Absolute Dating
• any method of measuring the age of an event or object in years
• radiometric dating (which uses the concept of radioactive
decay) is the most common method of absolute dating
• used to determine the age of rocks and fossils
• uses the concept of isotopes
• uses the concept of half-life
What is an isotope?
• Most elements have the same number of protons and neutrons
• An isotope is an element that has the same number of protons
as other atoms of the same element do but have a different
number of neutrons
• Some isotopes are stable – meaning that they do not change
over time
• Other isotopes are unstable
o Scientists call unstable isotopes radioactive
o These unstable isotopes tend to break down into stable
isotopes over time
How are isotopes used to calculate the age of rocks?
• The process of the break down of radioactive isotopes into
stable isotopes is called radioactive decay
• Unstable isotopes of one element break down or decay to form
a stable isotope of the same element or a different element
• Examples:
o Carbon-14 decays to form Carbon-12
o Uranium-238 decays to form Lead-206
• The unstable isotope is referred to as the parent material
• The stable material produced by the decay of parent material is
called the daughter isotope
• Radioactive decay occurs at a steady rate
• Scientists use the relative amounts of stable (daughter) and
unstable (parent) isotopes in an object to determine an object’s
age
• A old object will have more daughter isotopes compared to the
amount of parents isotopes
What is a “half-life” and how is it used to determine the age of
an object?
• A half-life is the time needed for half of a sample of a
radioactive substance to undergo radioactive decay
• In other words, a half-life is the time needed for half of the
parent material to become daughter material
• The rate of radioactive decay is predictable
• The rate of radioactive decay depends on what parent material
is decaying
• The rate of decay is different for different types of parent
material
• Radiometric Dating = the method of determining the age of an
object based on the ratio of parent material to daughter material
Radioactive parent isotopes and their stable daughter products
Radioactive Parent
Stable Daughter
Potassium 40
Argon 40
Rubidium 87
Strontium 87
Thorium 232
Lead 208
Uranium 235
Lead 207
Uranium 238
Lead 206
Carbon 14
Nitrogen 14
Each radioactive isotope has its own unique half-life.
A half-life is the time it takes for half of the parent radioactive
element to decay to a daughter product.
Half Lives for Radioactive Elements
Radioactive Parent
Stable Daughter
Half life
Potassium 40
Argon 40
1.25 billion yrs
Rubidium 87
Strontium 87
48.8 billion yrs
Thorium 232
Lead 208
14 billion years
Uranium 235
Lead 207
704 million years
Uranium 238
Lead 206
4.47 billion years
Carbon 14
Nitrogen 14
5730 years
Radioactive decay occurs at a constant rate.
The rate of decay is proportional to the number of parent atoms
present.
The proportion of parent to daughter tells us the number of halflives, which we can use to find the age in years.
For example, if there are equal amounts of parent and daughter, then
one half-life has passed.
If there is three times as much daughter as parent, then two halflives have passed.
Example:
• A rock sample contains an isotope that has a half-life of
100,000 years
• You analyze the sample and find that it has equal amounts of
the radioactive parent material and the stable daughter material
• This means that half of the radioactive isotopes have decayed –
so the rock sample must be 100,000 years old
• Knowing that it will take another 100,000 years for half of the
remaining parent material to decay
• You analyze a different sample and find that ¾ of the material
is made of daughter material and only ¼ is made of parent
material
• How old is this sample?
o 100,000 years + 100,000 years = 200,000 years
Potassium – Argon Dating Method
• Method for determining the age of igneous rocks based on the
amount of argon-40 in the rock
• Radioactive potassium-40 decays to argon-40 with a half-life of
about 1.3 billion years
• this method useful for dating rocks that are billions of years old
• Used to date rocks older than 100,000 years
Uranium – Lead Dating Method
• Uranium-238 is a radioactive isotope that decays in a series of
steps to lead-206
• Half-life of Uranium-238 is 4.5 billion years
• Method of dating very old rocks by means of the amount of
common lead they contain
• Used to date rocks more than 10 million years old
• By this method, the age of the Earth has been estimated to be
approximately 4.6 billion years
• The oldest rock sample on record is a metamorphic gneiss from
northern Canada, which is dated at 3.9 billion years old
Rubidium – Strontium Dating Method
• Method of estimating the age of rocks, minerals, and meteorites
• Uses measurements of the amount of the stable isotope
strontium-87 formed by the decay of the unstable isotope
rubidium-87 that was present in the rock at the time of its
formation
• This method is applicable to very old rocks because the
transformation from rubidium to strontium is extremely slow
• The half-life, or time required for half the initial quantity of
rubidium-87 to disappear, is approximately 49 billion years
• This method is used to date rocks older than 10 million years
Carbon 14 Dating Method
• Method of determining the age of once-living material
• Developed in 1947
• Depends on the decay of the radioactive isotope carbon-14
(radiocarbon) to nitrogen-14 (stable carbon)
• Cosmic rays from the sun strike Nitrogen 14 atoms in the
atmosphere and cause them to turn into radioactive Carbon 14,
which combines with oxygen to form radioactive carbon
dioxide
• All living plants and animals continually take in carbon - Some
of this carbon is radioactive carbon-14, which slowly decays to
the stable isotope nitrogen-14
o green plants absorb it in the form of carbon dioxide from
the atmosphere. Plants use this carbon dioxide to form
sugar through photosynthesis.
o it is passed to animals through the food chain
• All living things contain a constant ratio of Carbon 14 to
Carbon 12. (1 in a trillion)
• When an organism dies it stops taking in carbon, so the amount
of carbon-14 in its tissues steadily decreases
• Because carbon-14 decays at a constant rate, the time since an
organism died can be estimated by measuring the amount of
radiocarbon in its remains.
• The half-life of carbon-14 is only 5,730 years
• This method is a useful technique for dating fossils and
archaeological specimens from 500 to 50,000 years old
• It is widely used by geologists, anthropologists, and
archaeologists
Dating Method
Used to date objects . . .
Potassium-Argon
older than 100,000 years
Uranium-Lead
older than 10 million years
Rubidium-Strontium
older than 10 million years
Carbon-14
less than 50,000 years
What absolute dating method would you use to date the rocks at the
bottom of the Grand Canyon?
What absolute dating method would you use to date the rocks within
which dinosaur fossils are found?
What absolute dating method would you use to date ancient Native
American items found at an archeological dig?
Is radiometric dating used to determine the age of
sedimentary rocks?
•
•
•
Radioactive isotopes don'
t tell much about the age of
sedimentary rocks (or fossils).
The radioactive minerals in sedimentary rocks are derived
from the weathering of igneous rocks.
If the sedimentary rock were dated, the age date would be the
time of cooling of the magma that formed the igneous rock.
The date would not tell anything about when the sedimentary rock
formed.
Summary
• During radioactive decay, an unstable isotope decays at a
constant rate and becomes a stable isotope of the same or a
different element
• The unstable isotope is called the parent material
• The stable isotope is called the daughter material
• Half-life is the time it takes for half of the parent material to
decay to form daughter material
• Radiometric dating, based on the ratio of parent to daughter
material, used to determine the absolute age of a sample
• Depending upon the perceived age of a sample, scientists use
different methods of radiometric dating, including carbon-14
dating
• The Geologic Column was produced using both relative and
absolute dating methods