Module 22A - Geological Laws

Module 23Module 23Geological LawsGeological Laws

GEOLOGIC LAWSGEOLOGIC LAWS

Geologic LawsGeologic LawsSuperpositionSuperpositionOriginal HorizontalityOriginal HorizontalityOriginal ContinuityOriginal ContinuityUniformitarianismUniformitarianismCrossCross--cutting Relationshipcutting RelationshipInclusionsInclusionsFaunal SuccessionFaunal Succession

Missing strataMissing strataUnconformityUnconformityCorrelationCorrelation

Law of SuperpositionLaw of SuperpositionIn an undisturbed rock sequence, the bottom In an undisturbed rock sequence, the bottom layer of rock is older than the layer above it, orlayer of rock is older than the layer above it, orThe younger strata at the top in an undisturbed The younger strata at the top in an undisturbed sequence of sedimentary rockssequence of sedimentary rocks..

Law of SuperpositionLaw of Superposition

Undisturbed strataUndisturbed strata

Law of SuperpositionLaw of Superposition

Disturbed strataDisturbed strata

Law of original horizontalityLaw of original horizontality

Sedimentary rocks are laid down in horizontal or Sedimentary rocks are laid down in horizontal or nearly horizontal layers, ornearly horizontal layers, orSedimentary strata are laid down nearly Sedimentary strata are laid down nearly horizontally and are essentially paralel to the horizontally and are essentially paralel to the surface upon which they acummulatesurface upon which they acummulate

Law of Original ContinuityLaw of Original Continuity

The original continuity of waterThe original continuity of water--laid sedimentary laid sedimentary strata is terminated only by pincing out againts strata is terminated only by pincing out againts the basin of deposition, at the time of their the basin of deposition, at the time of their depositiondeposition


NOTE:NOTE:This law is considerable This law is considerable oversimplificationoversimplification. The last . The last discoveries indicate that the termination is not discoveries indicate that the termination is not necessarilynecessarily at a basin border. at a basin border. FaciesFacies changes may changes may terminated a strata.terminated a strata.

UniformitarianismUniformitarianism

James HuttonJames Hutton (1726(1726--1797) 1797) Scottish geologist Scottish geologist developed the laws of developed the laws of geologygeologyUniformitarianismUniformitarianism is a is a cornerstone of geologycornerstone of geologyConsidered the Considered the Father of Father of Modern GeologyModern Geology


ORIGINAL STATEMENTORIGINAL STATEMENT““In examining things present, we have data from In examining things present, we have data from which to reason with regards to that which is to which to reason with regards to that which is to happen hereafter. Therefore, upon the superposision happen hereafter. Therefore, upon the superposision that the operations on nature are equable and that the operations on nature are equable and steady, we find, in natural appearences, means for steady, we find, in natural appearences, means for concluding a certain portion of time to have concluding a certain portion of time to have necesserily elapsed in the production of necesserily elapsed in the production of tthese hese events of which we see the effects (Hutton, p.217)events of which we see the effects (Hutton, p.217)””


UniformitarianismUniformitarianism is based on the is based on the premisepremise that:that:the physical and chemical laws of nature have the physical and chemical laws of nature have remained the same through timeremained the same through timepresentpresent--day processes have operated day processes have operated throughout geologic timethroughout geologic timerates and intensities of geologic processes, rates and intensities of geologic processes, and their results may have changed with timeand their results may have changed with time

To interpret geologic eventsTo interpret geologic events from evidence from evidence preserved in rockspreserved in rocks

we must first understand presentwe must first understand present--day day processes and their resultsprocesses and their results

UniformitarianismUniformitarianism is a cornerstone of geologyis a cornerstone of geology


MODIFIED STATEMENTMODIFIED STATEMENT““The present is the key to the past"The present is the key to the past"

•• The processes (plate tectonics, mountain The processes (plate tectonics, mountain building, erosion) we see today are believed to building, erosion) we see today are believed to have been occurring since the Earth was formed.have been occurring since the Earth was formed. ??


The concept (The concept (uniformitarianismuniformitarianism), fundamental to ), fundamental to geology, that geology, that processesprocesses which operate at present which operate at present also operated in the past, and also operated in the past, and producesproduces the same the same results. These processes results. These processes need notneed not have operated have operated at the at the same ratesame rate, nor at the , nor at the same intensitysame intensity..

Saying Saying ““The present is the key to the pastThe present is the key to the past““, , which is which is commonly offered as a commonly offered as a ‘‘definitiondefinition’’ of of uniformitarianismeuniformitarianisme, is considerable , is considerable oversimplificationoversimplification..The degree of correlation between ancient and The degree of correlation between ancient and modern processes decreases as the time interval modern processes decreases as the time interval increases, e.g. in eras prior to the emergence of increases, e.g. in eras prior to the emergence of land vegetation, weathering and erosion must land vegetation, weathering and erosion must have been different in character and intensity have been different in character and intensity compared with today compared with today

Whitten, DGA and Brooks, JRV. 1977. The Penguin Dictionary of Geology. Middlesex: Penguin Books. p. 466.

Law of CrossLaw of Cross--cutting Relationshipcutting Relationship

The thing doing the cutting is younger than the The thing doing the cutting is younger than the thing being cut.thing being cut.

Find out the crossFind out the cross--cuttingcuttingrelationship!relationship!

Law of CrossLaw of Cross--cutting Relationshipcutting Relationship

How many crossHow many cross--cutting relationships are there?cutting relationships are there?

Law of InclusionsLaw of InclusionsThe included rock is older than the rock around itThe included rock is older than the rock around it

1.1. State the principle of inclusionsState the principle of inclusions2.2. In the above diagrams list the strata, their order of occurrenceIn the above diagrams list the strata, their order of occurrence from from

oldest to youngestoldest to youngest3.3. What is the nature of the contact What is the nature of the contact AA--AA’’??4.4. What is the nature of the contact What is the nature of the contact BB--BB’’??

Law of Faunal SuccessionLaw of Faunal SuccessionFossils occur in a definite, invariable sequence in Fossils occur in a definite, invariable sequence in the geologic record. the geologic record.

Unconformity and ConformityUnconformity and Conformity

UnconformityUnconformityGaps in the rock record caused by weathering, Gaps in the rock record caused by weathering, erosion, and volcanic action.erosion, and volcanic action.

UnconformityUnconformityDevelopment ofDevelopment ofangular unconformityangular unconformity

UnconformityUnconformity

Sedimentary rocksSedimentary rocks


UnconformityUnconformityDevelopment of a Development of a disconformitydisconformity

Deposition ofDeposition ofrocks 1, 2, 3, & 4rocks 1, 2, 3, & 4

Uplift, & erosionUplift, & erosionof rocks 3, & 4of rocks 3, & 4

Subsidence &Subsidence &deposition deposition

of rocks 6 & 7of rocks 6 & 7

Sea levelSea level

DisconformityDisconformity

UnconformityUnconformityDisconformityDisconformity



UnconformityUnconformityDevelopment of a nonconformityDevelopment of a nonconformity

Deposition ofDeposition ofrocks J & K.rocks J & K.Intrusion or Intrusion or

metamorphismmetamorphismof rocks IMof rocks IM

Uplift, & erosionUplift, & erosionof rocks J, K,of rocks J, K,

& IM& IM

Subsidence &Subsidence &deposition deposition

of rocks P & Qof rocks P & Q

Sea levelSea level


UnconformityUnconformityNonconformityNonconformity

Igneous rockIgneous rock


The matching of rock The matching of rock outcrops of one region outcrops of one region to those of another to those of another region.region.Geologists use fossils, Geologists use fossils, unique rocks, or unique rocks, or mineral features to mineral features to match the layers.match the layers.

CorrelationCorrelation

CorrelationCorrelationSequence of sedimentarySequence of sedimentaryrock with complete recordrock with complete recordof depositionof deposition

Sequence shows a breakSequence shows a breakin the record as indicatedin the record as indicatedby by correlatablecorrelatable fossilsfossils

Dashed lines indicateDashed lines indicatecorrelation of rock units correlation of rock units between the two areasbetween the two areas


CorrelationCorrelation

Module 22A - Geological Laws

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