Geologists have developed a geologic history of Earth by sequencing past geologic events and determining the relative and absolute ages of fossils and rocks. Relative age refers to the age of a fossil or rock in comparison to another, while absolute age is the real age. Index fossils and volcanic ash, which are widespread and lived or occurred over short time periods, are used to determine relative ages. Radioactive dating determines absolute ages by measuring the decay of radioactive elements like carbon-14 and uranium-238, which have half-lives of 5,700 and 4.5 billion years respectively.

1. L.O: EARTH’S HISTORY

2. EARTH’S EARLY ATMOSPHERE
FORMED FROM OUT GASSING OF
VOLCANOES.

3. Geologists have sequenced the
geologic events in past to develop a
geologic history of Earth.
Scientists use relative age and
absolute age to determine age of
fossils.

4. Relative age: the age of a fossil, rock
or geological event in comparison to
another.

5. absolute age: the real age of a
fossil.

6. Index fossils: fossils used to
determine relative age of other
fossils, rocks and events.

7. a good Index fossil: wide-spread &
lived short time period

9. Volcanic ash also is a good time
marker. Like index fossils wide-
spread. Short time period.

10. Now complete page 102-113

11. Radioactive
decay:

12. Radioactive elements:
• Radioactive element decay.
• The amount of time it takes for half of a
radioactive element to decay is half-life.
• The half-life of an element cannot be changed.
• The half-life of carbon 14 is 5,700 years.
• The half-life of uranium 238 is 4.5 billion
years.

13. Carbon 14
• Half-life 5,700
• Used to date recent fossils-less than 50,000
years.

14. Uranium 238
• Halflife 4,500,000,000 years.
• Uranium’s half life is how we know earth’s
age.

15. Complete page 1-7