Geologic Time

Which of the following describes the principle of original horizontality?

Folded sedimentary layers were originally laid down flat and later deformed.

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.

The Quaternary and Tertiary rocks are seperated by this type of unconformity:

Disconformity.

What happens during radioactive decay?

Parent isotopes turn into daughter isotopes.

An undeformed sequence of sedimentary rocks is exposed in a large rive canyon. Which two principles would be demonstrated by the rocks?

Principles of superposition and lateral continuity.

The dike dates at least to the _______ period.

Quarternary.

The principle of lateral continuity.

Sedimentary layers, when formed, extended horizontally in all directions.

What is the principle of original horizontality?

Sedimentary rocks are close to horizontal when deposited.

The principle of original horizontality.

Sedimentary rocks were originally deposited as flat-lying, horizontal layers.

What is the scientific definition of half-life?

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

The Triassic rocks must have been most likely tilted during or after the ________ period.

Triassic.

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.

Due to an unconformity, the _______ period is missing from the rock record.

Jurassic.

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.

What does the term unconformity mean?

A missing rock layer in a sequence that represents a period of erosion or non deposition.

Law of superposition.

A sedimentary rock bed on the bottom must be older than the rock bed on the top.

If a sequence of sedimentary units is cut by a fault, what does the principle of cross-cutting relationships tell a geologist?

All of the sedimentary units must have been deposited and lithified before being cut by the fault.

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

Angular unconformities separate rock layers along nonparallel surfaces.

The triassic and Cretaceous rocks are separated by this type of unconformity:

Angular unconformity.

Disonformity

Break between parallel sedimentary rock layers above and below. Disconformities represent times when sediments were not deposited or were eroded.

An igneous dike cuts through limestone, but not through the overlying sandstone. Which of the following statements is most accurate?

First, the limestone was laid down, then intruded by the igneous dike, and lastly the sandstone was deposited.

Nonconformity

Found where igneous or metamorphic rocks have eroded and then sedimentary rock layers are deposited above.

Angular unconformity

Found where sedimentary layers were tilted and eroded and younger and more flat-laying sedimentary layers were deposited above.

What is the principle of cross-cutting relationships?

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

A sandstone contains inclusions of metamorphic rock. An igneous dike cuts both the sandstone and inclusions. List the rocks from youngest to oldest.

Igneous dike, sandstone, metamorphic rock.

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

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

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

The principle of cross-cutting relationships.

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.

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.