Geologic Time Geologic Time

Chapter 9Chapter 9

Relative DatingRelative Dating

Determining the age of a rock in Determining the age of a rock in relation to one another.relation to one another. Does not give exact ageDoes not give exact age

Use a collection of strategies Use a collection of strategies to determine relative age of a to determine relative age of a rockrock

Relative Ages of EventsRelative Ages of Events Original Horizontality - Most Original Horizontality - Most

sediments are deposited in sediments are deposited in horizontal layers. horizontal layers. If they are not horizontal some If they are not horizontal some

event must have affected the event must have affected the layers after they formed.layers after they formed.

Law of SuperpositionLaw of Superposition

When a layer is deposited, any When a layer is deposited, any rock unit that it rests on must be rock unit that it rests on must be older than that layer.older than that layer. Ex- Stack of papers, the oldest Ex- Stack of papers, the oldest

paper is the one on the bottom. paper is the one on the bottom.

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Younger Units Deposited on Younger Units Deposited on Older UnitsOlder Units

Red layers deposited Red layers deposited over tanover tan

Tan sediment deposited Tan sediment deposited over older rockover older rock

Third layer is Third layer is youngest and youngest and is on topis on top

Law of Embedded Law of Embedded FragmentsFragments

As a new rock forms, older As a new rock forms, older fragments of rock can be fragments of rock can be embedded into it. embedded into it.

So…Embedded fragments are So…Embedded fragments are OLDER than the rock in which OLDER than the rock in which they are embedded. they are embedded.

Ex: Chocolate chip cookieEx: Chocolate chip cookie

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Younger Younger Sediment or Sediment or

Rock Can Rock Can Contain Contain Pieces of Pieces of

Older RockOlder Rock

Determine which rock is Determine which rock is younger in each imageyounger in each image

BrecciaBreccia

Gray layersGray layers

ConglomerateConglomerate

Dark basaltDark basalt

Gray Gray granitegranite

Dark metamorphic rocksDark metamorphic rocks

Igneous IntrusionsIgneous Intrusions

There are several types of There are several types of igneous intrusions.igneous intrusions.

Sill- horizontal intrusionSill- horizontal intrusion Dike- vertical intrusionDike- vertical intrusion Batholith-large scale Batholith-large scale

intrusionsintrusions

Law of Crosscutting Law of Crosscutting

When igneous intrusions or When igneous intrusions or faults cut through a rock faults cut through a rock layer. layer.

The intrusion is younger than The intrusion is younger than the rock that it cuts through.the rock that it cuts through.

Ex. Toothpick in a sandwichEx. Toothpick in a sandwich

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Younger Younger Rock or Rock or

Feature Can Feature Can Crosscut an Crosscut an Older Rock Older Rock or Featureor Feature

LimestoneLimestone

FracturesFractures

Red Red layerslayers

Dark dikeDark dike

Dark Dark igneous igneous rockrock

Tan Tan dikesdikes

Determine which rock or Determine which rock or feature is younger in feature is younger in each imageeach image

Unconformity Unconformity

A buried erosion surface over A buried erosion surface over a layered surface.a layered surface.

You cannot account for the You cannot account for the missing layers.missing layers.

General term for missing General term for missing layers.layers.

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What Does an Unconformity What Does an Unconformity Represent?Represent?

Limestone folded and Limestone folded and erodederoded

Gray limestone Gray limestone deposited under deposited under waterwater

Conglomerate Conglomerate deposited on deposited on top of eroded top of eroded surface forming an surface forming an unconformityunconformity

UnconformityUnconformity

NonconformityNonconformity

11stst type of an unconformity. type of an unconformity. Occur over rocks that are not Occur over rocks that are not

layered. Sedimentary over ig. layered. Sedimentary over ig. or metaor meta

DisconformityDisconformity

22ndnd type of unconformity type of unconformity When an unconformity occurs over When an unconformity occurs over

horizontal sedimentary layers and horizontal sedimentary layers and new layers form on top. new layers form on top.

Absolute DatingAbsolute Dating

Relative dating does not tell Relative dating does not tell you the actual age of the rock. you the actual age of the rock.

Absolute dating uses Absolute dating uses radioactive decay to determine radioactive decay to determine the actual age of the rock.the actual age of the rock.

Historical MethodsHistorical Methods

Tree rings- each ring usually Tree rings- each ring usually represents a single year.represents a single year. Size and color depend on Size and color depend on

environmental Conditions.environmental Conditions. Different trees have similar Different trees have similar

rings for same years.rings for same years. Used to date back to 2000 B.C.Used to date back to 2000 B.C.

VarvesVarves

Varves- sediment that is Varves- sediment that is deposited on a yearly cycle.deposited on a yearly cycle.Each annual varve is Each annual varve is distinctive.distinctive.

Varves in one lake can be Varves in one lake can be correlated to varves in other correlated to varves in other lakes.lakes.

Radioactive decayRadioactive decay Some particles are unstable Some particles are unstable

and break down emitting and break down emitting subatomic particles. subatomic particles.

Every radioactive element Every radioactive element has a specific rate at which it has a specific rate at which it breaks down. breaks down.

That rate can determine the That rate can determine the age of the substance. age of the substance.

Types of DecayTypes of Decay Radioactive isotopes emit or Radioactive isotopes emit or

capture tiny particles.capture tiny particles.Three types of decayThree types of decay

Alpha- positive particleAlpha- positive particleBeta- negative e- emittedBeta- negative e- emittedElectron capture- p+ captures Electron capture- p+ captures e- and becomes a neutron.e- and becomes a neutron.

IsotopesIsotopes

Isotope’s atomic number changes Isotope’s atomic number changes and becomes a new element.and becomes a new element.Parent isotope- original elementParent isotope- original elementDaughter isotope- product of Daughter isotope- product of decay.decay.Ex- uranium-238Ex- uranium-238 lead 206 lead 206

Half-lifeHalf-life

Half-life – The amount of time Half-life – The amount of time it takes for ½ of a sample of a it takes for ½ of a sample of a radioactive isotope to decay. radioactive isotope to decay. (1/2 of the radioactive atoms)(1/2 of the radioactive atoms)

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How Does Radioactive How Does Radioactive Decay Occur?Decay Occur?

Half the parent atoms Half the parent atoms decayed to daughter decayed to daughter atoms (time = half life)atoms (time = half life)

Before decay, Before decay, unstable unstable parent atomsparent atoms

After a second half After a second half life, only ¼ parent life, only ¼ parent atoms remainatoms remain

Example Example for 1000 for 1000 atomsatoms

Before Any Before Any DecayDecay

After One After One Half-LifeHalf-Life

After Two After Two Half-LivesHalf-Lives

Atoms of Atoms of ParentParent 1,0001,000 500500 250250

Atoms of Atoms of DaughterDaughter 00 500500 750750

Radiocarbon DatingRadiocarbon Dating uses carbon-14uses carbon-14 carbon-14 is radioactivecarbon-14 is radioactive half-life is 5370 yrshalf-life is 5370 yrs Produced naturally from reaction Produced naturally from reaction

between N-14 and cosmic raysbetween N-14 and cosmic rays Only works on organic material and Only works on organic material and

only on sample that are under only on sample that are under 70,000 years old.70,000 years old.

Uranium-lead datingUranium-lead dating

Used to date very old rocks Used to date very old rocks because uranium has a half-because uranium has a half-life of 4.5 billion years.life of 4.5 billion years.

Does not work on rocks Does not work on rocks younger than 10 million years younger than 10 million years old.old.

Rubidium-StrontiumRubidium-Strontium

Half-life of rubidium is 47 Half-life of rubidium is 47 billion years.billion years.

Only works with extremely Only works with extremely old rocks.old rocks.

Used mainly on igneous Used mainly on igneous rocks.rocks.

Potassium-Argon DatingPotassium-Argon Dating

Used to date metamorphic Used to date metamorphic and sedimentary rocks as and sedimentary rocks as well as igneous.well as igneous.

Can date rocks as young as Can date rocks as young as 50,000 years old.50,000 years old.

FossilsFossils Remains, traces, or imprints Remains, traces, or imprints

of a plant or animal that are of a plant or animal that are preserved in a rock. preserved in a rock.

Occurs several ways.Occurs several ways.

Original RemainsOriginal Remains

The original unchanged remains of plants The original unchanged remains of plants or animals. or animals.

Rare! Rare! Example: Woolly mammoths found Example: Woolly mammoths found

frozen in permafrostfrozen in permafrost Example: prehistoric insects trapped in Example: prehistoric insects trapped in

resin (a sticky sap that oozes from trees). resin (a sticky sap that oozes from trees). Resin hardens into amberResin hardens into amber

Replaced RemainsReplaced Remains

Remains are slowly replaced, molecule Remains are slowly replaced, molecule by molecule, by rock-forming minerals. by molecule, by rock-forming minerals.

Occurs in bones, teeth and shells. Occurs in bones, teeth and shells. Circulating groundwater removes the Circulating groundwater removes the

original organic material and replaces original organic material and replaces them with minerals from the water. them with minerals from the water.

Petrified wood is an examplePetrified wood is an example

Molds and CastsMolds and Casts After an organism (leaf, insect) gets After an organism (leaf, insect) gets

buried in the mud or sediments, its buried in the mud or sediments, its hard body parts become a fossil and hard body parts become a fossil and the mud/sediment becomes rock. the mud/sediment becomes rock.

When the body part dissolves out of the When the body part dissolves out of the rock, a hollow depression results called rock, a hollow depression results called a mold. a mold.

Minerals may seep into the mold and fill Minerals may seep into the mold and fill it forming a cast of the original fossil. it forming a cast of the original fossil.

Trace FossilsTrace Fossils

Any impression left in the Any impression left in the rock by an animal such as rock by an animal such as trails, footprints, tracks, trails, footprints, tracks, burrows and bite marks. burrows and bite marks.

Carbonaceous FilmsCarbonaceous Films A trace of a fossil that is a A trace of a fossil that is a

thin carbon film resembling a thin carbon film resembling a silhouette.silhouette.

The remains are affected by The remains are affected by high temperature and high temperature and pressure causing the tissues pressure causing the tissues of the animals and plants to of the animals and plants to undergo chemical changes.undergo chemical changes.

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How Are Fossils Preserved?How Are Fossils Preserved?

BonesBonesShells/hard partsShells/hard parts ReplacementReplacement

Cast or moldCast or mold Thin carbon filmThin carbon film ImpressionsImpressions

AmberAmber Constructed featureConstructed feature

Using Index FossilsUsing Index Fossils An index fossil is the remains of an An index fossil is the remains of an

organism that lived and died within a organism that lived and died within a particular time segment of Earth’s particular time segment of Earth’s history.history.

4 characteristics of an index fossil.4 characteristics of an index fossil. Unique so they are easily identifiedUnique so they are easily identified AbundantAbundant Found over a great distanceFound over a great distance Organism can only have existed for Organism can only have existed for

a brief period. a brief period.

Fossils as Environmental Fossils as Environmental IndicatorsIndicators

Fossils tell you the climate. Fossils tell you the climate. Since animals can only exist Since animals can only exist at certain temperatures, at certain temperatures, fossils tell you the climate.fossils tell you the climate.

Matching Key BedsMatching Key Beds

Key Bed – a single rock layer Key Bed – a single rock layer that is unique, easily that is unique, easily recognizable and recognizable and widespreadwidespread

Ex. volcanic eruptions Ex. volcanic eruptions

Stratigraphic MatchingStratigraphic Matching

Matching layers of rock. Matching layers of rock. Ex. A layer of limestone Ex. A layer of limestone

sandwiched between sandwiched between conglomerate rocks. conglomerate rocks.