Planet earth stratigraphy and geologic time notes
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Planet earth stratigraphy and geologic time notes Document Transcript

  • 1. 08 Time and GeologyUniformitarianism• People two or three centuries ago did not realize the expanse of geologic time. Christianity placed geologic events within a biblical chronology, and catastrophic events were blamed for features of the landscape. For example, Grand Canyon split open and remained that way ever since. Clams in the mountains was explained by a worldwide inundation drowning the earth’s mountains. No known physical laws could account for such events so they were attributed to divine intervention. Theories that explain scientific phenomena as the result of major catastrophes is considered catastrophism. The earth opened up and the ocean poured out of the crack, earth’s crust precipitated from the ocean basins.• In the 18 century, James Hutton concluded that the earth is a dynamic, ever- th changing place in which new rocks, lands and mountains arise continuously as a balance against their destruction by erosion and weathering. Taking a cyclic view of the planet, he believed that the past history of our globe must be explained by what can be seen to be happening now, the present is the key to the past. We use present observations, physical and chemical laws (natural laws are invariant with time and are an accumulation of our observations) to infer the origin of particular features seen today. The principle of Uniformitarianism states that forces are operating today in the same fashion as they have for millions of years. The geologic processes operating at present are the same processes that have operated in the past, they may vary in their geographic location and geologic intensity.• What made the idea of uniformitarianism so difficult to accept was the vast amount of time implied. For example, layers of shale 2000 meters thick exist, but at observed rates silt and clay settle at rates of less than a mm a year onto the ocean floor. 2000 meter thick deposits would therefore require more than a million years to form, much longer than people in the eighteenth century believed the earth existed. Acceptance of the principle of uniformitarianism led to the realization that geology involves time periods much greater than a few thousand years. Absolute age-age given in years or some other unit of time. Relative time-the sequence in which events took place.Relative TimeRelative age refers to the sequence in which events took place (not the number ofyears involved). The sequence of geologic events can be determined by piecingtogether the history of the individual parts. Most of the individual parts of thelarger problem are solved by applying several simple principles. In this way thesequence of events or the relative time involved can be determined. (1) original horizontality
  • 2. 08 Time and Geology (2) superposition (3) cross cutting relationshipsRefer to figure 8.2 representing an area similar to the grand canyonThe principle of original horizontallity states that beds of sediment deposited inwater form as horizontal layers. Thus, if we observe rock layers that are folded orinclined at a steep angle, they must have been moved into that position by crustaldisturbance after their deposition.The principle of superposition states that within a sequence of undisturbedsedimentary rocks, the layers get younger going from bottom to top. Ifsedimentary rock is formed by sediment settling onto the ocean floor, then thefirst (or bottom) layer must be there before the next layer can be deposited ontop of it. The principle also applies to layers formed by multiple lava flows andbeds of ash from volcanic eruptions.Cross-cutting relationships states that a disrupted pattern is older than thecause of disruption. When igneous intrusions or faults cut through other rocks,they are assumed to be younger than the rocks they cut.Inclusions are pieces of one rock unit that are contained within another. The rockmass adjacent to the one containing the inclusions must have been there first inorder to provide the rock fragments. Therefore the rock mass containinginclusions is the younger of the two.Layers of rock are said to be conformable when they are found to have beendeposited. Although particular sites may exhibit conformable beds representingsignificant spans of geologic time, there is no place on earth that contains a fullset of conformable strata. Throughout earth history, the deposition of sedimenthas been interrupted over and over again. All such breaks in the rock record aretermed unconformities. An unconformity represents a long period of time duringwhich deposition ceased, erosion removed previously formed rocks, and thendeposition resumed.Angular unconformities consist of tilted or folded sedimentary rocks that areoverlain by younger, more flat lying strata. Angular unconformities indicate thatduring the pause in deposition, a period of deformation (folding or tilting) as well aserosion occurred.Disconformities are a type of unconformity in which the strata on either side areessentially parallel. Many disconformities are difficult to identify because therocks above and below are similar and there is little evidence of erosion.Nonconformities are a type of unconformity in which the break separates oldermetamorphic or intrusive igneous rocks from younger sedimentary strata.Intrusive igneous masses and metamorphic rocks originate far below the surface.
  • 3. 08 Time and GeologyTherefore, for a nonconformity to develop, there must be a period of uplift andthe erosion of overlying rocks. Once exposed at the surface, the igneous ormetamorphic rocks are subjected to weathering and erosion prior to sedimentation.CorrelationIn order to develop a geologic time scale that is applicable to the whole earth,rocks of similar age in different regions must be matched up. Correlation meansdetermining the age relationships between rock units or geologic events in separateareas. Correlation is necessary for understanding the geologic history of a region, acontinent, or the whole earth. Substantiation of the plate tectonics theorydepends on intercontinental correlation of rock units and geologic events, piecingtogether evidence that the continents were once one great body. Part of theevidence supporting the theory of plate tectonics (and continental drift) dependedon correlating the age of rocks in South America and Africa.(1) Physical continuity - is tracing physically the course of a rock unit to correlate rocks between two different places.(2) Similarity of rock types - is correlation of two regions by assuming that similar rock types in two regions formed at the same time. This method must be used cautiously if the rocks being correlated are common ones. Correlation by similarity in rock types is more reliable when they are very unusual sequences of rock.(3) Correlation by fossils - Fossils are common in sedimentary rock. Plants and animals that lived at the time the rock formed were buried by sediment, and their fossil remains are preserved in sedimentary rock. Fossils in one layer of sedimentary rock often differ markedly from fossils in layers above and below. The significance of fossils as geologic tools was made evident by an English engineer and canal builder, William Smith. Smith discovered that each rock formation in the canals contained fossils unlike those in the beds either above or below it. Further, he noted that sedimentary strata in widely separated areas could be identified by their distinctive fossil content. Based upon Smith’s observations, one of the most important and basic principles in historical geology was formulate: Fossil organisms succeed one another in a definite and recognizable order, and therefore any time period can be recognized by its fossil content. This is known as the principle of faunal succession. When fossils are arranged according to their age by using the law of superposition on the rocks in which they are found, they do not present a random or haphazard picture. To the contrary, fossils show progressive changes from simple to complex and reveal the advancement of life through time. For example, in the fossil record there is represented, in succession, an age of trilobites. an age of fishes, an age of reptiles, and an age of mammals. These ages pertain to groups
  • 4. 08 Time and Geology that were especially plentiful and characteristic during particular time periods. Within each of the ages, there are many subdivisions based on certain species. This same succession of dominant organisms, never out of order, is found on every major landmass. No matter where on earth they are found, individual fossil species always occur in the same sequence relative to one another. By comparing fossils found in a layer of rock in one area with similar fossils in another area, we can correlate the two rock units.Since fossils were found to be time indicators, they became the most useful meansof correlating rocks of similar age in different regions. Most useful are indexfossils - a fossil from a very short lived species known to exist during a specificperiod of geologic time. A single index fossil allows the geologist to correlate therock in which it is found with all other rock layers in the world containing thatfossil. Rock formations, however, do not always contain a specific index fossil. Insuch situations, groups of fossils are used to establish the age of the bed. Severaldifferent fossils in a rock is referred to as a fossil assemblage. Fossil assemblagesare generally more useful for dating rocks than a single fossil because thesediment must have been deposited at a time when all the species representedexisted.Overlapping ranges of fossils help date rocks more exactly than using a singlefossil.^ Age of bed containing Age of only fossil A and C bed Age of bed containingTim Fossil Fossil Fossil containi fossils A, B, ande A B C ng C only Age of bed fossil A containing only^ fossil A and BThe Standard Geologic Time ScaleBased on fossil assemblages, the geologic time scale subdivides geologic time. Thegeologic time scale is a sort of calendar to which events and rock units can bereferred. The geologic time scale is a relative time scale representing an extensive
  • 5. 08 Time and Geologyfossil record. It consists of three eras, which are subdivided into periods, whichare in turn subdivided into epochs. Geologists can therefore use fossils in rock torefer the age of the rock to the geologic time scale.EON ERA- The PERIOD- EPOCH eras are Each period bound by is bound by profound less worldwide profound changes in changes in life forms. life forms.Phanerozoic Cenezoic Means ‘new Quaternary Recent We live inEon life’ (holocene) theMeans Pleistocene Holocene“visible Tertiary Epoch oflife.” Pliocene theThe rocks Miocene Quaternaryand Oligocene Period.deposits of Eocenethe Paleocene The mostPhanerozoic recent iceeon contain ages werean part of theabundance Pleistoceneof fossils epoch.thatdocumentmajorevolutionarytrends. Mesozoic Means Cretaceous The time of ‘middle life’. Jurassic the Reptiles Triassic dinosaurs were the dominant animals.
  • 6. 08 Time and Geology Paleozoic Means ‘old Permian life’. Began Pennsylvania with the n appearance Mississippia of abundant n and complex Devonian life Silurian including Ordovician creatures Cambrian with shells and other hard parts.Proterozoic PrecambriaEon n timeArchean collectivelyEon is the vastcollectively amount ofPrecambria time thatn Time preceded the Paleozoic Era. Contains few fossils.
  • 7. 08 Time and GeologyAbsolute AgeWidespread use of fossils for correlation led to the development of the standardgeologic time scale. Originally based on relative age relationships, the subdivisionof the standard geologic time scale have now been assigned absolute ages throughradioactive dating.Radioactive DatingAn atom is composed of electrons, protons, and neutrons. Protons and neutrons arefound in the center nucleus of the atom. By adding together the number ofprotons and neutrons in the nucleus, the mass number of the atom is determined.The atomic number is equal to the number of protons. Every element has adifferent number of protons, and thus a different atomic number. Atoms of thesame element may have different numbers of neutrons in the nucleus. Such atoms,called isotopes, have different mass numbers but the same atomic number.The forces that bind protons and neutrons together in the nucleus are strong.Some isotopes have unstable nuclei, and the forces that bind the protons andneutrons together are not sufficiently strong. As a result, the nucleispontaneously break apart, or decay, a process called radioactivity. Three types ofradioactive decay are common(1) Alpha particles may be emitted from the nucleus. An alpha particle is composed of 2 protons and 2 neutrons. Therefore the emission of an alpha particle reduces the mass number by 4 and the atomic number by 2.(2) When a beta particle, or electron, is given off from a nucleus, the mass number remains unchanged, because electrons have no mass. However, since the electron comes from a neutron (a neutron is a combination of a proton and electron), the proton is left in the nucleus, and therefore the nucleus contains one more proton than before. Therefore, the atomic number increases by 1.(3) Sometimes an electron is captured by the nucleus. The electron combines with a proton and forms a neutron. The mass number remains unchanged. However, since the nucleus now contains one less proton, the atomic number decreases by 1.The radioactive isotope is referred to as the parent, and the isotope resultingfrom the decay of the parent are termed the daughter products.Radioactivity provides a reliable means of calculating the ages of rocks andminerals which contain particular radioactive isotopes, a procedure referred to asradiometric dating. Radiometric dating is reliable because the rates of decay formany isotopes have been precisely measured and do not vary. Therefore, eachradioactive isotope used for dating has been decaying at a fixed rate since the
  • 8. 08 Time and Geologyformation of the rocks in which it occurs and the products of decay have beenaccumulating at a corresponding rate. For example, when uranium is incorporatedinto a mineral that crystallizes form magma, there is now lead (the stable daughterproduct) from previous decay. As the uranium in the newly formed mineraldisintegrates, atoms of the daughter product are trapped and measurable amountsof lead eventually accumulate. The time required for one-half of the nuclei in asample to decay, called half-life, is a common way of expressing the rate ofradioactive disintegration. When the quantities of parent and daughter are equal,we know that one half life has transpired. When one-quarter of the original parentatoms remain and three-quarters have decayed to the daughter product, theparent daughter ratio is 1:3 and we know that two half lives have passed.Therefore, if the half-life of a radioactive isotope is known and theparent/daughter ratio can be determined, the age of the sample can be calculated.Notice that the percentage of radioactive atoms that decay during one half-life isalways the same. However the actual number of atoms that decay with the passingof each half-life continually decreases. Thus as the percentage of radioactiveparent atoms declines, the proportion of stable daughter atoms rises, with theincrease in daughter atoms just matching the drop in parent atoms.It is important to realize that an accurate radiometric date can be obtained only ifthe mineral remained a closed system during the entire period since its formation.That is, a correct date is not possible unless there was neither the addition norloss of parent or daughter isotopes. Most successfully dated rocks are igneous,which, since they solidified from a melt, are unlikely to be contaminated bypreviously formed daughter products. Metamorphic rock can yield inaccurate agesbecause heat during metamorphism can drive out some of either the daughterproduct or the radioactive isotope. Sedimentary rock cannot be radioactivelydated.RadioCarbon: Dating “young” events.To date very recent events, carbon-14 (also called radiocarbon), the radioactiveisotope of carbon, is used. Because of its half life of 5730 years, it can be usedfor dating events of about 50,000 years to those from recent geologic history.Carbon-14 is continuously produced in the upper atmosphere as a consequence ofcosmic ray bombardment, in which cosmic rays shatter the nuclei of gasses,releasing neutrons. Some of the neutrons are absorbed by nitrogen (atomicnumber 7, mass number 14), causing its nucleus to emit a proton. As a result, theatomic number decreases by 1 (to 6), and a different element, carbon-14, iscreated. This isotope of carbon is incorporated into carbon dioxide, circulates in
  • 9. 08 Time and Geologythe atmosphere, and is absorbed by living matter. As a result, all organismscontain a small amount of carbon-14. As long as an organism is alive, the decayingradiocarbon is continually replaced, and the proportion of C-14 and C-12 (thestable and most common isotope of carbon) remain constant. However, when theplant or animal dies, the amount of carbon-14 gradually decreases as it decays tonitrogen-14 by beta emission. By comparing the proportions of C-14 and C-12 in asample, radiocarbon dates can be determined.Combining Relative and Absolute Ages.Radioactive dating can provide absolute time brackets for events whose relativeages are known. By combining many radioactive dates from sites carefully selectedfor well-known relative age relationships, absolute ages have been assigned to thegeologic time scale.