Presentation on theme: "Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship."— Presentation transcript:
1 Geologic TimeGeochronology – the study of the timing of geologic events and the ages of geologic materialsRelative dating – the space/time relationship between two or more geologic events or unitsNumerical dating – absolute time or measured dates through geochemical analysis
2 Principles of Relative Age Dating Uniformitarianism – James Hutton – The present is the key to the past.Horizontality/Superposition – Layers were originally laid down horizontally with oldest at bottom/youngest at topCross-cutting relationships – any intrusive formation must be younger than the rocks it intrudes or cuts.Inclusions – must be older material picked up during intrusionFaunal succession – there is a direct link between the procession of fossils and time (e.g. once a plant/animal becomes extinct it is never seen again)
3 UnconformitiesGaps in geologic record forming boundaries that separate rocks of different ages.Non-conformity – (pg. 251) boundary between unlayered intrusion and overlying sedimentary rocksAngular unconformity – (Fig. 9.6) nonconforming boundary between horizontal rocks that are rotated and overlying rocks that were deposited horizontallyDisconformity – (pg. 251) boundary between parallel layers of sedimentary rocks
4 Great Unconformity of the GC (angular unconformity)
5 Great Unconformity of GC (nonconformity – Tapeats SS over crystalline basement)
7 Disconformities in GC (gaps between sedimentary layers)
8 CorrelationRocks that are of the equivalent age but are separated spatially (Fig.9.9)Fossil evidence – biota of same ageKey beds – short duration event over a wide area (Ft. Apache Lm. In Sedona)Facies change within a unit
10 Correlating Stratigraphic Sections Vertical measurement of rock unit thicknessSection is a hypothetical column of rock units for a particular areaMultiple sections can be correlated by drawing lines between similar or identical beds or unitsFacies change can be identified if type of rock between correlated units changes between measured sections
13 Numerical Dating Techniques Isotope dating – rate of decay for radioactive isotopes – uses known rates to measure the time elapsed since mineral formationHalf life – time it takes half the atoms of the parent isotope to decay into daughter isotopes (Fig.9.15)Typically more useful for igneous rocksMost reliable in closed systemLimited by timeUranium-thorium-lead – 10million to 4.6 billion only for oldest rocksPotassium-argon – 100,000 to 4.6 billion very old and very young datesRubidium-strontium – 10 million to 4.6 billion half-life only rocks > 10myCarbon 14 – good for < 100,000 yrs old
16 How does Carbon-14 dating work? Cosmic rays from the sun strike Nitrogen 1 4 atoms in the atmosphere and cause them to turn into radioactive Carbon 14, which combines with oxygen to form radioactive carbon dioxide.Living things are in equilibrium with the atmosphere, and the radioactive carbon dioxide is absorbed and used by plants. The radioactive carbon dioxide gets into the food chain and the carbon cycle.All living things contain a constant ratio of Carbon 14 to Carbon 12. (1 in a trillion).At death, Carbon 14 exchange ceases and any Carbon 14 in the tissues of the organism begins to decay to Nitrogen 14, and is not replenished by new C-14.The change in the Carbon 14 to Carbon 12 ratio is the basis for dating.The half-life is so short (5730 years) that this method can only be used on materials less than 70,000 years old. Archaeological dating uses this method.) Also useful for dating the Pleistocene Epoch (Ice Ages).Assumes that the rate of Carbon 14 production (and hence the amount of cosmic rays striking the Earth) has been constant (through the past 70,000 years).
17 Geologic Time Scale (Fig. 9.17) *Read Box 22.3 EonsEras –Cenozoic – Age of mammals, early primates, Ice Ages, man ( mya), spread of modern humans last 10,000yrs.Mesozoic, age of dinosaurs, early flowering plants, formation of the Rocky MountainsPaleozoic, - Mt. Building in N.A. and Eur., early land plants, coal forming swampsPrecambrian 4.6by Earth forms, ~1.5by Pangaea forms, .5by first multi-celled organisms
18 Periods – Breakdown of Eras PaleozoicPermianPennsylvanianMississippianDevonianSilurianOrdovicianCambrian
Your consent to our cookies if you continue to use this website.