1. How long ago did the Earth form? ___ FOUR POINT FIVE BILLIONFOUR POINT FIVE BILLION

2. What did the water vapor condense into? ___ OCEANSOCEANS

3. Scientist think life has evolved over hundreds of ___ of years? MILLIONSMILLIONS

4. Evidence of the age of the Earth can be found by measuring the age of ___. ROCKSROCKS

6. How did our planet generate the internal heat that is present today? Gravity pulls everything to the center of the Earth while density causes heavy radioactive materials to be dense and sink to the center of the Earth.

7. ___ ___ is the estimation of the age of an object by measuring its content of certain radioactive isotopes. RADIOMETRIC DATINGRADIOMETRIC DATING

8. ___ are unstable isotopes that break down and give off energy in the form of charged particles called ___. RADIOISOTOPES AND RADIATIONRADIOISOTOPES AND RADIATION

9. What is this breakdown called? ___ RADIOACTIVE DECAYRADIOACTIVE DECAY

10. ___ ___ is the time it takes for one half of a given amount of a radioisotope to decay. HALF LIFEHALF LIFE

11. Half life from radioactive decayHalf life from radioactive decay

12. By measuring the proportions of certain radioisotopes and their products of ___, scientists can compute how many half lives have passed since a rock was formed. DECAYDECAY

13. The half life of uranium is 760 million years. If the age of the earth is 4.2 billion years, then 4.2/.76 = about 6 doublings or about 64 times as much uranium existed on earth at creation as exists now.

14. Half life from radioactive decayHalf life from radioactive decay