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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.

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absolute age

Absolute Age

Finding a rock’s birthday

absolute age1
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 age2
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
  • 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 Air

Water Rocks Cars Hair

what are atoms
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


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:
do all atoms look the same
Do all atoms look the same?

NO !!!

There are over 110 ways that atoms can look


This is an element made up of 4 of the same atoms

  • What is an ELEMENT?

A type of matter made up of one or more of the same atoms.

how many elements are there
How many elements are there?
  • Since there are over 110 different atoms, this means that there are over 110 different elements in the world.
  • 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
How do unstable elements decay?
  • Over time the atoms inside of the element give off:
  • Particles
  • 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
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.
how can we use this to find the birthday of an intrusion or extrusion
How can we use this to find the birthday of an intrusion or extrusion?
  • When the intrusion or extrusion hardens it is born.
  • 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.
the make up of the intrusion or extrusion changes

New element

Here’s the


Radioactive element

The make-up of the intrusion or extrusion changes.

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.

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

how to determine absolute age
How to determine absolute age
  • Radioactive dating:
  • Determine the amount of radioactive element still left in the rock.
  • Compare that with the amount of stable element the radioactive element decays into.
potassium argon dating



1.3 billion years





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.
carbon 14 dating
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 dating1
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
Which rocks can we date?
  • Igneous rocks. Because their birthdays are when the lava hardens.
  • Sedimentary rockscannot be dated because they are made of old rocks. Their birthdays would not be right.
why not sedimentary rocks

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

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

Why not Sedimentary Rocks?

Do you see the problem???

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

radioactive dating
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 dating1
Radioactive Dating

We can date the intrusion and extrusion and use their absolute ages to tell about how old the other layers are.