# Age of the Earth: How we can determine the Age of Rocks?

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The age of rocks is determined by radiometric dating, which looks at the proportion of two different isotopes in a sample. Radioactive isotopes break down in a predictable amount of time, enabling geologists to determine the age of a sample using equipment like this thermal ionization mass spectrometer.
1. Unit 5: Age of the Earth
Objective:
E5.3D Describe how index fossils can be used to determine time sequence.
E5.3g Identify a sequence of geologic events using relative-age dating principles.
2. Relative Dating
In the same way that a history book shows
an order of events, layers of rock (called
strata) show the sequence of events that
took place in the past. Using a few basic
principles, scientists can determine the
order in which rock layers formed. Once
they can know the order, a relative age
can be determined for each rock layer.
*Relative age indicates that one layer is
older or younger than another layer, but
does not indicate the rock’s age in years
(absolute age).
3. Law of Superposition
Sedimentary rocks form when new sediments are
deposited on top of old layers of sediment. As the
sediments accumulate, they are compressed and
harden into sedimentary rock layers.
Scientists use a basic principle called
the Law of Superposition to
determine the relative age of a layer
of sedimentary rock. *The Law of
Superposition is that an undeformed
sedimentary rock layer is older than
the layers above it and younger than
the layers below it. According to the
Law of Superposition, layer 1 was the
first layer deposited, and thus the
oldest layer. The last layer
deposited was layer 12, and thus it is
the youngest layer.
4. *Principle of Original Horizontality
Scientists also know that
sedimentary rock generally forms in
horizontal layers. The Principle of
Original Horizontality is that
sedimentary rocks left undisturbed
will remain in horizontal layers.
Therefore, scientists can assume that
sedimentary rock layers that are not
horizontal have been tilted or
deformed by crustal movements that
5. Unconformities
Movements of Earth’s crust can lift up rock
layers that were buried and expose them
to erosion. Then, if sediments are
deposited, new rock layers form in place of
the eroded layers. The missing rock layers
create a break in the geologic record in
the same way that pages missing from a
book create a break in a story.
*A break in the geologic record is
called an unconformity. An
unconformity shows that deposition
stopped for a period of time, and rock
may have been removed by erosion
6. *Unconformities
There are three types of
unconformities.
1. An unconformity in which stratified
(layers) of rock rests upon
unstratified rock is called a
nonconformity.
2. The boundary between a set of tilted
layers and a set of horizontal layers
is called an angular unconformity.
3. The boundary between horizontal
layers of old sedimentary rock and
younger, overlying layers that are
According to the Law of Superposition, all rocks beneath an unconformity are older than the
deposited
rocks on an eroded surface is
above the unconformity.
called a disconformity.
7. Crosscutting Relationships
When rock layers have been disturbed by faults (a break or
crack in Earth’s crust) or intrusions (a mass of igneous rock that
forms when magma is injected into rock and then cools and
solidifies), determining relative age may be difficult. In such
cases, scientists may apply the Law of Crosscutting
Relationships. *The Law of Crosscutting Relationships is that a
fault or intrusion is always younger than all the rocks it cuts
through above and below the unconformity.
8. Relative Age
Based on what you now know about the Law of Superposition, the
Principle of Original Horizontality, unconformities, and the Law of
Crosscutting Relationships can you place the layers indicated in
the diagram in the correct order, starting from the oldest layer?
The oldest layer is Q, followed
by O, then N, then M, then L. P
cuts across layers L-Q, so it is
the next layer since it does not
cut into layer H. Above the
unconformity we then have
layer H, followed by I, then J,
with K being the youngest
layer.
9. *Index Fossils
Paleontologists can use fossils to determine the
relative ages of the rock layers in which the
fossils are located. Fossils that occur only in rock
layers of a particular geologic age are called
index fossils.
To be an index fossil, a fossil must meet certain
1. It must be present in rocks scattered over a
large region.
2. It must have features that clearly distinguish it
from other fossils.
3. Organisms from which the fossil formed must
have lived during a short span of geologic time.
4. The fossil must occur in fairly large numbers
within the rock layers.