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