Transcript Absolute Age – The exact age of a fossil or rock formation

Absolute Age
The actual age of a
fossil or rock formation
What are atoms?
The smallest unit of matter that
makes up everything on Earth.
What are elements?
Matter that is made up of the
same atoms.
What is a radioactive element?
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The atoms of many
elements exist in
different forms.
Most of these forms are
stable and do not
change.
But some of these forms
are said to be unstable
elements.
It’s these unstable
elements that are called
radioactive.
What are some examples of
radioactive elements?
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Carbon -12 (stable): Carbon-14
(unstable)
Potassium – 39 (stable) : Potassium40 (unstable)
What is radioactive decay?
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Radioactive
(unstable) elements
want to be stable.
The unstable element
(parent material)
breaks down to
become another
element (daughter
material).
This is called
RADIOACTIVE
DECAY
What is radioactive dating?
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Radioactive
dating is how
scientists use
radioactive
elements to
determine the
absolute age of
fossils & rocks.
Why are radioactive elements used to
determine the absolute age?
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The radioactive
decay always
happens at the same
rate (speed).
If you know how fast
it breaks down, then
you can figure out
how much time has
passed by.
What is a half-life?
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The amount of
time it takes for
half of the
radioactive atoms
(unstable atoms)
to change into a
stable element.
Different
radioactive
elements have
different half-lives.
What are some examples of
radioactive elements’ half-lives?
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Carbon-14 –changes into Nitrogen-14
• half-life of 5,730 yrs
Potassium-40 – changes into Argon-40
• Half-life of 1.3 billion yrs
Urianium-235– eventually changes into Lead-207
• Half-life of 704 million yrs
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Urianium-238–
eventually changes into
• Half-life of 4.5 billion yrs
Lead-206
Half-life Example
The half-life of C-14 is about 5700 years.
(Every 5700 years, HALF of the C-14
changes!)
You have 60 grams of Carbon-14.
 After 5700 years pass, how much C-14 would
you have? N-14?
Now you have 30g of C-14 & 30g of N-14
Another 5700 years go by.
How much C-14 would you have? N-14?
15g of C-14 & 45g of N-14
This would continue until all the unstable
element becomes stable.
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How is absolute age found from
radioactive dating?
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In radioactive dating, scientists first
find out how much radioactive
element is in the sample.
Then, they compare the radioactive
amount to the stable amount.
The ratio between stable and unstable
lets scientists know how many halflives the fossil or rock has gone
through, and therefore, how old they
are.
YouTube:
Carbon Dating
Radioactive dating can only be
used on what?
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Absolute age can only be determined
for igneous rocks
Any fossils that have some organic
parts remaining
• for example: original remains, carbon
film, bones that are only partially
petrified.
When does radioactive decay
begin?
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In non-living things, radioactive decay
starts as soon as the magma or lava
hardens.
OR
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In living things, every time an organism dies
a stopwatch starts ticking.
Death starts the stopwatch.
Science can read it.
Why can’t we find the absolute age of
sedimentary rocks or of fossils made
from sediments?
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Sedimentary rocks and fossils made
from sediments are formed from
millions of pieces of particles that come
from rocks of all different ages.
Radioactive dating would find the ages of
the particles not the age of the whole
rock.
We’d never know how old the rock really
is!
How do scientists date sedimentary
rocks?
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To date sedimentary rocks, scientists
find the age of any intrusions or
extrusions that are near the
sedimentary rock.
 So, any layer of sedimentary rock
above an intrusion or extrusion, is
YOUNGER! Any sedimentary layer
found below an extrusion is older.
(RELATIVE AGE!!!)
How old is the Earth?
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The oldest rocks ever found on the
Earth have been dated at about 4.0
billion years old, but scientists
believe the Earth is really…
4.6 billion years old.
How do we know how old the Earth is?
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Scientist believe the Earth and the
moon formed at the same time.
Scientist have dated moon rock
brought back to Earth by astronauts.
Therefore the Earth is believed to be
at least as old as the moon rocks.