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Lecture Outlines Physical Geology, 14/e
Plummer, Carlson & Hammersley Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Time and Geology Physical Geology 14/e, Chapter 8
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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“The present is the key to the past”
Age of earth Prior to the 19th century, accepted age of Earth was based on religious beliefs Biblical: ~6,000 years for Western culture Chinese/Hindu: Old beyond comprehension James Hutton: “Father of Geology” realized geologic processes require vast amounts of time Charles Lyell popularized Hutton’s concepts in book Principles of Geology “The present is the key to the past” James Hutton Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Relative vs. numerical age
Relative age - the order of events or objects, from oldest to youngest original horizontality superposition, lateral continuity, cross-cutting relationships, inclusions, unconformities, correlation of rock units and fossils Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Relative vs. numerical age
Numerical age - the age of events or objects, expressed as a number or numbers determined using radiometric dating (determining how much radioactive decay of a specific element has occurred since a rock formed or an event occurred) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Relative age determination
Contacts - surfaces separating successive rock layers (beds) Formations - bodies of rock of considerable thickness with recognizable characteristics allowing them to be distinguished from adjacent rock layers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Relative age determination
Original horizontality - beds of sediment deposited in water are initially formed as horizontal or nearly horizontal layers Superposition - within an undisturbed sequence of sedimentary or volcanic rocks, layers get younger from bottom to top Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Relative age determination
Lateral continuity - original horizontal layer extends laterally until it tapers or thins at its edges Cross-cutting relationships - a disrupted pattern is older than the cause of the disruption Intrusions and faults are younger than the rocks they cut through Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Relative age determination
Baked contacts - contacts between igneous intrusions and surrounding rocks, where surrounding rocks have experienced contact metamorphism Inclusions - fragments embedded in host rock are older than the host rock Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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unconformities Unconformity: a surface (or contact) that represents a gap in the geologic record Disconformity - an unconformity in which the contact representing missing rock layers separates beds that are parallel to each other Angular unconformity - an unconformity in which the contact separates overlying younger layers from eroded tilted or folder layers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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unconformities Nonconformity - an unconformity in which an erosional surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock plutonic and metamorphic rocks exposed by large amounts of erosion typically represents a large gap in the geologic record Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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correlation The determination of the time-equivalency of rock units
Physical continuity—physically tracing a continuous exposure of a rock unit Similarity of rock types assumes similar sequences of rocks formed at same time can be inaccurate if very common rock types are involved Correlation by fossils fossil species succeed one another through the layers in a predictable order Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Geologic time scale Worldwide relative time scale
Subdivides geologic time based on fossil assemblages Divided into eons, eras, periods, and epochs Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Geologic time scale Precambrian Paleozoic era - Old Life
represents 87% of geologic time Paleozoic era - Old Life appearance of complex life Mesozoic era - Middle Life dinosaurs abundant era ended by mass extinction Cenozoic era - New Life mammals and birds abundant most recent ice ages occurred during the Pleistocene Epoch of the Quaternary Period Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Numerical age dating Numerical dating - puts absolute values (e.g., millions of years) on the ages of rocks and geologic time periods uses radioactive decay of unstable isotopes only possible since radioactivity was discovered in 1896 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Numerical Age Dating Half-life - time it takes for a given amount of radioactive isotope to be reduced by half Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Combining relative and Numerical ages
Radiometric dating gives numerical time brackets for events with known relative ages individual layers may be dated directly radiometric dating of units above and below brackets age of units in between Geologic Time Scale divided into four Eons Hadean Archean Proterozoic Phanerozoic Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Age of earth Numerical dating gives age for Earth of ~ 4.56 billion years oldest age obtained for meteorites, believed to have been unchanged since the formation of the solar system Earth and rest of solar system very likely formed at this time Geologic (deep) time is vast a long human lifetime represents only about % of geologic time Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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End of Chapter 8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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