Chapter 8 Section 2
Handout
Determining Absolute Age
1. What does relative
age indicate?
• Only that one rock formation is
younger or older than another
rock formation.
2. What is absolute age?
• The numeric age
• Actual age in years
3. In general, at about what rate
is sedimentary rock such as
limestone, shale, or sandstone
deposited?
• About 30cm of sedimentary
rock are deposited over a period
of 1,000 years.
4. What are two reasons a
sedimentary layer might not be
deposited at the average rate?
• A flood can deposit many
meters of sediment in a single
day
• Rates of deposition may change
over time
5. Small amounts of what
type of materials in rocks
can act as natural clocks?
• Radioactive materials
6. Atoms of the same
element that have different
numbers of neutrons are
called
• Isotopes
7. Radioactive isotopes
emit particles and energy
• At a constant rate regardless of
surrounding conditions.
8. In what way is the natural
breakdown of radioactive
elements most useful?
• It can accurately measure the
absolute age of rocks.
9. The method of using
radioactive decay to
measure the absolute age of
rocks is called
• Radioactive dating
10. The original
radioactive isotope in a
rock is called
• The parent isotope.
11. What are daughter
isotopes?
• A different isotope of the same
element or an isotope of a
different element into which a
radioactive atom has changed
as it emits particles and energy.
12. What is a half-life?
• The amount of time it takes for
half the mass of a given amount
of a radioactive isotope to
decay into its daughter isotope.
13. What is the half-life
of carbon – 14?
• about 5,730 years
14. Why does radioactive carbon
– 14 begin to decay after a plant
or animals dies?
• Because the organism is dead,
it no longer absorbs Carbon – 12
or Carbon – 14.
The End ???
Chapter 8 Section 2
Review Page 196
1. What is the difference
between relative and absolute
age.
• Relative age is the age of an
object compared to the age of
another object. (This rock layer is
younger than this fault.)
• Absolute age is the numerical age
of an object. (This rock layer is
125 million years old!)
Relative Age Dating
Absolute Age Dating
2. Explain why
calculations of absolute
age based on rates of
erosion and deposition
can be inaccurate.
• The rate at which sediment
is deposited or eroded
away can vary from year to
year due to many factors.
Google Earth.lnk
Rates of erosion and
deposition
3. Describe varves, and
describe how and
where they form.
• Varves are layers of sediment
made of one layer of sand
covered by one layer of silt.
They are deposited at the
bottom of glacial lakes and
represent one year of
deposition.
Varves
4. Explain how
radiometric dating is
used to estimate
absolute age.
• By comparing the percentage of a
radioactive (parent) isotope to a stable
(daughter) isotope in a sample of rock,
and based on the known rate of decay
of the parent, scientists can calculate
the length of time since the rock
formed.
Radiocarbon Dating: Parent to Daughter
5. Define half-life, and
explain how it helps
determine an object’s
absolute age.
• Half-life is the amount of time
that one half of a sample of a
radioactive isotope takes to
break down by radioactive
decay to form a daughter
substance.
6. Suppose you have a shark’s
tooth that you suspect is about
15,000 years old. Would you use
238U or 14C to date the tooth?
Explain your answer.
• You would use radiocarbon (14C) dating
because the object is organic and because it
is too young to be accurately dated by using
238U.
The End…