NOTES - Relative Dating.doc

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Pg xx-xx

Unit- Geologic Time

Intro: Rock layers and geologic features represent the historical record of the earth’s past (like a history book written by the earth).

Rock Record – Rock layers and other geologic features that represent earth’s geologic history.

I) Relative Ages

A) Relative Dating – Estimating the age of a rock or geologic event based upon the study of other rock layers and events around it.

Ex: Rock layer A is younger than rock layer B but older than rock layer C.

++ Numerical Date – Specific age of a rock or geologic event.

B) Principle of Uniformitarianism - All natural laws that operate now have also operated in the geologic past. In other words, “the present is the key to the past.”

C) The following principles relate to Sedimentary rock layers: 1) Principle of Superposition - In undisturbed rock layers,

younger layers are on top of older layers.

2) Principle of Original Horizontality - Sediments are deposited in horizontal layers.

[] Tilting or folding of Sedimentary rock layers that happen from the movement of tectonic plates (earth’s crust) happened after the sediments were deposited.

3) Principle of Original Lateral Continuity –Sedimentary rock layers or lava flows extend laterally in all directions until they thin out (pinch out) or reach the edge of their basin. D) These two principles relate to other geologic features and

to all three types of rocks.

1) Principle of Crosscutting Relationships – A cutting feature (layer, faults, cracks, etc.) is younger than any layer it cuts…. “Rock has to be there to be cut.”

2) Principle of Inclusion – A rock body or layer that contains inclusions (rock fragments, pebbles, etc.) is younger than the inclusions…. Inclusions are older. “Inclusions have to be there to be ‘picked up’ by a rock layer.”

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[] They represent a loss of geologic time like pages ripped from a book.

Three types of Unconformities:

1) Disconformity – Loss of rock record between parallel or nearly parallel layers.

2) Angular Unconformity – Loss of rock record that has tilted or folded sedimentary rocks on the bottom with younger flat (horizontal) layers on top.

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Introduction:

Rock

layers and geologic

features represent

the

historical record of the

earth’s past (like a

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Intro: Rock layers and geologic features represent the historical record of the earth’s past (like a history book written by the earth).

Rock Record

– Rock

layers and other

geologic features that

represent earth’s

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Rock Record – Rock layers and other geologic features that represent earth’s geologic history.

I) Relative Ages

A)

Relative Dating –

Estimating the age of a

rock or

geologic event

based upon

the study

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I) Relative Ages

Ex: “Rock layer A is

younger than layer B

and older than layer

C.”

A

C

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

Ex: Older than rock layer A but younger than rock layer B.

++ Numerical Date –

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++ Numerical Date – Specific age of a rock or geologic event.

B) Principle of

Uniformitarianism - All

natural laws that

operate now have also

operated in the

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B) Principle of Uniformitarianism - All natural laws that operate now have also operated in the geologic past.

In other words, “

the

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B) Principle of Uniformitarianism - All natural laws that operate now have also operated in the geologic past. In other words, “the present is the key to the past.”

C) The following

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C) The following principles relate to Sedimentary rock layers:

1)

Principle of

Superposition - In

undisturbed rock

layers, younger layers

are on top of older

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

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

1) Principle of Superposition - In undisturbed rock layers, younger layers are on top of older layers.

2) Principle of Original

Horizontality -

Sediments are

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2) Principle of Original Horizontality - Sediments are deposited in horizontal layers.

[] Tilting or folding of

Sedimentary rock

layers – (that happens

from the movement

of tectonic plates

(earth’s crust)) –

happened after the

sediments

were

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[] Tilting or folding of Sedimentary rock layers that happen from the movement of tectonic plates (earth’s crust) happened after the sediments were deposited.

3) Principle of Original

Lateral Continuity

–

Sedimentary rock layers

or lava flows extend

laterally in all directions

until they

thin out

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 3) Principle of Original Lateral Continuity –Sedimentary rock layers or lava flows extend laterally in all directions until they thin out (pinch out) or reach the edge of their basin.

D) These two principles relate

to other geologic features

and to all three types of

rocks.

1) Principle of

Crosscutting

Relationships – A

cutting feature (layer,

faults, cracks, etc.) is

younger than any layer

it cuts….

“Rock has to be

there to

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Youngest

___, _, __, _, ___ oldest

5

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1) Principle of Crosscutting Relationships – A cutting feature (layer, faults, cracks, etc.) is younger than any

layer it cuts…. “Rock has to be there to be cut.”

2) Principle of

Inclusion

– A rock body or layer

that contains inclusions

(rock fragments,

pebbles, etc.)

is

younger than the

inclusions

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Inclusions

are older

2) Principle of Inclusion – A rock body or layer that contains

inclusions (rock fragments, pebbles, etc.) is younger than the inclusions….

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A

B

C

D