PM 11 Geologic Time

The era of "ancient life" is the ________ era.

Paleozoic

What remains after a sample undergoes two half-lives?

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

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.

To what does the term Hadean apply?

The Hadean is an informal term for the time interval at the very beginning of Earth's history, which took place before the synthesis of Earth's oldest-known rocks.

Why is the geologic time scale more detailed in the Phanerozoic than in previous eons?

The Phanerozoic Eon is more detailed because of the presence of organisms with hard parts and the rapid increase in biodiversity.

Which of the following describes radioactive decay by beta particle emission?

The atomic number of the daughter isotope is one more than the parent; the mass numbers are the same.

Why is "-zoic" part of so many names of units in the geologic time scale?

The boundaries of geologic time units with names ending in -zoic have been determined by worldwide changes in life forms.

What describes all geologic time before the Phanerozoic eon?

The earliest span of geologic time is referred to as the Precambrian, which is an old, informal name for the first unit of geologic time. Details of its history are gradually emerging from every continent.

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.

Using a calendar year as an analogy to the geologic time scale, when do the oldest anatomically modern humans appear on Earth?

The oldest human fossils show up at about eleven minutes to midnight on December 31st.

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 of the following is an essential characteristic of an index fossil?

The organism only lived for a short period of geologic time.

What is the best definition of fossil range?

The period of time when an organism first and last appears in the sedimentary record.

How is the geologic time scale related to the fossil record?

The periods of the time scale were constructed based on ranges of fossil organisms.

Which geologic principles is incorrectly defined?

The principle of cross-cutting relationships states that the fault or intrusion that cuts across several rock layers is older than these layers.

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.

In the late 1700s James Hutton published his important work titled ________.

Theory of the Earth

A dike, which is a tabular intrusive igneous body that cuts across existing rock structures, is shown below. Notice that this feature cuts through horizontal layers of sandstone, or country rock (outlined in yellow). The inset photograph shows an inclusion, which is a fragment of unmelted crustal rock that was incorporated into a body of magma before it crystallized as a dike. During intrusion, magma caused the crustal rock to fracture and pieces were mixed into the magma. The magma then cooled, preserving the inclusion encased in igneous rock. You can see that the inclusion is different in composition than the surrounding rock. Using principles of relative dating, classify the statements according to whether they are "true," or " false". Drag the appropriate items to their respective bins.

True - The sandstone country rock is older than the dike. The intrusive igneous rock making up the dike is the youngest in age. False - The dike is older than the inclusion. The inclusion formed originally in the sandstone country rock.

Radioactive decay in mineral shows that two half-lives have elapsed, giving an age of approximately 1.4 billion years. Using the table in the video as a reference, what is the correct radioactive parent and stable daughter isotope pair?

U235, Pb207

Name the epoch, period, era and eon in which we currently live.

We live in the Holocene Epoch of the Quaternary Period, which is part of the Cenozoic Era and Phanerozoic Eon.

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

You go for a hike across an outcropping of sandstone and you see large footprints in the sandstone. You have just found ________.

a trace fossil

Which of the following includes all common types of radioactive decay?

alpha particle emission, beta particle emission, electron capture

Rare, but complete preservation of fossils occurs in ________.

amber

Paleontology is the study of ________.

ancient life

One type of fossil that can preserve details of organic soft parts is ________.

carbonized imprints

When a radioactive isotope decays by electron capture, the electron ________.

combines with a proton in the nucleus; the atomic number of the daughter is one less than the parent

Following the deposition of a sequence of sedimentary rocks, which event is the first to occur to produce an angular unconformity?

deformation

If you observe a disconformity, you would infer that the region had experienced ________.

deposition in shallow water, uplift or lowering of sea level enough to expose the rocks to erosion and non-deposition for a period of time, and subsequent resubmergence of the remaining sedimentary units where they experience new deposition

If you observe a nonconformity, you would infer that the region had experienced ________.

deposition of sedimentary layers on crystalline igneous or metamorphic rocks that are exposed at the surface of the earth

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

deposition, erosion, deposition, erosion, deposition

A(n) __________ exhibits sedimentary layers that are parallel to each other above and below an erosive surface.

disconformity

Which of the following denotes the divisions of the geologic time scale in correct order of decreasing lengths of time beginning with the longest time interval and ending with the shortest?

eon, era, period, epoch

Which of the following is a common feature that all unconformities exhibit?

erosive surface

The basic concept of Uniformitarianism is ________.

geology changes constantly but the rules that control those transformations do not change

What fundamental concept states that in a horizontal sequence of conformable sedimentary strata, each higher bed is younger than the bed below it?

law of superposition

You pick up a sandstone. In the rock sample you see an impression of a sea shell in the rock. This type of fossil is a ________.

mold

Which of the following characteristics is the most desirable for constraining the relative age of rocks?

organisms with overlapping ranges and geologically short lifespans

In the late 1800's various principles (original horizontality, inclusions, cross cutting relations, etc.) were established and used to ________.

organize the order in which events occurred on earth

Petrified wood is an example of a fossil formed by ________.

permineralization replacing original cellulose of the wood

The ________ is the idea or concept that ancient life forms succeeded each other in a definite, evolutionary pattern and that the contained assemblage of fossils can determine geologic ages of strata.

principle of fossil succession

An important tool in understanding the actual numerical dates of past geologic events is ________.

radioactivity

By applying the law of superposition, ________ dates can be determined.

relative

Which organism from the video is the least useful for relative dating?

scallop

An unconformity is a buried ________.

surface of erosion separating younger strata above from older strata below

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

Unconformities which represent a break in the geologic record of deposition are important because ________.

they represent a period of major activity

One of the primary goals of geology is to ________.

understand the history of the earth

If you observe an angular unconformity, you would infer that the region had experienced ________.

uplift and tilting of sedimentary units that exposes them to erosion over a period of time with subsequent renewed deposition

A worm would stand a poor chance of being fossilized because ________.

worms have no hard parts

The sedimentary layers above an erosive surface are __________ relative to all rocks present below the surface.

younger

Which of the following is an accurate description of a nonconformity?

younger sedimentary strata overlie uplifted and weathered igneous or metamorphic rocks

In southeast Utah, the San Juan River and its tributaries have cut through hundreds of feet of Paleozoic sedimentary rocks. The photo below shows the Honaker Trail Formation, the Halgaito Formation, and the Cedar Mesa Sandstone. Each unit is composed of rocks that were deposited in distinct environments, and record the transition from a marine to continental setting. The Honaker Trail Formation is made up of predominately carbonates, which record deposition in a shallow marine setting. The Halgaito Formation is located above the Honaker Trail Formation. This unit is composed of siltstones and sandstones deposited in channel and floodplain environments on a coastal plain. The layers of the Cedar Mesa Sandstone make up the steep cliffs of this region. This unit is composed of sandstones (sand particles), which accumulated mostly due to deposition in an eolian, or wind-blown environment. Use the information from the video and above on relative dating to rank the geologic events and processes from oldest to youngest. Pay particular attention to the sediments and depositional environments for each formation. Rank from oldest to youngest.

Carbonate sediments were deposited in a shallow marine depositional environment. Sediment was deposited in stream and floodplain depositional environments. An abundance of sand was transported and deposited by wind in a large dune field. Erosion of the sedimentary layers by The San Juan River by incision.

A major difference between Catastrophism and Uniformitarianism is ________.

Catastrophism supports rapid changes on the earth and a young age of the earth while Uniformitarianism requires long times for changes to be significant and thus a very old earth

The era known as the "age of mammals" is the ________ era.

Cenozoic

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

Each radioactive isotope used for dating has been decaying at a fixed rate since the formation of the minerals in which it occurs.

What is the category name for the largest division of time used on the geologic time scale?

Eon

In order of longest to shortest, what are the four basic units that make up the geologic time scale?

Eon, era, period, and epoch

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 fossil succession?

Fossil organisms succeed one another upward through rock layers in a definite and determinable order; therefore, a rock from any time period can be recognized by its fossil content.

Along with being important in correlation, how else are fossils useful to geologists?

Fossils can help determine the depositional environment in which the rock layer that contains them formed.

What is the principle of cross-cutting relationships?

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

Who is credited with formulating the doctrine of uniformitarianism?

Hutton

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

Inclusions, Cross-Cutting Relationships

The San Rafael Swell is a monocline, or large step-like fold, located in central Utah. The photos below were taken on the east and southeast portion of this fold, and show tilted sedimentary beds and canyons from different perspectives. Use the principles of relative dating that were discussed in the video to fill in the blanks of the sentences below. Drag the terms in the left-hand column to the appropriate blanks on the right.

1. The Principle of Cross-Cutting Relationships can be used to determine the relative ages of the sedimentary rocks versus canyon formation. 2. The sedimentary beds were initially deposited in a(n) horizontal orientation. 3. The process of stream incision is younger compared to the folding of the rock units. 4. The Principle of Original Horizontality says that folded sedimentary beds have been disturbed since deposition. 5. The sedimentary layers are older than both folding and stream incision.

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 is the age of the Earth accepted by most scientists today?

4.5 billion years

If one half-life has lapsed, what is the radioactive parent to stable daughter isotope ratio?

50:50

If a rock contains ten different kinds of fossils, which of the following must be true?

All of the organisms had to be alive when the rock was deposited.

________ is an erosional contact between tilted, older strata below and horizontal, younger strata above.

An angular unconformity

Which of the following correctly describes an angular unconformity, a disconformity, and a nonconformity?

An angular unconformity consists of tilted or folded rock layers that have been eroded and overlain by horizontal sedimentary layers; a disconformity consists of horizontal sedimentary layers that have been eroded and overlain by horizontal sedimentary layers; a nonconformity consists of metamorphic rocks or igneous intrusions that have been eroded and overlain by horizontal sedimentary layers.

Which accurately contrasts an index fossil with a fossil assemblage?

An index fossil is an individual fossil used to correlate different rock layers of the same age; a fossil assemblage is a specific combination of fossils that is used to more precisely date a rock layer.

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

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.

The Jurassic Period lies in the _____ Era, which was dominated by large terrestrial vertebrates, or _____.

Mesozoic; dinosaurs

Which of the following statements regarding radioactive decay is true?

More daughter products accumulate over time.

In the images below, which contains a disconformity?

Sedimentary 3 Sedimentary 2 Sedimentary 1

Which image is an example of an angular unconformity? SEE IMAGES BELOW FOR ANSWER SELECTIONS.

Sedimentary rock 6

What is the principle of original horizontality?

Sedimentary rocks are close to horizontal when deposited.