(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) evaluate relative dating methods using original
horizontality, rock superposition, lateral continuity, crosscutting 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
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
Let’s learn about some of the ways that geologists can
radioactive isotopes. date events using relative dating techniques.
Because of the work of many historical
geologists, the Earth’s timeline has been worked
out and established for us to consider.
Geologists have divided Earth’s history up
into a hierarchical set of divisions for
describing geologic time. The generally
accepted divisions are:
• Eon
• Era
• Period
• Epoch
• Age
To the ridiculously
specific and detailed.
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…
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 Steno’s 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 Steno’s 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 Steno’s day, it is now
known that not all sedimentary layers are deposited
purely horizontally, such as those laid down on a sand
dune.
1638-1686
Another of Steno’s 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 Steno’s 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.
Younger
Older
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.
Yet another of
Steno’s 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.
The red lines below show how the butte could connect to the
canyon walls and form a continuous stack of flat layers at one
time. If erosion by the Green and Colorado rivers had not
affected this region these beds would still be laterally
connected.
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
Sometimes magma pushes, or intrudes, into
cracks in existing rocks.
When the melted rock
Originally developed
by James
cools and solidifies, the resulting feature is
an igneous
This image shows
Hutton, andcalled
added
tointrusion.
by Charles
metamorphic rock in Death Valley, California,
cut by a darker igneous intrusion.
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.
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 rocks…but 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.
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.
Index fossils
belong to the
forms of life
which existed
during limited
periods of
geologic time and
thus are used as
A
B
guides to the age
of the rocks in
which they are
preserved. These
are some
examples of
index
As you can see, index fossils, once identified, can be usedcommon
to “relatively
fossils.
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)
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.
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!