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19 - Geologic time.docx

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Earth Sciences
Maureen Padden

19 – GEOLOGIC TIME UNIFORMITARIANISM  James Hutton’s principle  Geologic processes and natural laws from the past are those observed or inferred from observations as operating at present o Physical laws are independent of time and location  Geologists follow actualism which encompasses sudden geological events RELATIVE TIME  Sequence of events without reference to the age in years  Identifies characteristic formations: map-able rock units with distinctive mineralogical, structural, or textural characteristics  Contacts are used for deciphering time; contacts are surfaces separating two different rock types or rocks of diff. ages PRINCIPLES OF RELATIVE AGE Original Horizontality:  Beds of sediment deposited in water originally formed as horizontal layers  Geologic event – tilting of bed rock – occurs after (after deposition) Superposition:  In a sequence of sedimentary or volcanic rock layers, the layers are oldest near the bottom and youngest near the top Lateral Continuity:  Original sedimentary layer extends laterally until it tapers or thins at edges Cross-cutting Relationships:  A disrupted pattern is older than the cause of disruption (intrusion or fault) CORRELATION  Correlate local formations with those of other regions Technique #1: Correlation on the basis of physical characteristics  Correlation by tracing the bed o Spotting identical bedding; lateral continuity o Similar strata are not necessarily of the same age  Correlation of near-by units by physical similarities  Correlation of widely separated (km’s) units by physical similarities and similar sequence Technique #2: Stratigraphic Correlation o Principle of faunal succession (W. Smith): different sedimentary layers are characterized by distinctive fossil species and the fossil species succeed one another through the layers o Rocks with identical fossils are of identical age o Stratigraphic correlation tools: index fossils – fossil of short-lived, geographically widespread species known to exist during certain time interval o Allows from global rock-layer correlation o Note: evolution of organic life is the explanation for fossil records UNCONFORMITIES Type of contact - surface joining two or more layers – that represents gap in sedimentary/geologic record. This is b/c not all layers have been deposited continuously over time “Time” gap – missing rock – represented is measured by fossil (above and below) and radiometric dating (the absolute age) DISCONFORMITIES  The contact separates beds that are parallel to one another; there are layers in a sequence that are missing  Could be due to erosion followed by disposition ANGULAR UNCONFMITY  Younger strata overlie an erosion surface on tilted or folded layered  Rock undergoes: (1) deposition and lithification/solidifcation, (2) uplifted, (3) folding of layers, (4) erosion, (4) renewed
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