EXAM 2 GeoTime

EXAM 2 GeoTime - 2 ways to date geologic events Relative...

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Unformatted text preview: 2 ways to date geologic events Relative dating – Age of rocks compared to one another – Sequence of events – First method used to date rocks Absolute dating – Gives an age (years before the present) – Depends on natural radioactive decay of elements Relative Dating Nicolas Steno (1669) – Said fossils were remains of ancient life – Suggested first laws for relative dating of rock layers Relative Dating Logical tools are useful for defining relative age. – – – – – – – Principle of uniformitarianism. Principle of superposition. Principle of original horizontality. Principle of original continuity. Principle of cross-cutting relationships. Principle of inclusions. Principle of baked contacts. M F R V L When these sedimentary rocks formed, which layer was deposited first? A. R B. L C. V D. M E. F Layer above is younger than layer below. Principle of Superposition States that in undeformed sedimentary rocks, the youngest layer is on top Youngest rocks Oldest rocks Oldest Principle of Superposition When the rocks were originally deposited: A. They were deposited at this angle. B. They were deposited at a steeper angle (closer to vertical). C. They were deposited at a shallower angle (closer to horizontal). Principle of original horizontality Layers of sediment, when originally deposited, are fairly horizontal Sed rx may still be horizontal in Sed the rock record ... the ... or not ... When these rocks were deposited: A. B. C. No deposition occurred where the canyon is located. The rocks that make up the walls of the canyon continued across. Different rocks were deposited in the canyon. Principle of Original Continuity Layers of sediment naturally form a fairly continuous sheet across a basin. Layers on sides of a canyon were once continuous! Put the units in order from oldest to youngest: A. T, L, O, E B. L, T, O, E C. T, O, E, L E O L T Principle of Cross-Cutting Relations A geologic feature that cuts across another feature is younger than what it cuts. FAULTS Principle of Cross-Cutting Relations What happened last? The sedimentary rocks, the dike, or the fault? Principle of inclusions If a rock contains fragments of another rock, the fragments are older than the rock they are in. Contact Metamorphism Baked contacts. – Thermal metamorphism occurs when country rock is invaded by a plutonic igneous intrusion – The baked rock must have been there first (it is older) Geologic History Relative ages help to unravel a complicated history Simple rules permit one to decipher this diagram. Geologic History Deposition of horizontal strata below sea level in order 1, 2, 3, 4, 5, 6, 7, and 8 (oldest to youngest). Geologic History Igneous intrusion of a sill. Geologic History Tectonic compression and folding. – Beds had to be present to be folded. Uplift above sea level and erosion. Geologic History Intrusion of a granitic igneous pluton. Geologic History Extensional normal faulting. – Faulting cross-cuts the older granitic pluton. Geologic History Intrusion of a dike. – Dike cross-cuts the normal fault. Geologic History Erosion to present landscape configuration. – Erosion removed the volcano and cross-cuts the dike. Relative Age Determining relative ages empowers geologists to easily unravel complicated geologic histories. Missing time… Unconformities- Gaps in the rock record 3 ll unconformities A types of unconformities: “Gaps” in the rock record Missing time… Mising rock + Mising Missing time! Missing Angular unconformity Angular sedimentary over folded/ sedimentary tilted rocks A B Which surface is an unconformity? A. B. C. C Unconformity in the GC Angular unconformity in the Grand Canyon Nonconformity in the Grand Canyon Tapeats Sandstone (~550 million years old) Vishnu Schist (~1700 million years old) Nonconformity between Grenville + Potsdam ss (Orange Co, NY) Nonconformity Disconformity in Tennessee – 1 My hiatus closeup Each unconformity represents a hiatusperiod of time missing from rock record – Non-deposition – Erosion Principle of Faunal Succession Principle – Certain fossils are found in well defined time intervals (“range”) time – Relative ages of strata can be determined from fossil content determined Index fossils: Easy to identify Geographically widespread Geographically Limited to a short span of geologic time (evolves and goes extinct rapidly) time Biostratigraphy - using fossils to determine relative ages of rocks determine Paleozoic Brachiopods Index Fossil- Trilobites Correlation Lithologic correlation: Match units by rock type Fossil correlation: Match rock units by fossils correlation line Correlation among rock strata in 3 national parks. Construct relative geologic time scale: Construct - Determine relative ages of Determine stratigraphic columns around world stratigraphic - Correlate & stack columns Using all the tools at your disposal to date the rocks! What is missing?? Construction of a relative geologic time scale time Absolute Time and Radiometric Dating Radioactive (unstable) elements turn into non-radioactive (stable) elements over time Decay rate is constant and can be measured in a lab If amount of parent (radioactive) and daughter (stable) are know, we can calculate age Radioactive decay of Radioactive neutrons in 87Rubidium 87Rubidium to yield 87Strontium 87 Half Life = time it takes for 1/2 of parent isotopes Half to decay to daughter isotope to By Measuring the ratio Of P/D, you can derive an Age estimate if the ½ life is established Break down Break After one half­life, 50% or 1/2 of the parent decays, and you are left with an equal amount of daughter isotope. There is a 1:1 parent to daughter ratio. Radiometric clock starts when crystals cool enough for both parent & daughter isotopes to be locked into the crystal lattice. Provides age of mineral. Igneous rocks - date when lava or magma cooled Metamorphic rocks - date when rock cooled from high metamorphic temps Sedimentary rocks - (usually) can’t use radiometric dating What about sedimentary rocks? - We rely on relationships with datable igneous & metamorphic rocks. W. W. Norton Radioactive decay and half-lives β– 87 87 Rb Radioactive decay schemes 87 Sr • 87Rb-87Sr half-life 48 Ga • 147Sm-143Nd half-life 106 Ga • 238U-206Pb half-life 4.5 Ga • 235U-207Pb half-life 0.7 Ga • 232Th-208Pb half-life 14 Ga Organic matter Organic only Useful applications to archaeology Absolute dating using seasonal markings seasonal - Count rings to provide a chronometer Count for geologic time. for - Rings also preserve information on Rings climate & environmental changes through time. through • Tree rings Tree • Lake varves Lake • River deposits River • Chemically precipitated sedimentary rocks sedimentary • Shells Shells • Glacial ice Glacial Tree Rings: Dependent on Temperature, Precipitation, Nutrients… Ability to ID specific events VARVESVARVESGlacial Lakes, Paleoclimate Age of Mammals Multicellular life Age of Dinosaurs land Fig. 12.24 plants Age of diversification W. W. Norton Absolute geologic time scale Unicellular life ...
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