Test 2 Chapter 11

Question 1

Contrast catastrophism and uniformitarianism.

Answer 1

Catastrophism is the view that Earth has been shaped largely by sudden, drastic events.
Uniformitarianism embraces the idea that Earth has been shaped gradually by processes that have
been in action for a long time.

Question 2

How did each philosophy view the age of the Earth?

Answer 2

The philosophy of catastrophism viewed the Earth as being about 5000 or so years old, while
uniformitarianism recognizes the Earth as being much older.

Question 3

Distinguish between numerical dates and relative dates

Answer 3

Numerical dates specify the actual number of years that have passed since an event occurred, or
the actual numerical age of a rock.

Question 4

What is the significance of an unconformity?

Answer 4

An unconformity tells us that some period of time is missing in a rock layer, most likely due to
deformation, uplift, and/or erosion, and then resumed deposition.

Question 5

Distinguish among angular unconformity, disconformity, and nonconformity.

Answer 5

An angular unconformity occurs where tilted strata are overlain by horizontal strata, representing a
deformation and erosion event followed by renewed deposition. A disconformity occurs where a gap
in the record occurs between relatively flat-laying, parallel sedimentary layers, representing a
period of nondeposition or erosion. A nonconformity occurs where younger sedimentary strata
overlie older metamorphic or intrusive igneous rocks.

Question 6

Describe several ways a plant or animal can be preserved as a fossil.

Answer 6

Permineralization occurs when minerals precipitate out of groundwater and fill pores and empty
spaces in tissues of bones or wood. Molds occur when a shell or other structure is buried and
dissolved by groundwater, creating an imprint of the fossil in the sediment. If the mold is filled
with mineral matter, it is a cast—a representation of the shell or structure. Carbonization occurs
when leaves and delicate life-forms are preserved as a thin residue of carbon when encased in
fine sediment. If the film of carbon is lost, a replica of the surface may be retained in the fine
sediment, creating an impression. Amber is the hardened resin of ancient trees and can preserve
an organism that became trapped in it. Trace fossils are indirect evidence of past life, frequently
representing movement or eating habits of an organism.

Question 7

List three examples of trace fossils.

Answer 7

Examples of trace fossils include tracks, burrows, coprolites, and gastroliths.

Question 8

What conditions favor the preservation of an organism as a fossil?

Answer 8

Preservation of an organism as a fossil is favored by rapid burial and presence of hard parts on
the original organism.

Question 9

What is the goal of correlation?

Answer 9

The goal of correlation is to develop a time scale applicable to large regions and the entire
Earth, thus providing a more comprehensive view of Earth’s history.

Question 10

State the principle of fossil succession in your own words.

Answer 10

The principle of fossil succession tells us that past life evolved over time and specific organisms
succeeded one another in a certain order over geologic time. Using this idea, we can identify
rock layers, and their relative time periods, by distinctive fossils sequences contained in the rock.

Question 11

Contrast index fossil and fossil assemblage.

Answer 11

Index fossils are individual fossil organisms that are widespread geographically and are known
to be unique to a certain time period. Fossil assemblages are groups of fossils known to be
unique to a certain time period.

Question 12

Along with their value for correlation, how else are fossils useful to geologists?

Answer 12

Fossils can also provide important information about past environments and conditions of those
environments

Question 13

List four ways that unstable nuclei change. For each type, describe how the atomic number and
atomic mass change

Answer 13

The three ways a nucleus can change are alpha emission, beta emission, and electron capture. In
alpha emission, an alpha particle, composed of 2 protons and 2 neutrons, is emitted from an
atom’s nucleus. This reduces the mass number of the atom by 4, and the atomic number by 2. In
beta emission, a beta particle (an electron) is emitted from a neutron in the nucleus, converting
the neutron to a proton. Because electrons have no mass, the mass number remains unchanged.
This leaves one more proton in the nucleus and thus increases the atomic number of the atom by
1. With electron capture, an electron is captured by the nucleus and combines with a proton to
form a neutron. The mass number remains unchanged, but the atomic number decreases by 1
because the nucleus contains one less proton

Question 14

Why is radiometric dating a reliable method for determining numerical dates?

Answer 14

Radiometric dating is a reliable means of calculating ages of rocks because the rates of decay of
many isotopes have been precisely measured, and these rates do not vary under the physical
conditions of Earth’s surface layers. In other words, the decay rates are not affected by
temperature or pressure. This means that radioactive isotopes that exist in rocks have been
decaying at fixed rates since their formation, and their daughter products have been
accumulating at a corresponding rate. Precise measurements of parent and daughter allow us to
accurately determine the age of a sample.

Question 15

For what timespan does radiocarbon dating apply?

Answer 15

Because carbon-14 has a relatively short half-life (5730 years), we can use this isotope to
measure recent events as far back as 70,000 years.

Question 16

Briefly explain why it is often difficult to assign a reliable numerical date to a sample of
sedimentary rock

Answer 16

Sedimentary rocks are largely composed of grains of preexisting rocks. Therefore, the minerals in
a sedimentary rock are older than the sedimentary rock and are likely a mix of ages from a
diverse group of source rocks.

Question 17

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

Answer 17

Geoscientists can use datable igneous masses to bracket the ages of sedimentary strata. For
example, if we find a dike cutting through sedimentary strata, we know the strata are older than
the igneous dike. Alternatively, if we find an ash layer in a series of sedimentary beds, we know
the layers beneath the ash are older, and the layers above the ash are younger.

Question 18

List the four basic units that make up the geologic time scale.

Answer 18

The four basic units of the geologic time scale, from longest span of time to shortest, are eons,
eras, periods, and epochs.

Question 19

Why is -zoic part of so many names on the geologic time scale?

Answer 19

“-zoic” refers to life and many of the subdivisions of geologic time are based on notable changes
in life-forms on Earth.

Question 20

What term applies to all of geologic time prior to the Phanerozoic eon? Why is this span not
divided into as many smaller time units as the Phanerozoic?

Answer 20

All geologic time prior to the Phanerozoic is commonly referred to simply as Precambrian. This
time period is not divided into as many smaller units because we know much less about the
history of the Earth during this time span. Fossil records are largely absent, making correlations
difficult. In addition, many of the rocks of this time are greatly deformed, making environmental
interpretations difficult.

Question 21

To what does the term Hadean apply? Is it an “official” part of the geologic time scale.

Answer 21

The term Hadean refers to the portion of the time scale before the oldest-known rocks on Earth;
this is the period older than approximately 4 billion years. This is an informal term used by
geoscientists and has not been adopted as an official term on the geologic time scale.