Download Journal #23 - Mrs. Dawson`s Classroom

JOURNAL #25
Read
pages 191- 192 and answer
the following questions:
What is the difference between
relative and absolute age?
What are varves? How do they
form?
REVIEW
ABSOLUTE
DATING METHODS
TODAY’S LEARNING GOAL

Today, we will describe the process of radioactive
decay and how it can be used to determine the
absolute age of rocks
ABSOLUTE DATING METHODS
 To
learn about Earth’s history,
scientists often need to determine
the age of a rock formation.
 This is known as absolute age
(numeric age)
 Scientists use a variety of methods
to measure age.



Rates of Erosion
Rates of Deposition
Varve Count
RATES OF EROSION
 If
scientists measure the rate at
which a stream erodes its bed, they
can estimate the age of the stream.
 Example: Niagara Falls, Grand
Canyon.
RATES OF DEPOSITION
 Another
way to estimate absolute age
is to calculate the rate of sediment
deposition.
 By using data collected over a long
period of time, geologists can estimate
the average rates of deposition for
common sedimentary rocks.
 Not always accurate because a flood
(for example) can deposit meters of
rock in one day.
VARVE COUNT
A
tree’s age can be estimated by
counting the growth rings in its
trunk.
 Similarly, scientists look at layers of
sediment deposits called varves.
 They consist of light-colored band of
coarse particles and a dark band of
fine particles.
RADIOMETRIC DATING
 Rocks
generally contain small
amounts of radioactive material that
can act like natural clocks.
 Atoms of the same element that have
different numbers of neutrons called
isotopes.
 Radioactive isotopes have nuclei that
emit particles and energy at a
constant rate regardless of
surrounding conditions.
RADIOMETRIC DATING
 During
the emission of the particles,
large amounts of energy are released.
 The method of using radioactive decay
to measure absolute age is called
radiometric dating.
 Scientists measure the concentrations
of the original radioactive isotope and
the newly formed isotope.
 Comparing these proportions gives the
absolute age of the rock.
CARBON DATING
 Younger
rock layers may be dated
indirectly by dating organic material
found within the rock.
 Bones, shells, wood
 Cabron-14 or radiocarbon dating can
determine the age of these organic
remains
CARBON DATING
 Carbon-14
form CO2.

combines with oxygen to
Remember the carbon cycle?
 The
find the age of the organic
material, scientists determine the ratio
of C-14 to C12 in the sample
 While organisms are alive, the ratio
remains constant. The half life of C-14
is 5730 years.
 Since the organism is dead, it no longer
absorbs carbon.
REVIEW:
 What
is absolute age?
The known numeric age of a substance
REVIEW:
 What
is radiometric dating?
Method of using radioactive decay
to measure absolute age
REVIEW:
 What
are varves?
Layers of sediment deposits that consist
of light-colored band of coarse particles
and dark band of fine particles. They
help scientists estimate age.
RADIOACTIVE DECAY ACTIVITY
 Use
a stopwatch to record time.
 Wait 20 seconds and then use
scissors to carefully cut a sheet of
paper in half. Select one piece and
set the other piece aside.
 Repeat step 2 until nine 20 second
intervals have passed (9 times)
 Answer the questions on the activity
sheet.