Time and change: dating methods
Absolute time scales and absolute age
Q. Fill in the blanks:






It is possible to work out the absolute ages of rocks using
.
Some minerals in rocks contain radioactive elements. The
of these elements break
down at a steady rate to form new elements.
Isotopes are substances with the same atomic number but different mass number. e.g. The
isotopes of potassium are: K, K, K . The top number gives the
(no. of protons
+ no. of neutrons). The bottom number gives the atomic number (no. of protons = no. of
electrons)
Radioactive elements are those which have unstable
which break down and give off
alpha, beta or gamma radiation.
When an alpha or beta particle is lost from a radioactive atom it
into a
product which is different from its parent (the original material).
The rate of decay is steady. The time for the original number of atoms to be reduced by one half
is the
.
Original
content left
1
2
1
4
1
8
1
16
1
32
Number of
half lives
1
2
3
4
5

The age of a rock can be calculated if
(1) the proportion of parent to daughter product can be measured
(2) the
is known.

The age calculated is a true measure of the age of formation if:
(a) there have been no
or gains of daughter products by other processes
(b) there have been no losses or
of parent products by other processes
(c) the mineral or rock was formed in a
period of time compared to its age.
 Events which can be dated are: (1) crystallisation of
rocks from magma
(2) recrystallisation of rocks to form
rocks
(3) uplift & cooling of
mountain chains
(4) time of deposition of
rocks (provided the rock
contains one mineral formed at the time of deposition).
Potassium - Argon Dating Method ( K changes to


Ar)
This has a half life of
million years.
It is the most important dating method used today because many rocks contain
minerals.
- rich




It can be used to date
and metamorphic rocks which contain the minerals biotite,
hornblende, plagioclase feldspar, and K- rich feldspar. (All these minerals contain potassium).
The method can be used on
rocks which contain the mineral glauconite. (This
is a mineral which contains
& which forms at the time of deposition).
It can be used on whole
samples e.g. fine grained lavas & slates.
It can be used to date rocks from 1 million years to 3900
years old.
Problems in absolute dating
1. Isotopes with half lives within one or two orders of magnitude of the age of the Earth are ideal.
This is so that the amounts of
product can be measured accurately.
2. The calculated age is only correct if there is no loss or gain of material other than by radioactive
decay. (n.b. Argon is a
and it may be lost from the rock, or added, from other sources during
metamorphism when the rock is heated up).
3. The calculated age is correct only if the
is accurately known.
Completed exercise:
Time and change: dating methods
Absolute time scales and absolute age






It is possible to work out the absolute ages of rocks using radioactive dating.
Some minerals in rocks contain radioactive elements. The isotopes of these elements break
down at a steady rate to form new elements.
Isotopes are substances with the same atomic number but different mass number. e.g. The
40
41
isotopes of potassium are: 39
19 K , 19 K , 19 K . The top number gives the mass number (no. of protons
+ no. of neutrons). The bottom number gives the atomic number (no. of protons = no. of
electrons)
Radioactive elements are those which have unstable nuclei which break down and give off alpha,
beta or gamma radiation.
When an alpha or beta particle is lost from a radioactive atom it decays into a daughter product
which is different from its parent (the original material).
The rate of decay is steady. The time for the original number of atoms to be reduced by one half
is the half life.
Original
content left
1
2
1
4
1
8
1
16
1
32
Number of
half lives
1
2
3
4
5

The age of a rock can be calculated if (1) the proportion of parent to daughter product can be
measured
(2) the half life is known.

The age calculated is a true measure of the age of formation if:
(a) there have been no losses or gains of daughter products by other
processes
(b) there have been no losses or gains of parent products by other
processes
(c) the mineral or rock was formed in a short period of time compared to
its age.
 Events which can be dated are: (1) crystallisation of igneous rocks from
magma
(2) recrystallisation of rocks to form
metamorphic rocks
(3) uplift & cooling of fold mountain chains
(4) time of deposition of sedimentary rocks (provided the rock
contains one mineral formed at the time of deposition).
Potassium - Argon Dating Method ( 40 K changes to

This has a half life of 11900 million years.
40
Ar )





It is the most important dating method used today because many rocks contain K- rich
minerals.
It can be used to date igneous and metamorphic rocks which contain the minerals biotite,
hornblende, plagioclase feldspar, and K- rich feldspar. (All these minerals contain potassium).
The method can be used on sedimentary rocks which contain the mineral glauconite. (This is a
mineral which contains potassium & which forms at the time of deposition).
It can be used on whole rock samples e.g. fine grained lavas & slates.
It can be used to date rocks from 1 million years to 3900 million years old.
Problems in absolute dating
1. Isotopes with half lives within one or two orders of magnitude of the age of the Earth are ideal.
This is so that the amounts of daughter product can be measured accurately.
2. The calculated age is only correct if there is no loss or gain of material other than by radioactive
decay. (n.b. Argon is a gas and it may be lost from the rock, or added, from other sources during
metamorphism when the rock is heated up).
3. The calculated age is correct only if the half life is accurately known.