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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:

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Earth and Space Science


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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)  evaluate 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.

Let’s learn about some of the ways that geologists can date events using relative dating techniques.

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Earth’s Geologic Time Scale

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
  • 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.

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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 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

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Law of Superposition

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.

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Law of Lateral Continuity

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. 

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.

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

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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 magma pushes, or intrudes, into cracks in existing rocks. When the melted rock cools and solidifies, the resulting feature is called an igneous intrusion. This image shows metamorphic rock in Death Valley, California, cut by a darker igneous intrusion.

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.

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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 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.

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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.

A

B

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)

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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.

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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!