Deck 16: Geologic Time: Concepts and Principles

Question 1

How were absolute ages added to the geologic time scale in the 20^th century?

Answer 1

Thousands of absolute ages were added to the relative time scale that had been generated. Since sedimentary rocks cannot be radiometrically dated, interbedded volcanic rocks and igneous intrusions that could be dated provided information that was useful in establishing boundaries between the subdivisions in the geologic time scale.

Question 2

If you start with an element with 1,000,000 parent atoms, how many parent (P) and how many daughter (D) atoms will there be after 6 half-lives?

Answer 2

0: P = 1,000,000, D = 0;
1 half-life: P = 500,000, D = 500,000;
2 half-lives: P = 250,000, D = 750,000;
3 half-lives: P = 125,000, D = 875,000;
4 half-lives: P = 62,500, D = 937,500;
5 half-lives: P = 31,250; D = 968,750;
6 half-lives: P = 15,625, D = 984,315.

Question 3

What is carbon-14 dating useful for in terms of age and what type of sample?

Answer 3

Carbon 14 has a half-life of 5,730 years when it decays to carbon 12. Its short half-life makes it useful for specimens younger than 70,000 years, and it is generally used to date former living material. Carbon 14 is useful in archeology rather than geology and is crucial to understanding the events of the latter portion of the Pleistocene Epoch (Ice Age).

Question 4

How was the geologic time scale developed in the 19^th century?

Question 5

Why is correlation needed to construct a complete geologic history of a region? How does correlation work, e.g., in the Colorado Plateau?

Question 6

Why do igneous rocks yield the most accurate radiometric ages, what does the calculated numerical date represent?

Question 7

What is relative dating, and what can it tell us?

Question 8

Why are metamorphic rocks difficult to date accurately?

Question 9

What is the principle of uniformitarianism? Why does the principle of uniformitarianism lead to the conclusion that Earth is very old in age?

Question 10

What is the principle of superposition, and what is its importance? Who is credited with discovering it?

Question 11

What is the principle of fossil succession, and what is its importance? Who is credited with discovering it?

Question 12

What other attributes do ashfalls provide in serving as excellent marker beds?

Question 13

How do stalagmites enter into the debate on global warming?

Question 14

What is a closed system, and why is it necessary for a rock or mineral being dated to be a closed system? What is the result if it is not a closed system?

Question 15

What are the three types of unconformities, and what is each missing?

Question 16

What were some of the ways early scholars tried to determine the age of Earth?

Question 17

How is correlation accomplished using rocks and fossils?

Question 18

What is absolute dating, and what is it based on? What can absolute dating tell us?

Question 19

What do unconformities represent?

Question 20

What is the importance of a marker bed?

Question 21

What is the geologic time scale today based on compared to the past?

Question 22

What is the principle of lateral continuity?

Question 23

How does relative dating differ from absolute dating?

Question 24

What fact makes radiocarbon dating possible?

Question 25

What is the principle of inclusions?

Question 26

What is the principle of original horizontality?

Question 27

What are the six fundamental geologic principles that are used in relative dating?

Question 28

What criteria are used to correlate rock units in different areas?

Question 29

How are oxygen isotopes used to determine ancient climate?

Question 30

What are the requirements for a useful guide fossil?

Question 31

A clast of granite taken from a conglomerate is determined by radiometric dating to be one hundred million years old. What, if anything, does this tell you about the age of the conglomerate?

Question 32

How are radioactive elements different from stable elements?

Question 33

How does the sequence of events differ between an angular unconformity and a disconformity?

Question 34

What sort of instrument is used to determine the parent-daughter ratio of isotopes, and what does it measure?

Question 35

What is the principle of cross-cutting relationships?

Question 36

Explain why correlation by fossils is necessary to demonstrate time equivalence of strata.

Question 37

What does half-life measure?

Question 38

What are the three types of radioactive decay?

Question 39

Which of the major rock types were most important originally in developing the geologic time scale, and why?

Question 40

How are marker beds used to determine the age of strata in a region that has no marker beds but does contain strata with the same fossil assemblage zone?

Question 41

Different varieties of an atom that have a different number of neutrons in their nuclei are ____________________.

Question 42

When a stalagmite has an oxygen-18 to ____________________ ratio that is low, the climate at the time the stalagmite formed was relatively ____________________

Question 43

Fragments of a rock that are contained in a layer of another rock must be ____________________ in age than the layer of rock that surrounds them.

Question 44

A body of rock that is intruded by another rock must be ____________________ in age than the intrusion.

Question 45

The principle of original horizontality states that ____________________ rocks were deposited in horizontal layers. Thus, if rock layers are now ____________________, they must have been disturbed after lithification.

Question 46

In order to correlate age-equivalent rock units of dissimilar composition or to correlate rock units over a large area, ____________________ and the principle of ____________________ are used.

Question 47

Carbon 14 is useful for archeological purposes because it can effectively date samples back to approximately ____________________ years.

Question 48

The time necessary for half of the atoms of the unstable parent element to decay to the new, more stable daughter element is the isotope's ____________________.

Question 49

A sedimentary rock that shows evidence of contact metamorphism must be ____________________ in age than the igneous rock with which it is in contact, even if it lies above the igneous rock.

Question 50

After one half-life, ____________________% of the radioactive parent atoms will have decayed to stable daughter atoms.

Question 51

A surface of discontinuity representing a break in the depositional record; a discontinuity representing erosion or nondeposition is a(n) ____________________.

Question 52

Carbon-14 dating is based on the ratio of the parent isotope ____________________ to the daughter isotope ____________________.

Question 53

____________________ dating is used to obtain absolute age dates.

Question 54

An interval of geologic time not represented by strata in a particular area is a(n) ____________________.

Question 55

Long-lived radioactive isotope pairs used in radiometric dating have half-lives of the magnitude of ____________________ or ____________________ of years.

Question 56

The principle of ____________________ states that rock layers extend laterally in all directions unless or until they terminate in one of a number of natural means.

Question 57

The most accurate radiometric dates are obtained from ____________________ rocks. Only in very unusual circumstances can ____________________ rocks be radiometrically dated.

Question 58

The placement of events into a sequential order based upon their position in the rock record is referred to as ____________________ dating.

Question 59

Prior to the discovery of radioactivity in the late ____________________ century, geologists were able to develop a(n) ____________________ geologic time scale.

Question 60

The half-life of a radioactive element is the amount of time it takes for ____________________ of the atoms of the unstable ____________________ element to decay to atoms of a new, more stable ____________________ element.

Question 61

Which of the following was not among the early scientific attempts to determine Earth's age?

A) the cooling rate of a molten iron sphere of Earth's diameter
B) the rate of sediment deposition
C) the rate at which water precipitation accumulated in the ocean
D) the geologic principles and the inferred rates of change necessary to explain the fossil record
E) the rate of salt accumulation in the ocean

Question 62

Potassium 40 decays to argon 40 by a single electron capture.

Question 63

The time equivalence of strata is determined by:

A) correlation of unusual lithologies.
B) correlation of key beds.
C) correlation of fossils.
D) lateral tracing.
E) superposition.

Question 64

The loss of an electron from a neutron in the nucleus is called:

A) alpha decay.
B) beta decay.
C) electron decay.
D) electron capture.
E) half-life decay.

Question 65

The loss of two protons and two neutrons from the nucleus is:

A) alpha decay.
B) beta decay.
C) electron decay.
D) electron capture.
E) half-life decay.

Question 66

To determine the radiometric age of a rock, scientists must be able to measure very accurately the:

A) unstable parent element.
B) unstable daughter element.
C) stable parent element.
D) stable daughter element.
E) unstable parent element and stable daughter element

Question 67

Whose work was the basis for the principle of uniformitarianism and the unavoidable necessity of a great age for Earth?

A) Charles Lyell
B) William Smith
C) James Hutton
D) Lord Kelvin
E) John Joly

Question 68

The principle that states that an igneous intrusion or a fault is younger than the rocks that it intrudes or cuts is the principle of:

A) cross-cutting relationships.
B) inclusions.
C) fossil succession.
D) uniformitarianism.
E) superposition.

Question 69

When radiometric dating is applied to metamorphic rocks, the date obtained is:

A) the date of final crystallization of the magma.
B) the date at which metamorphism began.
C) a minimum age for the parent rock.
D) the date at which metamorphism ceased.
E) a maximum age for the daughter rock.

Question 70

James Hutton concluded that Earth must be very old and wrote that "we find no vestige of a beginning, and no prospect of an end."

Question 71

The age of the Earth is:

A) 12.6 billion years.
B) 12.6 million years.
C) 4.6 billion years.
D) 4.6 million years.
E) 10,000 years.

Question 72

A fossil that is particularly useful for correlation is an evolutionary fossil.

Question 73

Fossils are found almost exclusively in sedimentary rocks.

Question 74

Sources of uncertainty in radioactive dating do not include:

A) loss of daughter isotope.
B) uncertainty in decay constants.
C) experimental and sample error.
D) addition of parent isotope.
E) questions about metamorphism.

Question 75

A surface of erosion on metamorphic or igneous rocks below sedimentary rocks is a(n):

A) nonconformity.
B) disconformity.
C) angular unconformity.
D) anticonformity.
E) none of the above

Question 76

If an isotope has a half-life of 50 million years and an 1/8 of the sample remaining consists of that isotope, the age of the rock from which it was taken is:

A) 400 million years.
B) 300 million years.
C) 150 million years.
D) 800 million years.
E) none of the above

Question 77

The Colorado Plateau has a record of geologic events occurring over about 2 billion years that was put together:

A) in a sequence found in the Grand Canyon.
B) in a sequence from a drill core through the Plateau.
C) by correlating the uppermost rocks in one area with the lowermost equivalent rocks in another area.
D) in a sequence exposed in the Colorado Rockies.
E) in a sequence spanning the entire United States.

Question 78

All radioactive elements decay in only one decay step.

Question 79

Concurrent range zones are used in fossil correlation because most fossils:

A) have long geologic ranges.
B) are very rare.
C) are geographically restricted.
D) are not common.
E) are not well preserved.

Question 80

The process of placing geologic events in a sequential order as established by their position in geologic record is called ________ dating.

A) absolute.
B) numerical.
C) radiometric.
D) relative.
E) none of the above.