Transcript Relative Dating

Geologic Time

Rocks record geologic events and the changing
life forms of the past.
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Uniformitarianism: The forces and processes
that we observe today have been at work for a
very long time.
Relative Dating
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Tells us the
sequence (order) in
which events
occurred, but NOT
HOW LONG AGO
they occurred.
Relative Dating
*3 Laws/Principles:
1. Law of Superposition:
In an undisturbed
sequence of
sedimentary rocks,
each bed is older than
the one above it and
younger than the one
below it.
Stratigraphy
Relative Dating
2. Principle of Original Horizontality: Layers of
sediment are generally deposited in a
horizontal position.
Relative Dating
3. Principle of Cross-Cutting Relationships:
States that when a fault cuts through or when
magma intrudes into other rocks and
crystallizes, we can assume that the fault or
intrusion is younger than the rocks affected.
Relative Dating
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Inclusion: Pieces of rock units that are
contained within another (are younger than the
rock they are in).
Relative Dating
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Unconformity: a
gap in the geologic
record that occurs
when rock is
eroded, exposing
older rock and
then new rock
forms on the much
older exposed
rock.
Absolute Dating
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1.
2.
3.
Three Types:
Radiometric Dating
Tree Ring Dating
Ice Core Dating
Radiometric Dating
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Radiometric Dating: Calculating the age
of rocks and minerals that contain certain
radioactive isotopes.

Uses the half-lives of those isotopes to
calculate the age of rocks.
Radiometric Dating
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Half-Life: The time it takes for 50% (1/2)
of the nuclei in a radioactive sample to
decay to its stable isotope.
Multiply the number of half-lives by the
half-life time to get the age of a fossil.
Half-Life Practice

A bone of a T-Rex contains 1/16 the original parent
material. Haw many half-lives have occurred in the
T-Rex bone?

If 2 half lives have passed, what fraction of the
original parent material remains?

If the half-life time of an isotope is 10,000 years and
3 half lives have passed, what is the age of the fossil?
Radiometric Dating

This method is possible because each
radioactive isotope decays at a
CONSTANT rate.
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Cannot be used for sedimentary rocks
because they are formed from many
pieces of older rocks, so there would be
many ages of rocks.
Tree Ring Dating
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Tree rings are a glimpse
into the past.
Width of bands indicate
climate changes and
number of rings indicate
age.
Must use a sample to be
sure the results are
accurate.
Ice Core Dating

Each layer of ice in a core corresponds to a
single year, or sometimes even a single season
and most everything that fell in the snow that
year remains behind, including wind-blown
dust, ash, atmospheric gases and even
radioactivity.
Ice Core Dating