time introduction to earth science

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Lecture 10 Stratigraphy and Geologic Time       Stratigraphy Basic principles of relativ e age dating Unconformities: Markers of missing time Correlation of rock units A bsolute dating Geologic Time How old is the Earth? W hen did v arious geologic ev ents occur? Interpreting Earth history is a prime goal of geology Some know ledge of Earth history and geologic time is also required for engineers in order to understand relationships betw een geologic units and their impact on engineering construction  Stratigraphy :  Stratigraphy is the study of rock lay ers (strata) and their relationship w ith each other  Stratigraphy prov ides simple principles used to interpret geologic ev ents Two rock units at a cliff in Missouri (US Geological Survey)  Basic principles of relativ e age dating Relativ e dating means that rocks are placed in their proper sequence of formation A formation is a basic unit of rocks Below are some basic principles for establishing relativ e age betw een formations  Principle of original horiz ontality  Principle of superposition  Principle of faunal succession  Principle of cross-cutting relationships  Principle of original horiz ontality : Lay ers of sediment are generally deposited in a horiz ontal position Thus if w e observ ed rock lay ers that are folded or inclined, they must, w ith exceptions, hav e been mov ed into that position by crustal disturbances sometime after their deposition  Most lay ers of sediment are deposited in a nearly horiz ontal position Thus, w hen w e see inclined rock lay ers as show n, w e can assume that they must hav e been mov ed into that position after deposition Hartland Quay , Dev on, England by Tom Bean/DRK Photo  Principle of superposition: In an undeformed sequence of sedimentary rocks, each bed is older than the one abov e and y ounger than the one below The rule also applies to other surfacedeposited materials such as lav a flow s and v olcanic ashes Principle of superposition (W.W Norton)  A pply ing the law of superposition to the lay ers at the upper portion of the Grand Cany on, the Supai Group is the oldest and the K aibab Limestone is the y oungest (photo by Tarbuck)  A bsolute dating  The geologic time based on stratigraphy and fossils is a relativ e one: w e can only say w hether one formation is older than the other one  A bsolute dating w as made possible only after the discov ery of radioactiv ity  Radioactiv ity  A t the turn of the 20th century , nuclear phy sicists discov ered that atoms of uranium, radium, and sev eral other elements are unstable The nuclei of these atoms spontaneously break apart into other elements and emit radiation in the process know n as radioactiv ity  W e call the original atom the parent and its decay product the daughter For example, a radioactiv e 92 U 238 atom decay s into a stable nonradioactiv e 82 Pb 206 atom  example ty pes of radioactiv e decay  A lpha decay : an α particle (composed of protons and neutrons) is emitted from a nucleus The atomic number of the nucleus decreases by and the mass number decreases by  Beta decay : a β particle (electron) is emitted from a nucleus The atomic number of the nucleus increases by but the mass number is unchanged Illustration of alpha and beta decay s (adapted from Tarbuck and Lutgens)  The decay of U 238 A fter a series of radioactiv e decay s, the stable end product Pb 206 is reached (Tarbuck and Lutgents)  Decay constant  The rate of decay of an unstable parent nuclide is proportional to the number of atoms (N) remaining at the time t dN/dt=- λ *N  The reason that radioactiv e decay offers a reliable means of keeping time is that the decay constant λ of a particular element does not v ary w ith temperature, pressure, or chemistry of a geologic env ironment  Half-life  The half-life of an radioactiv e element is the time required for one-half of the original number of radioactiv e atoms to decay : T 1/ =0.693/ λ  The half-liv es of geologically useful radioactiv e elements range from thousands to billions of y ears The age of the Earth (4.6 billion y ears) w as first obtained using U/Th/Pb radiometric dating The half-life of U 238 is 4.5 billion y ears  The radioactiv e decay is exponential Half of the radioactiv e parent remains after one half-life, and one-quarter of the parent remains after the second half-life (Tarbuck and Lutgens) The concept of a half-life The ratio of parent-to-daughter changes with the passage of each successive half-life (W.W Norton)  Geologic Time The geologic time scale subdiv ides the 4.6-billion-y ear history of the Earth into many different units, w hich are linked w ith the ev ents of the geologic past  The time scale is div ided into eons: Precambrian and Phaneroz oic and eras: Precambrian, Paleoz oic ("ancient life"), Mesoz oic ("middle life"), and Cenoz oic ("recent life") The eras are bounded by profound w orldw ide changes in life-forms  The eras are div ided into periods  The periods are div ided into epochs The standard geologic time scale was developed using relative dating techniques Radiometric dating later provided absolute times for the standard geologic periods (W.W Norton)  The aw esome span of geologic time The geologic time represents ev ents of aw esome spans of time If the 4.6-billiony ear Earth history is represented by a 24hour day w ith the beginning at 12 midnight, the first indication of life w ould occur at 8:35am Dinosaurs w ould appear at 10:48pm and become extinct at 11:40pm The recorded history of mankind w ould represent only 0.2 sec before midnight    The K T extinction A t the boundary betw een Cretaceous (the last period of Mesoz oic) and Tertiary (the first period Of Cenoz oic) about 66 million y ears ago, know n as K T boundary , more than half of all plant and animal species died in a mass extinction The boundary marks the end of the era in w hich dinosaurs and other reptiles dominated and the beginning of the era w hen mammals became important The w idely held v iew of the extinction is the impact hy pothesis A large object collided w ith the Earth, producing a dust cloud that blocked the sunlight from much of the Earth’s surface W ithout sunlight for photosy nthesis, the food chains collapsed, w hich affected large animals most sev erely
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