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Chapter 7

GLG110 Textbook Notes Exploring Geology 2nd Ed Chapter 7

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Earth Sciences

[GLG Text Notes Chapter 7] 1 Chapter 8: Geologic Time • Geologic events by themselves have little meaning until they are put into a time perspective; among geology’s major contributions to science are the geologic time scale and the discovery that Earth history is exceedingly long • Scientists were seeking a numerical date • Relative dating means that rocks are placed in their proper sequence of formation o The law of superposition: states that in an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below o Principle of original horizontality: means that layers of sediment are generally deposited in a horizontal position; rocks like this have not been disturbed o Principle of cross-cutting relations: when a fault cuts through other rocks, or when magma intrudes and crystallizes, we can assume that the fault or intrusion is younger than the rocks it affected o Inclusions: fragments of one rock unit that have been enclosed within anther o Unconformities: deposition of sediment has been interrupted over and over again; all such breaks in the rock record are termed this way; produced by erosion and/or non-deposition; in each case uplift and erosion has been followed by subsidence and renewed sedimentation  Angular unconformity: consists of tilted or folded sedimentary rocks that are overlain by younger, more flat-lying strata; indicates that a period of deformation and erosion has occurred  Disconformities: sometimes this method is difficult to apply since underlying layers may or may not be older or younger than the since the rocks are similar  Nonconformity: the break separates older metamorphic rock or intrusive igneous rocks from younger sedimentary strata; for this to develop, an episode of uplift and the erosion of overlying rocks must occur • To develop a geo time scale that is applicable to the entire Earth, rocks of similar age in different regions must be matched up; known as correlation [GLG Text Notes Chapter 7] 2 o Correlation by physical criteria—often difficult for large distances and also since rocks are mostly covered by soil and vegetation o Correlation by fossils—fossil organisms succeed one another in a definite and determinable order, and therefore any time interval can be recognized by its fossil content aka principle of fossil succession  Geologists pay particular attention to fossils called index fossils, which are limited to a short span of geo time but is widespread geographically; they are used as the base o Radioactivity: in some isotopes the nuclei are unstable because forces binding protons and neutrons are not strong enough thus th
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