L.O: EARTH’S HISTORY

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

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.

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

absolute age: the real age of a fossil.

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

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

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

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Radioactive decay:

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.

Carbon 14

• Half-life 5,700• Used to date recent fossils-less than 50,000

years.

Uranium 238

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

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