Transcript Absolute-Age Dating

Earth Science
Chapter 21
Section 3
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?
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.
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 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?
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.
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?
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.
Vocabulary

 Absolute-age dating
 Radioactive decay
 Radiometric dating
 Half life
 Radiocarbon dating
 Dendrochronology
 Varve
 Isotope