地质中的时间尺度ppt课件

1、Understanding EarthChapter 8:The Rock Record and theGeologic Time ScaleCopyright 2007 by W. H. Freeman & CompanyJohn Grotzinger, Thomas H. Jordan Frank Press and Raymond SieverFifth EditionConcepts you will need to know for the examsCHAPTER 8Geologic Time ScaleRelative ageAbsolute agePrinciple of or

2、iginal horizontalityPrinciple of superpositionPrinciple of cross-cutting relationahipsUnconformityAngular unconformityDisconformityHalf-lifeGeologic Time ScaleGeologists differ from other professional scientists in their view of time. Geologists deal with periods of time that go as far back as the o

3、rigin of the universe.Before we had the ability to read the “atomic clocks of nature, geologists used the concept of relative ages. We can see changes in the landscape over time.We can see one type of organism replace another type of organism through the fossilized remains of these changes over time

4、.With the advent of atomic/radioactive decay techniques we began to obtain absolute dates.Absolute dates or ages have specific numbers Relative dating has been done in the past through the study of layers of sedimentary rock(aka “the field of stratigraphy)Principle of Original Horizontalitysediments

5、 deposited innearly flat layers. Cross-bedded layers are contained within overall flat beds.If layers are faulted (broken) or folded it means that they were deformed AFTER sediments were deposited.Principle of SuperpositionLowest layers are olderIf this is not so, then the layers have beenoverturned

6、 by tectonic activity:There are limitations for using stratigraphyto keep time because, (1) sedimentation rates are variablee.g. Mississippi can deposit 1m of sediment in 1000 yThe deep ocean may deposit 1mm / 1000 yearsWe need to estimate time in another way and be able to recognize when the repres

7、entation of time via sediments is incomplete.Paleontology is the study of ancient life from fossilized remains.William Smith in the 19th century was an engineer who noted that similar rocks in different areas had similar fossils and that different rock types had different fossils.He made a composite

8、 (ideal, pieced-together) stratigraphic succession as in the following figureMost of geologic time goes unrecordedSometimes NO SEDIMENT is DEPOSITED over an interval of time (hiatus)Other times sediment is eroded away.This is happening here: Mississippi Valley is filling whereas the higher ground is

9、 being eroded away.An unconformity in which the layers above an erosional surface are parallel to the layers below is known as a DISCONFORMITYOne way of producing a disconformity is to change global sea-level by glaciation and glacial melting.Cross-cutting relationships in sedimentary rocks allow is

10、 to date the relative timing of events such as faulting and igneous intrusionThe Geologic Time Scale.is still under constructionLife began only 2500 MaRadioactive Dating MethodsMining Uranium OxideU3O8 U-238 99.3%, U-235 0.7%Millingcrushedsoaked in sulfuric acidYellow Cake U3O8CONVERSION:yellowcake

11、is heated to about 147 F, converted into uranium hexafluoride gas (UF6). ENRICHMENT:The UF6 gas centrifuged heavier U-238 moves outside U-235 concentrates inwardConvert to UO2 powderNuclear Reactors: enriched 3-5% U-235Nuclear Weapons: enriched 90% U-235Rubidium exist Rb87 is unstableRb has an average atomic weight of 85.5Rb87 has an atomic mass 87Rb87 has 37 protons and 50 neutrons in its nucleus Sr87 is stableSr has an average atomic weight of 87.62Sr87 has an atomic mass 87Sr87 has 38 protons and 49 neutrons in its nucleus Source: wikipedia 11-5-2021Rb37p+50nSr38p+49n+ el