Dating Methods Flashcards
What is absolute dating?
Putting units or events into sequential order by saying one event is older or younger than another
How is radio metric dating carries out?
Naturally occurring radioactive isotopes are unstable, break down at statistically constant rate, decay measured as half life. If relative amounts of parent to daughter atoms in a rock is calculated we know how many half lives have occurred since radioactive atom was created
How are relative amounts of parent and daughter atoms calculated?
Measured using a mass spectrometer, new techniques being developed that use lasers or electron microprobes meaning rock doesn’t have to be crushed
Why is it difficult to use radio metric dating?
Radioactive isotopes only found in small quantities in some rock forming minerals, so only a few rocks can be dated radio metrically
What isotope of uridium is used and what does it decay to? What is the half life?
U238 to Pb206 4500Ma
U235 to Pb 207 710Ma
What isotope of potassium is used and what does it decay to?
K40 to Ar40 1260Ma
What isotope of rubidium is used and what doe it decay to?
Rb87 to Sr87 50 000Ma
Why might dates have a margin of error?
Due to equipment, contaminated samples or difference between individual minerals and whole rock dates
What problems can occur with dating sedimentary rocks?
Difficult as weathering and erosion break closes cycle, so loss of gases of daughter isotope, especially in glauconite where k is the only isotope
Sedimentary made up of fragments of older rocks, could be much older than rock being dated
What problems can occur with dating metamorphic rocks?
Different minerals closed system at different temperatures, parent or daughter atoms can be lost during heating events, so some might reset, giving conflicting dates.
What problems can occur with dating igneous rocks?
Large scale intrusions may take tens of millions of years to cool, different parts reach closure temperature at different times
Describe potassium argon dating
K40 to Ar40 half life of 1260Ma, also to Ca40 but not used as cannot be distinguished from calcium commonly occurring. Argon gas formed so may escape from rocks, leading to underestimate of age, to determine amount of gas, rock melted and isotopic composition of releases gas measures via mass spectroscopy
Describe rubidium to strontium dating
Rb87 to Sr87 half life 50,000Ma
Solid strontium so more useful for metamorphic rocks
What are the laws of stratigraphic Merida if dating?
Original horizontality Principle of superposition Way up criteria Includes fragments Cross cutting relationships Unconformities
How can fossils be used for relative dating of rocks?
Zone fossils, identification of fossils allows us to divide up geological record into divisions, based on their fossil content, Unknown rocks can be identified by fossil content, easier in rocks that had high rate of deposition, so less terrestrial, or before organisms with hard parts where common
What methods in the past where used in an attempt to calculate the age of the earth?
Salt content if oceans
Cooling rates
Rate of sedimentation
How was the geological column constructed?
Phranerozoic won divided into three eras, Mesozoic, Paleozoic, Cenozoic, mark drastic changes in fossil content by mass extinctions, 251Ma and 65Ma, further divided into periods on basis of important changes in fossil species
What is correlation?
Process of matching beds in geographically different area, often for economic processes such as coal and oil exploration. Mostly done via borehole data
What are the methods of biostratigraphic dating?
First or last appearance of a fossil
Range of a zone fossil and overlaps
Fossil assemblage
What are the problems with biostratigraphic correlation?
Fossils restricted to particular environment so only in some rock type
Some fossils long ranged
Some good zone fossils delicate so poorly preserved in some environments
Derived fossils may confuse sequence
Not all sedimentary rocks contain fossils
What are the methods of lithostratigraphic correlation?
Correlate a sequence of beds
Look at thickness of beds
Composition of beds
What are the problems with lithostratigraphic correlation?
Lateral variation due to:
Sedimentary deposits may change in thickness
Two different sedimentary rocks deposited at same time eg in river channel or delta
Diachronous beds due to cyclic sedimentation, one sediment type laid down at different times
What is chronostratigraphic correlation?
Matching events which may be possible over large areas
What events may provide evidence for chronostratigraphic correlation?
Tuffs from volcanic eruption
Using valves from glacial lakes
What is a zone fossil?
Range of fossil
What makes a good zone fossil?
Abundant Well preserved in many environments Widespread Easily identified Rapid evolution Commonly preserved Found in many rock types
What are corals a good zone fossil?
Widespread, rugoes and tabulate extinct at PT, Sclerectinian from Triassic, good environment indicators
Why are Brachipods good zone fossils?
Flourished in lower Paleozoic, long hinged mostly does out at PT, short hinged in Mesozoic, so long hinged indicates Paleozoic rocks
Why are Ammonoids a good zone fossil?
Nektonic so wide distribution, strong exoskeleton so well preserved, evolved rapidly, clearly distinguish between species, goniatites Devonian to Permian, ceratites in Permian to Triassic, then ammonites
Why are trilobites good zone fossils?
Variety of species, high in numbers, during Paleozoic, abundant in Ordovician
What are graptolites good zone fossils?
Early Ordivician to early Devonian(very rare) rapid evolution, clear changes and trends seen, abundant, nektonic so preserved in many Rick types, sclereoprotein so preserved below CDC
Why are Echinoids good zone fossils?
Used to distinguish between Paleozoic and younger rocks, rare in Cambrian until Carboniferous, irregular in lower Jurassic
