Principles of GeologyPrinciples of Geology

Essential QuestionsEssential Questions

�� What is Uniformitarianism?What is Uniformitarianism?

�� What is What is CatastrophismCatastrophism??

�� What is StenoWhat is Steno’’s Law?s Law?

�� What are the other geologic principles?What are the other geologic principles?

�� How is relative age determined using How is relative age determined using

these principles?these principles?

What is Uniformitarianism?What is Uniformitarianism?

Development of the TheoryDevelopment of the Theory

�� In the midIn the mid--seventeenth century, biblical scholar seventeenth century, biblical scholar and Archbishop James Ussher determined that and Archbishop James Ussher determined that the earth had been created in the year 4004 the earth had been created in the year 4004 BCEBCE

�� Just over a century later James Hutton, known Just over a century later James Hutton, known as the father of as the father of geologygeology, suggested that the , suggested that the earth was much olderearth was much older..

�� The 19th century scholar Sir Charles Lyell whose The 19th century scholar Sir Charles Lyell whose Principles of GeologyPrinciples of Geology (1830) popularized the (1830) popularized the concept of concept of uniformitarianismuniformitarianism..

UniformitarianismUniformitarianism

�� can be summarized by the phrase "the can be summarized by the phrase "the

present is the key to the past."present is the key to the past."

�� based on the slow, natural processes based on the slow, natural processes

observed on the landscapeobserved on the landscape

�� if given enough time, a stream could carve if given enough time, a stream could carve

a valley, ice could erode rock, sediment a valley, ice could erode rock, sediment

could accumulate and form new landformscould accumulate and form new landforms

What is What is CatastrophismCatastrophism??

CatastrophismCatastrophism

�� is the idea that many of Earthis the idea that many of Earth’’s crustal s crustal

features (strata layers, erosion, features (strata layers, erosion, polystratepolystrate

fossils, etc) formed as a result of past fossils, etc) formed as a result of past

cataclysmic activity.cataclysmic activity.

What is StenoWhat is Steno’’s Law?s Law?

The 3 principles of StenoThe 3 principles of Steno’’s Laws Law

�� The Principle of SuperpositionThe Principle of Superposition

�� The Principle of Original HorizontalityThe Principle of Original Horizontality

�� The Principle of Original Lateral ContinuityThe Principle of Original Lateral Continuity

The Principle of SuperpositionThe Principle of Superposition

�� In any series of layered rocks, the older In any series of layered rocks, the older

rocks are on the bottom and they become rocks are on the bottom and they become

younger as you move up through the younger as you move up through the

beds.beds.

The Principle of Original The Principle of Original

HorizontalityHorizontality

�� All sediments are originally deposited in All sediments are originally deposited in

flat layers.flat layers.

The Principle of Original Lateral The Principle of Original Lateral

ContinuityContinuity

�� Sedimentary beds form over a large area in a continuous Sedimentary beds form over a large area in a continuous sheet, gradually thinning toward the edges.sheet, gradually thinning toward the edges.

�� Often, coarserOften, coarser--grained material can no longer be grained material can no longer be transported to an area because the transporting medium transported to an area because the transporting medium has insufficient energy to carry it to that location. In its has insufficient energy to carry it to that location. In its place, the particles that settle from the transporting place, the particles that settle from the transporting medium will be finermedium will be finer--grained, and there will be a lateral grained, and there will be a lateral transition from coarsertransition from coarser-- to finerto finer--grained material. grained material.

What are the other geologic What are the other geologic

principles?principles?

�� The Principles of Intrusive RelationshipsThe Principles of Intrusive Relationships

�� The Principle of CrossThe Principle of Cross--cutting cutting

RelationshipsRelationships

�� The Principle of InclusionsThe Principle of Inclusions

�� The Principle of Faunal SuccessionThe Principle of Faunal Succession

�� Principle of UnconformitiesPrinciple of Unconformities

The Principles of Intrusive The Principles of Intrusive

RelationshipsRelationships

�� The sedimentary rocks are older than the The sedimentary rocks are older than the igneous rock which intrudes themigneous rock which intrudes them. . �� In other words, the sedimentary rocks had to be In other words, the sedimentary rocks had to be there first, so that the igneous rocks would have there first, so that the igneous rocks would have something to intrude. something to intrude.

�� Examples of types of igneous intrusions are:Examples of types of igneous intrusions are:�� DikesDikes

�� SillsSills

�� Laccoliths Laccoliths

�� StocksStocks

�� BatholithsBatholithsConcordant Concordant –– a differing type a differing type

of rock whichof rock which lies parallel to a lies parallel to a

formationformation

Discordant Discordant -- a differing type a differing type

of rock cutting across a formationof rock cutting across a formation

�� DikesDikes

�� is a type of is a type of sheet intrusionsheet intrusion

referring to any geologic referring to any geologic

body that cuts body that cuts discordantlydiscordantlyacross across

�� SillSill

�� is a tabular is a tabular sheet sheet

intrusionintrusion that has that has

intrudedintruded between between

older older layerslayers of of

sedimentary rocksedimentary rock, ,

beds of beds of volcanicvolcanic

lavalava, or even along , or even along

the direction of the direction of

foliationfoliation in in

metamorphic rockmetamorphic rock

�� StocksStocks

�� is a discordant is a discordant igneousigneous

intrusionintrusion having a having a

surface exposure of surface exposure of

less than 40 square less than 40 square

milesmiles

�� may have been may have been

feeders for volcanic feeders for volcanic

eruptions eruptions

�� BatholithsBatholiths�� is a large emplacement of is a large emplacement of

igneousigneous intrusiveintrusive rock that rock that

forms from cooled forms from cooled magmamagma

deep in the earth's deep in the earth's crustcrust

�� almost always made mostly almost always made mostly

of of felsicfelsic or intermediate or intermediate

rockrock--typestypes

�� very large intrusive bodies, very large intrusive bodies,

usually so large that there usually so large that there

bottoms are rarely exposedbottoms are rarely exposed

�� LaccolithsLaccoliths�� is a is a sheet intrusionsheet intrusion that has that has

been injected between two been injected between two

layers of layers of sedimentary rocksedimentary rock. .

�� The pressure of the The pressure of the magmamagma

is high enough that the is high enough that the

overlying strata are forced overlying strata are forced

upward, giving the upward, giving the laccolithlaccolith

a dome or mushrooma dome or mushroom--like like

form with a generally form with a generally

planar base planar base

�� Diagram (1): Dike B is younger than Sedimentary Rock Diagram (1): Dike B is younger than Sedimentary Rock A. Erosion surface C is younger than Dike B.A. Erosion surface C is younger than Dike B.Sedimentary Rock D is younger than Erosion Surface C.Sedimentary Rock D is younger than Erosion Surface C.

�� Diagram (2) Sill B is younger than Sedimentary Rock A. Diagram (2) Sill B is younger than Sedimentary Rock A. Dike C is younger than sill B.Dike C is younger than sill B.

�� Diagram (3) Stock B is younger than Sedimentary Rock Diagram (3) Stock B is younger than Sedimentary Rock A. Dike C is the youngest.A. Dike C is the youngest.

The Principle of CrossThe Principle of Cross--cutting cutting

RelationshipsRelationships�� Originally developed by Originally developed by James Hutton James Hutton in in Theory of the Theory of the

EarthEarth (1795) (1795)

�� embellished upon by embellished upon by Charles Lyell Charles Lyell in in Principles of Principles of GeologyGeology (1830), (1830),

�� the principle states that the fault which cuts a rock is the the principle states that the fault which cuts a rock is the younger of the two featuresyounger of the two features

�� if a fault is found that penetrates some formations but if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut are older than the fault, and the ones that are not cut must be younger than the fault must be younger than the fault

Types of Cross cuttingTypes of Cross cutting

�� StructuralStructural�� A fault or fracture cuts through an older rockA fault or fracture cuts through an older rock

�� StratigraphicStratigraphic�� An An erosionalerosional surface (or surface (or unconformityunconformity) cuts across older rock ) cuts across older rock

layers, geological structures, or other geological features.layers, geological structures, or other geological features.

�� SedimentologicalSedimentological�� Occur where currents have eroded or scoured older sediment in Occur where currents have eroded or scoured older sediment in

a local area to produce, for example, a channel filled with sanda local area to produce, for example, a channel filled with sand..

�� PaleontologicPaleontologic�� Occur where animal activity or plant growth produces Occur where animal activity or plant growth produces

truncation. This happens, for example, where animal burrows truncation. This happens, for example, where animal burrows penetrate into prepenetrate into pre--existing sedimentary deposits.existing sedimentary deposits.

�� GeomorphicGeomorphic�� Occur where a Occur where a surficialsurficial feature, such as a feature, such as a river, flows through a river, flows through a

gap in a ridge of rock. In a similar example, an impact crater gap in a ridge of rock. In a similar example, an impact crater excavates into a subsurface layer of rock.excavates into a subsurface layer of rock.

The Principle of InclusionsThe Principle of Inclusions

�� developed by developed by James HuttonJames Hutton

�� with sedimentary rocks, if inclusions (or with sedimentary rocks, if inclusions (or

clastsclasts) are found in a formation, then the ) are found in a formation, then the

inclusions must be older than the inclusions must be older than the

formation that contains themformation that contains them

The Principle of Faunal SuccessionThe Principle of Faunal Succession

�� developed by the English developed by the English geologistgeologist William SmithWilliam Smith

�� is based on the observation that sedimentary is based on the observation that sedimentary rock rock strata contain fossilized flora and faunastrata contain fossilized flora and fauna, , and that these fossils succeed each other and that these fossils succeed each other vertically in a specific, reliable order that can be vertically in a specific, reliable order that can be identified over wide horizontal distancesidentified over wide horizontal distances

�� As organisms exist at the same time period As organisms exist at the same time period throughout the world, their presence or throughout the world, their presence or (sometimes) absence may be used to provide a (sometimes) absence may be used to provide a relative age of the formations in which they are relative age of the formations in which they are found found

Principle of UnconformitiesPrinciple of Unconformities

�� Any place where there is missing time is Any place where there is missing time is

called an called an unconformityunconformity

�� Unconformities normally happen either Unconformities normally happen either

through erosion or nonthrough erosion or non--depositiondeposition

�� An unconformity represents An unconformity represents timetime during during

which no sediments were preserved in a which no sediments were preserved in a

regionregion

The Great Unconformity

Grand Canyon, AZ

Types of UnconformitiesTypes of Unconformities

Angular UnconformityAngular Unconformity�� is where horizontally parallel is where horizontally parallel stratastrata of sedimentary rock of sedimentary rock

are deposited on tilted and eroded layers, producing an are deposited on tilted and eroded layers, producing an

angular discordance with the overlying horizontal layers. angular discordance with the overlying horizontal layers.

�� younger sediments rest upon the eroded surface of tilted younger sediments rest upon the eroded surface of tilted

or folded older rocksor folded older rocks

DisconformityDisconformity

�� is an unconformity between parallel is an unconformity between parallel layerslayersof of sedimentary rockssedimentary rocks which represents a which represents a period of erosion or nonperiod of erosion or non--depositiondeposition

�� contact between younger and older beds is contact between younger and older beds is marked by a visible, irregular or uneven marked by a visible, irregular or uneven erosionalerosional surface surface

DisconformityDisconformity continuedcontinued……

NonconformityNonconformity

�� exists between sedimentary rocks and exists between sedimentary rocks and

metamorphicmetamorphic or or igneous rocksigneous rocks when the when the

sedimentary rock lies above and was sedimentary rock lies above and was

deposited on the predeposited on the pre--existing and eroded existing and eroded

metamorphic or igneous rockmetamorphic or igneous rock

�� develops between sedimentary rock and older develops between sedimentary rock and older

igneous or metamorphic rock that has been igneous or metamorphic rock that has been

exposed to erosion exposed to erosion

Nonconformity continuedNonconformity continued……

How is relative age determined How is relative age determined

using these principles?using these principles?

What is relative dating?What is relative dating?

�� is the science determining the relative is the science determining the relative

order of past events, without necessarily order of past events, without necessarily

determining their absolute agedetermining their absolute age

�� can only determine the can only determine the sequential ordersequential order in in

which a series of events occurred, not which a series of events occurred, not

whenwhen they occur they occur