Clues to Earth’s Past
Relative-Age Dating
Relative Ages of Rocks
You just remembered where you left the money that you
have been looking for. It is in the pocket of the pants you
wore to the movies last Saturday. Now imagine that the
pants are in your pile of dirty laundry. How can you tell
where your money is? There really is some order to your pile
of clothes. Every time you add clothes to the pile, you place
them on top. The clothes from last Saturday are on the
bottom. That is where your money is!
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There is order in a rock formation just as there is order in
a pile of clothes. In many rock formations, the oldest rocks
are in the bottom layer and the youngest rocks are in the
top layer.
If you have brothers and sisters, you might describe your
age by saying, “I’m older than my sister and younger than
my brother.” This tells how your age relates to others in your
family. It is your relative age. Geologists are scientists who
study Earth and rocks. They have developed a set of principles
to compare the ages of rock layers. These principles help them
organize rocks according to their relative ages. Relative age is
the age of rocks and geologic features compared with other rocks and
features nearby.
Superposition
Your pile of dirty clothes demonstrates the first principle
of relative-age dating—superposition. Superposition is the
principle that in undisturbed rock layers, the oldest rocks are on the
bottom.
Forces do sometimes disturb rock layers after they are
deposited. But if no disturbance takes place, each layer of
rocks is younger than the layer below it. The principle of
superposition is shown in the top part of the figure below.
Layer 1 in the figure is the oldest rock layer, while layer 4 is
the youngest.
Original Horizontality
The second principle of relative-age dating is called
original horizontality. It is shown in the middle part of
the figure below. Again, layer 1 is the oldest rock layer and
layer 4 is the youngest.
According to the principle of original horizontality, 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.
When you see rocks that are tilted, remember that all layers
were originally deposited horizontally.
Lateral Continuity
Superposition
Original horizontality
4
3
2
1
4
3
2
1
Lateral continuity
4
3
2
1
4
3
2
1
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Another principle of relative-age dating is that sediments
are deposited in large, flat sheets. The sheets, or layers,
continue in all lateral directions until they thin out or until
they meet a barrier. This principle, shown in the bottom part
of the figure below, is called the principle of lateral
continuity. For example, a river might erode the layers, but
the order of the layers does not change.
Inclusions
Sometimes, 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 the magma becomes rock, the broken
piece of rock becomes a part of it. A piece 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 the pieces. The first part of the figure
below shows sediments deposited in layers that have become
rock. The vertical intrusion shown in the middle part of the
figure below is called a dike. The dike formed when magma
flowed into the rock layers. The dike is younger than the
pieces of rock, or inclusions, inside it.
Fault
Sedimentary
rock
layers
Dike
Inclusions
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1
2
3
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.
According to the principle of cross-cutting relationships,
if one geologic feature cuts across another feature, the
feature that it cuts across is older. Notice in the figure above
that both faults and dikes cut across existing rock. In the
figure on the right, the fault cuts across rock layers and the
dike. Scientists conclude that the dike is older than the fault
because the fault is cutting across the dike. Both the fault
and the dike are younger than the rock layers.
Unconformities
After rocks form, they are sometimes uplifted and exposed
at Earth’s surface. As soon as rocks are exposed, wind and
rain start to weather and erode them. These eroded areas
represent a gap in the rock record.
A Gap in Time Often, new rock layers are deposited on top of
old, eroded rock layers. When this happens, an unconformity
(un kun FOR muh tee) 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 hole or a space in the rock. It is
a surface on a layer of eroded rocks with younger rocks on
top. An unconformity does represent a gap in time. It could
represent a few hundred years, a million years, or even
billions of years.
Types of Unconformities There are three major types of
unconformities, as shown in the figure below. In a
disconformity, younger sedimentary layers are deposited on
top of older, horizontal sedimentary layers that have been
eroded. In an angular unconformity, sedimentary layers are
deposited on top of tilted or folded sedimentary layers that
have been eroded. In a nonconformity, younger sedimentary
layers are deposited on older igneous or metamorphic rock
layers that have been eroded.
Disconformity
Younger sedimentary rock
Disconformity
➞
Older sedimentary rock
Unconformity
Younger sedimentary rock
Angular
Unconformity
➞
Older sedimentary rock
Nonconformity
Younger sedimentary rock
Nonconformity
➞
Older sedimentary rock
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Types of Unconformities
Correlation
Rock layers contain clues about Earth. Geologists use
these clues to build a record of Earth’s geologic history.
Many times the rock record is incomplete. For example,
unconformities create gaps in the geologic record. Geologists
fill in the gaps in the rock record by matching rock layers
and fossils from separate locations. Matching rocks and fossils
from separate locations is called correlation (kor uh LAY shun).
Matching Rock Layers
Another word for correlation is connection. Sometimes,
geologists can connect rock layers simply by walking along
rock formations and looking for similarities. Other times,
soil might cover the rocks, or rocks might be eroded. When
this happens, geologists correlate rocks by matching exposed
rock layers in different locations. As shown in the figure
below, geologists have used correlation to develop a
historical record for part of the southwestern United States.
Bryce Canyon
National Park
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Utah
Zion Bryce
N.P. Canyon
N.P.
Grand
Canyon
N.P.
Zion National Park
Grand
Canyon
National
Park
Navajo sandstone
Arizona
Moenkopi formation
Kaibab limestone
260 million
years old
Older rocks
not exposed
A
B
190 million
years old
Carmel
formation
230 million years old
Older rocks
not exposed
C
Index Fossils
Some correlated rock formations are within a few
hundred kilometers of one another, such as those in some
national parks. They are correlated based on similarities in
rock type, structure, and fossil evidence. 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 formations. Fossils of species that lived on Earth for
hundreds of millions of years are not helpful. They represent
time spans that are too long.
The most useful fossils represent species such as certain
trilobites. These trilobites existed for only a short time in
many different areas on Earth. The fossils of these trilobites
are index fossils. Index fossils represent species that existed on
Earth for a short length of time, were abundant, and inhabited many
locations. When an index fossil is found in rock layers at
different locations, geologists can infer that the layers are of
similar age. Trilobites, shown in the figure below, are
examples of index fossils.
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