Geology 101 Quiz 9 Flashcards

1
Q

two types of age determination

A

relative and absolute dating

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

relative dating

A

finding the age relative to another object or event (younger or older)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

absolute dating

A

finding the age in years

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

absolute dating has only been possible since the discovery of ? at the start of the ? century

A

radioactivity; 20th

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

principle of original horizontality

A

sedimentary layers, called strata, were deposited nearly horizontally

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

principle of superposition

A

younger sediments are deposited on top of older sediments

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

in undisturbed strata, the ? layer will always be the youngest

A

upper

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

the order of the layers repeats itself

A

thrust fault

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

when a fold has been pushed over onto itself - strata repeat, but in the middle of the repeating layers, an additional repeated set is upside down

A

overturned fold

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

principle of original lateral continuity

A

identical layers on opposite sides of a valley were once continuous

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

principle of inclusions

A

if a rock contains fragments of rock from an adjacent unit, the adjacent unit must be older

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

principle of cross-cutting relationships

A

younger rock or features cut across older rocks or features

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

uncomformity

A

an eroded surface

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

index fossil

A

a fossil organism with widespread distribution that existed for a short time span

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

use of index fossils

A

only organisms that are widespread and with a relatively short time of existence are useful for relative dating

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

index fossils appear out of order because of

A

alteration of the strata after deposition

17
Q

geologic column

A

early geologists had no knowledge of the absolute age of various layers in the sequence, so they assigned names to each time interval (Jurassic, Cretaceous, Cambrian)

18
Q

absolute dating synonyms

A

isotopic dating, radiometric (measurements using radioactivity) dating

19
Q

radioactive isotopes are atoms that have

A

either too many or too few neutrons in the nucleus that results in an unstable arrangement (some arrangements are more unstable than others, resulting in faster rates of decay)

20
Q

decay refers to

A

a spontaneous change in the number of protons and neutrons in the nucleus of the atom by either ejecting some of them, or by converting a proton into a neutron or a neutron into a proton (a new element is created b/c # protons changes)

21
Q

half life

A

the time required for half the radioactive atoms to decay

22
Q

is decay linear?

A

no, this is why we use half lives - decay is exponential

23
Q

the more unstable the nucleus, the (less/more) radioactive the isotope and the (shorter/longer) the half life

A

more; shorter

24
Q

limits on radioactive dating

A
  1. rock must contain a detectable concentration of the isotope being used
  2. the age of the rock must be within about 6 to 12 half lives of the isotope being used (depending on the method)
  3. enough time must pass for daughter products to appear in measurable quantity
  4. pay attention to what is being dated
25
Q

ways to date sedimentary rocks

A
  1. proximity to datable igneous rocks

2. use of index fossils

26
Q

proximity to datable igneous rocks to date sedimentary rocks

A

if datable igneous rocks are associated with the sedimentary rock, it may be possible to bracket the possible age of the sedimentary rock, or at least assign an upper or lower age limit

27
Q

proximity example: if sandstone is between a two layers of ash 11 to 15 million years old, its age is likely

A

within 11-15 million years old

28
Q

proximity example: if sandstone is between two sills 11 to 15 million years old, its age is likely

A

greater than 15 million years because sills intrude

29
Q

use of index fossils to date sedimentary rocks

A

if one of the index fossils is found in sedimentary rocks, the absolute age (or at least a range) can be assigned to the rocks even if there are no ash deposits or other directly datable rocks

30
Q

how do we know isotope dating methods are accurate?

A
  1. radioactive decay rates have been shown to be constant under a wide range of temperatures, pressures, and chemical reactions
  2. multiple isotope methods, based on different sets of assumptions, typically give comparable results
  3. in some cases, non radioactive methods can be used to estimate an age that can then be compared with isotope methods