CEE 1030 1st Edition Lecture 8Two ways to date geologic events Relative dating- determine whether a rock is older or younger than surrounding rocks field relationships, fossils Absolut dating- determine actual number of years since rock was formed radioactive decay = radiometric dating Structure of an atom- nucleus contains most of the mass of the atom - has protons positive charge - has neutrons no charge, neutral - electrons with negative charge: cloud around nucleus Atoms and Isotopes atomic number= number of protons - all atoms of a particular elements have the same atomic number mass number= number of protons and neutrons - atoms of a particular element can have different numbers of neutrons isotopes Radioactive decay radioactivity- spontaneous changes (decay) in the structure of certain unstable atomic nuclei parent- an unstable radioactive isotope daughter product(s)- the isotopes resulting from decay of a parent isotope: usually of a different element - unstable element will decay into a stable element, can change elements in order to be stable Three main types: alpha decay, beta decay, electron capture - alpha decay loss of 2n + 2p, atomic mass (top number on left of element) goes down by four, atomic number goes down two- beta decay n p+ electron, atomic number goes up by one, atomic mass stays the same - electron capture p+ electron n, opposite of beta, atomic number goes down by oneHow is radioactive decay useful for dating? the rate at which radioactive decay occurs for a particular unstable isotopeis constantif we know the rate of decay and the amount of parent and daughter isotopes present in a sample, we can calculate an age half-life of a radioactive isotope is the time taken for half of the atoms initiallypresent Requirements for isotope dating decay rates constant and precisely known: no evidence or theoretical basis toexpect decay rates to vary under the physical conditions within the Earth or over time initial concentration of daughter known: best to choose minerals that do not include daughter element during initial formation, can also use isochron techniques: separate and analyse several different minerals from rock sample closed system- parent and daughter elements must not diffuse (leak) in or out of the mineral - diffusion stops below a certain temperature (150o- 800o C) which varies for different elements and different minerals - chemical weathering of sample a problem - minerals in slowly-cooled metamorphic rocks can give different ages by different techniques - radiometric dating is a complex procedure that requires precise measurements - careful and detailed sample preparation- sophisticated and expensive equipment Carbon-14 or radiocarbon dating -14C: half- life is 5730 years, does not tell us anything about the age of the earth - only for very young samples, <40,000 years - is being formed actively in the atmosphere, atoms moving at the speed of light , by high-energy cosmic rats - incorporated into all living organisms- can date wood, charcoal, bone, shells - useful tool for archeologists and geologists who study very recent earth history Agreement between systems - two best dating systems40Ar- 39Ar and U-Pb done on the same samples: excellent agreement Uranium- Lead Dating Method- probably best method used -238U206Pb, 4470 million years -235U207Pb, 704 million years - minerals with high U and low/ no PbEarth’s oldest rocks- Arcasta Gneisses: Yellowknife area of the NW territories near Great Slave Lake in Canada - 4.03 Billion years old Absolute ages for the geological time scale - obtaining absolute ages for boundaries between units on the geological time scale is not simple - time scale is based mainly on fossils found in sedimentary rocks - absolute ages are best determined on igneous and some metamorphic rocks Sedimentary rocks: not easy! - detrital mineral grains will give age of original source rock - cement minerals give younger ages than sediment deposition age - date indirectly from volcanic igneous rocks interbedded with sedimentary rock units: pyroclastic ashes, lava flows Volcanic Ash Layers- blanket a wide geographical area, distinctive time marker horizon, often contain minerals that can be dated by radiometric
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