GEOL162 geologictime

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Ngày đăng: 29/11/2016, 23:14

Geological Time - really, really, really long! Motion pictures are generally projected at 32 frames per second Therefore, each frame (image) is on the screen for only split second- let each frame represent 100 years Start movie at present and go back in time •The Declaration of Independence would show up 1/16 of a second into the movie •The Christian era (BC-AD boundary) would be 3/4 of a second into the movie •The most recent Ice Age would be seconds into it •The movie would run about hours before we got to the end of the Mesozoic era (extinction of the dinosaurs) •We'd have to watch the movie for about days to see the beginning of the Paleozoic era (macroscopic life) •The whole movie (to the beginning of geologic time on QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture Geologic Time • Two ways to relate time in geology: > Relative: Placing events in a sequence based on their positions in the geologic record > Chronologic : Placing a specific number of years on an event or rock sample Geologic Time Scale • a combination of the two types of age determinations > a relative sequence of lithologic units - established using logical principles > measured against a framework of chronologic dates Geologic Time and the "geologic column" • Developed Developed usingusing logical logical rules to establish relative rules to establish sequences of events relative sequences of events - superposition - cross-cutting relationships - original horizontality - lateral continuity • Added to as new information is obtained and data is refined refined - Use of fossils for correlation and age determination • Numerical Dates attached to strata after the - development of Radiometric techniques Still being refined as more information becomes available The Geologic Time Scale (1:2) The Geologic Time Scale (2:2) Relative Dating Methods • determines the relative sequence of events > which came first, which came last > no numeric age assigned • Relative age principles: > Superposition > Lateral continuity > Inclusions > Original Horizontality, > Cross-cutting Relationships > Fossil succession Those in yellow are most useful History of Historical Geology • Niels Stensen (Nicolaus Steno) - Fundamental Principles of Relative Time > Principle of Superposition- see below > Principle of Original Horizontality- see below > Principle of Original Lateral Continuity- see below Law of Superposition • In In undisturbed undisturbed strata, strata, the the layer layer on on the the bottom bottom is is oldest, those above are younger Nonconformities A nonconformity is an erosional surface on igneous or metamorphic rocks which are overlain by sedimentary rocks Breakout in to groups and discuss the sequence observed here Age Estimates of Earth Counting lifetimes in the Bible Comparing cooling rates of iron pellets Determine sedimentation rates & compare Estimate age based on salinity of the ocean all age estimates were off by billions of years some were more off than others! Absolute Dating Methods Radioactive Decay sequences acts as an atomic clock we see the clock at the end of its cycle analogous to starting a stopwatch allows assignment of numerical dates to rocks > Radioactive isotopes change (decay) into daughter isotopes at known rates rates vary with the isotope 235 40 14 + + e.g., U , K , C, etc Decay unstable nuclei in parent isotope emits subatomic particles and transform into another isotopic element (daughter) does so at a known rate, measured in the lab • Half-life The amount of time needed for one-half of a radioactive parent to decay into daughter isotope Assumptions?-you bet Cross-checks ensure validity of method Rate of Decay t0 All atoms are parent isotope or some known ratio of parent to daughter t1 half-life period has elapsed, half of the material has changed to a daughter isotope (6 parent: daughter) t2 half-lives elapsed, half of the parent remaining is transformed into a daughter isotope (3 parent: daughter) t half-lives elapsed, half of the parent remaining is transformed into a daughter isotope (1.5 parent: 10.5 daughter) We would see the rock at this point Radioactive Isotopes % parent remaining • analogous to sand in an hour glass - we measure how much sand there is > represents themass of elements - we measure the ratio of sand in the bottom to sand in the top - at the end (present) > daughter (b) and parent (t) - we know at what rate the sand falls into the bottom > the half life of the radioactive element - how long would it take to get the amount sand in the observed ratio starting with all of it in the top? 100 Parent Daughter 50 25 13 time -> Five Radioactive Isotope Pairs Isotopes Parent Daughter Half-Life (Years) Effective Dating Range of Parent (Years) Uranium 238 Lead 206 4.5 billion 10 million to 4.6 billion Uranium 235 Lead 207 704 million Thorium 232 Lead 208 14 billion 48.8 billion Rubidium 87 Strontium 87 4.6 billion 10 million to 4.6 billion Potassium 40 Argon 40 1.3 billion 100,000 to 4.6 billion Minerals and Rocks That Can Be Dated Zircon Uraninite Muscovite Biotite Potassium feldspar Whole metamorphic or igneous rock Glauconite Muscovite Biotite Hornblende Whole volcanic rock Radiocarbon and Tree- Ring Dating Methods •• Carbon-14 dating is based on the ratio of C-14 to C-12 in an organic sample Valid only only for for samples samples less less than than 70,000 70,000 >> Valid years old old years Living things things take take in in both both isotopes isotopes of of >> Living carbon carbon When the the organism organism dies, dies, the the "clock" "clock" starts starts >> When Method can be validated by cross-checking with tree rings Carbon 14 Cycle Recognizing Patterns of change Walther's Law • The vertical sequence is repeated by the horizontal sequence - walking from A to B to C to the Coast you would encounter the rocks that would be encountered by drilling a core into the earth at any point (A, B, or C) Facies Diagram • distribution of lithofacies (rock-types) - these are associated with their respective EOD • biofacies are similar but refer to fossils rather than rock types Eustasy, relative sea-level, and relative position of lithofacies • Eustasy= changes in volume of water in ocean • lithofacies depend on - sea-level land level geometry of coast sediment supply Vail Curve • an attempt at global • correlation of lithologies - for better production - of petroleum resources Rock designations • Rock units called Lithostratigraphic units - described in terms of Group, Formation, & Member > each term has specific meanings in geological parlance • Formation - a mappable lithostratigraphic unit > has a location for identifying the type-section > has a rock designation describing the lithology - sometimes not all the same lithology > in which case the term "Formation" takes the place of lithologic type • Groups are composed of several formations • Members are distinctive units within a formation - group is largest and contains formations and members - formations are next and contain members Fundamental lithological units Formation- a rock layer with distinctive characteristics that is mappable over a large are at “typical” map scales 1:62,500 or more commonly 1:24,000 Formations have Members smaller layers that are unique that are not mappable over larger areas and won’t show up at typical map scales Groups have formations; formations have members
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