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Relative and
Absolute Dating
Question of the Day
Question: What are the 3 types of rocks and
what is the rock cycle?
Answer: … … …
Relative and Absolute Dating
What is Dating?
When geologists date rocks, they are
determining how long ago they formed.
Two ways to do this:
Relative Dating
Absolute Dating
Relative Dating
Determining how old something is
compared to something else
Use words like “older” or “younger”
instead of exact numbers
Rules of Dating
 two rules for dating rocks:
1. Stuff on the bottom is oldest and the stuff on
the top is youngest
 This is called the
Law of Superposition
 This only works for sedimentary rocks
(sometimes lava flows)
Order these layers from oldest to
Youngest layer
Oldest layer
Rules of Dating
two rules for dating rocks:
2. If a fault or another rock cuts through a layer it
happened after (and is younger than) the layer
This is called the
Law of Cross-Cutting Relations
Order the events from oldest to youngest
Gray Layer
Multicolored layer
Spotted Layer
Absolute Dating
Determining how old
something is
Use numbers (in
millions of years, mya)
Only works for
Igneous Rocks
How Absolute Dating Works
When magma/lava cools, radioactive elements
are incorporated into the minerals
 Examples:
-- Potassium 40
-- Uranium 235
How Absolute Dating Works
These elements begin to decay at a known rate
starting when the rock cools
We can measure how much of the element is
Tells us how much time has passed since the
rock formed.
What can we learn from this?
 Geologist
Basalt, dated to 80
 T-Rex Fossil
Basalt flow, dated
to 100 mya
So when did this T-Rex live?
Between 100 and 80 mya
This may not seem very accurate, but compared to
the 4,500 million years the earth has been around it
gives us a lot more information than we had before
What can we learn from this?
Absolute Dating
Helps us determine the age of the earth
Helps us determine when specific events in the
history of the earth happened (ex. Extinction of
the dinosaurs)
Relative Dating
Can help us estimate the time span between
major earthquakes, storms, tsunamis etc
Can help us determine the order that life forms
developed on earth
Lab: What’s Up?