Transcript Dating Fossils

Dating Fossils
Superposition
• Suggests that younger rocks
lie above older rocks in
undisturbed sequences
Relative Dating
• Used to determine whether
something is younger/older than
something else.
• Used for events and objects
Geologic Column
• Unconformities occur due to changes such
as faults, folds, tilts, and intrusions.
• Unconformities disrupt superposition, so
scientists must rely on the geologic
column to see where rock is missing and
to tell what should be there
Absolute Dating
• Determining the exact age of an event or
an object using radioactive isotopes
• Uses isotopes (most are unstable)
• If you know the rate of decay of an
isotope, you can see how much of an
element is in a fossil and compare that to
the half-life of the element’s isotope
Half-life
• The time it takes for ½ of a radioactive
sample to decay
• After every half-life, the radioactive sample
decays by ½
Isotopes
• Uranium 238 (decays to Lead 206)
– Half-life is 4.5 billion years
– Used to date rocks more than 10 my old
Other isotopes:
Potassium 40 has a half-life of 1.3 by
Carbon 14 has a half-life of 5,730 years
Faults
• Breaks in the Earth’s crust
• Causes blocks of crust to slide
Folds
• Rock layers fold and buckle from Earth’s
internal forces
Tilting
• Occurs when internal forces in the Earth
slant rock layers without folding them
Intrusions
• Molten rock and squeezes upward into the
existing rock and cools.