Presentation on theme: "Chapter 8: Geologic Time"— Presentation transcript:
1Chapter 8: Geologic Time PowerPoint PresentationStan Hatfield . SW Illinois CollegeKen Pinzke . SW Illinois CollegeCharles Henderson . University of CalgaryTark Hamilton . Camosun CollegeCopyright (c) 2005 Pearson Education Canada, Inc.
2Geological Time is of Vast Duration Sediments were buried under a Mountain Range, metamorphosed, half the crust was uplifted & eroded, glaciers carried it, lake storms removed all but the biggest rocks.
3Geologic TimeRelative age dates – placing rocks and events in their proper sequence of formation. e.g. Tertiary is younger than Cretaceous (rocks, fossils, climate)Numerical dates – specifying the actual number of years that have passed since an event occurred (known as absolute age dating using isotope clocks)Geologic time scale – Earth’s history is ~4.5 Ga long and written disproportionately by the rocks formed and processes which operated at different times and places.
5Relative Dating – Key Principles Law of SuperpositionDeveloped by the Danish physician Nicolaus Steno working in Italy in 1669In an undeformed sequence of sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottomMost sedimentary & volcanic rocks are deposited in sequences of essentially flat lying beds
6Relative Dating – Key Principles Principle of Original HorizontalityLayers of sediment are generally deposited in a horizontal positionRock layers that are flat have not been disturbedPrinciple of Cross-Cutting RelationshipsYounger features cut across older featuresPrinciple of InclusionsYounger rocks/features include older onesAll 3 types of rocks can include older rocks and minerals
7Permian Strata, South Rim Relative Dating – Key Principles: Horizontality & Law of SuperpositionCorrelationPermian Strata, South Rim
8Lower Paleozoic Strata: Devon, UK Fold HingeDeposition, DeformationAt the Closing of Iapetus
10Relative Dating – Key Principles UnconformitiesTypes of UnconformitiesDisconformity – strata on either side of the unconformity are parallel (least missing time)Angular unconformity – tilted rocks are overlain by flat-lying rocks (10’s of Ma missing)Nonconformity – metamorphic or igneous rocks in contact with sedimentary strata (most missing time, eroded down to crystalline basement rocks)
11Siccar Point, Scotland & The development of an angular unconformity.
14m h i g l k j e f d c What kind of unconformity is “i”? What kind of Figure out the sequence of geological events,Oldest on the bottom, youngest on the top.mhiglkjefdcWhat kind ofunconformity is “i”?What kind ofContact is “m”?
15m h i g l k j e f d c “i” is a disconformity “m” is intrusive, Dyke ADyke BIntr-lFault BkjIFault AhgfedcFigure out the sequence of geological events,Oldest on the bottom, youngest on the top.mhiglkjefdc“i” is a disconformity“m” is intrusive,& baked margins.
16Which type of unconformity here at the South Rim represents the greatest amountof missing time and why?
17Correlation of Rock Layers Matching of rocks of similar ages in different regions is known as correlationCorrelation often relies upon fossilsWilliam Smith (late 1700s and early 1800s) noted that sedimentary strata in widely separated areas could be identified and correlated by their distinctive fossil contentOther than fossils we often rely on widespread instantaneous events like volcanic ash fallsUnconformities bound packages of stratigraphy
18Correlation across slightly overlapping stratigraphy Arizona to Utah.
19Correlation of Rock Layers Correlation often relies upon fossilsPrinciple of Fossil (Faunal) Succession – fossil organisms succeed one another in a definite and determinable order that documents the evolution of life; therefore any time period can be recognized by its fossil contentIndex fossils – represent best fossils for correlation; they are widespread ecologically & geographically and are limited to a short time span (i.e., they evolved rapidly)
21Dating with Radioactivity Reviewing Basic Atomic StructureIsotope (only a few of these are radioactive)Variant of the same parent atom, same atomic numberDiffers in the number of neutrons (weight)Results in a different mass number than the common type of atom for this elementToo many additional neutrons makes a nucleus unstable and prone to radioactive decay
23Dating with Radioactivity Parent – an unstable radioactive isotopeDaughter product – the isotopes that result from the decay of a parentHalf-Life – the time required for one-half of the radioactive nuclei in a sample to decayHalf-life differs for every Parent-Daughter pair of isotopes
24Exponential Decay of Parent Isotopes (& exponential growth of daughters) The DecayEquation:A = A0 e –λtA0 is the initialamount of theparent isotope
25Dating with Radioactivity Radiometric DatingPrinciple of Radioactive DatingThe percentage of radioactive atoms that decay during one half-life is always the same (50 percent)However, the actual number of atoms that decay continually decreasesComparing the ratio of parent to daughter yields the age of the sample
27Dating with Radioactivity Radiometric datingUseful radioactive isotopes for providing radiometric agesRubidium-87 Strontium-87 ~PrecambrianFeldspar, micas, amphibolesThorium-232 Lead-207 ~PcZirconTwo isotopes of uranium (235 and 238) to Lead (208 & 206), 4.5 Ga & 713 Ma respectively, ~Pc to MesozoicPotassium-40 Argon-40 or relative to Argon-39 >2 MaFeldspar, micas, volcanic glass
28Slow U 238U 210Pb (½ life = 4.5 Ga) Used for Ancient Zircons & <200a 210Pb The whole span is used for dating zircons in igneous & metamorphic rocks usually of Mesozoic or older age and requires the measurement of both parent isotope 238U and 210Pb. With care and high precision Laser ablation Mass Spectrometry this can be done for zircons as young as ~10 Ma. Mainly this is the dating work horse for Precambrian rocks older than 1 Ga.Disequilibrium dating of Pleistocene Carbonates can be done using 226Ra. Muds younger than 200 years can be dated using 210Pb accumulation from atmospheric 222Rn.
29Dating with Radioactivity Rules for Radiometric DatingMineral contains both parent & daughter as for Zircon with 238U and 206Pb or feldspar with 87Rb and 87SrFormed at time of event to be dated as for a lava flow, dyke or contact metamorphismThe mineral is a closed system and has neither gained nor lost parent & daughter
30Dating with Radioactivity Radiometric datingSources of errorA closed system is requiredIf temperature is too high, daughter products may be lostTo avoid potential problems, only fresh, unweathered rock samples should be used
31Production of 14C in the upper atmosphere (& decay once organisms die) Cosmic raysExpel neutrons fromAtmospheric gases (N,O).This expels a protonFrom nitrogenForming radioactive 14C.The extra neutronin radioactive 14Cdecays back to 14NWith a ½ life of5730 years.
32Dating with Radioactivity Dating with carbon-14 (radiocarbon dating)Half-life of only 5730 yearsUsed to date very recent eventsCarbon-14 is produced in the upper atmosphereUseful tool for anthropologists, archaeologists, historians, and geologists who study very recent Earth historyOnly works for plant or animal tissue, not rocks
34Geologic Time Scale Structure of the geologic time scale Names of the eonsPhanerozoic (“visible life”) – the most recent eon, began just over 540 million years agoProterozoicArcheanHadean – the oldest eon
36Geologic Time Scale Structure of the geologic time scale Era – subdivision of an eonEras of the Phanerozoic eonCenozoic (“recent life”)Mesozoic (“middle life”)Paleozoic (“ancient life”)Eras are subdivided into periodsPeriods are subdivided into Epochs
38Precambrian time Nearly 4 billion years prior to the Cambrian period Not divided into smaller time units because the events of Precambrian history are not known in great enough detailAlso, first abundant fossil evidence does not appear until the beginning of the Cambrian
39Difficulties in dating the geologic time scale Not all rocks can be dated by radiometric methodsGrains comprising detrital sedimentary rocks are older than the rock in which they formedThe age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed as porphyroblasts may grow for many MaThe rock needs minerals with the right parent and daughter isotope pairs and in the right age span to be measureable.
40Difficulties in dating the geologic time scale Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth history and arrive at agesDates change as brackets become narrower and methods refined; e.g., base of Triassic is now 252 Ma, base of Permian is now 299 Ma, base of Cambrian is 543 Ma…
48Bolide impact & Deccan Trap Volcanism both occur at 65Ma Bolide impact & Deccan Trap Volcanism both occur at 65Ma. Biodiversity dwindled for 10 Ma prior to this. The K/T massive extinction is debated.