Chapter 18: Fossils
The information below refers to six isotopes. How many elements are represented in this group of isotopes?
Three
A _____ is formed when magma intrudes between existing sedimentary layers.
sill
What is the principle of cross-cutting relationships?
Geologic features that cut through rocks must form after the rocks that they cut through.
Below are a series of sketches that show rock units organized by color, and traced across valleys with dotted lines. The keys include information on rock types and relative ages. Remember creating a clear and accurate key is important when sketching in the field. Pay special attention to the keys in these drawings when you make your choice. Select the correct sketch below.
Image 4
Which of the following best describes why the Vishnu Schist is visible at Earth's surface?
The Vishnu Schist was exposed when sedimentary layers above it were eroded. Submit
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.
What does the term unconformity mean?
a missing rock layer in a sequence that represents a period of erosion or nondeposition
Which list best describes the events that would lead to the layering of sedimentary rocks in this diagram?
deposition, erosion, deposition, erosion, deposition
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
Use the principles of relative dating that were discussed in the video to fill in the blanks of the sentences below.
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.
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 C-14 isotope. The C-14 isotope, which has a half-life of 5730 years, begins to decay as the organism dies. What would be your estimation of the fossil's age?
17,190
What scenario requires a geologist to use the principle of cross-cutting relationships?
A dike truncates layers in a sedimentary succession.
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
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
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.
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.
(Oldest Event) -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.
What does the Vishnu Schist tell you about the history of the Grand Canyon prior to the formation of the canyon itself? What is the order of events in the Grand Canyon? Place the following events in the order in which they happened.
(Oldest event) -Formation of Vishnu Schist -Deposition of sedimentary layers found above the Vishnu Schist -Carving of the canyon by the Colorado River (Most recent event)
Where else might you expect to find metamorphic rocks like the Vishnu Schist of the Grand Canyon?
-Deep beneath Earth's surface, where sedimentary layers have been buried. -In a mountain range.
If one half-life has lapsed, what is the radioactive parent to stable daughter isotope ratio?
50:50
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
Based on the same principles as above, classify the radioactive decays as alpha emission, beta emission, or electron capture.
Alpha emission: -226Ra88 -218Po84 Beta Emission: -40K19 -234Th90 Electron Capture: -40K19
What characteristic most directly DISTINGUISHES an angular unconformity from a nonconformity?
Angular unconformities separate rock layers along nonparallel surfaces.
By referring to the periodic table of elements, determine the appropriate number of protons and neutrons for each isotope. Remember that atomic mass of an element is the number of protons (which is the atomic number) and the number of neutrons. Elements are defined by the number of protons they have.
Atomic number = 19 Mass number = 39 -K (potassium)20 neutrons Atomic number = 6 Mass number = 13 -C (carbon)7 neutrons Atomic number = 6 Mass number = 12 -C (carbon)6 neutrons Atomic number = 19 Mass number = 41 -K (potassium)22 neutrons Atomic number = 82 Mass number = 207 -Pb (lead)125 neutrons Atomic number = 82 Mass number = 206 -Pb (lead)124 neutrons
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 carbon-14 and uranium-235 (or U-235). All organic materials contain carbon and are dated using C-14; inorganic materials are dated using any radioactive element, such as uranium, rubidium, potassium, and thorium, except carbon. Now, help John group the samples.
Carbon-14: -Mummies -wood pieces U-235: -Gneisses -granite -volcano ash
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.
The Principles of ___________ and __________ were used to determine relative ages in the previous question.
Inclusions, Cross-Cutting Relationships
Examine the images of the Grand Canyon below. Notice that most of the canyon consists of layers of sedimentary rocks, but if you were to hike down into the inner gorge you would encounter the Vishnu Schist, a metamorphic rock. Which of the following processes were involved as the Vishnu Schist metamorphosed?
Intense pressure due to regional metamorphism.
Which of the following statements regarding radioactive decay is true?
More daughter products accumulate over time. Submit
What happens during radioactive decay?
Parent isotopes turn into daughter isotopes.
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.
In the images below, which contains a disconformity?
Rock 3 Rock 2 Rock 1
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
Which image is an example of an angular unconformity?
Sedimentary rock 6 with others angled underneith
What is the principle of original horizontality?
Sedimentary rocks are close to horizontal when deposited.
Which principles of relative dating did you use to answer the previous question?
Superposition and Cross-Cutting Relationships
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
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. Submit
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.
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. Submit
Using principles of relative dating, classify the statements according to whether they are "true," or " false".
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 inclusion formed originally in the sandstone country rock. -The dike is older than the inclusion.
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. Submit
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
What is the scientific definition of half-life?
the amount of time over which the number of parent isotopes decreases by half
