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Transcript
2/26/2016
Homework 5 Geologic Time
Homework 5 Geologic Time
Due: 11:59pm on Sunday, February 28, 2016
You will receive no credit for items you complete after the assignment is due. Grading Policy
Interactive Animation: Relative Geologic Dating
When you have finished, answer the questions.
Part A
Which of the following statements about relative and absolute age dating is most accurate?
ANSWER:
Relative age dating places rocks and events in chronological order and can provide information about absolute age.
Relative age dating provides information about absolute ages but does not place rocks and events in chronological order.
Relative age dating places rocks and events in chronological order but does not provide information about absolute age.
Relative age dating does not provide information about absolute ages, nor does it place rocks and events in chronological order.
Correct
Part B
What is the principle of original horizontality?
ANSWER:
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Metamorphic rocks are close to horizontal when deposited.
Sedimentary rocks are close to horizontal when deposited.
Sedimentary rocks are close to horizontal when eroded.
Metamorphic rocks are close to horizontal when eroded.
Correct
Part C
What is the principle of superposition? ANSWER:
Within a sequence of rock layers formed at Earth's surface, rock layers in the middle of a sequence are older.
Within a sequence of rock layers formed at Earth's surface, rock layers higher in the sequence are older.
Within a sequence of rock layers formed at Earth's surface, rock layers lower in the sequence are older.
Correct
Part D
What is the principle of cross­cutting relationships?
ANSWER:
Geologic features that cut through rocks must form at roughly the same time as the rocks that they cut through.
Geologic features that cut through rocks must form before the rocks that they cut through.
Geologic features that cut through rocks must form after the rocks that they cut through.
Correct
Part E
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?
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ANSWER:
Faults A and B are about the same age, and both are older than the five layers of rock.
Fault A is younger than fault B, and both are older than the five layers of rock.
Faults A and B are about the same age, and both are younger than the five layers of rock.
Fault A is younger than fault B, and both are younger than the five layers of rock.
Correct
Part F
Which principle of relative age dating is important for determining the relative age of igneous rock that has intruded into overlying rock?
ANSWER:
the principle of original horizontality
the principle of cross­cutting relationships
the principle of intrusive relationships
the principle of superposition
Correct
Part G
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?
ANSWER:
Faulting of rock along F is the youngest event. We know this because all three layers of sedimentary rock have been broken.
The intrusion of I2 is the youngest event. We can know this because I2 sits on top of all other rocks.
Deposition of the three sedimentary layers, S, is the youngest event. We know this because the fault underlies the igneous rocks.
The intrusion of I1 or I2 is the youngest event. Without more information, we cannot know which igneous rock is youngest.
Correct
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SmartFigure: Relative Dating
Launch the SmartFigure Video
When you have finished, answer the questions.
Part A
A sandstone contains inclusions of metamorphic rock. An igneous dike cuts both the sandstone and inclusions. List the rocks from youngest to oldest.
Hint 1.
Use your knowledge regarding the principles of cross­cutting relationships and dating by inclusions to answer this question.
ANSWER:
metamorphic rock, igneous dike, sandstone
igneous dike, sandstone, metamorphic rock
metamorphic rock, sandstone, igneous dike
sandstone, metamorphic rock, igneous dike
igneous dike, metamorphic rock, sandstone
Correct
Part B
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If a sequence of sedimentary units is cut by a fault, what does the principle of cross­cutting relationships tell a geologist?
Hint 1.
Recall what the principle cross­cutting relationships states and how it is used for relative age dating.
ANSWER:
The sedimentary units on the left side of the fault are the same as those on the right side.
All of the sedimentary units must have been deposited and lithified before being cut by the fault.
The fault is older than the sedimentary sequence.
Sedimentary layers are laid down horizontally.
The oldest sedimentary unit is located at the base of the sequence, while the youngest is at the top.
Correct
Part C
Which of the following describes the principle of original horizontality?
Hint 1.
The video showed a sequence of folded sedimentary rocks. What had to occur to form this feature?
ANSWER:
Inclusions within a sedimentary rock are older than the sedimentary rock itself.
Folded sedimentary layers were originally laid down flat and later deformed.
A fault or dike that cut a sedimentary sequence is younger than the sedimentary rocks it breaks through.
Undeformed sedimentary layers present on one side of a river­cut canyon are the same as those on the opposite side.
The oldest sedimentary unit is located at the base of the sequence, while the youngest is at the top.
Correct
Part D
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An undeformed sequence of sedimentary rocks is exposed in a large river canyon. Which two principles would be demonstrated by the rocks?
Hint 1.
Think back to the five principles you learned about in the video. Which two would be the most applicable to an undeformed rock sequence that has been eroded by a
large stream?
ANSWER:
principles of lateral continuity and inclusions
principles of superposition and lateral continuity
principles of cross­cutting relationships and superposition
principles of superposition and dating by inclusions
principles of lateral continuity and cross­cutting relationships
Correct
Part E
An igneous dike cuts through limestone, but not through the overlying sandstone. Which of the following statements is most accurate?
Hint 1.
Think about how the principles of superposition and cross­cutting relationships are used for this question.
ANSWER:
First, the sandstone was laid down, next the limestone was deposited, and finally was cut by the igneous dike.
The limestone and sandstone were deposited and then cut by the igneous dike.
First, the limestone was laid down, then intruded by the igneous dike, and lastly the sandstone was deposited.
The igneous dike represents the oldest rock, while the sedimentary rocks are relatively younger.
First, the limestone was laid down, folded and cut by an igneous dike, and finally the sandstone was deposited.
Correct
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GeoTutor: Constructing an Order of Sequence of Geologic Events ­ Geologic Time Scale
Geologists have divided the whole of history into units of increasing magnitude. This is called the geologic time scale. The entire time scale was originally based on relative dating,
since radiometric dating was not available at the time. Absolute dating techniques determine a numerical age of strata given in number of years. Relative dating techniques, on the
other hand, determine the age of a stratum relative to other strata (i.e., if it is younger or older), without providing any numerical data.
Geologists have been able to determine the relative ages of rocks and any fossils they contain to reconstruct a history that reveals the evolution of Earth's continents and living
organisms using four laws of stratigraphy:
1. Law of Superposition: Younger strata are deposited on top of older strata.
2. Law of Original Horizontality: Strata are deposited horizontally. Tilted strata had been tilted by some geologic event after the time of deposition.
3. Law of Lateral Continuity: Layers of sediment initially extend laterally in all directions. As a result, rocks that are otherwise similar, but are now separated by a valley or
other erosional feature, can be assumed to be originally continuous.
4. Law of Cross­Cutting Relationships: Magma intrudes and crystallizes (forming features such as faults and dikes). These features are younger than the strata they cut
through.
The geologic time scale subdivides the 4.6­billion­year history of Earth into several units, outlining the time frames of several events of the geologic past. See below for the geologic
time scale chart.
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Part A ­ Laws of stratigraphy
In the figure below, a series of geologic events, A­J, shows the configuration of rocks as seen from a road. Some strata have been tilted, and a volcanic dike has intruded some
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of the rocks. Use the laws of stratigraphy to rank these strata.
Rank the strata from oldest to youngest.
Hint 1. The Law of Cross­Cutting Relationships
The volcanic dike (H) must be older than any strata it does not cut through and younger than any strata it does cut through, because the strata it cuts through must have
been there before the intrusion of magma.
Hint 2. The Law of Original Horizontality
Pretend the tilted strata are horizontal. That is, "D" is above "A," "C" is above "A," and so on. The Law of Original Horizontality states that strata are deposited
horizontally in their original states. Tilted strata had been tilted by some geologic event after the time of deposition, but still retain their relative order.
ANSWER:
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All attempts used; correct answer displayed
Notice that the tilted strata are immediately overlain by horizontal strata. This can only occur if erosion has partially removed the tilted strata so they all terminate at the
same depth.
Part B ­ The geologic time scale and unconformities
Gaps in the rock record are called unconformities. Unconformities are caused by periods of erosion that have occurred between periods of deposition, which have erased a
portion of the rock record. There are three types of unconformities: (1) angular unconformities occur when tilted strata are overlain by horizontal strata—Click here to see an
angular unconformity; (2) disconformities occur when strata are separated by an erosional surface—Click here to see a disconformity); (3) nonconformities occur when strata
overlay igneous or metamorphic rocks that are resistant to erosion—Click here to see a nonconformity.
Now use the figure below, which has labeled each of the rock strata/layers from Part A with their respective geologic time periods, to fill in the gaps in the following sentences.
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Match the words in the left column to the appropriate blanks in the sentences on the right. Make certain each sentence is complete before submitting your answer.
Hint 1. How to determine the missing time period
Identify the youngest and oldest strata in the diagram, and use the geologic time scale provided above to find all of the geologic periods between these ages.
Hint 2. The types of unconformities
The volcanic dike terminating abruptly at a stratigraphic boundary would indicate that erosion has occurred.
Hint 3. The age of unconformities
An unconformity must be at least the age of the strata overlying it and can be as old as the strata below it.
ANSWER:
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Reset
1. The Quaternary and Tertiary rocks are separated by this type of unconformity: a disconformity .
Help
2. Due to an unconformity, the Jurassic period is missing from the rock record.
3. The Triassic rocks must have been most likely tilted during or after the Triassic period
4. The dike dates at least to the Quarternary period.
5. The Triassic and Cretaceous rocks are separated by this type of unconformity: an angular unconformity .
Correct
The tilting of the Triassic rocks could have occurred in the Triassic, Jurassic, or Cretaceous periods. This amounts to an uncertainty of at least 55 million years.
Interactive Animation: Angular Uncomformities, Noncomformities, and Discomformities
When you have finished, answer the questions.
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Part A
Which image is an example of an angular unconformity?
SEE IMAGES BELOW FOR ANSWER SELECTIONS.
ANSWER:
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Correct
Part B
In the images below, which contains a disconformity?
ANSWER:
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Correct
Part C
What does the term unconformity mean?
Hint 1.
un = NOT; conform = go along with
ANSWER:
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a missing rock layer in a sequence that represents a period of deposition
an extra rock layer that represents a period of deposition
a missing rock layer in a sequence that represents a period of erosion or nondeposition
an extra rock layer that represents a period of erosion
Correct
Part D
In the following rock sequence, how much erosion might have occurred between rock layer A and rock layer B?
ANSWER:
at least 10,000 years
none or only a very small amount (Time does not equate to erosion.)
more time than it took to deposit rock layer B
at least 1 million years
more time than it took to deposit rock layer A
Correct
Part E
What characteristic most directly DISTINGUISHES an angular unconformity from a nonconformity?
Hint 1.
The word angular is the key hint.
ANSWER:
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Angular unconformities represent missing time, whereas nonconformities do not.
Conformities represent missing rock layers.
Nonconformities separate parallel rock layers of the same rock type.
Nonconformities separate two different rock types, whereas angular unconformities form only between strata of the same rock type.
Angular unconformities separate rock layers along nonparallel surfaces.
Correct
Part F
Which list best describes the events that would lead to the layering of sedimentary rocks in this diagram?
ANSWER:
deposition, erosion, deposition, erosion, deposition
erosion, deformation, erosion, deformation, erosion
deposition, deformation, deposition, deformation, deposition
erosion, deposition, erosion, deposition, erosion, deposition, erosion
Correct
GeoTutor: Constructing an Order of Sequence of Geologic Events – Relative Dating
The ordering of events in geological history has long been a difficult task, but once simple principles were determined observation and logic could be used to determine the order of
events. With these principles, one cannot calculate the exact number of years ago an event occurred, but instead the sequence of events can be determined. This is referred to as
relative dating. The principles are as follows:
1. The law of superposition: In sedimentary rocks, the rock bed on the bottom must be older than the rock bed on the top.
2. The principle of original horizontality: Sedimentary rocks were originally deposited as flat­lying, horizontal layers.
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3. The principle of cross­cutting relationships: Any rock or feature, cutting through another rock or feature, must be younger than the material through which it cuts.
(For example, with faults, igneous intrusions such as dikes, or fractures, the first rock must be there for these secondary features to cut through.)
4. Inclusions: Any rock fragments included within another rock must be older than the rock in which they are included. (For example, if eroded fragments of one rock
layer become part of another sedimentary rock layer, the rock with the included fragments must be younger than the fragments themselves.)
Part A ­ Basic Principles for Relative Geologic Dating
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.
Drag the appropriate labels to their respective targets.
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 cut through must have been there first. This is the principle of cross­cutting relationships.
ANSWER:
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Correct
As you can see from above, using the logic of these principles when observing sedimentary rock, we can determine a sequence of events.
Now that we have investigated the principles of relative dating, we can use these principles to determine how to read the sequence of geologic events in a location.
Part B ­ Ordering of Geologic Events
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.
Refer to these relative dating principles:
1. Inclusions: Any rock fragments included within another rock must be older than the rock in which they are included. (For example, if eroded fragments of one rock
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layer become part of another sedimentary rock layer, the rock with the included fragments must be younger than the fragments themselves.)
2. The principle of cross­cutting relationships: Any rock or feature, cutting through another rock or feature, must be younger than the material through which it cuts.
(For example, with faults, igneous intrusions such as dikes, or fractures, the first rock must be there for these secondary features to cut through.)
3. Angular Unconformity: It consists of tilted or folded sedimentary rocks that are overlain by younger, more flat­lying strata. An angular unconformity indicates that
during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred.
4. Tilting or deformation could occur to an otherwise horizontally layered sedimentary rocks. Most layers of sediment are deposited in a nearly horizontal position.
Thus, when we see rock layers that are folded or tilted, we can assume that they must have been moved into that position by crustal disturbances after their
deposition. In such an instance, the tilted structure will be younger than the orginal horizontal layers.
Order the five images below along the timeline based on the sequence of geologic events. To find the oldest, look for the image that shows the least geologic changes. To find
the youngest, look for the picture that has the most geologic changes.
Rank from oldest to youngest.
Hint 1. Inclusions from rock layers above and below
In the picture where the gray layer first appears, the layer must be younger than the layers above and below because it has inclusions of both layers of rock within it
according to the principle of inclusions. Therefore, this event must have happened after the picture without the gray layer. This can occur when igneous rock intrudes
between layers of sedimentary rock and incorporates pieces of the rock layers above and below into the cooling magma.
Hint 2. The oldest and the youngest geologic features/events
The oldest geologic feature should have the least geologic changes and the youngest should have all features from the previous events.
ANSWER:
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All attempts used; correct answer displayed
As you can see, you can apply the logic of the principles of relative dating to successfully sequence the order of geologic events in a scene. The principles allow you to tell
the geologic story of a landscape.
Lab Activity 8.2.1 ­ Relative Dating
Now that you have practiced ordering geologic events that occurred within a scene or outcrop, you will relate the five geologic laws to this process. First, apply geologic laws to an
outcrop in the order that they are invoked by events within said outcrop. Then examine a second scene, where you will identify the geologic laws that explain the relative orders of
pairs of events.
Part A ­ Applying Geologic Laws in Order
Please rank from first to last the geologic laws that are used to determine the relative order of the four events that are labeled (but not ordered) in the drawing of the outcrop
below.
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Please rank the geologic laws used for the history of this outcrop from first to last.
You did not open hints for this part.
ANSWER:
Part B ­ Supporting an Outcrop’s History with Geologic Laws
For each rectangle associated with a pair of geologic structures or events, please identify the name of the geologic law that determines which of the two events within the pair
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occurred first.
Drag the appropriate labels to their respective targets.
You did not open hints for this part.
ANSWER:
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Gigapan: Virtual Fieldwork—Relative Dating and Unconformities
Geologists can determine the geologic history of an area by describing rock outcrops and analyzing the layers of rock. Today you will be a geologist visiting a rock outcrop virtually.
You will be able to zoom in and out of the Gigapan image to explore the outcrop and determine the relative ages of rock layers and the geologic history of the area by applying your
knowledge of the principles of geology and unconformities.
The principles of geology that you will use in this example are:
The law of superposition: A sedimentary rock bed on the bottom must be older than the rock bed on the top.
The principle of original horizontality: Sedimentary rocks were originally deposited as flat­lying, horizontal layers.
The principle of lateral continuity: Sedimentary layers, when formed, extended horizontally in all directions.
You will also use your knowledge of unconformities, features created when deposition stopped, uplifting and erosion occurred, and, after a period of time, sedimentation began anew
above the eroded layer. There are three main types of unconformities: A nonconformity is found where igneous or metamorphic rocks have eroded and then sedimentary rock layers are deposited above.
A disconformity is a break between parallel sedimentary rock layers above and below. Disconformities represent times when sediments were not deposited or were
eroded.
An angular unconformity is found where sedimentary layers were tilted and eroded and younger and more flat­laying sedimentary layers were deposited above. In this exercise, you will use Gigapan technology to:
become familiar with interpreting rock outcrops,
understand the sequence of events that occurred as these rocks formed and changed over time, and
identify the location of an unconformity in this outcrop and provide evidence for its type.
Gigapan technology mosaics thousands of photos together into a single image, allowing you to zoom in and see the tiniest of details. Imagine zooming in on a grain of sand on a
photo of a beach!
Instructions for all Parts:
1. Launch the Gigapan image http://www.gigapan.com/galleries/10030/gigapans/129421
2. You can zoom into the image to take a close look at the angular unconformity.
Instructions for Part A:
1. Scroll down and click on the Google Earth link on the Gigapan site to launch the Gigapan image in Google Earth.
2. Close the photo by clicking on Exit Photo to see your field site location in Google Earth.
3. Zoom in or out to determine your location. Also, on the upper right side, your will find the north arrow. If "N" is not aligned with "North" move it to North. This will ensure that the
alignment of your field site is directly facing you in an east­west direction.
4. You can reopen the Gigapan image by clicking on Angular Unconformity, west of El Paso, Texas on the left pane of Google Earth.
5. Do not close Google Earth.
Part A ­ Locating your field site
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 other nearby field sites. Now, determine where you are (your field site) in the world. Choose the map that best locates your field site.
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You did not open hints for this part.
ANSWER:
Instructions for Part B:
1. Go back to the Gigapan image for the Angular Unconformity, west of El Paso, Texas.
2. Examine the outcrop carefully. Make note of any features that would show up on a map (e.g., roads, trees, etc.).
3. Now switch back to Google Earth and zoom in or out to determine how the outcrop is oriented (runs north to south, runs northeast to southwest, etc.) compared to where you are
standing and viewing the outcrop. If "N" is not aligned with "North" move it to North.
Part B ­ The orientation of the outcrop
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Rock units tell us about Earth’s past, so 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. Also, in terms of the scientific method, it helps other geologists to be able
to recreate the field investigation step­by­step to confirm or refute any previous findings.
Imagine visiting this outcrop, standing at the location where the Gigapan image was taken, and observing the natural and built features around you. Choose the most accurate
representation of the outcrop’s orientation and your vantage point (where you are standing in relation to the outcrop). The representations below depict you and the outcrop as
viewed from above. Similar to how you identified the location of this outcrop in the previous part, use Google Earth at a multiple zoom levels. The yellow dot is the point where
the Gigapan image was taken.
You did not open hints for this part.
ANSWER:
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Instructions for Parts C and D:
1. Exit Google Earth and go back to the Gigapan image for Angular Unconformity, west of El Paso, Texas.
2. Examine the outcrop carefully, and make note of the orientation of the layers of sedimentation in this image. Are all of the rock layers running in the same direction? Does the formation
contain layers running at different angles?
3. Recall that angular unconformities refer to the junction between sedimentary rocks at an angle and rocks that are more horizontal and represent a time when the rocks were uplifted and
eroded. Can you see the evidence of uplift and erosion in the image?
Part C ­ Analysis of an outcrop sketch
Where you see layers of sedimentary rock at an angle in contact with rocks that are horizontal, they are separated by a surface called an angular unconformity. This erosion
surface represents a time when rocks were eroded before new layers of rock were formed. This can also occur during a pause in deposition, when a period of deformation (such
as folding or tilting) has occurred.
Choose the sketch that best represents the rock outcrop.
You did not open hints for this part.
ANSWER:
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Part D ­ Making observations I
Simple yet thoughtful observation exposes the history of an outcrop. The sedimentary rocks in the Gigapan image were formed as sediment accumulated as layers that stacked
atop older layers. As layers became lower in the stack sequence and covered by newer layers, they became rocks.
If this area had been under water, the shells of organisms would have become limestone, a rock that can't be identified visually but can be identified using field­based
tests. Underwater movement of sediment may also create mixes of fine and coarse grains. This sediment becomes conglomerate, a rock clearly identifiable given its combined
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coarse and fine grains. Over time, some layers would have become exposed as the water retreated and the rock layers above them were eroded. Additionally, some layers
would have been tilted by tectonic forces.
Classify the observations according to the rock that they describe, or choose “Not enough information to tell.”
Drag the appropriate items to their respective bins. Each item may be used only once.
You did not open hints for this part.
ANSWER:
Part E ­ Making observations II
Choose the location of the unconformity.
You did not open hints for this part.
ANSWER:
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Part F ­ Making observations III
Now that you have identified the unconformity in this outcrop, can you explain why it is an angular unconformity? Review the statements below, and indicate which are correct.
Select all that apply.
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You did not open hints for this part.
ANSWER:
It is an angular unconformity because layers of sedimentary rock are above and below the unconformity and the layers above and below are not parallel.
It is an angular unconformity because it is at an angle to the ground surface instead of parallel.
The tilting of the layers of rock occurred before erosion of the unconformity surface.
It is an angular unconformity because the layers of sedimentary rock above and below the unconformity are at the same angle.
Part G ­ Drawing conclusions from the timing of events
Review the outcrop again. Order the specific locations identified in the outcrop by their age. Note where the arrow, square, and circles are located.
Rank the areas identified in the cross section from oldest to youngest.
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ANSWER:
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Homework 5 Geologic Time
Part H ­ Forming a conclusion: Determining the geologic history of an area from an outcrop
Geologists collect observations from field sites and then summarize their interpretations. It’s your turn to take everything you learned while exploring the rocks in this formation
near El Paso, Texas, into a coherent story. Arrange the following geologic events in the order that they occurred.
Rank from oldest to youngest.
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ANSWER:
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Homework 5 Geologic Time
Interactive Animation: Radioactive Decay
When you have finished, answer the questions.
Part A
What happens during radioactive decay?
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ANSWER:
Daughter isotopes turn into energy.
Parent isotopes turn into energy.
Energy turns into daughter isotopes.
Parent isotopes turn into daughter isotopes.
Daughter isotopes turn into parent isotopes.
Part B
What is the scientific definition of half­life?
ANSWER:
the number of parent isotopes that will be lost during a single radioactive decay event
the number of daughter isotopes that will be gained during a single radioactive decay event
the amount of time over which the number of daughter isotopes increases by half
the amount of time over which the number of parent isotopes decreases by half
Part C
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?
ANSWER:
The rate of decay of atoms in container B is the same as the rate of decay of atoms in container A.
The rate of decay of atoms in container B is greater than the rate of decay of atoms in container A.
The rate of decay of atoms in container A is greater than the rate of decay of atoms in container B.
Part D
A container holds 100 atoms of an isotope. This isotope has a half­life of 1.5 months. How many total atoms will be in the container after 3 months?
ANSWER:
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Homework 5 Geologic Time
100 atoms
50 atoms
33 atoms
25 atoms
Part E
A container holds 100 atoms of an isotope. This isotope has a half­life of 1.5 months. How many atoms of the radioactive isotope will be in the container after 3 months?
ANSWER:
25 atoms
33 atoms
50 atoms
100 atoms
Part F
A rock sample contains 75 atoms of a parent isotope and 25 atoms of a daughter isotope. The half­life of the parent isotope is 100 years. How old is this rock?
ANSWER:
25 years old
50 years old
75 years old
100 years old
GeoTutor: Constructing an Order of Sequence of Geologic Events ­ Dating with Radioactivity ­ 2
You probably have read or seen stories about archeological findings that include organic remains of a 1000­year­old mummy or an ancient weapon made from stone, which is an
inorganic material. Geologists and paleontologists calculate the age of these organic (contain carbon) and inorganic (do not contain carbon) materials by radiometric dating using the
isotopes C­14 and U­235, respectively.
1. C­14 dating: This process is often known as radiocarbon dating. It is used to determine both historical and recent events of archeological artifacts of biological origin
such as bone, cloth, wood, and plant fibers.
2. U­235 dating: This is used to determine the age of inorganic substances such as ancient rocks and minerals.
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Homework 5 Geologic Time
Part A ­ Calculating the Age of a Fossil Based on the Number of Half­lives Elapsed
Each isotope has a unique half­life. The half­life of an isotope is the time taken for half of the starting quantity to decay (with a ratio of 1:1). After two half­lives, there will be one­
fourth of the original parent sample and three­quarters would have decayed to the daughter product (with a ratio of 1:3). After three half­lives, the ratio becomes 1:7, and so forth.
The graph, for instance, shows that assuming the half­life of a sample is 4 months, then in 4 months, there will be 0.5 gram of the parent element and 0.5 gram of the daughter
element will be produced. In month 8 (which is two­half­lives), there will be only 0.25 gram of parent element left and 0.75 gram of daughter element; that is, one­fourth of the
parent sample (in red) is left, and in month 12, there is only one­eighth of the parent element.
You attend a geology lab where you are asked to estimate the age of a fossil. The ratio of parent to daughter elements in the fossil sample is 1:7. You know that fossils are the
remains of living organisms, which have some amount of C­14 isotope. The C­14 isotope, which has a half­life of 5730 years, begins to decay as the organism dies.
What would be your estimation of the fossil's age?
You did not open hints for this part.
ANSWER:
22,920
5730
2865
11,460
40,110
17,190
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Homework 5 Geologic Time
Part B ­ Radiometric Dating of Organic and Inorganic materials
John is assisting a geologist who has traveled across the world and collected a few samples. He asks John to classify the samples that can be dated using carbon­14 and
uranium­235 (or U­235). All organic materials contain carbon and are dated using C­14; inorganic materials are dated using any radioactive element, such as uranium, rubidium,
potassium, and thorium, except carbon. Now, help John group the samples.
Drag the appropriate items to their respective bins. Each item may be used only once.
You did not open hints for this part.
ANSWER:
Chapter 18 Reading Quiz Question 2
Part A
Which geological principle states that even if most sedimentary rock layers are presently folded, they were deformed after deposition?
You did not open hints for this part.
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ANSWER:
principle of original horizontality
law of superposition
principle of cross­cutting relationships
principle of unconformities
principle of inclusions
Chapter 18 Problem 1 Multiple Choice
Part A
An unconformity is a buried ________.
ANSWER:
surface of erosion separating younger strata above from older strata below
surface of erosion with older strata above and younger strata below
fault or fracture with older rocks above and younger rocks below
fault or fracture with younger strata above and older strata below
Chapter 18 Problem 2 Multiple Choice
Part A
Which of the following best characterizes an angular unconformity?
ANSWER:
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Tilted strata lie below the unconformity, and bedding in younger strata above is parallel to the unconformity.
Horizontal lava flows lie below the unconformity, and horizontal, sedimentary strata lie above.
It is the discordant boundary between older strata and an intrusive body of granite.
Tilted strata lie below the unconformity with loose, unconsolidated soil above.
Chapter 18 Problem 6 Multiple Choice
Part A
By applying the law of superposition ________ dates can be determined.
ANSWER:
conventional
radiometric
relative
both relative and radiometric
Chapter 18 Problem 9 Multiple Choice
Part A
Sandstone strata and a mass of granite are observed to be in contact. Which of the following statements is correct geologically?
ANSWER:
The sandstone is younger if the granite contains sandstone inclusions.
The granite is older if the sandstone contains pebbles of the granite.
The granite is older if it contains inclusions of sandstone.
The sandstone is younger if it shows evidence of contact metamorphism.
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Chapter 18 Problem 28 True/False
Part A
A disconformity is an erosional unconformity with parallel beds or strata above and below.
ANSWER:
True
False
Chapter 18 Problem 12 Multiple Choice
Part A
A worm would stand a poor chance of being fossilized because ________.
ANSWER:
worms have been rare during the geologic past
worms have no hard parts
worms contain no carbon­14
all of these
Chapter 18 Problem 51 Short Answer
Part A
The remains or traces of prehistoric life are called ________.
ANSWER:
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Homework 5 Geologic Time
Chapter 18 Problem 16 Multiple Choice
Part A
Which of the following is not a very long­lived, radioactive isotope?
ANSWER:
C­14
K­40
U­238
Rb­87
Score Summary:
Your score on this assignment is 47.1%.
You received 7.06 out of a possible total of 15 points, plus 0 points of extra credit.
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