EES 1030 1st Edition Lecture 9Intro to Earth ScienceGeologic timeThere are 2 ways to date geological events:- Relative dating: determines whether a rock is older or younger than the rocks surrounding using relationships and fossils. - Absolute dating: Determines the number of years since the rock formed using radioactive decay and radiometric dating.Structure of an atom:- Nucleolus contains most of the mass of the atom. - Protons- positive charge- Neutrons with no charge- Electrons with a negative charge. These cloud around the nucleolusAtomic number: number of protons. All atoms of the same element have the same atomic number.Mass number: number of protons and neutrons added togetherRadioactivity: spontaneous changes in decay in the structure of a certain atomic nuclei.Radioisotopes: an unstable radioactive isotope.Alpha decay: loss of 2 N and 2PBeta decay: n > p + electronElectron capture: p + electron > nHow is radioactive decay useful for dating? - The rate at which radioactive decay occurs for a particular unstable isotope is a half life.- Half life: the amount of time for half the atoms originally present in a radioactive isotope to decayGeologically important isotopes:Radioactive Parent Stable daughter product Currently accepted half-life valuesUranium-238 Lead-206 4.5 billion yearsUranium-235 Lead-207 713 million yearsThorium-232 Lead-208 14.1 billion yearsRubidium-87 Strontium-87 47 billion yearsPotassium-40 Argon-40 1.3 billion yearsCarbon-14 Nitrogen-14 5730 yearsRequirements for isotopic dating:- Decay rates constant and precisely known: no evidence or theoretical basis to expect decay rates to vary under the physical conditions within the earth or over time- Initial concentration of daughter known: best to chose minerals that do not include daughter element during initial formation- 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 of preparation - Sophisticated and expensive equipmentCarbon-14 or radiocarbon dating?- 14C half-life is 5730 years- Only very young samples < 40,000 years- Formed continuously in upper atmosphere by high-energy cosmic rays- Incorporated into all living organisms- Can date wood charcoal bone and shellsObtaining absolute ages for boundaries between units on the geological time scale is not simpleTime scale is based mainly on fossils found in sedimentary rocksSedimentary rocks: - detrital mineral grains will give age of original source rock- Cement minerals give younger ages than sediment deposits age- Date indirectly from volcanic igneous rocks interbedded with sedimentary rock units: pyroclastic ashes, lava flows. - There are estimations of the age of sedimentary rocks Volcanic ash layers contain minerals that can be dated by radiometric techniques. Geologic time scale boundaries: Basalt that is 385 million years old has a time boundary above it that separates the older Devonian period from the carboniferous period which contains volcanic ash of 325 million years