chapter 9 Geology

What happens during radioactive decay?

Parent isotopes turn into daughter isotopes.

Petrified wood is an example of what type of fossil preservation?

Permineralization

________ is a term used to define 88% of Earth's history prior to the start of the Paleozoic era.

Precambrian

The ---- says that folded sedimentary beds have been disturbed since deposition.

Principle of Original Horizontality

Original horizontality

Principle stating that sedimentary rocks are deposited in horizontal or nearly horizontal layers.

What is the definition of relative dating?

Putting events in order from first to last

________ is a technique where the decay rate of radioactive isotopes is measured and used to determine the age of a geologic material.

Radiometric Dating

Which two conditions are the most important in order for an organism to become fossilized?

Rapid burial and possession of hard parts

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.

Which of the following is an example of a relative date?

The Silurian period came before the Devonian period.

How might a numerical date for a layer of sedimentary rock be determined?

The order of occurrence of the sedimentary layer is related to a local igneous body that has been radiometrically dated, thus yielding an age range for the sedimentary layer.

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 worm burrow is an example of which type of fossilization?

Trace fossil

________ was the English engineer and canal builder who observed that sedimentary strata in widely separated areas could be identified by their distinctive fossil assemblages.

William Smith

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.

The white granite of the Idaho Batholith near Boise, Idaho, contains dark gray xenoliths of metamorphic material. Which is older? How do you know?

Xenoliths; the principle of inclusions

What does the term unconformity mean?

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

Superposition

a principle that states that younger rocks lie above older rocks if the layers have not been disturbed

Inclusions

chunks of weathered rock are older than the rock they are contained in. −

When rocks are ________, several adjacent layers were deposited without interruption.

conformable

A(n) ________ is a type of unconformity with parallel beds above and below the erosional surface.

disconformity

Cross-Cutting Relationships

faults, fractures are younger than the rocks they cut through.

The principle of ________ states that organisms succeed one another in a definite, determinable order that can be used to recognize a specific time period.

faunal succession

Which type of rock is best suited for radiometric dating?

granite

The sedimentary beds were initially deposited in a(n) ------ orientation.

horizontal

What does "-zoic" in the eras of the geologic time scale refer to?

life

Unconformities

missing time

What kind of unconformity is visible in this figure? The unconformity in question is indicated by the Arrow A.

nonconformity

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

nonconformity

The sedimentary layers are ---- than both folding and stream incision.

older

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

original horizontality

________ is the scientific study of fossils.

paleontology

Which particle of an atom will determine the atomic number of an element?

proton

Two isotopes of uranium, 238U and 235U, have the same number of ___, but different numbers of ___.

protons; neutrons

Lateral Continuity

rocks extend in all directions until they reach the edge of a basin or pinch out.

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

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

What is the goal of correlation?

to gain a more comprehensive view of the geologic history of a region

The process of stream incision is ---------compared to the folding of the rock units

younger

A black, organic residue of an ancient fern would be an example of which type of fossilization?

Carbonization

Which era are we currently in?

Cenozoic

What scientific development made accurate numerical dating possible?

Discovery of radioactivity

What is a fossil?

Evidence of past life that can include skeletal as well as trace material

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.

When correlating rock between widely separated areas or between continents, what feature(s) do geologists rely on the most to link distant rock units of similar age?

Fossils

Which of the following is an example of a trace fossil?

Gastroliths

What is the principle of cross-cutting relationships?

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

The Principles of ___________ and __________ were used to determine relative ages in the previous question.

Inclusions, Cross-Cutting Relationships

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

If 235U has 92 protons, how many neutrons does it have?

143

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 remains after a sample undergoes two half-lives?

25 percent of the sample is composed of parent atoms, and 75 percent of the sample is composed of daughter atoms.

You are trying to determine the age of a lava flow using an isotope with a half-life of 1.5 million years. If the sample has gone through four half-lives, what is the age of the rock?

6 million years

For what time span does radiocarbon dating apply?

70,000 years ago or younger

What is a fossil assemblage?

A group of fossils used to establish the age of a rock unit

Which of the following is an example of a numerical date?

A meteorite impacted the Earth sixty-five million years ago.

Why is it difficult to assign a numerical age to a sedimentary rock?

A radiometric date taken from a grain within a detrital sedimentary rock only gives the age of that one grain, which likely came from a source different from the other grains within the rock.

What is an isotope?

An atom that has more or fewer neutrons than it should

What is an index fossil?

An organism that was geographically widespread but limited to a short span of geologic time

What is the significance of an unconformity?

An unconformity represents an interval of time that is missing from the rock record.

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

Angular unconformities separate rock layers along nonparallel surfaces.

Which principles of relative dating did you use to answer the previous question?

Superposition and Cross-Cutting Relationships

Which scientist is credited with developing the law of superposition?

Nicholas Steno

What is the difference between numerical dates and relative dates?

Numerical dates specify an absolute age in number of years, whereas relative dates determine the order of events in relation to one another.

Assume that Layer C is a lava flow that has been radiometrically dated to 97 million years ago. What is the age of Layer Z?

Older than 97 million years old

Which of the following would make the best index fossil: olenellus (522-510 million years ago), flexicalymene (~490-418 million years ago), and phacops (~418-360 million years ago).

Olenellus

What materials are best dated with carbon-14?

Organic materials

Principle of Cross-Cutting Relationships

The ----- can be used to determine the relative ages of the sedimentary rocks versus canyon formation Principle of Cross-Cutting Relationships

What is stratigraphic correlation?

Matching up rocks of similar age in different regions

In what era is the Jurassic period?

Mesozoic

What is the principle of original horizontality?

Sedimentary rocks are close to horizontal when deposited.