Geologic time( الجيولوجي (الزمن

Determining geological Determining geological agesages

Relative age dates (Relative age dates ( النسبي النسبي التأريخ – ( – (التأريخplacing rocks and geologic events in their placing rocks and geologic events in their proper sequenceproper sequence

of formationof formation Numerical datesNumerical dates ( ( المطلق المطلق التأريخ – – ((التأريخ

define the actual age of a particular define the actual age of a particular geologic event (termed absolute age geologic event (termed absolute age dating)dating)

Steno’s LawsSteno’s Laws

Nicolaus Steno, 1669:Nicolaus Steno, 1669:• Principle of original horizontalityPrinciple of original horizontality• Principle of superpositionPrinciple of superposition• Principle of original continuityPrinciple of original continuity

• Powerful tools for:Powerful tools for:• Analysis of outcrops.Analysis of outcrops.

Principle of Original Principle of Original Horizontality (Horizontality ( التأفق التأفق قانون قانون

((المبدئيالمبدئي

Sedimentary rocks are formed in layers Sedimentary rocks are formed in layers ((stratastrata) which were originally horizontal) which were originally horizontal

• Flat strata are probably undisturbed by Flat strata are probably undisturbed by folding and faultingfolding and faulting

• Tilted strata have been affected by tectonics.Tilted strata have been affected by tectonics.

Principle of Original Principle of Original HorizontalityHorizontality

Principle of SuperpositionPrinciple of Superposition(( الطبقات تعاقب الطبقات قانون تعاقب ((قانون

Developed by Nicolaus Steno in 1669Developed by Nicolaus Steno in 1669

In an undeformed sequence of In an undeformed sequence of sedimentary or volcanic rocks, oldest sedimentary or volcanic rocks, oldest rocks at base; youngest at toprocks at base; youngest at top

Principle of Principle of SuperpositionSuperposition

Oldest Oldest sedimentarysedimentary rocks at the bottom: C rocks at the bottom: C deposited firstdeposited first

Younger Younger sedimentarysedimentary rocks on top: B then rocks on top: B then A.A.

Superposition illustrated Superposition illustrated by strata in the Grand by strata in the Grand

CanyonCanyon

Principle of cross-cutting Principle of cross-cutting relationshipsrelationships(( التقاطع التقاطع قانون ((قانون

Magma under pressure squeezes into nearby rock Magma under pressure squeezes into nearby rock whenever fractures form in the rockwhenever fractures form in the rock

Then the magma slowly cools and becomes rock itself. Then the magma slowly cools and becomes rock itself. This newly formed igneous rock is, of course, younger This newly formed igneous rock is, of course, younger

than the rock it intruded than the rock it intruded Younger features cut across older ones.Younger features cut across older ones. When a fault cuts through other rocks, or when magma When a fault cuts through other rocks, or when magma

intrudes and crystallizes, we can assume that the fault or intrudes and crystallizes, we can assume that the fault or intrusion is younger than the rocks affected. intrusion is younger than the rocks affected.

Principle of cross-cuttingPrinciple of cross-cutting

• Cross cutting Cross cutting igneousigneous rocks are younger rocks are younger than what they than what they intrude.intrude.

FaultsFaults are younger are younger than what they cut.than what they cut.

Principle of InclusionPrinciple of Inclusion(( القاطعة الجذة القاطعة قانون الجذة ((قانون

• InclusionsInclusions are pieces of one are pieces of one rock unit that are contained rock unit that are contained within another.within another.

• Erosion surfaces exist in the Erosion surfaces exist in the rock record...rock record...

• Fragments within strata above Fragments within strata above are derived from the older are derived from the older strata below strata below they are older they are older than the strata containing them.than the strata containing them.

• The strata containing the The strata containing the fragments are younger than the fragments are younger than the strata the fragments came fromstrata the fragments came from

Included Fragments Included Fragments

If a rock contains a fragment or particle of If a rock contains a fragment or particle of another rock, then the fragment is older than another rock, then the fragment is older than the rock.the rock.

ExampleExample A pebble that is part of a conglomerate rock is A pebble that is part of a conglomerate rock is

older than the rock. The pebble had to exist older than the rock. The pebble had to exist first in order to become part of the first in order to become part of the conglomerate when it formed. conglomerate when it formed.

Conglomerate Conglomerate (sedimentary)(sedimentary)

Breaks in stratigraphic Breaks in stratigraphic recordrecord

Sometimes the sedimentation stops and the strata become Sometimes the sedimentation stops and the strata become exposed at the surfaceexposed at the surface

The beak in deposition may result from a regional uplift or sea The beak in deposition may result from a regional uplift or sea level fallinglevel falling

The strata exposed at the surface are subjected to weathering The strata exposed at the surface are subjected to weathering and erosionand erosion

Sedimentation restarts when the area becomes submerged in Sedimentation restarts when the area becomes submerged in water againwater again

The breaks (gaps) are marked by a rough and eroded surface The breaks (gaps) are marked by a rough and eroded surface called unconformitycalled unconformity

Unconformity is an indication of environmental conditions Unconformity is an indication of environmental conditions which cause the sedimentation to stop.which cause the sedimentation to stop.

Unconformities (loss of Unconformities (loss of rock record)rock record)(( التوافق التوافق عدم ((عدم

An unconformity is a break in the rock An unconformity is a break in the rock record produced by erosion and/or record produced by erosion and/or nondepositionnondeposition

Types of unconformitiesTypes of unconformities Angular unconformityAngular unconformity – tilted rocks overlain by flat- – tilted rocks overlain by flat-

lying rockslying rocks ( ( الزاوي التوافق الزاوي عدم التوافق ((عدم DisconformityDisconformity – strata on either side of the – strata on either side of the

unconformity are parallel unconformity are parallel Nonconformity Nonconformity – sedimentary rocks deposited above – sedimentary rocks deposited above

metamorphic or igneous rocks (basement)metamorphic or igneous rocks (basement)

Conformable ContactConformable Contact((متوافقةمتوافقة))

• Layers of rock that Layers of rock that have been deposited have been deposited without any without any interruption.interruption.

• No gaps in time.No gaps in time.• No missing record No missing record

due to erosion, non-due to erosion, non-deposition, etc.deposition, etc.

Formation of an angular Formation of an angular unconformityunconformity

An angular unconformity at Siccar An angular unconformity at Siccar Point, EnglandPoint, England

Unconformity

Cross Cutting Relationships in strata

Development of a Development of a NonconformityNonconformity

Nonconformity in the Grand Nonconformity in the Grand Canyon - Strata deposited over Canyon - Strata deposited over

SchistSchist

DisconformityDisconformity

• A break in the rock A break in the rock record across which record across which there is little change in there is little change in stratal orientation.stratal orientation.

• Often just a pause in Often just a pause in deposition (subtle).deposition (subtle).

• May also be obvious May also be obvious erosion surface.erosion surface.

Correlation of rock layers Matching strata of similar ages in different Matching strata of similar ages in different

regions is called regions is called correlationcorrelation

Correlation of strata in southwestern United States

Sections areincomplete

Correlation of rock layers Correlation of rock layers with fossilswith fossils

Correlation relies upon fossilsCorrelation relies upon fossils Principle of fossil successionPrinciple of fossil succession – fossil – fossil

organisms succeed one another in a organisms succeed one another in a recognizable order - thus any time period is recognizable order - thus any time period is defined by the type of fossils in itdefined by the type of fossils in it

Determining the ages of rocks using fossils

Geologic time scaleGeologic time scale The geologic time scale – a “calendar” of The geologic time scale – a “calendar” of

Earth historyEarth history Subdivides geologic history into unitsSubdivides geologic history into units Originally created using relative datesOriginally created using relative dates

Structure of the geologic time scaleStructure of the geologic time scale EonEon – the greatest expanse of time – the greatest expanse of time

Geologic Timescale

Divisions based on fossils

Geologic time scaleGeologic time scale

Structure of the geologic time scaleStructure of the geologic time scale Names of the eonsNames of the eons

PhanerozoicPhanerozoic (“visible life”) – the most recent (“visible life”) – the most recent eon, began about 540 million years agoeon, began about 540 million years ago

ProterozoicProterozoic ArcheanArchean HadeanHadean – the oldest eon – the oldest eon

Geologic time scaleGeologic time scale

Structure of the geologic time scaleStructure of the geologic time scale EraEra – subdivision of an eon – subdivision of an eon Eras of the Phanerozoic eonEras of the Phanerozoic eon

CenozoicCenozoic (“recent life”) (“recent life”) MesozoicMesozoic (“middle life”) (“middle life”) PaleozoicPaleozoic (“ancient life”) (“ancient life”)

Eras are subdivided into Eras are subdivided into periodsperiods Periods are subdivided into Periods are subdivided into epochsepochs

Geologic time scaleGeologic time scale

Precambrian timePrecambrian time Nearly 4 billion years prior to the Nearly 4 billion years prior to the

Cambrian periodCambrian period Not divided into small time units because Not divided into small time units because

the events of Precambrian history are not the events of Precambrian history are not know in detailknow in detail

Immense space of time (Earth is ~ 4.5 Ga)Immense space of time (Earth is ~ 4.5 Ga)

Radioactivity (Used to age date rocks)Radioactivity (Used to age date rocks) Spontaneous changes (decay) in structure of Spontaneous changes (decay) in structure of

atomic nucleiatomic nuclei Types of radioactive decayTypes of radioactive decay

Alpha emissionAlpha emission Emission of 2 protons and 2 neutrons (an alpha Emission of 2 protons and 2 neutrons (an alpha

particle)particle) Mass no.= +4Mass no.= +4 Atomic no.=-2Atomic no.=-2

Beta emissionBeta emission An electron (beta particle) is ejected from the nucleusAn electron (beta particle) is ejected from the nucleus

Mass no.= constantMass no.= constant Atomic no.=+1Atomic no.=+1

Electron captureElectron capture An electron is captured by the nucleusAn electron is captured by the nucleus The electron combines with a proton to form a neutronThe electron combines with a proton to form a neutron

Mass no.= constantMass no.= constant Atomic no.=-1Atomic no.=-1

Neutron capture Neutron capture (A) (A)

and Beta emission and Beta emission (B)(B)

Using radioactivity in Using radioactivity in datingdating

ParentParent – an unstable radioactive isotope – an unstable radioactive isotope Daughter productDaughter product – isotopes resulting from – isotopes resulting from

decay of parentdecay of parent Half-lifeHalf-life – time required for one-half of the – time required for one-half of the

parent isotope in a sample to decay parent isotope in a sample to decay

A radioactive decay A radioactive decay curvecurve

Dating with carbon-14 (Dating with carbon-14 (radiocarbon datingradiocarbon dating))

Half-life only 5730 yearsHalf-life only 5730 years Used to date very young rocksUsed to date very young rocks Carbon-14 is produced in the upper Carbon-14 is produced in the upper

atmosphereatmosphere Useful tool for geologists who study very Useful tool for geologists who study very

recent Earth historyrecent Earth history

Using radioactivity in datingUsing radioactivity in dating

Importance of radiometric datingImportance of radiometric dating Allows us to calibrate geologic timescaleAllows us to calibrate geologic timescale Determines geologic historyDetermines geologic history Confirms idea that geologic time is immenseConfirms idea that geologic time is immense