Outline: Chapter 16.3: Absolute Age Dating
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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. *This outline follows the Chapter 16.3 presentation slides.
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- Chapter 16 Lesson 3: Absolute Age Dating p582589 page 1
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 Absolute Age of
Rocks Absolute age means the ______________________ age, in years,
of a rock or object. o What is your absolute age? o 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 ___________________________ century. o Once
______________________________ had been discovered. o Radioactivity
is the release of _________________________ from unstable atoms.
Atoms You are all familiar with atoms. o What are the parts of an
atom? o What determines the element of an atom? o What is in the
nucleus of an atom? o What surrounds the nucleus? Review of
Isotopes All atoms of a given element have the same number of
_______________________. o How many protons does a hydrogen atom
have? However, an elements atoms can have different numbers
of_____________________. Atoms of the same element that have
different numbers of neutrons are called isotopes. o We name
isotopes with the element name and the number of particles
(protons+neutrons) in its nucleus.
- Chapter 16 Lesson 3: Absolute Age Dating p582589 page 2
Radioactive Decay Most isotopes
are________________________________. o Stable isotopes do not
change under normal conditions Unstable isotopes are called
_______________________________ isotopes. o Radioactive isotopes
_______________________, or change, over time. o As they decay,
they release energy and form _______________, stable atoms.
Radioactive decay is the process by which an unstable element
naturally changes into another element that is
________________________. The unstable isotope that decays is
called the ______________________ isotope. The new element that
forms is called the ____________________________ isotope. o In the
figure, the atoms of an unstable isotope of hydrogen (parent) decay
into atoms of a stable isotope of helium
(__________________________) Half-Life The rate of decay from
parent isotopes into daughter isotopes is different for
_________________________ radioactive elements. o Rate of decay is
constant for a given __________________________________ o This rate
is measure in _______________________ units called half-lives
- Chapter 16 Lesson 3: Absolute Age Dating p582589 page 3 An
isotopes half-life is the time required for ____________________ of
the parent isotopes to decay into daughter isotopes. o Half-lives
of radioactive isotopes range from a few microseconds to
_______________________ of years. As time passes, more and more
unstable parent isotopes decay and form _______________________
daughter isotopes. The means the ratio of parent and daughter
isotopes is always ________________________. When
______________________ the parent isotopes have decayed into
daughter isotopes, the isotope has reached one half-life.
Half-lives Time Percentageofremaining parentatoms 0 1 2 3 4 100 50
25 12.5 6.25 After one half-life, _________% of the isotopes are
parents and _________% of the isotopes are daughters After two
half-lives, 50% of the remaining parent isotopes have decayed so
that only ___________% of the original parent isotopes remain. This
process continues until nearly all parent isotopes have decayed
into daughter isotopes. Radiometric Ages Because radioactive
isotopes decay at a constant rate, they can be used like
_____________________ to measure the age of the material that
contains them.
- Chapter 16 Lesson 3: Absolute Age Dating p582589 page 4 In this
process, called _________________ ____________________, scientists
measure the amount of parent isotope and daughter isotope in a
sample of material they want to date. o From this ratio, they can
determine the materials _________________. Review What is measured
in radiometric dating? Radiocarbon Dating One important radioactive
isotope used for dating is an isotope of carbon called
_____________________________. o Radiocarbon is also known as
carbon-14 or C-14. How many protons and neutrons does C-14 have?
Radiocarbon forms in Earths upper atmosphere where it mixes with a
stable carbon isotope called carbon-_______ or C-12. The ratio of
the amount of C-14 and C-12 in the _________________________ is
constant. All _____________________ 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 ______________________ to the ratio
in the atmosphere. However, if an organism dies, it stops taking in
C-______. o The C-14 present in the organism starts to decay to
_________________-14 (N-14). o As the dead organisms C-14 decays,
the ratio of C-14 to C-12 _____________________. 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. o The
half-life of carbon-14 is _________________________ years. That
means radiocarbon dating is useful for measuring the age of remains
of organisms that died up to about _________________________ years
ago. o In remains older than this, there is not enough
_____________ left to measure accurately. 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?
- Chapter 16 Lesson 3: Absolute Age Dating p582589 page 5 Should
we expect more C-14 or N-14 in an organism that has been dead for
40,000 years? Dating Rocks Radiocarbon dating is useful only for
dating _____________________ material material from once-living
organisms. o This material includes bones, _________________,
parchment, and charcoal. Most rocks do not contain
__________________________ material. Even most
________________________ are no longer organic. o Their living
tissue has been replaced by rock-forming _______________________.
So, for dating rocks, geologists use different kinds of radioactive
isotopes. Dating Igneous Rock One of the most common isotopes used
in radiometric dating is ___________________________ or U-235.
U-235 is often trapped in the minerals of ____________________
rocks that crystallize from hot, molten magma. As soon as it is
trapped in a mineral, U-235 decays into ____________________ or
Pb-207. o What ratio would scientists use to determine how much
time has passed since the mineral was formed? o Which isotope
should there be more of it the rock is older than one
half-life?
- Chapter 16 Lesson 3: Absolute Age Dating p582589 page 6 Draw a
graph of U-235 decay. One half-life equals .704 billion years
Dating Sedimentary Rock How does sedimentary rock form? In order to
be dated by radiometric means, that ___________________ that formed
the rock must contain U-235. o The grains of sedimentary rocks come
from a variety of weathered rocks from different
_________________________. 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? Radioactive isotopes
within these grains generally record the ages of the
______________________ not when the sediment was deposited. For
this reason, sedimentary rock is not as __________________ to date
as igneous rock
- Chapter 16 Lesson 3: Absolute Age Dating p582589 page 7 More
radioactive isotopes used in radiometric dating Which has the
shortest half-life? _____________________________________________
Which has the longest?
____________________________________________________ Different
Types of Isotopes The half-life of U-235 is
____________________________________ million years. o This makes it
useful for dating rocks that are very _______________. Many
different ______________________ are also used. o However, would
isotopes with short half-lives be useful in dating old rocks? o
Which isotope would be too small to measure, the parent or the
daughter? Geologists often use a _________________________ of
radioactive isotopes to measure the age of a rock to make it more
accurate. The Age of Earth The oldest known rock formation dated by
geologists using radiometric means is in _________________________.
o It is estimated to be between _______________________________
billion years old. However, individual crystals of the mineral
zircon in igneous rocks in Australia have been dated at
________________________ billion years. Radiometric dating of rocks
from the Moon and meteorites indicate that Earth is
___________________ billion years old. o Scientists accept this age
because evidence suggests that Earth, the ___________, and
___________________ all formed at about the same time.
- Chapter 16 Lesson 3: Absolute Age Dating p582589 page 8 ***
Extra Figures***
- Chapter 16 Lesson 3: Absolute Age Dating p582589 page 9