Transcript The Fossil Record - Porterville Unified School District

The Fossil Record
How old is that fossil?
• There are two major
ways of dating objects
or fossils:
– Relative Dating
– Absolute Dating
How old is that fossil?
• Relative Dating
– Uses the relative position of fossils for age
comparison
Relative Dating
• Cannot be used to determine the
actual age of a fossil
• Can only be used to compare fossil
age
Example:
Archaeologists measure the fluorine content in
bones. Fluorine is from underground water.
Fluorine eventually replaces other things that
are in bones; so the more fluorine the bones
contain, the older they are.
Radiometric Dating
• A technique used to date materials
• A comparison between the amount of a
naturally occurring radioactive isotope and
its decay products
• Each radioactive isotope has a unique,
known half-life (time for ½ of the
substance to decay)
– Example: Radioactive Carbon-14 has a halflife of 5730 years
• The choice of dating material is based on
estimated age of the artifact or fossil to be
dated.
Radiometric Dating
• Radioactive samples decay at a predictable rate
• Estimates of the original composition of a
radioactive sample are made
• The amounts of the radioisotopes still present
can provide a measurement of time elapsed
– Example: Carbon dating, limited to organic materials
– Longer lived radioisotopes in minerals are used for
longer time scales
• “clocks in the rocks”
Table 1. Some Naturally Occurring Radioactive Isotopes and their half-lives
Radioactive Isotope
(Parent)
Product
(Daughter)
Half-Life
(Years)
Samarium-147
Neodymium-143
106 billion
Rubidium-87
Strontium-87
48.8 billion
Rhenium-187
Osmium-187
42 billion
Lutetium-176
Hafnium-176
38 billion
Thorium-232
Lead-208
14 billion
Uranium-238
Lead-206
4.5 billion
Potassium-40
Argon-40
1.26 billion
Uranium-235
Lead-207
0.7 billion
Beryllium-10
Boron-10
1.52 million
Chlorine-36
Argon-36
300,000
Carbon-14
Nitrogen-14
5715
Uranium-234
Thorium-230
248,000
Thorium-230
Radium-226
75,400
Radioactive Decay
• The amount of decay
in one half-life is used
to measure the time