Fossil Evidence

Fossil evidence. unit 2. Topic 9.5

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FOSSIL EVIDENCE
Dating Fossils
CARBON DATING
Used to date organic materials.
A small amount of 14C is present in carbon dioxide molecules in the air. This is then taken in by
plants in photosynthesis. THEREFORE, the 14C becomes incorporated in the plant molecules.
Animals gain the 14C when they eat the plants.
When an organism dies no more 14C is incorporated and the 14C starts to decay.
HALF LIFE FOR 14C = 5700 years.
Scientists can measure the ratio of 14C to normal 12C and work out the age of the sample.
CANT BE USED TO DATE FOSSILS OLDER THAN 40-50,000 YEARS OLD.
POTASSIUM ARGON DATING
Potassium-40 decays into Argon-40 and Calcium-40 at a known rate.
HALF LIFE FOR 40K = 1.25 Billion years.
40K is only present in volcanic rocks so it's only useful to date volcanic rocks.
BUT... Fossils are usually found in sedimentary rocks.
STRATIGRAPHY
Stratigraphy is the study of the layers in the rock.
The lower down the fossil the older it is.
A fossil is found is a sedimentary rock sandwiched between volcanic rock. THEREFORE,
Potassium Argon dating can be used to find out the age of the two volcanic layers.
Doesn't give us an exact age, just an estimate.
Skilled geologists needed because of earthquakes and volcanic eruptions which may bend,
split or tilt the rock layers.

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