Homework Ch. 10: Earth and Space Science Dr. Wang - Geologic Time

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?

25 atoms

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

Angular unconformities separate rock layers along nonparallel surfaces.

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?

Fault A is younger than fault B, and both are younger than the five layers of rock.

What is the principle of cross-cutting relationships?

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

What happens during radioactive decay?

Parent isotopes turn into daughter isotopes.

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

The principle of superposition ("younger on top") applies to __________.

Sedimentary rocks, volcanic rocks, and Extrusive igneous rocks

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.

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 (half-life) in container B is the same as the rate of decay of atoms (half-life) in container A.

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 does the term unconformity mean?

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

Which list best describes the events that would lead to the layering of sedimentary rocks in this diagram?

deposition, erosion, deposition, erosion, deposition

All types of unconformities represent missing time periods, or gaps in the geologic record. Nonconformities represent an erosional surface on __________ overlain by younger layered rocks.

metamorphic/ igneous rocks

What term describes an erosional surface bounded above by flat-lying sedimentary rocks and below by igneous or metamorphic rocks?

nonconformity

In the following rock sequence, how much erosion might have occurred between rock layer A and rock layer B?

none or only a very small amount (Time does not equate to erosion.)

Features that are "cross-cut" by another geologic feature must be __________ the cross-cutting feature.

older than

The fact that sedimentary rocks are almost always deposited in a horizontal position reflects which law or principle of geology?

original horizontality

Range zones for fossils allow geoscientists to determine the age of sediments in which they are found. Fossils with _____ range zones are better index fossils for figuring out precisely how old rocks are.

short

Which principle or law of geology states that in a sequence of undeformed sedimentary rocks, each bed is older than the one above?

superposition

What is the scientific definition of half-life?

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

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

While working in the field, you encounter an outcrop in which sandstone, shale, and limestone have been intruded by a basaltic dike. You know that the basaltic dike is younger than the sedimentary rock by applying which law or principle?

the principle of cross-cutting relationships

A crystal that has a parent/daughter ratio of 25%/75% for its radioactive isotope has undergone radioactive decay for __________ half-lives.

two