1、第四章 地球的年齡和地質年代學Two Conceptions of Earth HistoryuCatastrophism（災變說）（災變說） Assumption: Great Effects Require Great Causes Earth History Dominated by Violent Events uUniformitarianism（均變論）（均變論） Assumption: We Can Use Cause And Effect to Determine Causes of Past Events Finding: Earth History Dominated by

2、 Small-scale Events Typical of the Present. Catastrophes Do Happen But Are Uncommon 4.1 相對地質年代uN.Steno 地層層序律 層序地層學方法uW.Smith 生物地層學方法u標準化石 時間短、演化快、分布廣u地層的缺失不整合（角度不整合、假整合）u相互切割關系u地質年代表relative age and absolute ageuRelative age(相對年齡) - Relative means that we can determine if something is younger than o

3、r older than something else. Relative time does not tell how old something is, all we know is the sequence of events. For example: the sandstone in this area is older than the limestone. uAbsolute age(絕對年齡)- Absolute age means that we can more or less precisely assign a number (in years, minutes, se

4、conds, or some other units of time) to the amount of time that has passed. Thus we can say how old something is. For example: The sandstone is 300 million years old. PKU Trash PitTHU Trash PitWhat can we say and learn from these excavations? Relative age of trash layers Similar cultures must have ex

5、isted in both areas and lived at the same timeThe trash pits contain some clues to absolute ageRecognize a hiatus, or break in the depositional sequence at the THU siteuIn geology, we use similar principles to determine relative ages, correlations, and absolute ages. uRelative ages - Principles of S

6、tratigraphy uCorrelations - Fossils, key beds, physical criteria uAbsolute ages - Radiometric dating. Principles of StratigraphyuStratigraphy（地層學）（地層學） = the study of strata (layers) in the Earths crust. uLaws of Stratigraphy （地層學定律）（地層學定律）: 1. Original Horizontality - sedimentary strata are deposit

7、ed in layers that are horizontal or nearly horizontal, parallel to or nearly parallel to the Earths surface. 2. Stratigraphic Superposition - Because of Earths gravity, deposition of sediment will occur depositing older layers first followed by successively younger layers. 水平地層相對地質年代Breaks in the St

8、ratigraphic Record-unconformity （不整合）uAngular Unconformity（角度不整合）（角度不整合）Disconformity （平行不整合、假整合）Nonconformity （非整合、不整合）不整合關系示意圖Variation in Unconformities Stratigraphic Classification uTwo types of stratigraphic classification are used, one based on physical characteristics or material properties o

9、f the rocks - Rock Stratigraphic Units, and the other based on the time over which the material was formed - time stratigraphic units. uRock Stratigraphic Units（巖石地層單位）（巖石地層單位）: Distinctive bodies of rocks that differ from the rocks above and below in the general characteristics. The basic unit is a

10、 formation. uTime Stratigraphic Units（年代地層單位）（年代地層單位） : A bodies of rocks that were deposited during the same geologic time interval. The basic unit is a system. Correlation of Rock UnitsuIn order for rock units to be correlated over wide areas, they must be determined to be equivalent. Determinatio

11、n of equivalence is based on: uRelative Age - if two rock units are equivalent they must have the same age relative to rocks that occur below and above. The laws of stratigraphy enable this determination. uPhysical Criteria - Similarity in rock type, but only if relative age is equivalent. Key beds,

12、 like widespread volcanic ash layers that are the same over wide areas are often used to establish equivalence.ufossils present - Fossils are key indicators of relative age (life has evolved through time) and environments of depositionLaw of Cross-cutting relationships （切割關系定律）- the zucchini concept

13、uif one body of rock cuts across another body of rock, the latter must be older than the former.uTherefore, the thing being acted upon has to be older than the thing doing the acting4.2 絕對地質年代u放射性同位素法 同位素年齡u 1 Nt u t = ln( +1)u N0 t為衰變經歷的時間（即巖石形成年齡）， 為衰變常數，N0為母體原始數量（按原 子數計），Nt為N0經時間t后剩下的數量Absolute G

14、eologic TimeuAlthough geologists can easily establish relative ages of rocks based on the principles of stratigraphy, knowing how much time a geologic Eon, Era, Period, or Epoch represents is a more difficult problem without having knowledge of absolute ages of rocks. In the early years of geology,

15、many attempts were made to establish some measure of absolute geologic timeAbsolute Ages: Early AttemptsThe BibleuAdd up Dates in Bible uGet an Age of 4000-6000 B.C. For Earth uJohn Lightfoot and Bishop Ussher - 4004 B.C. (1584) uToo Short Absolute Ages: Early AttemptsSalt in OceanuRivers bring diss

16、olved solids to ocean uIf we know rate salt is added, and how much salt is in ocean, can find age of oceans. uGave age of about 100 million years. uProblems Is rate at which salt is added constant? How much salt leaves ocean? Absolute Ages: Early AttemptsSediment ThicknessuAdd up Thickest sediments

17、for each period uEstimate rate of deposition to find age uProblem: rates of deposition very variable! uIndicated ages of at least 100 million years Radiometric Dating uRadiometric dating relies on the fact that there are different types of isotopes. uRadioactive Isotopes - isotopes (parent isotopes)

18、 that spontaneously decay at a constant rate to another isotope.uRadiogenic Isotopes - isotopes that are formed by radioactive decay (daughter isotopes). Radiometric Dating: Half-LifePresent Radiometric Dating MethodsCosmogenicuC-14 5700 Yr. uBe-10 2.5 M.Y. PrimordialuK-Ar (K-40) 1.25 B.Y. uRb-Sr (R

19、b-87) 48.8 by uU-235 704 M.Y. uTh-232 14 B.Y. uU-238 4.5 B.Y.PrimordialuNd-Sm (Sm-147-Nd-143) 106 B.Y. uRe-187 43 B.Y. uLu-Hf (Lu-176) 36 B.YuFission TrackHow Do We Determine the age of Rocks?uN = N0exp(-kt)1ln10NNttThe Geologic Time ScaleAbsolute Dating and Geologic Time Scale The Oligocene rocks a

20、re younger than the 30 m.y old lava flow and older than the 20 m.y. old lava flow.The Eocene rocks are older than the 57 m.y. old dike and younger than the 36 m.y. old dike that cuts through them.The Paleocene rocks are older than both the 36 m.y. old dike and the 57 m.y. old dike (thus the Paleocene is younger than 57 m.y.地磁反向年表地質年代表宙最大的地質年代單位 顯生宙、元古宙、太 古宙、冥古宙代新生代、中生代、古 生代、元古代、太古代紀根據最早研究的標準 地區等給以命名，國 際通用世每個紀按早中晚分為 三個世地層一定地質時代形成 的成層巖石組合，相 對應于時代為宇、界、 系、統國際地層 單位4.3 地球的年齡u臆想和估計（圣經6000年，布豐75000年）u季節-氣候方法u沉積法 5億年u海水的含鹽量 10億年地球的年齡u地球上獲得的最大年齡42億年u月巖年齡46-47億年u隕石在46-47億年之間u同一性原理u地球