Grade 8 Integrated Science Chapter 12 Lesson 1 on relative-age dating of fossils and rock layers. This lesson explains how scientists use rock layers to determine a age of a rock or fossil compared to others. The goal of this lesson is for students to be able to correctly order rock layers by age and to know the different disconformities and nonconformities.

1. RELATIVE-AGE DATING Chapter 16 Lesson 2
Pages 574-580

2. NEW VOCABULARY
Relative Age – the age of rocks and geologic features compared with
other rocks and features nearby.
Superposition – the principle that in undisturbed rock layers, the
oldest rocks are on the bottom.
Inclusion – a piece of an older rock that becomes part of a new rock.
Unconformity – a surface where rock has eroded away, producing a
break, or gap, in the rock record.
Correlation – matching rocks and fossils from separate locations.
Index Fossil – Fossils that represent species that existed on Earth for
a short length of time, were abundant, and inhabited many locations.

3. FIND RELATIVE AGE
A.
B. C. D.

4. FIND THE RELATIVE AGE

5. FIND RELATIVE AGE

6. RELATIVE AGES OF ROCKS
Geologists, or scientists that study Earth and rocks, have developed a
set of principles to compare the ages of rock layers.
They use these principles to organize the layers of rock according to
their relative age.
Relative age is the age of rocks and geologic features compared with
other rocks and features nearby.
*How might you define your relative age?

7. RELATIVE AGE AND
SUPERPOSITION
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Tuesday
Wednesday
Thursday
Friday

8. PRINCIPLES OF RELATIVE
AGE DATING
1. Superposition
2. Original Horizontality
3. Lateral Continuity
4. Inclusion
5. Cross-cutting Relationships

9. SUPERPOSITION
Superposition is the principle that in
undisturbed rock layers, the oldest rocks
are on the bottom.
Unless some force disturbs the layers after
they were deposited, each layer of rocks is
younger than the layer below it.

10. ORIGINAL HORIZONTALITY
The second principle of relative-age
dating is original horizontality.
According to this principle, most
rock-forming materials are deposited
in horizontal layers.
Sometimes rock layers are deformed
or disturbed after they form.
For example, the layers might be
tilted or folded.
Even though they might be tilted, all
the layers were originally deposited
horizontally.

12. LATERAL CONTINUITY
Another principle of relative-age dating
is that sediments are deposited in large
continuous sheets in all lateral directions
The sheets, or layers, continue until they
thin out or meet a barrier.
A river might erode the layers. But their
placements do not change

13. CROSS-CUTTING
RELATIONSHIPS
Sometimes, forces within Earth
cause rock formations to break, or
fracture.
When rocks move along a fracture
line, the fracture is called a fault
Faults and dikes cut across existing
rock
According to the principle of cross-
cutting relationships, if one
geologic feature cuts across
another, the feature that is cuts
across is older.
A dike is a sheet of rock that
formed in a crack in a pre-existing
rock.

14. INCLUSION
Occasionally when rocks form they
contain pieces of other rocks.
This can happen when part of an
existing rock breaks off and falls into
soft sediment or flowing magma
When the sediment or magma
becomes rock, the broken pieces
become a part of it.
A pieces of an older rock that
becomes part of a new rock is called
an inclusion.
According to the principle of
inclusions, if one rock contains
pieces of another rock, the rock
containing the pieces is younger than

15. UNCONFORMITIES
After rocks form, they are sometimes uplifted and exposed at Earth’s
surface
 When rocks are exposed, wind and rain start to weather and erode them.
 These eroded areas represent a gap in the rock record
 Often, new rock layers are deposited on top of old, eroded layers.
 When this happens, an unconformity occurs.
 An unconformity is a surface where rock has eroded away, producing a break, or gap,
in the rock record.
An unconformity is not a hollow gap in the rock.
It is a surface on a layer of eroded rocks where younger rocks have
been deposited.
However, an unconformity does represent a gap in time.

16. UNCONFORMITIES
There are three types of
unconformities.
Disconformity
Angular Unconformity
Nonconformity

17. DISCONFORMITY
Younger sedimentary layers are deposited on top of older, horizontal
sedimentary layers that have been eroded

18. NONCONFORMITY
Younger sedimentary layers are deposited on older igneous or
metamorphic rock layers that have been eroded.

19. ANGULAR UNCONFORMITY
Sedimentary layers are deposited on top of titled or folded
sedimentary layers that have been eroded.

20. CORRELATION
When geologists are faced with
unconformities, gaps in the rock record,
they fill in the gaps by matching rock
layers or fossils from separate locations.
Matching rocks and fossils from separate
locations is called correlation.

21. MATCHING ROCK LAYERS
Another word for correlation is
connection.
Sometimes it is possible to connect rock
layers simply by walking along rock
formations and looking for similarities.
At other times, soil might cover the
rocks, or rocks might be eroded away
In these cases, geologists correlate
rocks by matching exposed rock layers
in different locations
Through correlation, geologists have
established a historical record for part
of the southwestern United States.

22. INDEX FOSSILS
If scientists want to learn the relative
ages of rock formations that are very
far apart or on different continents,
they often use fossils.
If two or more rock formations
contain fossils of about the same age,
scientists can infer that the
formations are also about the same
age.
Not all fossils are useful in
determining the relative ages of rock
layers
 Fossils of species that lived on Earth for
hundreds of millions of years are not helpful
for identifying when a rock was formed

23. INDEX FOSSILS
Scientists use fossil that existed
for only a short time in many
different areas on Earth, like
trilobites
 These are called index fossils
When an index fossil is found in
rock layers at different
locations, geologists can infer
that the layers are of similar age