Dating Methods

Question 1

What is absolute dating?

Answer 1

Putting units or events into sequential order by saying one event is older or younger than another

Question 2

How is radio metric dating carries out?

Answer 2

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

Question 3

How are relative amounts of parent and daughter atoms calculated?

Answer 3

Measured using a mass spectrometer, new techniques being developed that use lasers or electron microprobes meaning rock doesn’t have to be crushed

Question 4

Why is it difficult to use radio metric dating?

Answer 4

Radioactive isotopes only found in small quantities in some rock forming minerals, so only a few rocks can be dated radio metrically

Question 5

What isotope of uridium is used and what does it decay to? What is the half life?

Answer 5

U238 to Pb206 4500Ma

U235 to Pb 207 710Ma

Question 6

What isotope of potassium is used and what does it decay to?

Answer 6

K40 to Ar40 1260Ma

Question 7

What isotope of rubidium is used and what doe it decay to?

Answer 7

Rb87 to Sr87 50 000Ma

Question 8

Why might dates have a margin of error?

Answer 8

Due to equipment, contaminated samples or difference between individual minerals and whole rock dates

Question 9

What problems can occur with dating sedimentary rocks?

Answer 9

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

Question 10

What problems can occur with dating metamorphic rocks?

Answer 10

Different minerals closed system at different temperatures, parent or daughter atoms can be lost during heating events, so some might reset, giving conflicting dates.

Question 11

What problems can occur with dating igneous rocks?

Answer 11

Large scale intrusions may take tens of millions of years to cool, different parts reach closure temperature at different times

Question 12

Describe potassium argon dating

Answer 12

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

Question 13

Describe rubidium to strontium dating

Answer 13

Rb87 to Sr87 half life 50,000Ma

Solid strontium so more useful for metamorphic rocks

Question 14

What are the laws of stratigraphic Merida if dating?

Answer 14

Original horizontality
Principle of superposition
Way up criteria 
Includes fragments 
Cross cutting relationships 
Unconformities

Question 15

How can fossils be used for relative dating of rocks?

Answer 15

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

Question 16

What methods in the past where used in an attempt to calculate the age of the earth?

Answer 16

Salt content if oceans
Cooling rates
Rate of sedimentation

Question 17

How was the geological column constructed?

Answer 17

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

Question 18

What is correlation?

Answer 18

Process of matching beds in geographically different area, often for economic processes such as coal and oil exploration. Mostly done via borehole data

Question 19

What are the methods of biostratigraphic dating?

Answer 19

First or last appearance of a fossil
Range of a zone fossil and overlaps
Fossil assemblage

Question 20

What are the problems with biostratigraphic correlation?

Answer 20

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

Question 21

What are the methods of lithostratigraphic correlation?

Answer 21

Correlate a sequence of beds
Look at thickness of beds
Composition of beds

Question 22

What are the problems with lithostratigraphic correlation?

Answer 22

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

Question 23

What is chronostratigraphic correlation?

Answer 23

Matching events which may be possible over large areas

Question 24

What events may provide evidence for chronostratigraphic correlation?

Answer 24

Tuffs from volcanic eruption

Using valves from glacial lakes

Question 25

What is a zone fossil?

Answer 25

Range of fossil

Question 26

What makes a good zone fossil?

Answer 26

Abundant
Well preserved in many environments
Widespread
Easily identified 
Rapid evolution
Commonly preserved 
Found in many rock types

Question 27

What are corals a good zone fossil?

Answer 27

Widespread, rugoes and tabulate extinct at PT, Sclerectinian from Triassic, good environment indicators

Question 28

Why are Brachipods good zone fossils?

Answer 28

Flourished in lower Paleozoic, long hinged mostly does out at PT, short hinged in Mesozoic, so long hinged indicates Paleozoic rocks

Question 29

Why are Ammonoids a good zone fossil?

Answer 29

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

Question 30

Why are trilobites good zone fossils?

Answer 30

Variety of species, high in numbers, during Paleozoic, abundant in Ordovician

Question 31

What are graptolites good zone fossils?

Answer 31

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

Question 32

Why are Echinoids good zone fossils?

Answer 32

Used to distinguish between Paleozoic and younger rocks, rare in Cambrian until Carboniferous, irregular in lower Jurassic