Introduction � The Grand Canyon - Major John Wesley Powell, in 1869, led a group of explorers down the Colorado River Chapter 17 Chapter 17 � Powell returned to map the region. � Powell was impressed with the geologic strata and thus began an Geologic Time & investigation that continues today into the Geohistory: immense amount of geologic time presented in the canyon. � It is this vastness of Concepts and geologic time that sets geology apart from the Principles other sciences. Introduction How Is Geologic Time Measured? � Geologic time provides an immense contribution to � Time is defined by the methods other sciences used to measure it. � The logic used in applying the principles of relative � Relative dating is accomplished dating “involves basic reasoning skills” that are useful by placing events in a logical, sequential order. in almost any profession or discipline. � Absolute dating provides � The geologic time scale is fundamental to specific dates for geologic rock understanding the physical and biological history of units or events using our planet radiometric dating . � An accurate and precise geologic calendar is critical in determining the onset, duration, and possible causes of such past events as global climate change and their potential effects on humans. Fig. 17.1, p. 437 Early Concepts of Geologic � A world-wide relative Time and the Age of Earth time scale of Earth's rock record was � James Ussher, in the early 1600’s asserted that God established by the created Earth on Sunday, October 23, 4004 B.C. work of many geologists applying the principles of � Many early Christians analyzed historical records and historical geology and genealogies found in the scripture to try and determine the age correlation to strata of of the Earth. all ages throughout � During the 18th and 19th centuries, attempts were made to the world. determine Earth’s age based on scientific evidence rather than The Geologic Time revelation. Scale � Although some attempts were ingenious, they yielded a variety of ages that now are known to be much too young. Fig. 17.1, p. 437 1
James Hutton and the James Hutton and the Recognition of Geologic Time Recognition of Geologic Time � Scientific attempts to estimate Earth's age were first � Charles Lyell argued convincingly for Hutton's conclusions. made by naturalists during the 18th and 19th centuries. � He established the principle of uniformitarianism as � They formulated some of the basic principles used for the guiding principle of geology. deciphering the age of the earth. � James Hutton, the father of modern geology, first � This principle holds that the laws of nature have suggested that present day processes operating over been constant through time and long periods of time could explain all geologic features. � That the same processes operating today have � His observations were instrumental in establishing operated in the past, although not necessarily at the the principle of uniformitarianism and the fact that same rates. Earth was much older than earlier scientists thought. Relative Dating Methods Relative Dating Methods � Fundamental Principles of Relative Dating � Before the development of radiometric dating, there Besides uniformitarianism, several principles were was no reliable method for absolute dating, developed for relative dating: therefore relative dating methods were used. � 1. Superposition � Relative dating places events in sequential order � 2. Original horizontality but does not tell us how long ago an event took � 3. Cross-cutting relationships place. � 4. Lateral continuity � The principles of relative dating provided � 5. Inclusions geologists with a means to interpret geologic � 6. Fossil succession. history and develop a relative geologic time scale. � These principles are used to determine the relative geologic ages and for interpreting Earth history. 1. Superposition 2. Original Horizontality � Superposition states that , in an undisturbed � Original horizontality states that sediment is succession of sedimentary layers, the oldest layer is originally deposited in horizontal layers. on the bottom � and the youngest � Steno noted that layer is on the top. sedimentary particles settle from water under the influence of gravity. Fig. 17.2 a, p. 439 Fig. 17.12 a , p. 448 2
3. Lateral Continuity 4. Cross-cutting relationships � Sediment extends laterally in all directions until it � Based on detailed studies by James Hutton, Hutton thins and pinches out or terminates against the recognized that edge of a depositional basin. � an igneous intrusion must be younger than the rock it intrudes. � Ash beds make excellent correlation markers! � Also, faults must be younger than the rocks they displace. Fig. 17.3, p. 439 Fig. 17.13, p. 449 5. Principle of Inclusions 4. Cross-cutting relationships � Inclusions in a rock are older than the � Differentiating between a buried lava flow and a sill rock layer itself. Fig. 17.4, p. 440 Fig. 17.5, p. 442 6. Principle of Fossil Succession Relative Dating Methods � William Smith, an engineer working in the coal Unconformities are surfaces of discontinuity canals of England, independently recognized in the rock deposition sequence which superposition. encompass significant periods of time. � He observed that the fossils on the bottom of a � Unconformities may result sequence must be older than those at the top of the from nondeposition and/or sequence. erosion. � These surfaces encompass long periods of geologic time for which there is no geologic record at that location. Fig. 17.6, p. 443 Fig. 17.7, p. 443 3
MYA Relative Dating Methods 0 0 1 1 � Unconformities 2 2 � Three types of unconformities are recognized. 3 3 Unconformity 6 7 4 � A disconformity separates younger from older Amount Hiatus of rock 8 5 removed sedimentary strata that are parallel to each other. by erosion � An angular unconformity is an erosional surface 9 6 on tilted or folded rocks, over which younger 10 7 sedimentary rocks were deposited. 11 8 � A nonconformity is an erosional surface cut into 9 12 igneous or metamorphic rocks and overlain by younger sedimentary rocks. 10 11 12 Stepped Art Fig. 17-7, p. 443 Relative Dating Methods Format ation of a ion of a Dis Disconfor onformity � A disconformity separates younger from older sedimentary strata that are parallel to each other. Fig. 17.8b, p. 444 Fig. 17.8, p. 444 Relative Dating Methods Uplift and erosion � An angular unconformity is an erosional surface on tilted or folded rocks, over which younger rocks were deposited. Deposition Disconformity Uplift and erosion Jurassic rocks Mississippian rocks Stepped Art Deposition Fig. 17-8, p. 444 Fig. 17.9, p. 445 4
Uplift and erosion Deposition Angular unconformity Formation of an Formatio of an Angul Angular Unco r Unconformity nformity Erosion 4 Uplift and tilting Stepped Art Deposition Fig. 17.9, p. 445 Fig. 17-9a, p. 445 Relative Dating Methods � A nonconformity is an erosional surface cut into igneous or metamorphic rocks and overlain by younger sedimentary rocks. Format ation of a ion of a Nonconfor onformit mity Fig. 17.10, p. 446 Fig. 17.10, p. 446 a. Formation of a nonconformity. Uplift and erosion A nonconformity in the making! Ayers Rock, Australia Deposition Nonconformity Uplift and erosion of overlying sediments Stepped Art Intrusion of magma Fig. 17-10a, p. 446 Geo-inSight 1-3, p. 456 5
Relative Dating Methods Relative Dating Methods � Applying the Principles of Relative Dating � The principles of relative Applying the dating can be used to principles of reconstruct the geologic history of an area. relative dating � Although no specific dates can be applied, the relative sequence of events can be determined by using the principles of relative dating. Fig. 17.11, p. 447 Fig. 17.12, p. 448 Correlating Rock Units Uplift, tilting, and Erosion � Correlation is the demonstration of equivalency of faulting rock units from one area to another. Sedimentary deposition � Time equivalence is usually demonstrated by the Sedimentary deposition occurrence of similar fossils in strata. Sedimentary deposition Intrusion Lava flow Uplift and erosion Stepped Art Fig. 17.6, p. 443 Intrusion Sedimentary deposition Fig. 17-12, p. 448 Correlating Rock Units Correlating Rock Units � Guide fossils (or index fossils) are fossils that: � Correlation of Rock Units � Are easily identified and geographically widespread on the Colorado Plateau � Lived for brief periods of geologic time . � Use of concurrent ranges of fossils is the most accurate method of using index fossils Fig. 17.14, p. 450 Fig. 17.16, p. 451 Fig. 17.15, p. 451 6
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