Presentation on theme: "Deciphering Earth History and Creating a Time Scale"— Presentation transcript:
1 Deciphering Earth History and Creating a Time Scale Geologic TimeDeciphering Earth History and Creating a Time Scale
2 Let’s take a field trip and decipher Earth history. John Powell – 1869 – Geological Expedition through the Grand CanyonCoined the expression –”The Grand Canyon would be theBook of Revelation in the Bible of Geology.”evidence of ancient Earth revealed in the rocks“Like the pages of long, complicated history book”Rocks record geological events.Some “early chapters” are missing.Some chapters are torn and tattered.Interpreting Earth history is a prime goal among scientists.Geologists seek clues from the rock record.The history of Earth needs a calendar.The Geologic Time Scale – “The Earth’s history book”Let’s take a field trip and decipher Earth history.
3 Uniformitarianism Think back to chapter 1 – Intro to Geology, and write down the philosophical meaningbehind the concept of uniformitarianism.“The Present is the Key to the Past”As we take our “virtual” field trip, ponder theconcept of uniformitarianism.
4 Which stratigraphic layer is oldest? Let’s take a field trip and decipher some Earth history.Which stratigraphic layer is oldest?FormationContactsFormation(mappable units)EDCBA
6 Is the fault younger or older than the strata? Let’s take a field trip and decipher some Earth history.Is the fault younger or older than the strata?Fault (displacing strata)332211
7 Geologists use two types of dating methods to interpret Earth history. Relative Dating Techniques:Geologic events are arranged inchronological sequences using relative datingprinciples which came first?No numerical values are applied.Absolute Dating Techniques:Radioactive isotopes (unstable elements) decayinto stable atoms rate of decay is measureablewith a numerical valueAn actual number (numerical age) can beapplied.
8 I Geologic Dating. Describe the difference between: Relative Dating techniquesAbsolute Dating techniques2. What is meant by John Powell’s phrase:“The Grand Canyon is the Book of Revelation ?”3. What role does uniformatarianism playwhen interpreting the previous field tripslides?I will get an A on my exams and quizzes.
9 I>clicker question Uniformatarianism’s role in relatively dating a sequenceof strata is:All strata is datable.Layers of strata that formed in the past are subject toerosive forces.c. Earthquake faults displacing strata are always olderthan the displaced strata.d. Observing geological processes today, ensures theaccuracy of dating a sequence of strata that has formedin the geological past.
10 Relative Dating - placing the geologic occurrence in the proper sequenceWhich came first and WHY?To construct a “relative” geologic time scale, rules wereestablished (principles of relative dating).Nicholas Steno ( )Principle of Original HorizontalityLaw of SuperpositionPrinciple of Cross-Cutting RelationsPrinciple of InclusionsPrinciple of Unconformities
11 The Principle of Original Horizontality: Let’s unravel some geologic history from observations of various formationsand their contacts.Nicholas Steno – 1669 proposed the following relative dating principles:The Principle of Original Horizontality:Sedimentary rock layers are deposited as horizontal strata.Any observed non-horizontal strata have been disturbed.Sediment inputCBbasinA
12 Original Horizontal Strata Limestone (ls)Shale (sh)Sandstone (ss)granitic rock
13 The Principle of Superposition In any undisturbed sequence of strata, the oldest stratum is at the bottom of the sequence, and the youngest stratumis on top.Unit 1 = oldestUnit 5 = youngest54321
14 Which strata is oldest?54321youngest54oldest321
15 The principle of Cross-Cutting Relationships Any geologic feature that cuts across another geologic feature is younger.5Unit 1 = oldestUnit 6 = youngest43Which came first:Unit 5 or Unit 6?261
16 Which is older, the fault Which is younger, the dike or volcanic layer?Which is younger, the dikeor country rock?faultdikeVolcanic layercountry rockDetermine the relative age of the two dikes.12
17 The Principle of Inclusions: A piece of rock (clast) that has become “included”in another rock body is older than the rock bodyit has become part of – why?Rock body AAAAOlder (Rock A was there first.)Intrusion of pluton B
19 Which rock body is older?: A??CCan you identify the inclusionsfound in this Sierra NevadaMountain batholitic material?
20 Superposition Original Horizontality Principle of Inclusions YoungestSuperpositionOldestPrinciple ofInclusionsCross-Cutting RelationshipWhich granite is older?OlderABCAsp VnYounger
21 I this geology class. 3. Explain the concept of relative dating. 4. Draw a diagram, and explain each of thefollowing dating principles:Original HorizontalitySuperpositionCross-Cutting RelationsInclusion PrincipleI will get an A on my exams and quizzes.
22 In the diagram below, unit D is younger than ______ I>clickerIn the diagram below, unit D is younger than ______Units B and C because oforiginal horizontalityB. Units E and F because ofcross-cutting relationsC. Units E and F because ofinclusions principleD. Units B and A because of
23 Ok – given the principles, what is wrong with this stratigraphic section?youngest765321oldestMissing time – or does time really stop?
24 The principle of Unconformities rock surface that represents a period of erosion or non- depositionreferred to as “missing time”three major types of unconformities:disconformityangular unconformitynon-conformitydisconformity – unconformity in non-disturbedsedimentary layersangular unconformity – uncon. lies between angledstrata and overlyinghorizontal stratanon-conformity – sedimentary strataoverlies crystallinerocks (ig and met)UnconformityIgneous or metamorphic rock
26 Time Formation of an angular unconformity Deposition of layers (original horizontality)Tectonic activity producesuplift of strata causing tiltedlayering.Active erosional processesshaping the surfaceA period of erosion creatingthe unconformityContinued deposition buryingthe erosional surface to createan angular unconformity
27 Grand Canyon Stratigraphy Types of Unconformities
28 I this geology class. 5. Describe an unconformity and what what it represents regarding geologichistory.6. Diagram pictures that represent thethree types of unconformitiesI will get an A on my exams and quizzes
29 How do we get a fossil? – preservation of past life Fossils – evidence of past life or “time pieces,”the remains or traces of prehistoric lifePaleontology – study of fossilsHow do we get a fossil? – preservation of past life2 conditions must exist for preservationrapid burialpossession of hard partsPrehistoric bugRapid burial ofsediment coversthe bug – fossilBug diesBug soft parts areeaten or dissolved
30 Fossils – evidence of past life or “time pieces,” the remains or traces of prehistoric lifePreservation of fossilsSmall percentage of fossils preservedthroughout geologic time – WHY?Most organisms composed of soft parts.Organisms with hard parts and withina sedimentary environment are favored.Very rare to see vast array of other lifeformsHow do fossils help scientists relatively date layersof rock (strata)?
31 William Smith – Principle of Fossil Succession Fossil organisms succeed one another in adefinite and determinable order, and ,therefore,any time period can be recognized by its fossilcontent.“Fossils are arranged according to their age byusing the law of superposition.”Fossil succession:allows geologists to age date wide geographicalareasdocuments the evolution of lifeAge of mammalsAge of reptilesAge of fishYoungestOldest
32 How do fossils help date rocks? 1200 miles7766Disconformity543322Which fossils are the youngestand oldest?1
34 I this geology class. 7. Give 2 reasons why many organisms are not fossilized.8. Explain the law of fossil succession andhow this law allows dating of strata.9. How has fossil succession helped geologistsunravel earth history?I will get an A on my exams and quizzes.
35 I>clickerThe principle of fossil succession states:a. Fossils discovered in lower sections ofrock layers are younger than fossils above.b. Most fossil soft parts are preserved.c. The age of fossils is marked by thepreservation of all hard parts.d. Fossils can be dated by using the principleof superposition.
36 OK – We have relative dating and fossils – How do we get “absolute” ages on the rocks (numbers)?Radiometric dating – applying a numberradioactive atoms (isotopes) decay at aconstant rate over timeReview of the atom:Atomic number =Atomic mass =Isotope =number of protons (+) chargednumber of protons + neutronsunstable atoms with varying numbers ofneutrons (atomic masses)Unstable atoms will decay into stable atomsby losing neutrons and protons at aconstant rate.U238Pb206Decays
37 Mass # remains unchanged Radio active decay U Pb206Alpha emissionMass # reduced by 4Atomic # reduced by 2Beta emissionMass # remains unchangedAtomic # increases by 1Isotopes decay at a fixed rate.Decay rate is measureable.Isotope decay is not influencedby weathering.One isotope will decay intoanother isotope.
38 Stable Daughter Product Currently Accepted Half-Life Values How does radiometric dating work, and where does the age (number) come from?Parent element: the “beginning” element that contains 100%of radioactive particlesDaughter element: the element that the parent element decaysinto (or turns into over time)Half life: the time required for ½ of the parent to decay intothe daughter elementParent IsotopeStable Daughter ProductCurrently Accepted Half-Life ValuesUranium-238Lead-2064.5 billion yearsUranium-235Lead-207704 million yearsThorium-232Lead-20814.0 billion yearsRubidium-87Strontium-8748.8 billion yearsPotassium-40Argon-401.25 billion yearsSamarium-147Neodymium-143106 billion years
39 U235 Pb207 704 m.y. 1.4 b.y. 2.1 b.y. U-3 Daughter element 1/2 1/4 1/8 1 half life = 704 million yearsDaughterelement1/21/41/8704 m.y.1.4 b.y.2.1 b.y.Parentelement
40 I absolute dating. 11. Define the following absolute dating terms: parent/daughter elements, half-life12. Explain how the half-life is used tocalculate an absolute age.I will get an A on my exams and quizzes.
41 I>ClickerA certain material contains 1/64 of parent material. Ifthe half-live for decay is 2.3 million years, what is theage of the material?A m.y.B m.y.C m.y.D b.y.
42 What is the importance of radiometric dating? produced thousands of dates for eartheventsrocks have been dated at more than 3 b.y.granite in South Africa dated at 3.2 b.y.granite contains inclusions of quartzitequartzite inclusions must be olderAcasta gneiss in Northern Canada – 4.0 b.y.Earth believed to be 4.55 (4.6) b.y. oldRadiometric dating:vindicated the ideas of Hutton, Steno, and othersconsistent with relative dating techniquesallowed “absolute” dating on the Geologic Time Scale
43 Relative dating + Absolute dating Lets make a Geologic Time Scale!Relative dating + Absolute datingThe Geologic Time Scale:It combines both relative and absolute dating.Created during the nineteenth century in Western Europeand Great BritainSub-divides the 4.6 billion-year history of the earthEonsErasPeriodsEpochs
44 Precambrian Building the Geologic Time Scale Phanerozoic Proterozoic “visible life”fossil record becomes moredetailedanimals have hard shellsand skeletonsBuilding theGeologic Time ScaleProterozoicMulti-celled, soft bodyorganisms“early life”PrecambrianArcheanSingle-cell life developedmost “ancient” rocks foundpreserved rocks at the baseof the ArcheanHadeanrepresents the earth’stime of formationno rocks are represented“hellish” conditions
45 Cenozoic Era Mesozoic Era Paleozoic Era birds and mammals flourished appearance of manMesozoic Eramarks the rise in dinosaursdominant vertebratesfirst flowering plantsfirst shrew-likemammalsPaleozoic Eraknown as ancient lifelife progressed from marineinvertebrates to fish,amphibians, and reptiles
46 Cretaceous, Jurassic, Triassic Periods based on:fossil typesmassive extinctionsgeographical locationscharacteristics of strataCretaceous, Jurassic, Triassicage of reptilesdinosaurs dominantmassive dinosaur extinctionat 65 m.y. –Cretaceous“Jurassic Park”Cambrian periodanimals with hard shellsdiversification of life“the Cambrian explosion”
47 Epochs Age of Reptiles Age of fish Invertebrates How accurate is the not defined by extinctionevents, but % of fossilsstill livingplants and animals foundin the Pliocene epochhave living species todayEocene-few speciessurviving todayHolocenehuman’s timeAge ofReptilesAmphibiansAge of fishInvertebratesHow accurate is theGeologic Time Scale?
48 the Geologic Time Scale. 44Ithe Geologic Time Scale.13. You should be able to draw the GeologicTime Scale and label it with the following:Eons, Eras, Periods, and Cenozoic/Tertiary epochs.14. List major characteristics of eachperiod.15. How did the strength of both absolute andrelative dating techniques contribute to thedevelopment of the Geologic Time Scale?
49 I>clickerWhat time “slots” below arrange the geologic timescale from largest to smallest divisions?a. period, era, eon, epochb. era, eon, period, epochc. eon, period, epoch, erad. eon, era, period, epoch
50 Cenozoic, Mesozoic, Paleozoic The Geologic Time Scale – How much of Earth history isrepresented?Geologic Time ScaleCenozoic, Mesozoic, PaleozoicEras12%88%Precambrian Eon
51 Difficulties in Dating the Geologic Time Scale Not all rocks can be dated radiometrically.all minerals must contain 100% parent atoms.Sedimentary rocks can only rarely be dated.some parent atoms come from pre-existing rocksthat have been weathered and transported.sedimentary rocks are dated in proximity ofigneous bodies.Metamorphic rocks are challenging.some minerals do not necessarily represent thetime when the rock was formed