Geology 101 Quiz 9

Question 1

two types of age determination

Answer 1

relative and absolute dating

Question 2

relative dating

Answer 2

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

Question 3

absolute dating

Answer 3

finding the age in years

Question 4

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

Answer 4

radioactivity; 20th

Question 5

principle of original horizontality

Answer 5

sedimentary layers, called strata, were deposited nearly horizontally

Question 6

principle of superposition

Answer 6

younger sediments are deposited on top of older sediments

Question 7

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

Answer 7

upper

Question 8

the order of the layers repeats itself

Answer 8

thrust fault

Question 9

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

Answer 9

overturned fold

Question 10

principle of original lateral continuity

Answer 10

identical layers on opposite sides of a valley were once continuous

Question 11

principle of inclusions

Answer 11

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

Question 12

principle of cross-cutting relationships

Answer 12

younger rock or features cut across older rocks or features

Question 13

uncomformity

Answer 13

an eroded surface

Question 14

index fossil

Answer 14

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

Question 15

use of index fossils

Answer 15

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

Question 16

index fossils appear out of order because of

Answer 16

alteration of the strata after deposition

Question 17

geologic column

Answer 17

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)

Question 18

absolute dating synonyms

Answer 18

isotopic dating, radiometric (measurements using radioactivity) dating

Question 19

radioactive isotopes are atoms that have

Answer 19

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)

Question 20

decay refers to

Answer 20

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)

Question 21

half life

Answer 21

the time required for half the radioactive atoms to decay

Question 22

is decay linear?

Answer 22

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

Question 23

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

Answer 23

more; shorter

Question 24

limits on radioactive dating

Answer 24

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

Question 25

ways to date sedimentary rocks

Answer 25

1. proximity to datable igneous rocks

2. use of index fossils

Question 26

proximity to datable igneous rocks to date sedimentary rocks

Answer 26

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

Question 27

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

Answer 27

within 11-15 million years old

Question 28

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

Answer 28

greater than 15 million years because sills intrude

Question 29

use of index fossils to date sedimentary rocks

Answer 29

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

Question 30

how do we know isotope dating methods are accurate?

Answer 30

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