Transcript Unit 7 Lesson 3

Unit 7 Lesson 3 Absolute Dating
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Unit 7 Lesson 3 Absolute Dating
Florida Benchmarks
• SC.7.N.1.5 Describe the methods used in the
pursuit of a scientific explanation as seen in
different fields of science such as biology, geology,
and physics.
• SC.7.E.6.3 Identify current methods for
measuring the age of Earth and its parts,
including the law of superposition and radioactive
dating.
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Unit 7 Lesson 3 Absolute Dating
It’s About Time!
How can the absolute age of rock be
determined?
Determining the age of an event or object in years
is called absolute dating.
• Scientists often use radioactive isotopes to find
the absolute age of rocks and other materials.
• An isotope is an atom of an element that has a
certain number of neutrons.
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Unit 7 Lesson 3 Absolute Dating
How can the absolute age of rock be
determined?
• Isotopes decay at set rates, so they can be used
to date rocks
Radioactive isotopes are isotopes that are
unstable and break down into other isotopes in a
process called radioactive decay.
• Radioactive decay takes place when an unstable
isotope breaks down into a more stable isotope
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• The radioactive isotope is called the parent
isotope, and the stable isotope formed by its
breakdown is called the daughter isotope.
Half-life is the time needed for half of a sample
of a radioactive element to decay and form
daughter isotopes.
Unit 7 Lesson 3 Absolute Dating
How can the absolute age of rock be
determined?
• Scientists study the amounts of parent and
daughter isotopes to date samples.
Finding the absolute age of a sample by
determining the relative percentage of a
radioactive parent isotope and a daughter isotope
is called radiometric dating.
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Unit 7 Lesson 3 Absolute Dating
What is the best rock for radiometric
dating?
Igneous rocks are the best types of rock samples
to use for radiometric dating.
• When they form, minerals in igneous rocks often
contain only a parent isotope and none of the
daughter isotope.
• This makes the isotope percentage more accurate
and easier to interpret.
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Unit 7 Lesson 3 Absolute Dating
Time for a Change
What are some radiometric dating
methods?
• Scientists use many different isotopes for
radiometric dating.
• The type of isotope used depends on the type of
material being dated.
• The half-life of the isotope used is also very
important. It can’t be too short or too long
compared to the age of the sample.
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Unit 7 Lesson 3 Absolute Dating
What are some radiometric dating
methods?
Radiocarbon dating is a method used for dating
wood, bones, shells, and other organic remains
that are less than 45,000 years old.
• All living things have a constant ratio of
radioactive carbon-14 to carbon-12.
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• Once a plant or animal dies and no more carbon-12 is
taken in, the ratio between the isotopes changes.
• The number of half-lives of carbon-14 that have
passed gives the absolute age.
carbon-14 has a short half-life compared to potassium40, which is why it has a shorter time range.
Unit 7 Lesson 3 Absolute Dating
What are some radiometric dating
methods?
Potassium-argon dating is often used to date
igneous volcanic rocks that are 100,000 years to
billions of years old. In order to find the age of a
mineral you would need to know what the half-life
of potassium-40 is.
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 Uranium-lead
dating can be used to determine the
age of igneous rocks that are between 100 million
years and a few billion years old.
 It
measures the relative amounts of uranium-238 and
lead-206 that is present in a sample.
Unit 7 Lesson 3 Absolute Dating
Time Will Tell
How is radiometric dating used to
determine the age of Earth?
Radiometric dating can be used to find the age of
Earth, though not by dating Earth rocks.
Meteorites that fall to Earth are the same age as
the solar system, including Earth.
The absolute age of meteorites and other rocks in
the solar system is about 4.6 billion years old.
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Unit 7 Lesson 3 Absolute Dating
Showing Your Age
How can fossils help to determine the
age of sedimentary rock?
• Fossils can help in estimating relative ages of
some rocks.
• Fossils in older layers of rock are older than fossils
in younger rock layers.
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Unit 7 Lesson 3 Absolute Dating
Showing Your Age
How can fossils help to determine the
age of sedimentary rock?
• Certain types of fossils can be used to estimate
the absolute age of the sedimentary rock they are
found in.
An index fossil is a fossil used to estimate the
absolute age of rock layers.
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Unit 7 Lesson 3 Absolute Dating
How can fossils help to determine the
age of sedimentary rock?
• Index fossils have some specific characteristics.
• They are found over a large area.
• They have features that make them different from
other fossils.
Index fossils live for a short amount of time.
• They are abundant in the rock layers.
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Unit 7 Lesson 3 Absolute Dating
How are index fossils used?
• Index fossils act as markers for the time that the
organisms were alive on Earth.
• Index fossils can also be used to date rocks in
separate areas.
The appearance of the same index fossil in rock of
different areas shows that the rock layers formed
at the same time.
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