chapter 23

Relative dating does not tell the actual date of rock formation, it tells the _______.

time of rock formation in relation to other events in Earth's past

To match various rock outcrops over a large area, geologists use the principle of _______.

lateral continuity

With radiometric dating, scientists measure the _______.

ratio of a radioactive isotope (parent material) to its decay product (daughter material)

The geologic time scale is based on _______.

the relative ages of rock layers and their fossils

In a thick bed of sedimentary rocks we find an igneous dike. The order of formation is

sedimentary before igneous.

In a sequence of rock layers we find trilobite fossils at the bottom of rock layer 1 and dinosaur fossils directly above the fossil trilobites in layer 2. There are inclusions of trilobites in layer 2. We conclude that

by inclusion and faunal succession, layer 1 is older than layer 2

An assumption that Earth processes occurring today have always occurred is called

uniformitarianism

The theory that different forms of animals throughout Earth's past occurred in a definite order is called the principle of

faunal succession.

In any sequence of sedimentary rock layers that have not been subjected to stress, you would expect to find

older horizontal layers at the bottom with younger horizontal layers at the top.

The principle of faunal succession states that

fossil organisms follow one another in a definite, irreversible time sequence.

The principle of original horizontality states that each

new layer of sediment is laid down nearly horizontally over older sediment.

The rather simple idea that, unless disturbed, younger sedimentary layers overlie older layers, is referred to as the principle of

superposition.

In a sedimentary rock that contains inclusions of metamorphic rock, the inclusions must be

older than the sedimentary rock.

The five principles used to determine the relative ages of rock include original

horizontality, superposition, cross cutting, inclusion, and faunal succession.

Radiometric dating is based on

proportions of radioactive isotopes and their decay products.

Radiometric dating assumes that, once a mineral has crystallized, any daughter product results only from the decay of the original unstable parent. Some complications are

\n "time clock resetting" due to metamorphism, and product leakage that yields false age estimation.

The process where an element spontaneously changes into a different element (transmutation) by a change in the nucleus of an atom is called

radioactive decay.

Radiocarbon dating is limited to the remains of plants and animals that died no longer than about

5700 years ago.

If a radioactive element has a half-life of 704 million years, and the amount of parent material remaining is one-eighth of the original amount, the specimen is about

2,112,000,000 years old.

In radioactive decay, when the parent material decays to one-quarter of its original amount, the time is

two half-lives.

On the Geologic Time Scale, the time unit representing the largest span of time is the

Precambrian.

Which of the following statements about relative and absolute age dating is most accurate?

Relative age dating places rocks and events in chronological order but does not provide information about absolute age.

What is the principle of original horizontality?

Sedimentary rocks are close to horizontal when deposited.

What is the principle of superposition?

Within a sequence of rock layers formed at Earth's surface, rock layers lower in the sequence are older.

What is the principle of cross-cutting relationships?

Geologic features that cut through rocks must form after the rocks that they cut through

Five layers of rock are cut by two faults. Both faults cut through all five layers of rock. Fault A breaks through to the surface, whereas fault B does not. Which of the following statements about faults A and B is most accurate?

Faults A and B are about the same age, and both are younger than the five layers of rock.

Which principle of relative age dating is important for determining the relative age of igneous rock that has intruded into overlying rock?

the principle of cross-cutting relationships

A fault (F) breaks three layers of sedimentary rock (S). An igneous intrusion (I1) has broken through the bottommost layer of rock. A second igneous intrusion (I2) has moved up the fault and pooled on top of the uppermost layer of rock. Which event would be considered the youngest?

The intrusion of I1 or I2 is the youngest event. Without more information, we cannot know which igneous rock is youngest.

What does the term *unconformity* mean?

a missing rock layer in a sequence that represents a period of erosion or nondeposition

What characteristic most directly DISTINGUISHES an angular unconformity from a nonconformity?

Angular unconformities separate rock layers along nonparallel surfaces.

What happens during radioactive decay?

Parent isotopes turn into daughter isotopes.

What is the scientific definition of *half-life*?

the amount of time over which the number of parent isotopes decreases by half

Two containers hold the same radioactive isotope. Container A contains 1000 atoms, and container B contains 500 atoms. Which of the following statements about containers A and B is true?

The rate of decay of atoms in container A is greater than the rate of decay of atoms in container B.

A container holds 100 atoms of an isotope. This isotope has a half-life of 1.5 months. How many total atoms will be in the container after 3 months?

100 atoms

A container holds 100 atoms of an isotope. This isotope has a half-life of 1.5 months. How many atoms of the radioactive isotope will be in the container after 3 months?

\n 25 atoms

A rock sample contains 75 atoms of a parent isotope and 25 atoms of a daughter isotope. The half-life of the parent isotope is 100 years. How old is this rock?

50 years old