Introduction The Grand Canyon - Major John Wesley Powell, in 1869, led a group of explorers down the Colorado River Chapter 17 Powell returned to map the region. Powell was impressed with the geologic strata and thus began an investigation that Geologic Time & continues today into the immense amount of Geohistory: geologic time presented in the canyon. It is this vastness of geologic time that sets Concepts and 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, in almost any profession or discipline. sequential order. 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 established by the James Ussher, in the early 1600’s asserted that God work of many created Earth on Sunday, October 23, 4004 B.C. geologists applying the principles of historical geology and Many early Christians analyzed historical records and correlation to strata of genealogies found in the scripture to try and determine the age all ages throughout of the Earth. the world. During the 18th and 19th centuries, attempts were made to 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 made by naturalists during the 18th and 19th centuries. conclusions. 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 suggested that present day processes operating over This principle holds that the laws of nature have long periods of time could explain all geologic features. been constant through time and His observations were instrumental in establishing That the same processes operating today have the principle of uniformitarianism and the fact that operated in the past, although not necessarily at the Earth was much older than earlier scientists same rates. thought. Relative Dating Methods Relative Dating Methods Before the development of radiometric dating, there Fundamental Principles of Relative Dating was no reliable method for absolute dating, Besides uniformitarianism, several principles were therefore relative dating methods were used. developed for relative dating: Relative dating places events in sequential order 1. Superposition but does not tell us how long ago an event took 2. Original horizontality place. 3. Cross-cutting relationships 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 thins and pinches out or terminates against the Based on detailed studies by James Hutton, Hutton edge of a depositional basin. recognized that Ash beds make excellent correlation markers! an igneous intrusion must be younger than the rock it intrudes. Also, faults must be younger than the rocks they displace. Fig. 17.3, p. 439 Fig. 17.13, p. 449 4. Cross-cutting relationships 5. Principle of Inclusions Differentiating between a buried lava flow and a sill Inclusions in a rock are older than the 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 sequence must be older than those at the top of the Unconformities may result sequence. from nondeposition and/or 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 0 0 Relative Dating Methods 1 1 2 2 Unconformities Three types of unconformities are recognized. 3 3 Unconformity 6 4 7 Amount Hiatus of rock A disconformity separates younger from older 5 removed 8 by erosion sedimentary strata that are parallel to each other. 6 9 An angular unconformity is an erosional surface 7 10 on tilted or folded rocks, over which younger sedimentary rocks were deposited. 8 11 A nonconformity is an erosional surface cut into 9 12 igneous or metamorphic rocks and overlain by 10 younger sedimentary rocks. 11 12 Stepped Art Fig. 17-7, p. 443 Relative Dating Methods Formation of a A disconformity separates younger from older Disconformity sedimentary strata that are parallel to each other. Fig. 17.8b, p. 444 Fig. 17.8, p. 444 Up lift an Relative Dating Methods d ero sio n 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 Angular unconformity Deposition Erosion Stepped Art Fig. 17-9a, p. 445 Uplift and tilting Formation of an Deposition Angular Unconformity Fig. 17.9, p. 445 4 Formation of a Nonconformity Fig. 17.10, p. 446 is an erosional surface cut into Relative Dating Methods A nonconformity igneous or metamorphic rocks and overlain by younger sedimentary rocks. Fig. 17.10, p. 446 A nonconformityAyers Rock, in Australia the making! a. Formation of a nonconformity. Geo-inSight 1-3, p. 456 Uplift and erosion 5 Nonconformity Deposition Stepped Art Fig. 17-10a, p. 446 Uplift and erosion of overlying sediments Intrusion of magma Relative Dating Methods Relative Dating Methods Applying the Principles of Relative Dating The principles of relative Applying the dating can be used to reconstruct the geologic principles of history of an area. Although no specific relative dating 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 faulting Erosion Correlation is the demonstration of equivalency of Sedimentary deposition rock units from one area to another. Sedimentary deposition Time equivalence is usually demonstrated by the occurrence of similar fossils in strata. Sedimentary deposition Intrusion Lava flow Uplift and erosion Intrusion Sedimentary deposition Stepped Art Fig. 17.6, p. 443 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. 17.16, p. 451 Fig. 17.15, p. 451 Fig. 17.14, p. 450 6 Correlating Rock Units Absolute Dating Methods Subsurface Correlation - Identifying rock properties thru well cuttings, electrical resistivity logs Radioactivity was discovered during the late and radioactivity logs and seismic profiles 19th century by Marie and Philip Curie These techniques are widely used to correlate Soon after the discovery of radioactivity, subsurface units. geologists used radioactive isotope decay to develop a method for determining absolute ages of rocks. Fig. 17.17, p. 451 Fig. 17.18a, p. 452 Absolute Dating Methods Absolute Dating Methods Atoms, Elements, and Isotopes Radioactive Decay All matter is made up of chemical elements and Half-Lives Atoms are the smallest units of matter that retain Radioactive decay is the process the characteristics of an element. in which an unstable atomic An element is a substance composed of atoms nucleus is spontaneously that all have the same properties. transformed into an atomic Isotopes of an element behave the same nucleus of a different element. chemically but have different atomic mass The decay rate of unstable numbers. Some isotopes are radioactive and are isotopes to determine absolute useful for radiometric dating. ages of rocks Fig. 17.18, p. 452 Fig. 17.18b, p. 452 Fig. 17.19, p. 453 Parent Alpha Daughter Changes in atomic number Parent Daughter nucleus particle nucleus and atomic mass number nucleus Beta nucleus particle Atomic number = –2 Atomic number = +1 Atomic mass number = –4 Atomic mass number = 0 Proton Neutron Electron Proton Neutron Electron Fig. 17-18a, p. 452 Fig. 17-18b, p.