Earth ScienceEarth Science

Chapter 21Chapter 21

Section 3Section 3

Scientists can determine the numerical age of

rocks and other rocks Examining radioactive isotopes Examining the remains of fossils

What is an isotope?

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.

Radioactive Isotopes

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?

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?

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?

Dating Rocks

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.

Radiocarbon Dating

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?

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.

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.

Other Ways to Determine Absolute

Age

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

Vocabulary