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Absolute Dating
Age of Rocks
How do we determine the age of Rocks?
Age of Rocks
Relative
Dating
Absolute
Dating
Law of Superpositio
n
Ordering of rock
layersIndex Fossils
Radio-Active Dating
•The actual age of a rock or fossil•Absolute dating occurs through the process of radioactive dating
Absolute Dating
Radioactive dating is a way to measure the age
of a material by comparing the amount of
decayed radioactive element present in the
item
Radioactive Dating or Age
• The universe is full of naturally occurring radioactive elements.
• Radioactive atoms are inherently unstable; meaning over time, radioactive “parent atoms” decay into stable “daughter atoms.”
How do scientist tell the actual age of rocks?
• When molten rock cools, forming an igneous rock, K40 (potassium radioactive atoms) are trapped inside.
• Once formed K40 begins to decay at a predictable rate, and changing into Ar40 (Aragon).
• K40 is parent atom and Ar40 is daughter cell.
How do scientist tell the actual age of rocks?
• Scientist will measure the quantity of unstable atoms(K40) left in a rock and compare it to stable atoms(Ar40) and be able to determine the age of that sample.
• Carbon is used to date once living organisms.
• http://www.youtube.com/watch?v=vYvUSro7YUM
How do scientist tell the actual age of rocks?
• The amount of time required for half of the radioactive material to decay in an item or in our case the rock.
Half-life
• Rock 1st forms 100% of K40
• 1 half-life 50% of K40, 50% of Ar40
• 2 half-life 25% of K40, 75% of Ar40
• 3 half-life 12.5% of K40, 87.5 of Ar40
• 4 half-life 6.25% of K40, 93.75 of Ar40
• 5 half-life 3.125% of K40, 96.875 of Ar40
Half-life
Half-life Chart
1. If you have 10g of K40 and 2 half-life’s have passed how many grams of K40 do you have left?
10g of K40
1 half-life; 5g of K40
2 half-life; 2.5g of K40
Half-Life Questions
1. If 50% of the K40 is gone, how old is the rock?
• 1.3 million years old
2. If this rock is 3.2 million years old how much K40 is still present in this rock?
• About 10%
3. How much Ar40 is present if the rock is 3.2 million years old?
• About 90%
Half-life Questions