2Early Efforts on Dating the Age of the Earth James Ussher (mid 1600's)4004 B.C.Georges Buffon (mid 1700’s)75,000 yrCharles Walcott (1893)75 m.y.Lord Kelvin (late 1800's)100 m.y.
3II. Constancy of Natural Laws Early Efforts on Dating the Age of the EarthII. Constancy of Natural LawsCatastrophismGeorges Cuvier (late 1700’s)UniformitarianismJames Hutton (late 1700's)
4Radiometric Dating of Rocks Radioactivity was first discovered by Henri Becquerel in 1896 and Polish-French chemist Marie Curie discovered that radioactivity produced new elements (radioactive decay).Ernest Rutherford first formulated the law of radioactive decay and was the first person to determine the age of a rock using radioactive decay methods.
5Absolute Dating of Rocks The main method used to get the exact age of a rock is called radioactive or radiometric dating. This method studies the decay rates of radioactive isotopes in order to determine the age of the rock.But FirstWhat is an atom?What is an isotope?
6} ATOMS Protons Neutrons Nucleus Electrons Nucleus Fundamental unit of matter made up of subatomic particles}Protons (positive charge)NucleusNeutrons (no electrical charge)Electrons (negative charge)ProtonsNeutronsNucleusElectrons
8Types of Radioactive Decay protonsalpha particle4H2neutronselectrons238U234Th9290ALPHADECAYalpha decayParent NucleusChanges in atomic numberand atomic mass numberDaughter Nucleus92 protons90 protonsAtomic Number = - 2146 neutronsAtomic Mass Number = - 4144 neutronsAlpha decay occurs when the nucleus has too many protons which cause excessive repulsion. In an attempt to reduce the repulsion, a helium nucleus is emitted.
9Types of Radioactive Decay protonsbeta particlep +neutronselectronelectrons137Ba137Cs5655BETADECAYbeta decayParent NucleusDaughter NucleusChanges in atomic numberand atomic mass number55 protonsAtomic Number = + 156 protons82 neutronsAtomic Mass Number = 081 neutronsBeta decay occurs when the neutron to proton ratio is too great in the nucleus and causes instability. In basic beta decay, a neutron is turned into a proton and an electron. The electron is then emitted.
10Types of Radioactive Decay protonsneutronselectronselectron capture204Po204Bi8483ELECTRONCAPTUREChanges in atomic numberand atomic mass numberParent NucleusDaughter NucleusAtomic Number = - 184 protonsAtomic Mass Number = 083 protons120 neutrons120 neutronsElectron capture also occurs when the neutron to proton ratio in the nucleus is too small. The nucleus captures an electron which basically turns a proton into a neutron.
11U Pb206 SeriesThis process is called radioactive decay, and eventually uranium (parent) decays to lead (daughter product).
12Radioactive DatingDuring radioactive decay, the original material or “parent material” decays at a set rate into the new material or “daughter material”As the number of parent material decreases, the number of daughter material increases.The amount of time that it takes for exactly one-half of the parent to turn into daughter is known as a half-life.For example – the half-life of Carbon-14 is 5,700 years. Carbon-14 is the parent material and Nitrogen-14 is the daughter material it decays into.Therefore, this means that it will take 5,700 years for one-half of the Carbon-14 to turn into Nitrogen-14.
13Radioactive DatingSo, why does Radioactive Dating allow geologist to get absolute dates?The half-lives of radioactive isotopes are set in stone and DO NOT change.For example, the half-life of Carbon-14 is always 5,700 years, no matter what.A table of some common half-lives are given in the next slide.
14Radioactive Decay Rates IsotopeDisintegrationHalf-LifeIodine-131131I131Xe8.02 daysCobalt-6060Co60Ni5.3 yearsCarbon-1414C14N5.7 x 103 years40Ar40K1.3 x 109 yearsPotassium-4040Ca4.5 x 109 yearsUranium-238238U206Pb
15Half Life How can we tell age based on the number of parent isotopes? Radioactive isotopes “decay” at a particular rate. We express this rate as the “HALF-LIFE”, which is the time it takes for HALF of the parent isotopes to decay.U238Half Life = Number of years for 1/2 of the original number of atoms to decay from U to PbPo218
16Half Life Parent Isotope (100) Daughter Isotope Half-Life = 10 seconds Time (s)Half-LifeParentDaughter1001015050202257530312.587.54046.2593.755053.12596.875
17Start with a million parent atoms… 1005025184.108.40.206PARENT=1,000,000DAUGHTER=0
24Example #1100502520%220.127.116.11234562.3 half-livesIf a sample has 20% 14C, how old is the sample?Number of parent isotopesHalf-Life 14C = 5,700 yearsNumber of daughter isotopes
25Example #2100502512.58%6.253.131234563.6 half-livesA sample is found to have 131I at a concentration of 8%.How old is the sample?Number of parent isotopesHalf-Life 131I = 8 daysNumber of daughter isotopes
28Radioactive DatingThe smaller the half-life, the less useful the radioactive isotope is for dating really old stuff.For example, Carbon-14 half life is only 5,700 years. Therefore, it can only accurately date rocks no older than about 100,000 years old.Therefore, to date rocks that formed back at the beginning of the Earth, 4.6 billion years ago, we would use an isotope with a much larger half-life.Since Uranium-238 half-life is 4.5 billion years, anything containing it has undergone 1 half-life since the Earth first formed!