Download 3 Absolute Dating: A Measure of Time

Name
CHAPTER 6
Class
Date
The Rock and Fossil Record
SECTION
3 Absolute Dating: A Measure of Time
National Science
Education Standards
BEFORE YOU READ
After you read this section, you should be able to answer
these questions:
ES 2b
• How can geologists learn the exact age of a rock?
• What is radiometric dating?
What Is Radioactive Decay?
Geologists can use the methods of relative dating to
learn whether a rock is older or younger than another
rock. However, they often also need to know exactly how
old a rock is. Finding the exact age of an object is called
absolute dating. One way to learn the age of a rock is to
use unstable atoms.
All matter, including rock, is made of atoms. All atoms
are made of three kinds of particles: protons, neutrons,
and electrons. All of the atoms of an element, such as
uranium, have the same number of protons. However,
some atoms of an element have different numbers of neutrons. Atoms of an element that have different numbers
of neutrons are called isotopes.
Many isotopes are stable and are always in the same
form. However, other isotopes are unstable and can
break down into new isotopes of different elements. An
unstable isotope is also called a radioactive isotope.
Radioactive decay happens when a radioactive isotope
breaks down into a new isotope.
STUDY TIP
Learn New Words As you
read, underline words that
you don’t understand. When
you learn what they mean,
write the words and their
definitions in your notebook.
READING CHECK
1. Define What are isotopes?
Proton
Neutron
After radioactive
decay, an isotope
of a new element
is left. The new
isotope is stable.
Electron
This isotope is unstable,
or radioactive.
TAKE A LOOK
2. Compare How is a
radioactive isotope different
from a stable isotope?
Radioactive isotopes can
decay in different ways.
During one kind of radioactive
decay, a neutron becomes a
proton and an electron.
The electron moves to a
different part of the atom.
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The Rock and Fossil Record
Name
SECTION 3
Class
Date
Absolute Dating: A Measure of Time continued
Critical Thinking
3. Infer What happens to
the amount of parent isotope
in a rock with time? What
happens to the amount of
daughter isotope?
RADIOMETRIC DATING
A radioactive isotope is also called a parent isotope.
Parent isotopes break down into daughter isotopes.
Because of radioactive decay, the amounts of parent and
daughter isotopes in a rock are always changing. However,
they change at a constant, known rate. Therefore, scientists can learn the age of a rock by studying the amounts of
parent and daughter isotopes in it.
Radiometric dating is the process of determining the
absolute age of a sample based on the ratio of parent isotope to daughter isotope. In order to use radiometric dating, you need to know the half-life of the parent isotope.
The half-life of a radioactive isotope is how long it
takes for half of a sample of the isotope to decay. For
example, imagine that a parent isotope has a half-life of
10,000 years. A sample of this isotope has a mass of 12 mg.
After 10,000 years, only one-half, or 6 mg, of the sample
will be left.
YEARS
0ARENTISOTOPEMG
4HISSAMPLECONTAINS
MGOFAPARENT
ISOTOPE4HEISOTOPE
HASAHALFLIFEOF
YEARS
YEARS
0ARENTISOTOPE
MG??????
!FTERONEHALFLIFE
OFTHEORIGINAL
MASSOFPARENT
ISOTOPEISLEFT
YEARS
0ARENTISOTOPE
MG???????
!FTERTWOHALFLIVES
OROF
THEORIGINALMASSOF
PARENTISOTOPEISLEFT
YEARS
0ARENTISOTOPE
MG
????????????
!FTERTHREEHALFLIVES
OROFTHEORIGINALMASSOF
PARENTISOTOPEISLEFT
Math Focus
4. Calculate Fill in the blank
lines in the figure with the
mass of parent isotope that is
left at each step.
The half-lives of different isotopes can be very different. Some parent isotopes have half-lives of more than
4 billion years. Others have half-lives of only about 6,000
years. Very old rocks can be dated only if isotopes with
long half-lives are used. Very young rocks can be dated
only if isotopes with short half-lives are used.
How do scientists know which isotope to use to find
the age of a rock? They use information about the relative age of the rock to guess about how old the rock is.
Then, they find its age, using an isotope that is useful for
dating rocks of that age.
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The Rock and Fossil Record
Name
SECTION 3
Class
Date
Absolute Dating: A Measure of Time continued
What Isotopes Can Be Used for
Radiometric Dating?
Remember that different parent isotopes have different
half-lives. Each parent isotope can be used to date rocks of
different ages.
POTASSIUM-ARGON METHOD
Potassium-40 is one isotope that is often used in
radiometric dating. It has a half-life of 1.3 billion years.
It decays to produce the daughter isotope argon-40.
Scientists usually use the potassium-argon method to
date rocks that are older than about 1 million years.
URANIUM-LEAD METHOD
Uranium-238 is also used for radiometric dating. It
has a half-life of 4.5 billion years. It decays to produce
lead-206. Scientists use the uranium-lead method to date
rocks that are older than about 10 million years.
READING CHECK
5. Explain Using relative
dating, a scientist learns that
a rock is about 50,000 years
old. Can the scientist use the
potassium-argon method
to find the exact age of this
rock? Explain your answer.
RUBIDIUM-STRONTIUM METHOD
Rubidium-87 is also used for radiometric dating. It has
a half-life of about 48 billion years. It decays to produce
the daughter isotope strontium-87. The half-life of
rubidium-87 is very long. Therefore, this method is only
useful for dating rocks older than about 10 million years.
CARBON-14 METHOD
Carbon-14 is a radioactive isotope of the element carbon.
Carbon-14, along with the other isotopes of carbon, combines with oxygen to form the gas carbon dioxide. Plants
use carbon dioxide to make food. Therefore, living plants
are always taking in small amounts of carbon-14. Animals
that eat plants also take in carbon-14 from the plants.
When a plant or animal dies, it stops taking in carbon-14.
The carbon-14 already in its body starts to decay to produce
nitrogen-14. Carbon-14 has a short half-life: only 5,730 years.
Therefore, this method can be used to date the remains of
organisms that died in the last 50,000 years.
Parent isotope
Daughter isotope
Half-life
Potassium-40
READING CHECK
6. Describe How do animals
take in carbon-14?
TAKE A LOOK
7. Identify Fill in the spaces
in the chart to show the
features of different parent
isotopes.
Uranium-238
Rubidium-87
Carbon-14
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The Rock and Fossil Record
Name
Class
Date
Section 3 Review
NSES
ES 2b
SECTION VOCABULARY
absolute dating any method of measuring the
age of an event or object in years
half-life the time required for half of a sample
of a radioactive isotope to break down by
radioactive decay to form a daughter isotope
isotope an atom that has the same number of
protons (or the same atomic number) as other
atoms of the same element do but that has a
different number of neutrons (and thus a
different atomic mass)
radioactive decay the process in which a
radioactive isotope tends to break down into a
stable isotope of the same element or another
element
radiometric dating a method of determining
the absolute age of an object by comparing the
relative percentages of a radioactive (parent)
isotope and a stable (daughter) isotope
1. Describe How is radioactive decay related to radiometric dating?
2. Calculate A parent isotope has a half-life of 1 million years. If a rock contained
20 mg of the parent isotope when it formed, how much parent isotope would be
left after 2 million years? Show your work.
3. List What are two radioactive isotopes that are useful for dating rocks that are
older than 10 million years?
4. Apply Concepts A geologist uses relative dating methods to guess that a rock is
between 1 million and 5 million years old. What is one radioactive isotope the
geologist can use to learn the exact age of the rock? Explain your answer.
5. Infer Why can’t geologists use the carbon-14 method to date igneous rocks? Why
can’t they use the carbon-14 method to date dinosaur bones?
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The Rock and Fossil Record
Earth Science Answer Key continued
2. After 1 million years, there would be (1/2) 8. Possible answers: There was no deposition
9.
10.
11.
12.
(20 mg) 10 mg of parent isotope remaining.
After 2 million years, (1/2) (1/2) (20 mg) 5 mg of parent isotope would remain.
3. uranium-238, rubidium-87
4. Potassium-40, because it can be used to date
rocks that are older than about 1 million years.
5. C-14 dating can be used only on the remains
of living organisms. Igneous rocks do not
contain these remains. C-14 dating can be
used only on remains that are less than
50,000 years old. Dinosaur bones are older
than this.
happening at that time; there was a lot of
erosion happening at that time.
erosion, nondeposition
a place where part of a sequence of parallel
rocks is missing
Rocks are pushed up and eroded. Later, sediment is deposited on top of the eroded rock.
The rock layers below an angular unconformity are tilted and may be any kind of rock.
Review
1. An unconformity can form if no sediment is
2.
3.
4.
5.
deposited for a long time. An unconformity can
also form if layers of rock are eroded away.
an angular unconformity
The youngest rock layers are at the top, and
the oldest rock layers are at the bottom.
to interpret rock sequences and to identify
rock layers
A sequence of rock layers is uplifted and
eroded. Then, sediment is deposited on the
exposed rock layers. After a while, the sediment turns into rock.
SECTION 4 LOOKING AT FOSSILS
1. Body fossils are fossilized parts of an
2.
3.
4.
5.
6.
SECTION 3 ABSOLUTE DATING: A
MEASURE OF TIME
7.
1. atoms of an element with different numbers
2.
3.
4.
5.
6.
7.
of neutrons
Radioactive isotopes can break down. Stable
isotopes do not break down.
The amount of parent isotope decreases, and
the amount of daughter isotope increases.
8 mg
No, because K-Ar dating can be used only for
rocks that are older than about 1 million years.
by eating plants
Parent isotope
Daughter
isotope
8.
9.
10.
Review
1. tracks, burrows, coprolites
2. Only a small fraction of the organisms that
Half-life
Potassium-40
argon-40
1.3 billion years
Uranium-238
lead-206
4.5 billion years
Rubidium-87
strontium-87
48 billion years
Carbon-14
nitrogen-14
5,730 years
organism. Trace fossils are signs that an
organism once existed.
shells, teeth, bones
when minerals replace an organism’s tissue
They are not made of parts of an organism,
but they show that an organism once existed.
an impression left in sediment
the kinds of organisms that lived in the past;
how the environment has changed; how
organisms have changed
Organisms that had hard parts or lived in
certain environments were more likely to be
fossilized when they died.
They compare fossils. They also compare
fossils to living organisms.
Answers include: according to their age, by
absolute and relative dating methods
their shells
have existed in Earth’s history have been
fossilized. Many fossils have not yet been
discovered.
3. A beetle, because fossils in amber are made
when an organism gets stuck in tree sap.
Smaller organisms that can be found on
trees are more likely than other organisms
to become fossils in amber.
4. The climate was probably much warmer
when the plant was alive.
5. It must be common throughout the world. It
must have existed for a relatively short geologic time. It must be easy to identify.
Review
1. Radiometric dating uses known rates of
radioactive decay to determine the age of a
rock sample.
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Earth Science