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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.
09.01.a
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
09.01.a
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
09.01.a
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.
09.04.a
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)
09.02.a
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.
09.05.a
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.
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