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Relative and Relative and Absolute DatingAbsolute Dating
Relative and Relative and Absolute DatingAbsolute Dating
Question of the DayQuestion of the DayQuestion of the DayQuestion of the Day
Question: What are the 3 types of rocks and what is the rock cycle?
Answer: … … …Turn In:
-Mineral ID Lab-Rock ID Lab
-Rocks Webquest-p. 582 CYU 1 - 6
What is Dating?
When geologists date rocks, they are determining how long ago they formed.
Two ways to do this:Relative DatingAbsolute 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)
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
Fault
Gray Layer
Multicolored layer
Spotted Layer
OLDEST LAYER/EVENT
YOUNGEST LAYER/EVENT
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 left
Tells us how much time has passed since the rock formed.
What can we learn from this?
T-Rex Fossil
Geologist
Basalt, dated to 80 mya
Basalt flow, dated to 100 mya
Sandstone
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 DatingHelps us determine the age of the earthHelps us determine when specific events in the
history of the earth happened (ex. Extinction of the dinosaurs)
Relative DatingCan help us estimate the time span between
major earthquakes, storms, tsunamis etcCan help us determine the order that life forms
developed on earth
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