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 Radiometric Dating

Matter?

• What is MATTER?

•Anything that has mass and takes up space.

Can you think of some things that could be called matter?

You Me Books AirWater Rocks Cars 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

Atoms…

• Stable atoms– they don’t change, they stay the same.

• Unstable or Radioactive atoms– they turn into stable atoms over time.

How do unstable atoms decay?

•Over time the atoms give off:

1.Particles

2.Energy

This is called

RADIOACTIVE DECAY

Radioactive Decay

• This change from a radioactive atom to a stable atom is called radioactive decay

How fast do elements decay?

• A certain atom will decay at a constant speed. The speed never changes for that type of atom.

• HALF-LIFE = the time it takes for half of the radioactive element in a substance to decay into its stable element.

Radioactive decay

The make-up of the intrusion or extrusion changes.

Radioactive element

New element

Here’s the Half-Life

As time goes by, the red element decays and becomes the blue element.

The intrusion or extrusion’s make-up has changed because of this decay. We will soon see that we can determine the exact time it was born by seeing how much of each of the elements is left.

What can we date?

• IGNEOUS, METAMORPHIC, FOSSILS ONLY – because they have a true birthday.

• Once it’s born, the radioactive elements start to decay and the clock starts.

What can we date?

• Sedimentary rocks cannot be dated because they are made of sediment (tiny pieces of rocks with all sorts of different birthdays). We would find lots of birthdays of the sediment, and not the rock layer.

Why not Sedimentary Rocks?

Let’s say this Sedimentary rock is actually 20 million years old.

Radioactive Dating would say these rock particles are 50 million years old

Radioactive Dating would say these rock particles are 100 million years old

Do you see the problem???

Here are the elements used in radioactive dating

Radioactive Decay of Potassium-40

0

10

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40

50

60

70

80

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100

0 1.3 2.6 3.9 5.2

TIme gone by (in billions of years)

% o

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Potassium-40

Argon-40

Potassium-40

Argon-40

Radioactive Decay of Potassium-40

0

10

20

30

40

50

60

70

80

90

100

0 1.3 2.6 3.9 5.2

TIme gone by (in billions of years)

% o

f ele

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t in

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ck

Potassium-40

Argon-40

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

ROCK1.3 billion years

(half-life)

Potassium

Argon

ROCK

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.

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.