Geology - Chapter 9

To fully understand the new epoch we are living in, it is useful to compare it to past epochs and consider how human activity is shaping Earth. Sort the events below based on whether they occurred before or during the Anthropocene.

Before the Anthropocene Sixty-six million years ago, a massive asteroid struck Earth, resulting in the extinction of three-quarters of all plant and animal life, including dinosaurs. The first oxygen-producing bacteria evolved, creating the oldest fossils on Earth. During the Anthropocene In the last decade, the black rhinoceros of West Africa became extinct. Atmospheric CO2 has increased more quickly, and to higher concentrations, than during any other part of geologic time. Drainage patterns across the globe are altered due to dams and road building. Submit

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

disconformity

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

The half-life of an isotope is the amount of time it takes for one-half of the atoms in a sample to decay, which remains constant for each unique sample. The graph below shows the decrease in the amount of parent element and the increase in the amount of daughter element as the half-life increases. An isotope sample initially weighing 500 grams decays to produce a daughter element. What is the amount of parent element remaining after two half-lives?

125 grams

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

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

50:50

What scenario requires a geologist to use the principle of cross-cutting relationships?

A dike truncates layers in a sedimentary succession.

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.

Based on the same principles as above, classify the radioactive decays as alpha emission, beta emission, or electron capture.

Alpha emission 226Ra88226Ra88 decays to 222Rn86222Rn86 218Po84218Po84 decays to 214Pb82 Beta emission 234Th90234Th90 decays to 234Pa91234Pa91 40K1940K19 decays to 40Ca20 Electron capture 40K1940K19 decays to 40Ar18

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

Jurassic

Select the drawing that best represents the orientation of rock layers beneath the surface of the formation.

Drawing with lines \\\\\\\\\

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.

Below is a geologic structure that illustrates the various principles of relative dating. You will identify the basic principles used in relative geologic dating by dragging labels to their corresponding targets in the image below.

Hint 1. Inclusions in sedimentary rock layers According to the principle of inclusions, the layer of rock that has inclusions from another rock layer must be younger. Hint 2. A dike cutting through sedimentary rock layers The rock layers that the dike cuts through must have been there first. This is the principle of cross-cutting relationships.

Which of the following statements regarding radioactive decay is true?

More daughter products accumulate over time.

The principles of relative dating can be used to understand the order of geologic events. A geologic event can be anything: the deposition of horizontal layers of sedimentary rock, the faulting or folding of rock layers, the tilting of rock layers, the erosion (or wearing away) of rock, the intrusion of volcanic rock within existing rock layers, and so on.

Oldest to youngest oldest horizontal layers with ocean water at top layers with inclusions from above and below layers are cut by an igneous dike layers are tilted, erosion of tilted surface youngest

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

Original horizontality

Which represents the correct sequence from oldest to most recent or current eras on the Geologic Time Scale?

Paleozoic, Mesozoic, Cenozoic

What happens during radioactive decay?

Parent isotopes turn into daughter isotopes.

Which eon translates to "visible life" from the Greek?

Phanerozoic

The Anthropocene represents the first time that humans have played a pivotal role in the functioning of Earth as a system. There are both positives and negatives of this fact, and it is our responsibility to ensure that the positive impacts outweigh the negative ones in the future. For the options below, decide whether each is a negative or a positive outcome of humans' role in the Anthropocene.

Positive impact Humans have figured out how to selectively breed crops so that they have higher yields, allowing more people on the planet to be fed. Now that humans have recognized how destructive unrestrained growth can be, they have set aside vast swaths of natural areas to be protected, such as Yosemite National Park. As cities grow, more people have access to amenities that are harder to access in rural areas, such as state-of-the-art health care. Negative impact Clearing forested land so that cities can expand has resulted in many plants and animals becoming extinct as their habitat is destroyed. Burning fossil fuels by driving cars has led to large increases in air pollution, which has altered atmospheric chemistry and led to warmer air temperatures.

It is important to understand how the new landscape came about in order to find clues for the relative ages of rocks in an area. Keeping in mind the principles of geology and understanding the landscape that used to be in this area before weathering wore parts away, choose the oldest layer identified by a red dot in the section of the Gigapan image shown below.

Red dot in the foreground slightly to the left

Choose the layer most resistant to weathering and erosion in the section of the Gigapan image shown below.

Red dot in the middle

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.

When looking for fossils, it is important to identify whether fossils are found in the same formation and, thus, may have lived at the same time (or nearly the same time) or if they existed long before or after. Imagine that you have found dinosaur fossils at the location indicated with the star in the section of the Gigapan image below. Sort the other locations to identify whether the locations of the layers indicated by red dots are the same age, younger, or older than the layer indicated by the star (which possesses dinosaur fossils). It will be helful to recall the earlier parts of this exercise where you figured out the geometry and relative ages of these layers.

Same age potential fossil beds: -Red dot in the middle Younger age: -Two pictures with red dots to the right Older age: -Red dots on the left side

What is the principle of original horizontality?

Sedimentary rocks are close to horizontal when deposited.

Which principle or law of geology states that in a sequence of undeformed sedimentary rocks, each bed is older than the one above?

Superposition

The dike dates to the ___________ period.

Tertiary

An igneous dike cuts across existing sedimentary layers and erupts at the surface, creating a layer of basalt. The basalt, as well as all the layers below, are then cut by a fault. Which statement is true?

The fault is the youngest because it cuts through all the sedimentary layers and basalt.

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.

If a limestone, a shale, and a sandstone are deposited in that order without interruption, which of the following is true?

The limestone is the oldest and the succession is conformable.

As a geologist, you always want to first locate your field site on a map. It helps other geologists to locate the field site for future studies and helps you look for relationships with data from nearby field sites. Determine where you are (your field site) in the world. Choose the map that best locates your field site.

The outcrop is located in the northeast corner of Minnesota. Take a moment to reflect on what you know about this area already. Is it an active plate boundary? Does it have mountains, volcanoes, glaciers, etc?

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.

Rock units tell us about Earth's past. If we find evidence of geologic processes that have directional components (direction of plate movement, folds and faults, mineral foliation, wind and water currents, etc.), we need to be able to accurately reconstruct those directions. In terms of the scientific method, documenting outcrop orientation helps other geologists recreate the field investigation step-by-step to confirm or refute previous findings and associated interpretations. Choose the most accurate representation of the outcrop's orientation given your vantage point (as viewed using the Gigapan in relation to the outcrop).

The photographer/image is looking towards the west. In other words, the outcrop faces east at the lake behind the photographer.

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.

The tilted rocks must have been most likely tilted during or after the _______ period

Triassic

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

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 Quaternary and Tertiary rocks are separated by this type of unconformity:

a disconformity

Which of the following is associated with an erosive surface?

a stream running across an irregular surface

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

alpha particle emission, beta particle emission, electron capture

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

an angular unconformity.

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

deformation

Which is the shortest interval of geologic time?

epoch

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

erosive surface

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

nonconformity

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

organisms with overlapping ranges and geologically short lifespans

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

scallop

The question geologists attempt to answer when they approach an outcrop for the first time is "What kind of rocks are these?" Knowing the rock type helps to put the exposure into a geologic framework, from which additional questions can be asked, and answers determined. Now that you've had an opportunity to examine the Gigapan image "up close," identify the type of rock present in the outcrop.

sedimentary

A _____ is formed when magma intrudes between existing sedimentary layers.

sill

While working in the field, you find some sedimentary rocks that include hundreds of fossils of crinoids, numerous brachiopods, and a few trilobites. Realizing that these ancient organisms are some of the first with hard parts, and (when taken as a group) are not very similar to animal communities living today, in which unit of geologic time would you place these rocks?

the Paleozoic

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

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

For this component, you are asked to make observations of the sedimentary bedding characteristics of the outcrop. Bedding is defined as the arrangement of sedimentary rocks in beds or layers of varying thickness and character, oftentimes separated by bedding planes, which are planar or nearly planar surfaces that visibly separate each successive layer of stratified rock (of the same or different lithology) from the preceding or following layer. Now, based on your observations of the Gigapan outcrop and the following classifications of bedding thickness, estimate what type of bedding best describes the majority of the outcrop? Note that bed thickness is provided in both metric (cm) and US Standard (inch) units of measurement. Use the width of the Hospital (H) sign post visible in the foreground of the Gigapan image as an approximate guide against which to compare the thickness of the sedimentary layers. Let's assume that the sign post is 5 cm (2 inches) wide. While some of the rock layers are thicker than the sign post, many are about the same thickness, or even a little bit thinner.

thin

The subdivisions of the Geologic Time Scale were based on which criteria or characteristics?

types of fossils within the units

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