21.3 – Absolute-Age Dating
Objectives
 describe how scientists date rocks and other objects using
radioactive elements
 Explain how scientists can use certain non-radioactive
material to date geologic events
Main Idea: radioactive decay and certain kinds
of sediments help scientists determine the
numeric age of many rocks
Radioactive Isotopes
Measure the decay of the radioactive isotopes in
igneous and metamorphic rocks
Radioactive Decay
 Radioactive isotopes emit nuclear particles at a constant rate
 Identity of an element is due to the number of protons is has
Radioactive decay is constant:
• Regardless of pressure,
temperature, or any other
physical changes
• Allows scientists to determine
absolute age of object in which
radioactive element occurs
Radiometric Dating
The ratio of parent isotope to
daughter product indicates
the amount of time that has
passed since the object
formed
• When scientists date an
object using radioactive
isotopes, they are using a
method called radiometric
dating
Half-life and dating rocks
Half-life: length of time it takes for one-half of
the original isotope to decay
 After one half-life, 50% of the parent
remains, resulting in a 1:1 ratio of parent-to
–daughter product.
 What would happen after 2 half-lives?
Dating Rocks
to date using radiometric dating, scientists examine the parent-daughter
ratios of the radioactive isotopes in the minerals within the rocks
• Scientists might use U235, which has a half-life
of 700 million yrs, to date
a rock that is a few tens of
millions of years old
• If you wanted to date a
rock that is hundreds of
millions of years old,
scientists might use U-238
due to its longer half-life
• Might not work well if you
use an element with a
short half life to date
ancient rocks…why?
Radiocarbon Dating
Radioactive parent isotope
Approximate Half Life
Potassium-40 (K-40)
1.3 billion years
Uranium-238 (U-238)
4.5 billion years
Uranium-235 (U-235)
700 million years
Carbon-14 (C-14)
5730 years
Scientists use C-14 to determine the age of organic materials, which contain abundant
carbon, in a process known as radiocarbon dating
Radiocarbon Dating
Tissues of all living organisms contain small
amounts of C-14 and large amount of C-12
(most abundant form of carbon)
 Throughout life, C-14 decays but is continually replenished
through respiration
 Therefore, when organism dies, it no longer takes in C-14, so
over time, the ratio of C-14 to C-12 decreases
 Measuring the amount of C-14 can help determine how much
time has passed since death
Other ways to determine absolute age
Tree rings
 Many trees contain a record of time in the rings of their
trunks (annual tree rings)
 Each annual tree ring consists of a pair of early season and
late season growth rings
Science of using tree rings to determine
absolute age is called dendrochronology
• helps confirm the results from
radiocarbon dating
Matching
tree rings
from
different
wood
samples
Ice Cores
Analogous to tree rings
 Like tree rings, they contain a record of past environmental
conditions in annual layers of snow deposition
 Summer ice tends to have more bubbles and larger crystals
than winter ice