Geologic Time ChapterThe Key to the PastRelative TimePrinciples Used to Determine Relative AgeOriginal HorizontalitySuperpositionLateral ContinuityCross-cutting RelationshipsOther Time RelationshipsCorrelationPhysical ContinuitySimilarity of Rock TypesCorrelation by FossilsThe Standard Geologic Time ScaleUnconformityDisconformitiesAngular unconformitiesNonconformitiesNumerical AgeIsotopic DatingIsotopes and Radioactive DecayRadiocarbon DatingUse of Isotopic DatingHow Reliable is Isotopic Dating?Geologic Time ChapterPrepared by Iggy Isiorho forDr. IsiorhoTime and GeologyThe Key to the PastUniformitarianism – Principle that geologic processes operating at present are the same processes that operated in the past. The principle is stated more succinctly as “The present is the key to the past.”Actualism – The principle that the same processes and natural laws that operated in the past are those we can actually observe or infer from observations as operating at present. Under present usage, uniformit arianism has the same meaning as actualism for most geologists.Numerical 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). Relative TimeThe geology of an area may seem, at first glance, to be hopelessly complex. A non-geologist might think it impossible to decipher the sequence of events that created such a geologic pattern. However, a geologist has learned to approach seemingly formidable problems by breaking them down to a number of simple problems. As an example, the geology of Grand Canyon can be analyzed in four parts: (1) horizontal layers of rock; (2) inclined layers; (3) rock underlying the inclined layers (plutonic and metamorphic rock); and (4) the canyon itself, care into these rocks. Principles Used to Determine Relative AgeOriginal HorizontalitySuperpositionLateral continuityCross-cutting relationships Original HorizontalityOriginal Horizontality – The deposition of most water-laid sediment in horizontal or near-horizontal layers that are essentially parallel to the earth’s surface.BackSuperpositionSuperposition – A principle or law stating that within a sequence of undisturbed sedimentary rocks, the oldest layers are on the bottom, the youngest on the top.BackLateral ContinuityLateral Continuity – Principle that states that an original sedimentary layer extends laterally until it tapers or thins at its edge.BackCross-cutting RelationshipsCross-cutting Relationships – A principle or law stating that a disrupted pattern is older than the cause of disruption.BackOther Time RelationshipsInclusions – A fragment of rock that is distinct from the body of igneous rock in which it is enclosed. CorrelationIn geology correlation usually means determining time equivalency of rock units. Rock units may be correlated within a region, a continent, and even between continents. Various methods of correlation are described below along with examples of how the principles we described earlier in this chapter are used to determine whether rocks in one area are older or younger than rocks in another area. Physical ContinuityFinding physical continuity – that is, being able to trace physically the course of a rock unit – is one way to correlate rocks between two different places. In Fig. 8.13, the prominent white layer of cliff-forming rock, the Coconino Sandstone, exposed along the upper part of Grand Canyon can be seen all the way across the photograph. You can physically follow this unit for several tens of kilometers, thus verifying that, wherever it is exposed in Grand Canyon, it is the same rock unit. Similarity of Rock TypesUnder some circumstances, correlation between two regions can be made by assuming that similar rock types in two regions formed at the same time. This method must be used with extreme caution, especially if the rocks being correlated are common ones.Correlation by similarity of rock types is more reliable if a very unusual sequence of rocks is involved. If you find in one area a layer of green shale on top of a red sandstone that, in turn, overlies basalt of a former lava flow, and then find the same sequence in another area, you probably would be correct in concluding that the two sequences formed at essentially the same time.When the hypothesis of continental drift was first proposed (see Chapter 1) important evidence was provided by correlating a sequence of rocks (Fig. 8.16) consisting of glacially deposited sedimentary rock. Correlation by FossilsFaunal succession – 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 species known to have existed during a specific period of geologic time.Fossil assemblage – Various different species of fossils in a rock. The Standard Geologic Time ScaleStandard Geologic Time Scale – A worldwide relative scale of geologic time divisions.The geologic time scale, representing an extensive fossil record, consists of three eras, which are subdivided into periods, which are, in turn divided into epochs. (Remember that this is a relative time scale.) UnconformityUnconformity – A surface that represents a break in the geologic record, with the rock unit immediately above it being considerably younger than the rock beneath.3 types of unconformity:DisconformitiesAngular unconformities Nonconformities DisconformitiesDisconformities – A surface that represents missing rock strata but beds above and below that surface are parallel to one another.BackAngular unconformitiesAngular unconformities – An unconformity in which younger strata overlie an erosion surface on tilted or folded layered rock.BackNonconformitiesNonconformities – An unconformity in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock.BackNumerical AgeCounting annual growth rings in a tree trunk will tell you how old a tree is. Similarly, layers of sediment deposited annually in glacial lakes can be counted to determine how long