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DISP Earth Science > Geological Time > Flashcards

Geological Time Flashcards

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1
Q

What steps are taken to determine the age of the MIssissippi River Delta?

A

1) calculate volume of sediment that comes out (km cubed)
2) calculate average annual discharge (km cubed per year)
3) divide volume of sediment by average annual discharge - gives time it took for all the sediment to get there

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2
Q

What sort of measurement is relative dating?

A

qualitative

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3
Q

How does relative dating work, in basic terms?

A

compare two features and ask if one is older, younger, or the same age as any other geological feature

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4
Q

What are the main rules of relative geological time?

A

superposition, cross-cutting relationships, unconformity surfaces, and flaural/faunal succession (and original horizontality, lateral continuity, baked contacts, inclusions)

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5
Q

What is uniformitarianism?

A

principle that the physical processes we observe today also operated in the past at around the same rates and are responsible for the formation of geological features we see today

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6
Q

What is the principle of superposition?

A

layered rocks laid down one on top of another; older below younger - things are deposited in vertical sequence

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7
Q

What is the principle of cross-cutting relationships?

A

the geological feature that is cut is older than the feature that cut it

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8
Q

What do inclusions indicate about relative age?

A

the inclusion must be older than the geological feature that included it (it had to exist to be included)

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9
Q

What do instursive contacts indicate about relative age?

A

instrusion is younger than host rock (cross-cutting)

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10
Q

What do faults/folds indicate about relative age?

A

geological feature must be older than fault/fold - was originally deposited straight and continuously, so if now fractured/folded, fracturing/folding happened after

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11
Q

What is an example of an inclusion?

A

xenolith

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12
Q

What is an unconformity surface?

A

surface representing a period of nondeposition and possible erosion that represents a gap in the geologic record

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13
Q

Why do unconformity surfaces represent a gap in time?

A

because the sequence of strata have a relative time significance

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14
Q

What is a hiatus?

A

a gap in time

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15
Q

What are angular unconformities?

A

unconformity surface in which rocks below it were tilted/folded before development of unconformity; AU cuts across underlying layers, so orientations of layers above and below are different (tilted rock underneath; flat lying rock on top)

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16
Q

What do angular unconformity imply?

A

1) deposition of underlying rock
2) deformation of underlying rock
3) uplift and erosion ot flatten top
4) deposition of flat-lying sediment on top of the unconformity surface

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17
Q

What are the principles of faunal and floral sucession?

A

fossils vary with time; thus fossils used in rock layers of different age will be different

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18
Q

What are some indications of unconformities?

A

difference in orientation;
differences in dip;
gap in fossil succession;
pebbly layer of debris (from erosion)

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19
Q

How can faunal/floral sucession be sused to establish relative time scales over broad areas?

A

a particular group of fossil species can only be found in a limited interval of strata; when one species disappears at end of a strata, it never reappears; therefore can match up fossils over large areas

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20
Q

What are index fossils?

A

fossils of species that only existed for a short interval of geological time and are thus diagnostic of a particular time interval (like old cell phones = old movies)

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21
Q

What are the characteristics of index fossils?

A 
easily identified;
only lived for a very short time;
widely-distributed geographically;
widely-distributed environmentally;
abundant
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22
Q

Why was the geological time scale developed?

A

to correlate and standardize everyonne’s findings

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23
Q

What are the charactersitics of the geological time scale?

A

arbitrary, subjective, and utilitarian

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24
Q

What are the general divisions of the geological time scale, from most broad to smallest?

A

eon, era, period, (epoch)

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25
Q

How were the units of the geological time scale originally defined?

A

developed from well developed stratigraphy, paleontology, and structural relations (mostly in western Europe)

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26
Q

What are the eons of the geological time scale, from oldest to yongest?

A

precambrian, phanerozoic

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27
Q

What are the eras of the phanerozoic eon, from oldest to youngest?

A

paleozoic, mesozoic, cenozoic

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28
Q

What are the periods of the mesozoic era?

A

Triassic, Jurassic, Cretaceous

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29
Q

In what era didd the dinos live?

A

mesozoic (Triassic and Cretaceous)

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30
Q

What is the cenozoic era?

A

recent life

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31
Q

What is the mesozoic era?

A

middle life

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32
Q

What is the paleozoic era?

A

ancient life

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33
Q

In what era can we find fossils similar to those today?

A

cenozoic

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34
Q

What are the eras of the precambrian eon, from oldest to youngest?

A

archean, proterozoic

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35
Q

How much of the earth’s history is taken up by the precambrian eon?

A

7/8

36
Q

Are the units of the geological time scale equal in length?

A

no! (arbitrarily defined)

37
Q

How old was the oldest rock found?

A

over 4 billion years

38
Q

What is requried to determine the absolute age of something, and how is it done (in general)?

A 
a = rate of some process;
b = number of times this process has repeated since the even we want to date;
age = a/b
39
Q

What are the assumptions used in absolute dating?

A

ubiquity of process;
unvarying rate;
accurate measurement

40
Q

What are some examples of repeating events that can be used for absolute aging?

A

sedimentation (like Mississippi River Delta), radioactive decay

41
Q

What does dendochronolgoy do?

A

quantifies plant growth, using tree rings

42
Q

What limiting factors affect tree ring growth?

A

plant growth rate dependendent on most limiting factor;
plants at the edge of their range are the most sensitive;
cold winters prevent growth;
ring width is wider when treee growth is better

43
Q

How does the principle of uniformitarianism relate to dendochronology?

A

tree rings form today just like they did in the past

44
Q

In our climate, what does one tree ring represent and why?

A

1 ring = 1 year, since the freezing winter weather prevents growth

45
Q

Why do tree ring widths vary from year to year?

A

difference in climate - grow thicker rings in better climate years

46
Q

What does correlating sequences of tree rings allow?

A

extension of the chronology beyond the age of a single tree

47
Q

WHat trees are especially good for dendochronology and why?

A

Bristle Cone Pine;

average age 1000 years, so can correlate and get dendochronology for about 9000 years

48
Q

Why do Brislte Cone Pines last so long?

A

very resinous, so things don’t like to eat it, so not much decomposes it

49
Q

What are some characterstics of bristle cone pines?

A

evergreen, grow on limestone (with little else), nothing likes to eat it, old

50
Q

How can sediment accumulation be used for absolute dating?

A

sedimentation rates vary with seasons, forming annual sediment layers

51
Q

What are varves?

A

annual layer of sediment

52
Q

Why are varves formed - alternating light and dark bands?

A

organic productivity of lakes/seas varies with season; light bands form from spring runoff; dark from organic mateiral that grows over summer and dies in winter (lake bottom freezes over)

53
Q

What does a light and dark band of sediment together represent?

A

one year

54
Q

How do varves relate to glaciation?

A

in winter, more snow falls than dust; in summer, more dust - creates alternating bands

55
Q

How is C-14 created?

A

cosmic rays bombard N-14 in the upper atmosphere, changing a neutron to a proton that’s released

56
Q

Why does the ratio of C-14 to C-12 in living plants constant?

A

the ratio in the atmosphere is constant and they continuously respire atmospheric CO2, assimilating it into their structures

57
Q

How is carbon dating used?

A

1) living organism has constant ratio of C14/C12
2) when it dies, ratio decreases (since stop accumulating atmospheric CO2)
3) measure amount of C-14 remaining and use knowledge of its half life to calculate how many have happened

58
Q

How far back is carbon dating useful and why?

A

about 50 000 years - Carbon’s half life is 5730 years, so inssuficient C-14 remains after 10 half lives to be reliably detected (since already such a small percentage of world’s carbon)

59
Q

What has been done on bristle cone pine tree rings to create a calibration curve?

A

C-14 dating

60
Q

Why is the calibration curve between bristle cone pine rings and C-14 wiggly?

A

pollution in the atmosphere

61
Q

What does U-238 decay to?

A

Pb (through 14 intermediate isotopes)

62
Q

What are the general steps of radiometric dating?

A

1) collect rocks (mainly unweathered ones, to avoid isotope loss)
2) separate minerals in the rocks
3) extract parent and daughter isotopes
4) analyze parent-daughter ratio (using mass spectrometer)
5) calculate mineral age from ratio

63
Q

Why is U/Pb dating useful?

A

Pb can’t usually get into rocks except from decaying U (which is in feldspar, etc.), so the amount of Pb measured is proportional to the rock’s age

64
Q

What is mass spectrometry used for?

A

separating isotopes (using a magnet) and counting the atoms of each - used to measure the amounts of parent and daughter isotopes for radiometric dating

65
Q

How does mass spectrometry work?

A

sample chamber on left, detector on right, magmet deflects different isotopes different amounts

66
Q

What are some ways in which isotopes can be extracted from minerals?

A

dissolving in acid, evaporating portions with lasers, etc.

67
Q

What is the half life of K-40?

A

1 277 000 years

68
Q

How much of the world’s potassium is radioactive, and which isotope is radioactive?

A

0.012% K-40

69
Q

How much of the world’s carbon is radioactive, and which isotope is that?

A

1.3E-10 %

70
Q

What does K-40 decay to?

A

Ar-40

71
Q

What rock is potassium abundant in?

A

potassium-feldspar

72
Q

Why is K/Ar dating useful?

A

Ar (gas) in unlikely to be in the original rock (and unlikely to escape), so can measure the amount in the radiated unknown to determine how old the rock is, based on the original amount of K-40 in the sample

73
Q

For what ages does K/Ar dating work best?

A

a few million years to a little over a bililion years

74
Q

When does the “radiometric dating clock” reset?

A

when a rock is heated above its “blocking temperature” (~ 350 deg C for Ar; different for different minerals)

75
Q

Why is the “radiometric dating clock” reset at high temperatures?

A

above the blocking temperature, isotopes in a mineral’s crystal lattice are free to move and therefore able to escape/enter crystals easily, so the parent-daughter ratios are meaningless

76
Q

What rocks are best dated with radiometric dating and why?

A

igneous rocks - only time they were heated above blocking etmperature was when the formed (cooling after hot igneous event), so radiometric age = age of rock

77
Q

What does radiometric dating age in metamorphic rocks?

A

date of metamorphism (last time rock was heated above blocking temperature)

78
Q

What does radiometric dating age in igneous rocks?

A

dates of cooling of hot igneous events (rapid cooling ater eruption or cooling of plutons after they crystallize)

79
Q

What does ratiometric dating age in sedimentary rocks?

A

date of the source rock (when the mineral sediments crystallized) NOT the date of sedimentation

80
Q

Why are dino bones trapped in sandstone hard to age?

A

radiometric dating doesn’t give age of sandstone’s sedimentation, so sandstone provides no clues to dino’s age (unless there’s volcanic ash from an eruption and nearby volcanic rock to date)

81
Q

What is the most common dating method between 5 and 1000 million years (5 Ma - 1 Ga)?

A

K/Ar dating

82
Q

What is the most common dating method back 50 000 years?

A

carbon dating

83
Q

What minerals are usually dated using K/Ar dating?

A

K-bearing minerals in igenous rocks (plutons, lava flows, ash beds…)

84
Q

What is used to date things between 50 000 years and 5 million years?

A

no universally-accepted metthod

85
Q

How can relative and absolute dating be correlated?

A

use igneous rocks to calibrate the relative ages of sedimentar rocks - in a region, determine relative ages of features (sedimentary rocks, intrusions, etc.) then compare to known absolute ages of certain features (lava flows, plutons, etc.) to get age ranges for things that can’t be absolutely dated (sedimentary rocks, etc.)

DISP Earth Science (7 decks)

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