# c1.5 radioactive dating

```Relative & Absolute Dating
C 1.5- PINPOINTING TIME
DATING TECHNIQUES
Dating techniques refer to methods scientists
use to figure out the age of something, such as
a rock or a fossil
 There are two ways of doing this:

1.
2.
Relative dating
Absolute dating
RELATIVE DATING

Recall, relative dating is a technique used for
fossils or rocks when you already know some
information about the history of fossils and
rocks in the area
 It
uses the relative position of the item (e.g. on top
of fossil A but below fossil B) to make conclusions
about its age (e.g. older than fossil A but below
fossil B)
ABSOLUTE DATING
Relative dating is useful for coming up with an
approximate date, but is not very exact
 Absolute dating involves analyzing the chemical
content of the rock to come up with a more
exact date.



It employs the idea of radioactivity, originally discovered by
Marie Curie in the early 20th century.
Radioactivity studies the emission of energy from the
nucleus of an atom as is becomes more stable.
THE CLOCKS IN ROCKS



Rocks, such as those examined by geologists,
contain radioactive elements like rubidium and
uranium.
Over time, these parent isotopes decay, releasing
energy and forming stable daughter isotopes.
This breaking down of the parent occurs at a
predictable rate, so by comparing the ratio of parent to
daughter isotopes in a particular rock, it is possible to
determine its age (in millions of years).

Why is this useful information?
THE CLOCKS IN ROCKS
By finding the age of a rock, we can also, by
default, age any fossils found in that rock.
 Radioactive dating is the process of finding the
age of a mineral based on the rate of decay of
the elements found in that mineral.
 It involves two steps:

 counting
the number of daughter isotopes in the mineral
 using the known decay rate to calculate the length of
time required to produce that number of daughters.
THE CLOCKS IN ROCKS
How is the composition of the
rock analyzed?
 We’ll use zircon crystals, such
as those formed during a
volcanic eruption, as an
example

From the moment a
zircon crystal forms, it
is tough, dense, inert
and nonmagnetic
 It resists weather,
withstands extreme
temperatures, and is
incredibly stable



From the moment a zircon
crystal forms, unstable
uranium (within the crystal)
decreases, and the amount
When scientists study a
zircon crystal, they
measure the ratio of


This is like a very precise
stopwatch that can date
events from a few hundred
thousand years ago to
several billion years ago
It takes 4.468 billion years
for half of the uranium to
change to lead (called the
half life)


When a volcano erupts, gases
originally dissolved in the magma
form bubbles so quickly that they
make the magma explode into
crystals
Common minerals found in volcanic
ash are feldspar, quartz, mica and
zircon
 Unlike the others, zircon is
extremely durable
 It contains radioactive uranium,



After the volcanic ash falls
down on an area, the ash
could be washed away, or
become trapped in a crack,
or in a lake or swamp
Over time, this ash is buried
and becomes an identifiable
layer in the sedimentary rock
Zircon crystals buried in the
ash is like a mineral
stopwatch, ticking away as
the uranium decays into
In the lab, geologists crush
the ash sample and wash
away the fine dust
 Zircon’s unique properties
make it possible to
separate it out

 it
is dense, but not magnetic
 a magnet will separate out
the magnetic minerals
 because of its density the
zircon will settle at the
bottom, while the rest will
float



After removing the zircon from the heavy liquid, scientists dissolve
the crystals and separate the uranium and lead from the other
elements in the zircon
to read the stopwatch, the lead and uranium are put in a mass
spectrometer, a machine that separates and counts individual
atoms
the ratio of uranium to lead atoms allows scientists to calculate
how many years have passed since the volcano erupted
CALCULATIONS

The graph to the left
is referred to as a
decay curve.


along the x-axis is
time, given in
millions of years (or
number of half-lives
elapsed)
along the y-axis is
the percentage of
the parent isotope
present in the
sample
CALCULATIONS

notice that over time,
the percentage of the
parent decreases, and
the daughter’s
percentage increases
correspondingly

the total percentage in
the ratio of parent to
daughter is always
100%
CALCULATIONS

an element’s half life
is the amount of time
it takes for half the
parent to decay into
the daughter



after one half life, the
ratio is 50:50
after two half lives,
the ratio is 25:75
after three half lives,
the ratio is 12.5:87.5
and so on…
STEPS TO CALCULATING THE AGE OF A
MINERAL USING RADIOACTIVE DATING:

Step one: Find the element on the “elements
for radioactive dating” table, and determine if it
is the parent or daughter nuclide
STEPS TO CALCULATING THE AGE OF A
MINERAL USING RADIOACTIVE DATING:

Step two: Determine the percentage of
remaining parent material
 in
some cases, this will be given to you
 in other cases, you will be given the percentage of
the decay (daughter nuclide), and the parent’s
percentage can be found by subtracting the
daughter’s percentage from 100%
STEPS TO CALCULATING THE AGE OF A
MINERAL USING RADIOACTIVE DATING:

Step three: Using the decay curve, determine
the number of half-lives this percentage
corresponds to

Step four: Multiply that number of half-lives by
the “approximate half-life” listed on the
“elements for radioactive dating” for that
element
EXAMPLE #1:

A rock containing a fossil is found to contain
25% lead-206. What is the most likely age of
the fossil?
EXAMPLE (SOLUTION)

Step one: Find the element on the “elements
for radioactive dating” table, and determine if
it is the parent or daughter nuclide

lead-206 is the daughter nuclide
EXAMPLE (SOLUTION)

Step two: Determine the
percentage of remaining
parent material

If the sample contains
25% daughter nuclide, it
must contain 75% parent
nuclide (100% - 25%)

Step three: Using the
decay curve, determine
the number of half-lives
this percentage
corresponds to
75%
0.5 half-lives
EXAMPLE (SOLUTION)

Step four: Multiply that number of half-lives by
the “approximate half-life” listed on the
“elements for radioactive dating” for that
element



Age of the fossil = (half-life) x (# of half-lives)
Age of the fossil = (4.47x109 years per half-life) x (0.5 half-lives)
Age of the fossil = 2.2x109 years or 2.2 billion years
EXAMPLE #2:

A rock is analyzed
and contains 10%
carbon-14. What is
the age of the rock?
EXAMPLE (SOLUTION)
1.
2.
3.
4.
carbon-14 is the parent
nuclide
the sample contains
10% carbon-14 (given in
question)
10% corresponds to 3.5
half-lives
the age of the fossil is…
Age of the fossil = (half-life) x (# of half-lives)
Age of the fossil = (5.73x103 yrs/hl)(3.5 hl)
= 20 020 years
= 2.0x104 years
10%
3.5 half-lives
ASSIGNMENT


Practice Problems 21-23
(pg 323) & 24 & 25 (pg
324)