Transcript Relative vs. Absolute Dating

Relative vs. Absolute Dating


Magazines can be piled up
and you can know the
relative age of the magazine
by observing the order in
which the magazines were
stacked
Absolute dating is a process
that uses the properties of
atoms in rocks and other
materials to determine a
more accurate age of rock
layers
What are Atoms and Elements?




Atoms are the particles or “building blocks” of
which all matter is composed
Atoms combine, like blocks snapping
together, to form many different types of
matter.
Specific types of atoms are called elements
(i.e. oxygen, carbon, gold, etc.)
Some atoms “fall apart” or change over time
Three basic particles make up an atom
Protons, Neutrons, and
Electrons:

In the nucleus:
1.
2.


Protons: Particles that have a positive
electrical charge Larger than neutrons
Neutrons: Particles that have no electrical
charge Smaller than protons
The nucleus has a positive charge
Around the Nucleus:
3.

Electrons: Negatively charged particles that
move around the nucleus Smallest of
particles
There is one electron for each proton
Atom Diagram
Nucleus
(contains Protons
& Neutrons)
Electrons
(move around the Nucleus)
Special Atoms:

If the amount of protons in an atom changes,
a new element is formed



e.g. Carbon has 6 protons; Nitrogen has 7 protons
However, if the number of neutrons are
changed, the type of element is not changed


Isotopes
e.g. Carbon can continue to exist with different
amounts of neutrons (Carbon-14 indicates the total
of protons and neutrons. It still has 6 protons)
Atoms of the same element that have an
unusual number of neutrons are called
isotopes
Isotopes are unstable atoms

e.g. Carbon-14 is an unstable Carbon atom or
isotope
Unstable Isotopes
break down

In a process known as radioactive decay,

One of the isotopes neutrons breaks down,
changing into a proton and an electron

Some isotopes give off two protons and two
neutrons in the form of an alpha particle (see diagram-one
after next slide)

The electron leaves the atom as a beta particle
(see
diagram-one after next slide)


The nucleus loses a neutron but gains a proton
In the end, a new element is formed
Radioactive Decay Diagram
1
Decay of:
2- neutrons
2-protons
2
Uranium-238
3
Lead-206
(eventually)

Uranium-238 decays by emitting alpha particles (two
protons and two neutrons) and beta particles (one
electron). A beta particle is produced when a neutron
decays and becomes a proton.
Similar Chart: Uranium-238 to Lead-206


The decaying process is complex but scientists believe the process follows a
pattern
You do not need to memorize or understand this chart
Radioactive Decay Example

When atoms of the radioactive
isotope uranium-238 decay, they
eventually form atoms of lead206


Lead-206 is not radioactive, so it
will not decay any further
In this example, uranium-238 is
known as the parent material,
and lead-206 is known as the
daughter product
Radioactive decay is
measured by the
material’s “Half-Life”

The half-life is the rate, or speed,
at which the radioactive parent
material decays into its daughter
product


The half-life is the total amount
time it takes for half of the atoms in
the material to decay
Different isotopes decay at different
rates
Half-Life Example



1st
5,730 yrs.
Parent material: Carbon-14
It takes 5,730 years for half of
the Carbon-14 material to decay
into its daughter product
Nitrogen-14
Then, it takes another 5,730
years for ½ of the remaining
2nd
Carbon-14 material to decay
5,730 yrs.


Therefore, after 2 half lives, ¼ of
the original carbon-14 atoms still
remain
And, after 3 half lives, 1/8 of the
original carbon-14 atoms still
remain
Carbon-14
Nitrogen-14
Carbon-14
Nitrogen-14
Nitrogen-14
Half-Life Sequence diagram
Eventually,
such a small
amount of
the parent
material is
left that it
may not be
measureable
.
Note: There is no time period identified in this example. Yet,
the half-life, or period of decay, remains the same between
each half-life
Radiometric Dating



To a geologist, the decay of radioactive
isotopes is like a clock ticking away,
keeping track of time that’s passed since
rocks have formed
As time passes, the amount of parent
material in a rock decreases as the
amount of daughter product increases
A geologist can calculate the absolute age
of a rock by measuring:


the amount of parent and daughter materials
in a rock
and knowing the half-life of the parent
Radiometric Dating is Absolute Dating…with
Limitations


Carbon-14 is useful for dating fossils, bones, and
wood up to 50,000 years old
Rock that can be radiometrically dated are mostly
igneous and some recrystallized metamorphic
rocks




Only some sedimentary rock
The amount of starting material can come into
question
Decay rates are assumed to be uniform over time
Radiometric dating has been used to date the
oldest rocks found on Earth (3.96 billion years
old)

Scientists have estimated the age of Earth at 4.6
billion years