(7)

1
Earth and Space Science
(7)  Earth in space and time. The student knows
that scientific dating methods of fossils and
rock sequences are used to construct a chronology
of Earth's history expressed in the geologic time
scale. The student is expected to (a)  evaluat
e relative dating methods using original
horizontality, rock superposition, lateral
continuity, cross- cutting relationships,
unconformities, index fossils, and biozones
based on fossil succession to determine
chronological order  Unit Vocabulary Relative
dating Unconformities Nonconformities
Disconformities, Eon, Epoch, Era, Period,
Superposition Biozones Index fossils Relative
Dating Chronology Hiatus
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Relative Dating Techniques
Relative dating methods use geological principles
to place events in chronological order. Prior to
the discovery of radiometric dating which
provided a means of absolutely dating objects
based on the rates of radioactive decay of
certain elements they contain, geologists were
limited to the use of relative dating techniques
to determine the sequence of geological
events. Though relative dating can only determine
the sequential order in which a series of events
occurred, not when they occur, it remains a
useful technique especially in materials lacking
radioactive isotopes.
Lets learn about some of the ways that
geologists can date events using relative dating
techniques.
3
Earths Geologic Time Scale
Because of the work of many historical
geologists, the Earths timeline has been worked
out and established for us to consider.

• Geologists have divided Earths history up into a
  hierarchical set of divisions for describing
  geologic time. The generally accepted divisions
  are
• Eon
• Era
• Period
• Epoch
• Age

In the time scale shown at left, only the two
highest levels of this hierarchy are represented.
There are examples of far more detailed time
scales
To the ridiculously specific and detailed.
4
Principle of Original Horizontality
This principle was originally proposed by
Nicolas Steno, and states that layers of sediment
are originally deposited horizontally under the
action of gravity. The principle is especially
important to the analysis of folded and tilted
strata.
Because of Stenos observations of these rock
layers, he established that Earth had not been
static or unchanging, and that great forces had
been at work over long periods of time. Because
of Stenos Principle, conclusions were able to be
made that movement and collisions of large plates
of the Earth's crust were the cause of folded
strata, centuries later. While this was a big
deal back in Stenos day, it is now known that
not all sedimentary layers are deposited purely
horizontally, such as those laid down on a sand
dune.
5
Law of Superposition
Another of Stenos Laws is that of Superposition.
In a nutshell, this law states that as sediments
build up, the older layers are laid beneath the
newer layers. By Stenos reckoning, this would
mean that fossilized remains that were found in
older layers, represented life forms that were
older than the fossilized remains that were found
in the younger layers.
Steno believed that most rocks in the area where
he lived were laid down during the flood
mentioned in the Bible. Since the lower layers
contained no fossils, and the upper layers did,
he thought this meant the lower layers were
deposited before the creation of life. While
this seems like an odd mix of religion and
science, it was the first use of geology to
distinguish different time periods in the Earth's
history.
6
Law of Lateral Continuity
Yet another of Stenos contributions to the
science of geology is that sedimentary rocks are
composed of fragments of preexisting rocks that
have been both mechanically and chemically
weathered.
Before lithification (rock forming process),
these tiny fragments are transported and
deposited in areas where they are spread out over
continuous and sometimes quite extensive
geographic areas
7
Principle of Cross-Cutting Relationships
Originally developed by James Hutton, and added
to by Charles Lyell, the principle of
cross-cutting relationships states that a
geologic feature which cuts through another one
is younger.
Sometimes, the cross-cutting feature is an actual
intrusion, such as above. Other times, it may be
a fault or fracture. The cross-cutting may have
been created by animal activity, (such as
burrowing animals), or even meteorite impacts.
8
Unconformities
An unconformity is a gap in the geologic record
due to some kind of crustal deformation,
erosional event, or sea-level variations. Since
they are found in stratified rocks, they are
usually associated with sedimentary rocksbut can
occasionally be found in stratified volcanic
rocks as well.

• There are typically three types of unconformities
  described by geologists
• angular unconformities
• disconformities
• nonconformities

Unconformities represent times when deposition
stopped, an interval of erosion removed some of
the previously deposited rock, and finally
deposition was resumed. The break in between
times of deposition is known as a hiatus.
9
Inclusions
Any included pebbles and fragments must be older
than the host rock containing them.
The sand grains that the sandstone is made of
MUST be older than the rock they appear within.
These river rocks are older than the conglomerate
they will appear in.
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Index Fossils
Index fossils belong to the forms of life which
existed during limited periods of geologic time
and thus are used as guides to the age of the
rocks in which they are preserved. These are some
examples of common index fossils.
As you can see, index fossils, once identified,
can be used to relatively date the sediments in
which they are found. Since the organisms are
presumed to have lived at the same time, we can
say that layer A is the same age as layer B, give
or take.
(Which in geological terms can be a VERY long
time)
11
Geological Biozones Biostratigraphy
Biozones are intervals of geological strata that
are defined on the basis of their characteristic
fossils.
As species survive for a relatively short period
before becoming extinct, if the same fossil is
found in widely scattered rock units, it is most
likely that those rock units were all laid down
at about the same time. This becomes
particularly important in the petroleum/coal
exploration industry, as biozones provide the
primary time framework used for exploration and
production of fossil fuels.
12
Design a thinking map

• Using the template of your choice, create a
  thinking map that organizes each of the kinds of
  relative dating techniques that we discussed
  today.
• Use the type that you think is best for the job.
• Your thinking map should name and explain a
  little something about each relative dating
  method!