Relative dating
 Relative dating is a
method of
sequencing events
in the order they
happened.
Law of superposition
 The law of
superposition states
that the bottom layer
of a rock formation
is older than the
layer on top.
Which event happened
first?
Law of horizontality
 The law of original horizontality which
refers to how sediment particles settle
to the bottom of a body of water in
response to gravity.
 Horizontal layers of rock might become
tilted or folded by a geological event.
Original horizontality
The relative age of a rock
 The principle of crosscutting relationships
states that a vein of rock
or a fault that cuts across
a rock’s layers is younger
(more recent) than the
layers.
Layers might be tilted at any angle
and can even be upside down.
 The middle and top layers
formed after the bottom
layer but before the vein.
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Fossil succession
 The principle of fossil succession means that
fossils can be used to identify the relative age
of the layers of a rock formation.
 The organisms
found in the top
layers appeared
after the
organisms found
in the layers
below them.
Index Fossil
 An index fossil is any fossil that is both
widespread in rocks, and that existed
for a fairly short period of time.
 Allows us to correlate rock layers in
different places.
Absolute Dating
 Absolute dating is a method of measuring the
age of an object such as a rock or fossil in
years.
 Scientists use both absolute and relative
dating to develop the geologic time scale.
 If rocks from 2 different places contain the same
index fossils then they must have been laid down
at about the same period on history.
Absolute Dating
 Scientists know that it
takes 4.5 billion years
for one half of the
uranium atoms in a
specimen to turn into
lead.
 Radioactive decay refers
to how unstable atoms
lose energy and matter
over time.
 As a result of radioactive
decay, an element turns
into another element over
a period of time.
The half life of uranium
Carbon turns in to
nitrogen over time.
 We say that 4.5 billion
years is the half-life for
the radioactive decay of
uranium.
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