Chapter 12 Geologic Time

If the relative age of two formations (with respect to one another) is known, then the numerical ages for each formation can be inferred. a. true b. false

Which statement best summarizes the development of the geologic time scale? a. Names of relative ages (such as Silurian) and accurate numerical dates for these ages appeared at about the same time. b. Relative ages for sedimentary strata were known well before accurate numerical dates for these rocks could be provided. c. Numerical ages for rocks were known well before the relative sequence of sedimentary layers was established.

If the lithology and fossil content of two bodies of rock on opposite sides of a canyon are identical, then these remaining outcrops were likely physically connected at one time and formed part of an extensive, sheet-like layer of rock. This idea summarizes the principle of ____________. a. uniformitarianism b. original horizontality c. original continuity d. superposition

Concerning the relative ages of the granite and sandstone, the ____________. a. sandstone must be older, according to the principle of superposition b. granite must be older, according to the principle of components c. granite must be older, according to the principle of superposition d. Their relative ages cannot be determined from the information given.

Attempts to measure the age of Earth by extrapolating modern riverine influx rates into the past to produce modern marine salinity from an initially freshwater ocean failed to take into account that ____________. a. salts derived from the weathering of rock are insoluble in water b. dissolved salts from weathering of rock only rarely are transported all the way to the ocean c. salt is removed through precipitation, balancing riverine input

Magnetostratigraphy takes advantage of the fact that ____________. a. Earth's magnetic field only arose 10,000 years ago b. Earth's magnetic field has experienced numerous polarity reversals, with periods of normal and reversed polarity of varying lengths c. Earth's magnetic field has been uniformly strong, with constant polarity, throughout Earth's history

In an unweathered sample of detrital sedimentary rock, the ratio of an unstable isotope to its stable daughter isotope is 1/15. If no daughters were present at the time the rock cooled below closure temperature, and the half-life of the reaction is 50 million years, how old is the rock? a. 200 million years b. 400 million years c. 750 million years d. None of the above are correct

In a undisturbed sequence of sedimentary rocks, younger layers overlay older layers, according to the principle of ____________. a. superposition b. uniformitarianism c. original horizontality d. original continuity

The basalt body is best described as a ____________. a. pluton b. sill c. dike d. laccolith

Basaltic clasts within the basal layers of the conglomerate imply which of the following statements (choose the most correct answer)? a. The conglomerate must be the older of the two. b. The basalt must be the older of the two. c. The section must not have been overturned by tectonic activity at any time. d. B and C are both correct.

Two atoms of a single element that differ in number of neutrons are said to represent two distinct ____________ of that element. a. atomic species b. isotopes c. isomers d. isotherms

The surface below sedimentary rocks that overlie igneous or metamorphic rocks is termed a (an) ____________. a. nonconformity b. angular unconformity c. disconformity

James Hutton, the "father of geology," put forth the principle of ____________. a. uniformitarianism b. superposition c. original continuity d. original horizontality

Contact #3 is both a ____________ and a (an) ____________ at various points. a. nonconformity; angular unconformity b. fault; unconformity c. disconformity; nonconformity d. conformable contact; unconformity

As understood by modern geologists, the principle of uniformitarianism implies that ____________. a. physical processes observed today (such as erosion and volcanic eruption) have been active in the past and are responsible for the geologic history we see preserved in the rock record b. the Earth has always had the same basic appearance that it has today, with the distributions of ocean basins and continents identical to what we see today c. igneous, metamorphic, and sedimentary rocks are uniformly mixed throughout the crust of the Earth d. physical processes observed today (such as erosion and volcanic eruption) have been occurring throughout Earth history at constant rates; for example, no eruption in geologic time would have been greater than the 1815 Tambora eruption

Radiometric dates applied to sedimentary rocks produce ages that are ____________. a. just as accurate as when the technique is applied to igneous rocks b. too old (pre-date sedimentary deposition) c. too young (postdate sedimentary deposition)

Relativeages expressed on the geologic time scale primarily resulted from the study of ____________. a. radiometric dating of sedimentary rocks b. fossil content and spatial relationships among sedimentary rocks c. fossil content and spatial relationships among igneous rocks d. radiometric dating of igneous rocks

The surfaces named contacts #2 and #3 are examples of ____________. a. faults b. conformable sedimentary contacts c. unconformities d. baked contacts

Dendrochonology involves dating of historic and geologic events through the study of ____________. a. annual growth rings in trees b. remnant magnetism in iron-rich minerals c. growth layers in shells d. oxygen isotope profiles in glacial ice

In the 1800s Lord Kelvin estimated the age of the Earth, utilizing a number of assumptions. Which wrong assumption was the primary reason why his estimate (approximately 20 million years) was so wildly inaccurate? a. that no new heat has been added to the Earth since its initial formation b. that the Earth would lose heat over time through radiation into outer space c. that the Earth may have been molten when first formed d. that the Earth was once hotter than it presently is today

Which of the following geologic principles is a direct result of gravity? a. cross-cutting relationships b. original horizontality c. inclusions d. baked contacts

Why is radiocarbon dating only rarely applied in geological work? a. The half-life of carbon-14 is so long that it is effectively a stable isotope. b. The half-life of carbon-14 is so short that it can only be used to date materials that are less than 70,000 years old. c. No substances on Earth contain significant amounts of carbon-14

Contact #1 is an example of a (an) ____________. a. conformable sedimentary contact b. baked contact c. fault d. unconformity

When an unstable isotope decays, the daughter isotope that results is always a stable isotope. a. false b. true

Buried erosional surfaces between parallel sedimentary strata are termed ____________. a. nonconformities b. disconformities c. angular unconformities

Basaltic clasts within the conglomerate have been radiometrically dated to 50 million years ago (Eocene epoch of the Tertiary period). Is this a reliable age for the conglomerate? a. No, basalt never contains minerals bearing radioactive isotopes. b. No, this age is likely too young. c. Yes. d. No, this age is likely too old

Which of the following is always the same for multiple isotopes of a single element? a. atomic number b. atomic mass number c. tendency to decay radioactively d. None of the above are correct

Concerning the relative ages of the basalt and the fault, the ____________. a. fault must be older, according to the principle of cross-cutting relationships b. basalt must be older, according to the principle of cross-cutting relationships c. basalt must be older, according to the principle of original horizontality d. Their relative ages cannot be determined from the information given

The youngest geologic unit visible in the cross section is the ____________. a. marble b. limestone c. granite d. conglomerate

If horizontal sedimentary strata overlie tilted strata (and no fault is present), the surface between the horizontal and tilted strata must be a (an) ____________. a. nonconformity b. angular unconformity c. disconformity d. conformable sedimentary contact

In the area immediately surrounding an igneous intrusion, a host limestone is locally metamorphosed to produce marble. Which of the following statements is correct? a. The limestone must be older than the marble. b. The intrusive igneous rock must be older than the limestone. c. The relative ages of the three units cannot be determined with the information given. d. The marble must be older than the intrusive rock

The oldest geologic unit in the cross section is the ____________. a. limestone b. granite c. marble d. sandstone

Which statement about contact #3 is correct? a. It was once elevated farther above contact #2, but faulting has brought them closer together. b. It and contact #2 once formed a continuous unconformity. c. It presents an example of an angular unconformity. d. It must be younger than the conglomerate, due to the principle of baked contacts

If all of Earth history were compressed into a single year, Homo sapiens would first appear on ____________. a. October 31 (just in time for Halloween) b. December 31, at 11:59 P.M. (just before the ball drops on New Year's Eve) c. February 26 (close to Mardi Gras) d. April 15 (just before taxes are due)

The age of Earth CANNOT be reliably estimated from sediment thicknesses because ____________. a. sedimentation rates are not likely to remain constant at any one locality over all of Earth's history b. sedimentary rocks can be metamorphosed or melted c. locally, much of Earth's history is represented by unconformities between strata rather than the strata themselves d. All of the above are correct

Uniformitarianism is succinctly summarized by which phrase? a. The past is the key to the present. b. The present is the key to the future. c. The present is the key to the past. d. The future is the key to the present

Which of the following geologic principles is NOT a result of gravity? a. superposition b. cross-cutting relationships c. original continuity d. original horizontality

Which method of correlation is more reliable for determining age equivalence among bodies of rock that are physically isolated by vast distances? a. fossil correlation b. lithologic correlation

If the numerical ages of two formations are known, then the relative age of each with respect to one another can be inferred. a. true b. false

Which proportion of a radioactive isotope is expected to remain in an unaltered (unreacted) state after the passage of three half-lives? a. one-sixth b. one-eighth c. three-halves d. one-third

Which of the following always varies among multiple isotopes of a single element? a. atomic number b. atomic mass number c. tendency to decay radioactively d. None of the above are correct

Which eon of geologic time is represented by rocks containing abundant shelly fossils? a. Proterozoic b. Phanerozoic c. Archean

The marble rind surrounding the granite pluton must be younger than the limestone according to the principle of ____________. a. original continuity b. original horizontality c. baked contacts d. superposition

The principle of ____________ explains the occurrence of young rock within a newer igneous body. a. original horizontality b. inclusions c. cross-cutting relationships d. baked contacts

In an unweathered sample of igneous rock, the ratio of an unstable isotope to its stable daughter isotope is 1/15. If no daughters were present at the time the rock cooled below closure temperature, and the half-life of the reaction is 50 million years, how old is the rock? a. 200 million years b. 400 million years c. 750 million years d. None of the above are correct

Period names on the geologic time scale, such as Devonian and Permian, provide examples of ____________. a. numerical age b. relative age

Within the world's sedimentary rocks, fossils ____________. a. are randomly distributed b. are rarely, if ever, found c. occur in an ordered sequence

Precisely speaking, a measured radiometric age for a mineral crystal within an igneous rock denotes the amount of time that has passed since the ____________. a. temperature of the crystal became equal to the Curie point for the mineral b. temperature of the crystal became equal to the closure temperature for the mineral c. atoms within the crystal were part of a body of molten magma d. crystal solidified

Numericalages for boundaries between time units on the geologic time scale primarily resulted from the study of ____________, in conjunction with relative age data. a. fossil content and spatial relationships among igneous rocks b. radiometric dating of igneous rocks c. fossil content and spatial relationships among sedimentary rocks d. radiometric dating of sedimentary rocks

Concerning the relative ages of the shale and sandstone, the ____________. a. Their relative ages cannot be determined from the information given. b. shale must be older, according to the principle of superposition c. sandstone must be older, according to the principle of superposition d. shale must be older, according to the principle of components