Transcript Rock Dating PowerPoint

Rock Dating Methods
• Radiometric Dating: using known decay
rates (half-lives) of radioactive elements to
determine the ages of rocks.
• Relative Dating: putting rocks in order,
according to age
Radiometric Dating uses the known decay rates of radioactive atoms to
determine the ages of rocks.
One “half-life” is
the time it takes
for half of a
quantity of parent
atoms to decay
into daughter
atoms.
Half Lives for Radioactive Elements
Radioactive Parent
Stable Daughter
Half life
Potassium 40
Argon 40
Rubidium 87
Strontium 87
48.8 billion yrs
Thorium 232
Lead 208
14 billion years
Uranium 235
Lead 207
704 million years
Uranium 238
Lead 206
4.47 billion years
Carbon 14
Nitrogen 14
5730 years
1.25 billion yrs
When magma solidifies, parent and daughter atoms separate. Some of the
crystals that form contain 100% parent atoms. This “re-sets” the radiometric
dating clock.
Time = 0
After 1 half-life (50%
parents, 50% daughters
Relative Dating – Putting Rock Strata In Order
Relative Dating: Principles:
Superposition: The youngest rock strata are…
…on top
Faunal Succession: Strata with similar fossils…
… are of similar ages (true as long as the species only existed over a
short time span)
Crosscutting Relationships: If one feature cuts through another…
…the one that does the cutting is younger.
Original Horizontality: Rock strata are laid down…
…horizontally. Any strata that are not horizontal have been perturbed.
Letter “A” is the oldest item in this diagram. T is newest. Some letters
represent occurrences, rather than rock samples (N represents erosion or
glaciation, for example.) This order was determined by applying the principles
on the previous page.
Notice that H,
R, and M are
younger than
everything that
they “cut
through.”
Radiometric dating
is usually combined
with relative dating
to give us a more
complete
understanding of
when different rock
strata were formed.
Based on the dating of
volcanic ash, we may
infer that rocks with this
particular type of fossil
are between 520 and
545 million years old