Grade 8 Integrated Science Chapter 16 Lesson 3 on absolute age dating of fossils. This lesson follows the last lesson about relative age dating. This chapter describes radiometric age dating with explanations of radioactive decay and half-life. There is also a short explanation of igneous, metamorphic, and sedimentary age dating. The goal is that students understand radioactive decay, half-life, and how this can be used to determine the age of carbon fossils and different types of rocks.

1. Absolute Age Dating
Chapter 16 Lesson 3
p 582-589

2. Vocabulary
• Absolute age (583) – the numerical age, in years,
of a rock or object
• Isotope (584) – atoms of the same element that
have different numbers of neutrons
• Radioactive decay (584) – the process by which
an unstable element naturally changes into
another element that is stable
• Half-life (585) – the time required for half of the
parent isotopes to decay into daughter isotopes

3. Absolute Age of Rocks
• Absolute age means the numerical age, in years,
of a rock or object.
– What is your absolute age?
– How is absolute age different from relative age?
• Scientists have been able to determine the
absolute ages of rocks and other objects only
since the beginning of the twentieth century.
– Once radioactivity had been discovered.
– Radioactivity is the release of energy from unstable
atoms

4. Atoms
• You are all familiar with atoms.
– What are the parts of an atom?
– What determines the element of an atom?
– What is in the nucleus of an atom?
– What surrounds the nucleus?

5. Review of Isotopes
• All atoms of a given element have the same
number of protons
– How many protons does a hydrogen atom have?
• However, an element’s atoms can have different
numbers of neutrons.
• Atoms of the same element that have different
numbers of neutrons are called isotopes.
– We name isotopes with the element name and the
number of particles (protons+neutrons) in its nucleus.

7. Radioactive Decay
• Most isotopes are stable.
– Stable isotopes do not change under normal conditions
• Unstable isotopes are called radioactive isotopes.
– Radioactive isotopes decay, or change, over time.
– As they decay, they release energy and form new, stable atoms.
• Radioactive decay is the process by which an unstable
element naturally changes into another element that is stable.

8. Radioactive Decay
• The unstable isotope that decays is called the parent isotope.
• The new element that forms is called the daughter isotope.
• In the figure, the atoms of an unstable isotope of hydrogen
(parent) decay into atoms of a stable isotope of helium
(daughter)

9. Half-Life
• The rate of decay from parent isotopes into
daughter isotopes is different for different
radioactive elements.
– Rate of decay is constant for a given isotope
– This rate is measure in time units called half-lives
• An isotope’s half-life is the time required for half
of the parent isotopes to decay into daughter
isotopes.
– Half-lives of radioactive isotopes range from a few
microseconds to billions of years.

10. • As time passes, more and more unstable parent
isotopes decay and form stable daughter isotopes.
• The means the ratio of parent and daughter isotopes
is always changing.
• When half the parent isotopes have decayed into
daughter isotopes, the isotope has reached one half-
life.

11. Half-lives Time
Percentageofremaining
parentatoms
0 1 2 3 4
100
50
25
12.5
6.25

12. • After one half-life, 50% of the isotopes are
parents and 50% of the isotopes are daughters
• After two half-lives, 50% of the remaining
parent isotopes have decayed so that only a
quarter of the original parent isotopes remain.
• This process continues until nearly all parent
isotopes have decayed into daughter isotopes.

13. Radiometric Ages
• Because radioactive isotopes decay at a constant
rate, they can be used like clocks to measure the
age of the material that contains them.
• In this process, called radiometric dating,
scientists measure the amount of parent isotope
and daughter isotope in a sample of material they
want to date.
– From this ratio, they can determine the material’s age.

14. Review
• What is measured in radiometric dating?
– The amount of the parent isotope and daughter
isotope.

15. Radiocarbon Dating
• One important radioactive isotope used for
dating is an isotope of carbon called radiocarbon.
– Radiocarbon is also known as carbon-14 or C-14.
• How many protons and neutrons does C-14 have?
– 6 protons and 8 neutrons
• Radiocarbon forms in Earth’s upper atmosphere
where it mixes with a stable carbon isotope called
carbon-12 or C-12.
• The ratio of the amount of C-14 and C-12 in the
atmosphere is constant.

17. Radiocarbon Dating
• All living things use carbon as they build and repair
tissues
• As long as an organism is alive, the ratio of C-14 to C-12
in its tissues is identical to the ratio in the atmosphere.
• However, if an organism dies, it stops taking in C-14.
– The C-14 present in the organism starts to decay to
nitrogen-14 (N-14).
– As the dead organism’s C-14 decays, the ratio of C-14 to C-
12 changes.
• Scientists measure the ratio of C-14 to C-12 in the
remains of the dead organism to determine how much
time has passed since the organism died.

21. Radiocarbon Dating
• The half-life of carbon-14 is 5,730 years.
• That means radiocarbon dating is useful for
measuring the age of remains of organisms
that died up to about 60,000 years ago.
• In remains older than this, there is not enough
C-14 left to measure accurately.

22. Review
• What two isotopes of carbon are present in our
atmosphere?
• Is the ratio of carbon isotopes in the atmosphere constant
or changing?
• C-14 decays into what isotope?
• Should we expect more C-14 or N-14 in an organism that
has been dead for 40,000 years?
C-12 and C-14
The ratio of C-12 to C-14 is constant.
C-14 decays into N-14.
It should have more N-14 because I has been dead
for longer than C-14’s half-life (5,730 yrs).

23. Dating Rocks
• Radiocarbon dating is useful only for
dating organic material – material from
once-living organisms.
– This material includes bones, wood,
parchment, and charcoal.
• Most rocks do not contain organic
material.
• Even most fossils are no longer organic.
– Their living tissue has been replaced by
rock-forming minerals.
• So, for dating rocks, geologists use
different kinds of radioactive isotopes.

24. Dating Igneous Rock
• One of the most common isotopes used in
radiometric dating is uranium-235 or U-235.
• U-235 is often trapped in the minerals of igneous
rocks that crystallize from hot, molten magma.
• As soon as it is trapped in a mineral, U-235
decays into lead-207 or Pb-207.
– What ratio would scientists use to determine how
much time has passed since the mineral was formed?
– Which isotope should there be more of it the rock is
older than one half-life?

26. One half-life equals .704 billion years

28. Dating Sedimentary Rock
• How does sedimentary rock form?
– From sediment and a lot of pressure over a long time.
• In order to be dated by radiometric means, that
sediment that formed the rock must contain U-
235.
– The grains of sedimentary rocks come from a variety
of weathered rocks form different locations.
• However, by measuring U-235 would scientist be
getting the date that the sedimentary rock
formed or the date that the grain of sediment
formed?

29. Dating Sedimentary Rock
• Radioactive isotopes within these grains
generally record the ages of the grains – not
when the sediment was deposited.
• For this reason, sedimentary rock is not as
easy to date as igneous rock

30. More radioactive isotopes used in
radiometric dating
• Which has the shortest half-life?
• Which has the longest?

31. Different Types of Isotopes
• The half-life of U-235 is 704 million years.
– This makes it useful for dating rocks that are very old.
• Many different isotopes are also used.
– However, would isotopes with short half-lives be
useful in dating old rocks?
– Which isotope would be too small to measure, the
parent or the daughter?
• Geologists often use a combination of radioactive
isotopes to measure the age of a rock to make it
more accurate.

32. The Age of Earth
• The oldest known rock formation dated by geologists
using radiometric means is in Canada.
• It is estimated to be between 4.03 and 4.28 billion
years old.
• However, individual crystals of the mineral zircon in
igneous rocks in Australia have been dated at 4.4
billion years.
• Radiometric dating of rocks from the Moon and
meteorites indicate that Earth is 4.54 billion years old.
– Scientists accept this age because evidence suggests that
Earth, the Moon, and meteorites all formed at about the
same time.