric dating Uniformitarianism

Radiometric Dating• By measuring the ratio of parent isotope to daughter product

in a mineral AND knowing the half-life of the parent, you can calculate the absolute age of a rock• This is called radiometric dating

• A scientist must decided which parent isotope to use when measuring the age of a rock.• If it’s old, the scientist will use an isotope with a long half-life

• Example: potassium-40 takes 1.25 billion years to decay to argon-40

• To avoid errors, conditions must be met for ratios to give a correct indication of age:1. The rock being studied must retain ALL of the argon-40 that

produced by the decay of potassium-402. It cannot contain any contamination of daughter product from

other sources

Page 11: Absolute Ages of Rock Mr. Perez. Important vocabulary Isotopes Radioactive decay Half-life Radiometric dating Uniformitarianism

Radiometric Dating- Radiocarbon Dating• Carbon-14 is useful for dating bones, wood and charcoal up to

75,000 years old.• Living things take in carbon from the environment to build

their bodies• Most of this carbon is carbon-12, but some is carbon-14• The ratio of these two isotopes is always the same

• After an organism dies, the carbon-14 decays slowly• By determining the amounts of isotopes in a sample, scientists

can evaluate how much the isotope ratio differs from that in the environment

Page 12: Absolute Ages of Rock Mr. Perez. Important vocabulary Isotopes Radioactive decay Half-life Radiometric dating Uniformitarianism

Radiometric Dating- Radiocarbon Dating

Page 13: Absolute Ages of Rock Mr. Perez. Important vocabulary Isotopes Radioactive decay Half-life Radiometric dating Uniformitarianism

Radiometric Dating-• Age Determinations• Rocks that can be radiometrically dated are mostly IGNEOUS and

METAMORPHIC rock• Sedimentary rocks cannot be dated by carbon-14 because they

are made up of particles eroded from older rocks• The Oldest Known Rocks• Using radiometric dating, the oldest rocks on Earth are about

3.96 billion years old• The Earth is about 4.5-4.6 billion years old

Page 14: Absolute Ages of Rock Mr. Perez. Important vocabulary Isotopes Radioactive decay Half-life Radiometric dating Uniformitarianism
Page 15: Absolute Ages of Rock Mr. Perez. Important vocabulary Isotopes Radioactive decay Half-life Radiometric dating Uniformitarianism

Uniformitarianism• James Hutton (Scottish scientist in the 1700s)• Estimated that the Earth is much older than we think• Uniformitarianism- principle that states that Earth’s processes

occurring today are similar to those that occurred in the past• “The present is the key to the past”

• Today, scientists recognize that Earth has been shaped by two types of change1. Slow, everyday processes that take place over millions of years2. Violent and unusual events

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Resources• Florida Science Grade 7

Glencoe Science & McGraw Hill Publishing• Google Images


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