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EESA06H3 (234)
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Chapter 19

Chapter 19 - Time and Geology.docx

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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, Noah’s 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 Earth’s 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 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 Earth’s 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 fossi
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