William E. Ferguson. Geologic Time A major difference between geologists and most other scientists is their attitude about time. A "long" time may not.
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Geologic Time A major difference between geologists and most other scientists is their attitude about time. A "long" time may not be important unless it is > 1 million years.
Amount of Time Required for Some Geologic Processes and Events
Some geologic processes can be documented using historical records (brown area is new land from 1887-1988)
Uniformitarianism The present is the key to the past. — James Hutton Natural laws do not change— however, rates and intensity of processes may.
Two ways to date geologic events 1 RELATIVE DATING (relative position of fossils, structure, geomagnetics) 2 ABSOLUTE DATING (isotopic, tree rings, varves, etc.)
RELATIVE GEOLOGIC TIME Steno Laws (1669) developed to arrange rock units in time-order Principle of Superposition Principle of Original Horizontality Law of Cross -Cutting Relationships Law of Inclusions Laws apply to both sedimentary and volcanic rocks.
Principle of Superposition In a sequence of undisturbed layered rocks, the oldest rocks are on the bottom.
Jim Steinberg/Photo Researchers Oldest rocks Youngest rocks Principle of Superposition
Principle of Original Horizontality Layered strata are deposited horizontal or nearly horizontal or nearly parallel to the Earth’s surface.
Principles of original horizontality and superposition
Law of Cross-Cutting Relationships A rock or feature is younger than any rock or feature it cuts across.
LAW OF INCLUSIONS Included rocks are older than surrounding rocks.
PRINCIPLE OF FAUNAL SUCCESSION Principle of Faunal Succession - groups of fossil plants & animals have followed one another in a definite & discernable order so certain fossil assemblages characterize a specific time. INDEX FOSSILS - fossils used to correlate a specific time period Based on distinct preservable parts, lived a short time, in a specific environment with wide distribution - MICROFOSSILS
Ammonite FossilsPetrified Wood Chip Clark Tom Bean
CORRELATION Process used to tie separated strata together Based on matching physical or fossil features such as –Physical continuity - trace of rock unit –Similar rock types - marker beds, coal seams, rare minerals, odd color –Fossils
The Geologic Time Scale Divisions in the worldwide stratigraphic column based on variations in preserved fossils Built using a combination of stratigraphic relationships, cross- cutting relationships, and absolute (isotopic) ages
dinosaurs in plants, fish animals with skeletons dinosaurs out ice ages oldest Emperor seamount, ~80 Ma Kaua‘i
Absolute geochronology Adds numbers to the stratigraphic column based on fossils. Based on the regular radioactive decay of some chemical elements.
Isotopic dating Radioactive elements (parents) decay to nonradioactive (stable) elements (daughters). The rate at which this decay occurs is constant and knowable. Therefore, if we know the rate of decay and the amount present of parent and daughter, we can calculate how long this reaction has been proceeding.
Isotopes Different forms of the same element containing the same number of protons, but varying numbers of neutrons. i.e. i.e.: 235 U, 238 U 87 Sr, 86 Sr 14 C, 12 C
Half-life The half-life of a radioactive isotope is defined as the time required for half of it to decay.
Proportion of Parent Atoms Remaining as a Function of Time
Geologically Useful Decay Schemes ParentDaughterHalf-life (years) 235 U 207 Pb4.5 x 10 9 238 U 206 Pb0.71 x 10 9 40 K 40 Ar1.25 x 10 9 87 Rb 87 Sr47 x 10 9 14 C 14 N5730
PROBLEMS NEED A CLOSED SYSTEM!!! –MINERAL MAY LEAK PARENT OR DAUGHTER –MINERAL MAY BE CONTAMINATED WITH EITHER PARENT OR DAUGHTER
Another Clock Paleomagnetism Earth’s magnetic field reverses every half million years Reversals are recorded in rocks that are forming at that time - seafloor Time scale calibrated by both relative & absolute time methods