psc 121 prince george’s community college reading the rock record

PSC 121 Prince George’s Community CollegePSC 121 Prince George’s Community College

Reading Reading the Rock the Rock RecordRecord


Rock LayersRock Layers

Sediments and lava Sediments and lava flows form distinct flows form distinct layers as they are layers as they are depositeddeposited

A sedimentary layer A sedimentary layer that can be that can be distinguished from distinguished from the layer above and the layer above and below is called a below is called a stratumstratum (pl. strata) (pl. strata)

http://www.joshushund.com/terragen_01-06/wall_of_strata.jpg


What Information do Strata What Information do Strata Provide to a Geologist?Provide to a Geologist?

To answer this question we need to look To answer this question we need to look at some geologic principles.at some geologic principles.

#1 Processes that occurred many #1 Processes that occurred many millions of years ago probably millions of years ago probably happened the same way as they do happened the same way as they do today (because the science principles today (because the science principles are the same).are the same).

UniformitarianismUniformitarianism



#2 Sedimentary deposits or lava flows #2 Sedimentary deposits or lava flows generally form horizontal layers.generally form horizontal layers.

Principle of Original HorizontalityPrinciple of Original Horizontality



#3 Unless disturbed by an unusual #3 Unless disturbed by an unusual process, younger layers will be on process, younger layers will be on top of older layers. top of older layers.

Principle of SuperpositionPrinciple of Superposition

youngest

oldest


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Here are the layersin the Grand Canyonshown graphically.

The Bright Angelshale is youngerthan the TapeatsSandstone but older than the Muav Limestone.

The Vishnu Schistis the oldest formation (made ofmetamorphic rocks).



#4 When an igneous intrusion or fault #4 When an igneous intrusion or fault cuts across other rocks of any type cuts across other rocks of any type the intrusion is younger than the the intrusion is younger than the layers that are cut across.layers that are cut across.

Principle of Cross-Cutting RelationshipsPrinciple of Cross-Cutting Relationships


http://www.winona.edu/geology/MRW/mrwimages/Earth%20History/cross_section.jpg

1

2

3

4

5

6

Layers 1-4 are positionedoldest to youngest (superposition)

Formation 5 is youngerthan 1-4 since it cuts through them

Fault 6 is youngest because it cuts through 5


The fault and dikeare called non-conformities.

#6 When a feature contains pieces of other layers it has to be younger than the layers it contains.

Principle of Inclusion

Notice that the dikehas incorporated pieces of other layers.



#5 Fossils can be used to determine the #5 Fossils can be used to determine the approximate age of a stratum since specific approximate age of a stratum since specific living organisms lived over identified time living organisms lived over identified time periods. If a particular fossil is present in a periods. If a particular fossil is present in a layer, the layer must be the same age as layer, the layer must be the same age as other layers containing the same fossil, no other layers containing the same fossil, no matter where the layers occur.matter where the layers occur.

Principle of Faunal SuccessionPrinciple of Faunal Succession


How Can We Determine Age How Can We Determine Age More Accurately?More Accurately?

Radioactive elements are unstable Radioactive elements are unstable and their nuclei will release particles and their nuclei will release particles or energy to stabilize.or energy to stabilize.

Each radioactive element has a Each radioactive element has a half-half-lifelife – – the time it takes for one half of the time it takes for one half of the atoms in a sample to disintegratethe atoms in a sample to disintegrate

Some half-lives are fractions of a Some half-lives are fractions of a second while some are as much as second while some are as much as 4.5 million years.4.5 million years.


After 1half-life haspassed, half the atoms have changedor emitted energy.

After a 2nd half-life has passed half ofthe remaining atomshave also changed

Radiometric Dating


We can use radioactive elements in We can use radioactive elements in the rocks to help determine age.the rocks to help determine age.

If the rock contains a radioactive If the rock contains a radioactive element like potassium-40, we can element like potassium-40, we can measure how much of this element is measure how much of this element is present. Knowing how much should present. Knowing how much should be there we can determine the rock be there we can determine the rock age.age.

Determining Rock AgesDetermining Rock Ages


Suppose a rock sample has 1/64Suppose a rock sample has 1/64thth of the of the expected amount of radioactive material.expected amount of radioactive material.

1 1 1/2 1/2 1/4 1/4 1/8 1/8 1/16 1/16 1/32 1/32 1/64 1/64 This means that the sample has been This means that the sample has been

through through 66 half-lives. half-lives. If each half-life is 100,000 years then the If each half-life is 100,000 years then the

rock layer is rock layer is 600,000600,000 years old. years old.

Determining Rock AgesDetermining Rock Ages


Why Aren’t All Strata Straight?Why Aren’t All Strata Straight?

Geologic processes generate pressures Geologic processes generate pressures that cause layers to become deformed.that cause layers to become deformed.

The first type of deformation is The first type of deformation is folding.folding.

Click on imageto beginanimation

http://www2.wwnorton.com/college/geo/egeo/flash/9_1.swf


FoldsFolds

http://www.glossary.oilfield.slb.com/DisplayImage.cfm?ID=10


anticline

syncline


Why Aren’t All Strata Straight?Why Aren’t All Strata Straight?

Geologic processes generate pressures Geologic processes generate pressures that cause layers to become deformed.that cause layers to become deformed.

The second type of deformation is The second type of deformation is faulting.faulting.


FaultsFaults

Faults occur when sections of Earth’s Faults occur when sections of Earth’s surface shift position with respect to surface shift position with respect to anotheranother

One side of the fault is designated as a One side of the fault is designated as a footwall block and the other a hanging footwall block and the other a hanging wall blockwall block


Normal FaultsNormal Faults


Hanging wall moves down

Hanging wall moves down

NORMAL FAULT


Reverse FaultsReverse Faults


Hanging wall moves upward

REVERSE FAULT


Strike-Slip or Transform Strike-Slip or Transform FaultsFaults


TRANSFORM FAULTS – aerial view


Fault AnimationsFault Animations


Can you date the features from oldest to youngest?

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psc 121 prince george’s community college reading the rock record

Documents

younger layers

older layers

distinct layers

horizontal layers

rock record slide

strata http

principle of cross

principle of inclusion