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Absolute Age
Finding a rock’s birthday
Absolute Age?
• Give an example of an absolute age of a
rock layer…
•That rock layer is 200 million years old.
•Remember, absolute age is the exact
age of a rock layer.
Absolute Age
• Think of determining the absolute age of a
rock layer as finding the rock layer’s
birthday.
• Often impossible to do
• Uses Radioactive Dating
Matter- Lets Review?
• What is MATTER?
•Anything that has mass and takes up
space.
Can you think of some things that could
be called matter?
You
Water
Me
Rocks
Books
Cars
Air
Hair
What are atoms?
• All matter is made up of ATOMS.
• Atoms are so small that we cannot see
them.
• Rocks and Rock layers are made of atoms
too since they are types of matter.
What do atoms look like?
• We’re not sure.
• They are too small to see even with
microscopes.
• We think they look something like this:
Particles
Do all atoms look the same?
NO !!!
There are over
110 ways that
atoms can look
Elements?
• What is an ELEMENT?
A type of matter made up of one or more
of the same atoms.
This is an
element made up
of 4 of the same
atoms
How many elements are there?
• Since there are over 110 different atoms,
this means that there are over 110
different elements in the world.
Elements…
• Most elements are STABLE – they don’t
change, they stay the same.
• But some elements are UNSTABLE or
RADIOACTIVE – they decay or break
down over time.
How do unstable elements decay?
• Over time the
atoms inside of the
element give off:
1.Particles
2.Energy
This is called
RADIOACTIVE
DECAY
Radioactive Decay
• During Radioactive Decay, atoms from
one element break down to form atoms of
another element.
• Radioactive elements occur in IGNEOUS
ROCKS (INTRUSIONS & EXTRUSIONS).
How fast do elements decay?
• A certain element will decay at a constant
speed. The speed never changes.
• This rate of decay is known as the
element’s HALF-LIFE.
• HALF-LIFE = the time it takes for half of
the radioactive element to decay into the
new element.
Radioactive decay
How can we use this to find the
birthday of an intrusion or
extrusion?
1. When the intrusion or extrusion hardens it
is born.
2. Once its born, the radioactive elements
start to decay and become new elements.
• So, the make-up of the intrusion or
extrusion changes over time in a
measurable way.
How to determine absolute age
• Radioactive dating:
1.Determine the amount of radioactive
element still left in the rock.
2.Compare that with the amount of stable
element the radioactive element decays
into.
Here are the elements used in
radioactive dating
Potassium – Argon Dating
• Potassium to Argon Dating: It takes
potassium (radioactive element) 1.3 billion
years for half of it to turn into argon (stable
element). Useful for dating the oldest rocks
because of the long half-life.
Potassium
Potassium
1.3 billion years
ROCK
(half-life)
ROCK
Argon
Carbon 14 Dating
• Carbon 14 to Nitrogen 14 Dating: All
plants and animals contain Carbon 14
(radioactive element). As the organism
grows, more Carbon 14 is added to it.
When the organism dies, the Carbon 14
starts to decay into Nitrogen 14 (stable
element).
• It is used to date fossils, wood, and bone.
Carbon 14 Dating
• Carbon 14 Dating is useful for dating plant
and animal remains that lived from 500 to
50,000 years ago.
• It cannot be used for very old fossils or
rocks.
Which rocks can we date?
• Igneous rocks. Because their birthdays
are when the lava hardens.
• Sedimentary rocks cannot be dated
because they are made of old rocks. Their
birthdays would not be right.
Radioactive Dating
• To deal with this problem, we date the
intrusions and extrusions near the
sedimentary rock layers than use their
absolute ages to find the relative age of
the nearby sedimentary rock layers.
Radioactive Dating
We can date the intrusion and
extrusion and use their absolute ages
to tell about how old the other layers
are.