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Questions and Answers

What is the principle that states that sedimentary rock layers become younger from bottom to top?

Law of Original Horizontality

Law of Superposition (correct)

Principle of Cross-Cutting Relationships

Principle of Fossil Succession

What is the term for a gap in the geologic record, often represented by an uneven surface between two rock layers?

Nonconformity

Angular Unconformity

Disconformity

Unconformity (correct)

What is the term for the process of correlating rock layers between different locations based on their lithology?

Chronostratigraphic Correlation

Lithologic Correlation (correct)

Geochemical Correlation

Biostratigraphic Correlation

What is the term for a layer of volcanic ash that can be dated and used as a time marker in the geologic record?

Tephrochronology Signup and view all the answers

What is the term for a fossil that is found in a specific geologic time period and can be used to date the rock layer in which it is found?

Index Fossil Signup and view all the answers

What is the term for the study of the arrangement of rock layers and their relationships to each other?

Stratigraphy Signup and view all the answers

What is the term for the process of determining the relative ages of rocks based on their position in the geologic record?

Relative Dating Signup and view all the answers

What is the term for the scale that divides the Earth's history into distinct time periods?

Geologic Time Scale Signup and view all the answers

What type of correlation refers to the age equivalence of rocks?

Time correlation Signup and view all the answers

What is a characteristic of an index fossil?

It is a fossil of a species that was abundant and widespread Signup and view all the answers

What is an example of a key bed?

A layer of volcanic ash Signup and view all the answers

Why may the age of a rock unit vary by millions of years over a long distance?

Because the rock unit was deposited at different times in different places Signup and view all the answers

What is the purpose of correlating rocks of the same age?

To construct a record of Earth history and a geologic time scale Signup and view all the answers

What type of correlation refers to the continuity of a rock unit?

Lithologic correlation Signup and view all the answers

What is a characteristic of volcanic ash chronology?

It provides a precise age of the rocks at a specific point in time Signup and view all the answers

Why are index fossils useful for correlating rocks of the same age?

Because they are easily identified and existed for a short time Signup and view all the answers

Why are marine mammals considered good index fossils?

Because they spread rapidly and widely throughout the seas Signup and view all the answers

What is the purpose of using an assemblage of several fossils in dating and correlating rocks?

To provide a more precise estimate of the age of the rock Signup and view all the answers

What is a key bed, and how is it used in correlation?

A thin, widespread sedimentary layer, used to correlate rocks Signup and view all the answers

What is the advantage of using index fossils to determine the age of a rock?

They provide a precise estimate of the age of the rock Signup and view all the answers

What is the relationship between the time span of a species' existence and the precision of the age of a rock determined by an index fossil?

The shorter the species existed, the more precise the age determination Signup and view all the answers

What is the significance of volcanic eruptions in geologic time scale?

They eject great volumes of fine, glassy particles that can be used for correlation Signup and view all the answers

What is the purpose of correlating rocks?

To establish a timeline for Earth's history Signup and view all the answers

What is the advantage of using multiple lines of evidence, such as index fossils and key beds, to determine the age of a rock?

It allows for more precise age determination Signup and view all the answers

Study Notes

Fossil Record and Correlation

Marine mammals are useful index fossils because they spread rapidly and widely throughout the seas.
The shorter the time span that a species existed, the more precisely the index fossil reflects the age of the rock.
The presence of a single type of index fossil can establish the age of the rock, but an assemblage of several fossils is commonly used to date and correlate rocks.

Key Beds

A key bed is a thin, widespread, easily recognized sedimentary layer that was deposited rapidly and simultaneously over a wide area.
Key beds are used in correlation to establish the age of rocks.

Igneous Rocks

Igneous rocks are dated according to whether they caused metamorphism in the surrounding rock, whether they cross-cut preexisting rocks, or whether sediments were deposited on them after they were formed.

Geologic Time Scale

Fossils are useful in determining relative ages of rocks because different animals and plants lived at different times in Earth history.
The principle of superposition states that sedimentary rock layers become younger from bottom to top.
Rocks containing dinosaur bones must be younger than those containing trilobite remains.

Correlation

There are two kinds of correlation: time correlation (age equivalence) and lithologic correlation (continuity of a rock unit).
The two types of correlation are not always the same because some rock units, such as sandstone, were deposited at different times in different places.

Index Fossils

Index fossils can accurately indicate the ages of sedimentary rocks because they were produced by an organism that:
is abundantly preserved in rocks
was geographically widespread
existed as a species or genus for only a relatively short time
is easily identified in the field

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