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EESA06H3 (234)
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Chapter 19&20

Chapter 19 & 20 Textbook Notes

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University of Toronto Scarborough
Environmental Science
Nick Eyles

Chapter 19: Time and geology What is Uniformitarianism? Uniformitarianism principle that geologic processes operating at the present are the same processes that operated in the past. The present is the key to the past (uniform rate) Actualism physical laws are independent of time and location (same meaning as uniformitar..) Geological Time Scale a sort of calander to which events and rock units can be referred Earth was only 6,000 years old biblical chronology, Noahs flood features we observed in rocks and landscapes where supernaturally and catastrophically James Hutton father of modern day geology Charles Lyell Principles of Geology book o geological features could be explained by present-day processes How can the Sequence of Past Geological events be determined? Numerical Age (absolute age) age given in years or some other unit of time Relative Time the sequence in which events took place (not measured in time units) ex: Grand Canyon can be analyzed in 4 parts: o horizontal layers of rock o inclined layers o rock underlying the inclined layers (plutonic + metamorphic rock) o the canyon itself, carved into these rocks Principles Used to Determine Relative Age Contacts boundary surface between two different rock types or ages of rocks Formations bodies of rock of considerable thickness with recognizable characteristics that make each distinguishable from adjacent rock units, named after local towns or landmarks Stratigraphy subdiscipline of geology, that uses interrelationships between layered rock or sediment to interpret the history of an area or region uses 4 principles: 1) Original Horizontality the disposition of most water-laid sediment in horizontal or near horizontal layers that are essentially parallel to the Earths surface 2) Superposition a principle or law stating that within a sequence of undisturbed sedimentary rocks, the oldest layers are on the bottom and the youngest on the top 3) Lateral Continuity principle that states that an original sedimentary layer extends laterally until it tapers or things at its edges 4) Cross-Cutting Relationships a principle or law stating that a disrupted pattern is older than the cause of disruption Other Time Relationships: Inclusion fragments included in a host rock are older than the host rock Unconformities - a surface that represents a break in the geological record, with the rock unit immediately above it being considerable younger than the rock beneath it 1) Disconformity a surface that represents missing rock strata, but beds above and below that surface are parallel to one another www.notesolution.com 2) Angular Unconformity an unconformity in which younger strata overlie an erosion surface on tilted or folded layered rock, implies the following sequence of events from oldest to youngest: o Deposition and lithification of sedimentary rock o Uplift accompanied by folding or tilting of layers o Erosion o Renewed deposition on top of eroded surface 3) Nonconformity an unconformity in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock o Crystallization of igneous or metamorphic rock at depth o Erosion of a least several kilometers of overlying rock (the great amount of erosion further implies considerable uplift of this portion of Earths crust) o Deposition of new sediment, thick eventually becomes sedimentary rock, on the ancient erosion surface How can rock units be traced from one area to another? Correlation determining time equivalency of rock units, rock units may be correlated within a region, a continent, and even between countries Physical Continuity being able to physically (visually) follow a rock unity between two places Similarity of Rock Types same sequence of rocks Tillites glacially deposited sedimentary rock Key bed a very distinctive layer Correlation by Fossils Paleontologist specializes in the study of fossils Faunal succession (William Smith) a principle or law stating that fossil species succeed one another in a definite and recognizable order; in general, fossils in progressively older rock show increasingly greater differences from species living at present Index fossil a fossil from a very short-lived, geographically widespread species known to exists during a specific period of geological time Fossil assemblage various different species of fossils in a rock layer How do we use Relative Dating to Understand Geological Time? The Standard Geologic Time Scale a worldwide relative time scale based on fossil assemblages Relative time scale: 3 eras which are subdivided into periods which are subdivided into epochs Precambrian the vast amount of time that preceded the Paleozoic Era Paleozoic Era (old life) began with the appearance of complex life, as indicated by fossils Mesozoic Era (middle life) dinosaurs became the dominant animals Cenozoic Era (new life) o Quaternary Period the youngest geologic period, includes present time Pleistocene Epoch an epoch of the Quaternary Period characterized by several glacial ages www.notesolution.com
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