James Ussher (mid 1600's) Georges Buffon (mid 1700s) Lord Kelvin (late 1800's) 75,000 yr 100 m.y. Charles Walcott (1893) 75 m.y. 4004 B.C.
II. Constancy of Natural Laws Catastrophism Georges Cuvier (late 1700s) Uniformitarianism James Hutton (late 1700's)
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.
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 First What is an atom? What is an isotope?
Fundamental unit of matter made up of subatomic particles Electrons Nucleus ATOMSNeutrons Protons } Nucleus Protons (positive charge) Electrons (negative charge) Neutrons (no electrical charge)
alpha particle Parent Nucleus Daughter Nucleus alpha decay Changes in atomic number and atomic mass number Atomic Number = - 2 Atomic Mass Number = - 4 4H4H 2 ALPHA DECAY 238 U 92 234 Th 90 92 protons 146 neutrons 90 protons 144 neutrons protons neutrons electrons Alpha 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.
beta particle Parent Nucleus Daughter Nucleus beta decay Changes in atomic number and atomic mass number Atomic Number = + 1 Atomic Mass Number = 0 BETA DECAY 137 Cs 55 137 Ba 56 55 protons 82 neutrons 56 protons 81 neutrons electron e - protons neutrons electrons Beta 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. p +
electron capture Parent Nucleus Daughter Nucleus Atomic Number = - 1 Atomic Mass Number = 0 ELECTRON CAPTURE 204 Po 84 204 Bi 83 84 protons 120 neutrons 83 protons 120 neutrons e - protons neutrons electrons Electron 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. Changes in atomic number and atomic mass number
U 238 Pb 206 Series This process is called radioactive decay, and eventually uranium (parent) decays to lead (daughter product).
During 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.
So, 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.
Radioactive Isotope Disintegration Half-Life Carbon-14 14 C 14 N 5.7 x 10 3 years Uranium-238 238 U 206 Pb 4.5 x 10 9 years Potassium-40 40 K 40 Ar 40 Ca 1.3 x 10 9 years Iodine-131 Cobalt-60 8.02 days 131 I 60 Co 5.3 years 131 Xe 60 Ni
Half Life = Number of years for 1/2 of the original number of atoms to decay from U to Pb How can we tell age based on the number of parent isotopes? Radioactive isotopesdecay at a particular rate. We express this rate as theHALF-LIFE, which is the time it takes for HALF of the parent isotopes to decay. U 238 Po 218
123456 Number of parent isotopes Number of daughter isotopes 2.3 half-lives 20% 100 50 25 12.5 6.25 3.13 If a sample has 20% 14 C, how old is the sample? Half-Life 14 C = 5,700 years
123456 Number of parent isotopes Number of daughter isotopes 3.6 half-lives 8% 100 50 25 12.5 6.25 3.13 A sample is found to have 131 I at a concentration of 8%. How old is the sample? Half-Life 131 I = 8 days
The 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!