1. Earth Science Chapter 21 Section 3
Absolute-Age Dating
Earth Science
Chapter 21
Section 3

2. Radioactive Isotopes
Scientists can determine the numerical age of rocks and other rocks
Examining radioactive isotopes
Examining the remains of fossils
What is an isotope?

3. Radioactive Isotopes Radioactive decay
Definition: the emission of radioactive particles and the resulting change into other isotopes
Decay is constant so scientists can determine absolute age.
Radioactive isotopes emit nuclear particles at a constant rate
As number of protons change, the original radioactive (parent) isotope is converted to a new element (daughter)
Compare Absolute-Age dating to Relative-Age dating.

4. Radiometric Dating
As parent atoms decrease during radioactive decay, the number of daughter atoms increase
Ratio of parent to daughter atoms indicate how much time has passed
What percent of daughter isotopes would exist in a sample containing 50 percent parent isotope?

5. Half-Life
The length of time it takes for one-half of the original isotope to decay
After 1 half-life 50% of the parent remains (1:1 ratio with daughter)
After 2 half lives, one half of the remaining parent decays (25:75 ratio of the original parent to daughter)
What would the ratio be for 3 half-lives?

6. Dating Rocks
Dating igneous or metamorphic rock requires the examination of parent-daughter ratios of minerals that make up the rock.
Best isotope to use for dating depends on the appropriate age of rock being dated.
Uranium 235 has a half life of 700 million years
Used for dating rocks 10s of million years old.
The dating isotope must have a half life that is longer than the rock being tested.
Explain why radiometric dating is not used for sedimentary rocks?

7. Radiocarbon Dating
Carbon-14 (C-14) is used to determine the age of organic materials.
Plant and animal material: bones, charcoal, amber
C-14 has a shorter half-life than other isotopes.
Living organisms replenish C-14 when alive
At death the C-14 begins to decrease
Measuring the amount of C-14 in a sample indicates how much time has passed since its death
Used for dating recent geological events.
Why is radiocarbon dating used for recent geological events?
Compare and contrast the use of U-238 and C-14 in absolute age dating.

8. Other Ways to Determine Absolute Age
Tree Rings
Annual rings in trees consist of two seasonal rings
Early and late season
Rings show conditions
Trees in same geographical region have similar ring patterns
Dendrochronology: the science of using tree rings to determine absolute aging.
Describe how archaeologists use dendrochronology to calculate the age of finds?

9. Other Ways to Determine Absolute Age
Ice Cores
Contain a record of past environmental conditionals in annual layers of snow.
Summer ice has more bubbles/larger crystals
Study glacial cycles
How could ice cores be used to study climate change? Give exmples.

10. Other Ways to Determine Absolute Age
Varves
Bands of alternating light and dark colored sediments of sand, clay and silt
Represent the seasonal deposition of sediments
Lakes
Summer deposits: Sand sized particles with traces of living matter
Winter deposits: Thin fine grained sediments
Describe the usefulness of Varves to geologists who study glacial lake deposits.

11. Vocabulary Absolute-age dating Radioactive decay Radiometric dating
Half life
Radiocarbon dating
Dendrochronology
Varve
Isotope