Transcript ppt - LILT
Sedimentary rocks and their depositional environments • How can sedimentary rocks be used to figure out what the environment was in the past? Depositional environments *How does energy of the environment relate to grain size??? Sedimentary environments • Sea level rising, shoreline moves inland transgression • Sea level falling, shoreline moves off land regression • What can cause changes in sea level? • Transgression and regression animation Sea level rising: transgression Transgression How to recognize a transgression Fining upward Transgression facies From: http://www.purdue.edu/eas/mesozoic/lab_05.html Sea level dropping: regression Regression How to recognize a marine regression Coarsening upward How to recognize a transgression Regressive facies From: http://www.purdue.edu/eas/mesozoic/lab_05.html Geologic time: absolute dating • relative dating animations • We’ve talked about rocks and how to tell the relative age of them using Steno’s laws and sedimentary structures. • Radioactive decay of certain elements allows us to date rocks more precisely. Absolute dating • Radioative elements are geologic “clocks” • Amount of decay=age of the element and age of rock (have to be careful here!) Isotopes of Hydrogen e- • Hydrogen • 1 proton, 0 neutrons • Deuterium • 1 proton, 1 neutron en • Isotope are elements with different atomic masses (neutron #’s differ) • When atoms are unstable they eject particles from their nucleus and become an ISOTOPE! Radioactive decay • Decay rate in terms of half-life (time) • Half-life: time for 1/2 of all atoms to decay from parent element to daughter element Radioactive Decay • Mineral forms with only parent atoms • Through time, the parent decays to form daughter product How to use half-life (example) • Half-life of potassium 40 is 1.3 billion years • After 3.9 billion years,how many half-lives have occurred • What fraction of potassium 40 is left? • 1/2 * 1/2 * 1/2 = 1/8 All rocks are not created equal for radiometric dating use! • Expensive and may not have good minerals to date • Sedimentary rocks are almost impossible to date…why? (think of what sed rocks are made of) • Best rocks: igneous • Complicated rocks: metamorphic Problems with dating • Errors • Usually 0.5% of age, as good as 0.1% • E.g. -- 2 billion year rock is +/- 2-20 million • Dating detrital sed rocks • Is the age found of the sedimentary deposit or igneous formation? • Dating metamorphic rocks • Did isotope ratio survive metamorphism? Common isotopes used to date rocks! Parent Daughter 1/2 life Use 238U 206Pb 4.5 billion Old rocks 10Ma - 4.6Ga 40K 40Ar 1.3 billion Young rocks <10 Ma 14C 14N 5730 years Young artifacts 100-70,000 years Carbon-14 dating • How does Carbon-14 dating work? • Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere and cause them to turn into radioactive Carbon 14, which combines with oxygen to form radioactive carbon dioxide. • Living things are in equilibrium with the atmosphere, and the radioactive carbon dioxide is absorbed and used by plants. The radioactive carbon dioxide gets into the food chain and the carbon cycle. • All living things contain a constant ratio of Carbon 14 to Carbon 12. (1 in a trillion). • At death, Carbon 14 exchange ceases and any Carbon 14 in the tissues of the organism begins to decay to Nitrogen 14, and is not replenished by new C-14. • The change in the Carbon 14 to Carbon 12 ratio is the basis for dating. • The half-life is so short (5730 years) that this method can only be used on materials less than 70,000 years old. Archaeological dating uses this method.) Also useful for dating the Pleistocene Epoch (Ice Ages). • Assumes that the rate of Carbon 14 production (and hence the amount of cosmic rays striking the Earth) has been constant (through the past 70,000 years). Carbon-14 Dating • For more see this site carbon-14 animations and info • And this site pdf article Structure of the Carbon Atom • Nucleus • Protons (+) • Neutrons (n) • Electrons (-) • # protons: atomic # • # protons + neutrons: atomic mass • Isotope: element with different # neutrons