8.E.2.1
Infer the age of Earth and
relative age of rocks and fossils
from index fossils and ordering
of rock layers (relative dating
and radioactive dating).
8.E.2.2
Explain the use of fossils, ice
cores, composition of
sedimentary rocks, faults and
igneous rock formations found
in rock layers as evidence of the
history of the Earth and its
changing life forms.
Notebooks today!
Entry #4
Dating Methods
2 Pages to glue in……
How do we know the age of the Earth?
• The fossil record give us important information
about past life and environments on Earth.
• Certain fossilized organisms could only live in
specific environments or under certain
climate conditions.
• Scientist use a variety of methods to determine
the age of the Earth…
Relative dating
Absolute dating
Relative Dating (not dating your relatives!)
Age of one object compared to another, not an exact age
1. Index Fossils
• Fossil of an organism that only lived during a short period of time, must be
found over a wide area of Earth and must be unique.
 Example: TRILOBITES were a group of hard shelled animals with 3 body
sections, lived in shallow seas and became extinct 245 million years ago.
2. Ordering of rock layers
• See foldable!
TRILOBITES
Relative Dating:
Which Came First?
(Not dating your
relatives!)
Sediments are
originally
deposited in
horizontal
layers.
Sedimentary layers
or lava flows
extend sideways in
all directions until
they thin out or
reach a barrier.
In undisturbed
rock layers, the
oldest layer is at
the bottom and
the youngest
layer is at the
top
Any part of a
previous rock
layer, like a piece
of stone, is older
than the layer
surrounding it.
In some rock formations, layers or parts of layers may be
missing. This is often due to erosion. Erosion by water or
wind removes sediment from exposed surfaces. Erosion
often leaves a new flat surface with some of the original
material missing.
Any feature that
cuts across rock
layers is younger
than the layers.
A fault is a crack or
break occurs across
rock layers.
A fault is always
younger than the
layers that have
faulted.
Let’s try some
examples!
absolute Dating
Exact age of object can be determined
1. Ice cores
• Cylinders of ice that are drilled out of glaciers and polar ice sheets.
• Give important information on Earth’s climate changes over time.
• When snow falls it carries compounds that are in the air, snow builds up
layer upon layer and traps the climate record.
2. Carbon-14 dating
• radioactive atoms (unstable) begin to break down over time at a
predictable rate.
• Scientists can estimate the amount of time that has passed since the
rock formed.
Ice cores
Carbon-14 Dating
Found in
ALL living
things!
What happens??
C
CC
C
C
C
C
C
C
C
CC
CC
C
CC
C
C
C
C
C
C
C
CC
CC
C
CC
C
C
C
C
C
C
C
CC
CC
Half-life of carbon
Half-life of potassium
How
old?
Scientists have measured the amount of
Carbon -14 remaining in this elephant skull
to be 50%.....
5,700 years old
How old?
Scientists have found what they think is the oldest fossil on Earth,
a remnant of life from when Earth's skies were orange and its
oceans green. The levels of potassium in the fossil were
measured and determined that 12% of the original potassium
remained.
3.7 million years old