Title:
Exploring the Earth 2: Hawaii and Measuring Geologic Time
Description:
We have explored the evidence for plate tectonics and found that earthquakes and volcanoes provide basic evidence about the lithospheric plates that move around on Earth’s surface. For example, we found that most volcanoes occur at the boundaries between plates. The volcanoes of Hawaii, however, occur in the center of a plate. In this exploration, we will see how Hawaii has formed and how radiometric dating can be used to determine the ages of Hawaiian volcanism and other geologic events.
Instructions:
You must complete this assignment online before 9:30 AM on ??. Before you begin this assignment, make sure that your browser window is the size you want, because resizing the window while you are submitting your answers will cause the work you have already done to be lost.
Question 1 [multiple choice; set so that the correct answer is displayed when students receive their scores; ONE POINT]
[image: copy pacific.basin.gif after the question and before the answer box.)
You have learned that most volcanoes occur at plate boundaries. However, if we look at the Hawaiian volcanoes, we see that they are located in the middle of the Pacific plate, not at its edge. The map below (from a U.S. Geological Survey online publication about Hawaii: shows the north-central part of the Pacific Ocean sea floor. The boundaries of the Pacific plate are around the edges of the Pacific Ocean. The Hawaiian islands, in the middle of the ocean, are part of a long chain of seamounts (Hawaiian Ridge and Emperor Seamount Chain) that extends westward and northward to the Aleutian trench. The seamounts are volcanoes that are no longer active and that have sunk below the surface of the sea. Which of the following statements best describes the appearance of the Hawaiian / Emperor islands and seamounts on the sea floor?
A)They form a random pattern, with no recognizable shape.
B)They form a circular pattern on the sea floor.
C)They form a continuous line extending from east to west.
D)They form two lines with a bend between them.
Question 2 [multiple choice; set so that the correct answer is displayed when students receive their scores; ONE POINT]
[image: copy hawaiian.chain.gif after the question and before the answer choices.]
The map below is similar to the map in Question 1, but it also includes ages of some of the islands and seamounts. The ages shown on the map (<1 my to 70 my) are the ages of the volcanoes; they indicate when lava extruded from the sea floor to form the labeled island or seamount (my = millions of years). Use the ages on the map to estimate the age of the curve between the Hawaiian Ridge and the Emperor Seamounts.
A)20 million years ago
B)40 million years ago
C)50 million years ago
D)70 million years ago
Question 3 [multiple choice; set so that the correct answer is displayed when students receive their scores; ONE POINT]
[image: copy hotspot.gif after the question and before the answer choices.]
The diagram below (also from the USGS site referenced in Question 1) explains the progression of ages observed in the Hawaiian / Emperor Seamount chain. Hot spots are places in the asthenosphere (deep beneath the earth’s surface) that are hotter than normal and that produce large volumes of melted rock called magma. (Note that when magma erupts on the sea floor or on land, it is called lava.) Hot spots are fixed in the asthenosphere (they do not move). Magma produced at a hot spot punctures the overlying lithospheric plate and forms a volcano on the sea floor. The lithosphere is not fixed in one place and when it moves over the asthenosphere, away from the hot spot, the volcano stops erupting and a new one is formed in its place. With time, the extinct volcanoes keep moving northwestward (with the Pacific plate) and new active volcanoes form over the hot spot. The white arrows show the direction of the lithospheric plate moving over the underlying asthenosphere. Which Hawaiian Island is the oldest (that is, farthest away from the hot spot)?
A)Kauai
B)Oahu
C)Maui
D)Hawaii
Question 4 [multiple choice; set so that the correct answer is displayed when students receive their scores; ONE POINT]
[image: copy islands.map.gif after the question and before the answer choices.]
The map below shows the Hawaiian Islands in more detail (map from web site of the Hawaii Center for Volcanology: Note that Loihi is an active volcano that has not yet built up above sea level. Given that the volcanoes in the Hawaiian / Emperor chain are progressively younger toward the southeast, which volcano on the big Island of Hawaii is the youngest?
A)Hualalai
B)Kilauea
C)Mauna Kea
D)Mauna Loa
Question 5 [multiple choice; set so that the correct answer is displayed when students receive their scores; ONE POINT]
[image: copy volc.age.gif after the question and before the answer choices.]
We can use the Hawaiian / Emperor Seamount chain to determine the direction of motion of the Pacific plate and how fast it is moving. (Because the eastern edge of the Pacific plate is the San Andreas fault, understanding the rate of motion of the Pacific plate helps us to calculate the frequency of earthquakes in our own state!) The diagram below shows the age of volcanoes in the chain (in millions of years) with distance from Kilauea, the active volcano on the southeast end of the big island of Hawaii. Use only the data for the past 10 million years (10,000,000 yrs). These data tell us that 10 million years ago, the Pacific plate was located 1000 kilometers (km) from its present location. How fast is the Pacific plate is moving? (Note: 1 km = 1000 m; 1 m = 100 cm)
A)1 cm/yr
B)10 cm/yr
C)100 cm/yr
D)1000 cm/yr
Question 6 [multiple choice; set so that the correct answer is displayed when students receive their scores; ONE POINT]
Establishing the ages of geologic events, such as the eruption of volcanoes in the Hawaiian Islands, is critical to understanding the processes that shape our planet. But how are these ages obtained. The link below, to the Hawaii Center for Volcanology web site, contains more information about the Hawaiian Islands. Read this to become more familiar with the region. At the bottom of this page is a chart with the age data used to create the diagram of volcano ages you viewed in Question 5. Look at the chart, which is called: Ages of some of the Hawaiian Islands and outer seamounts. What kind of ages are presented in this chart?
[Make a link to this external site; set to bring up a second window if that doesn’t happen automatically]
A)C-14
B)U-Pb
C)K-Ar
D)Rb-Sr
Question 7 [multiple choice; set so that the correct answer is displayed when students receive their scores; ONE POINT]
The methods listed in the answers for Question 6 are ways to obtain numerical dates for geologic events by radiometric dating. Minerals from rocks are used to measure the time when the rock formed. However, not all types of rocks can be used for radiometric dating. The link below is to a U.S. Geological Survey web site that explains how radiometric dating is used to calibrate the geologic time scale. Read through this page until you find a list of rocks and other materials that have been dated using radiometric (atomic clock) methods. The first items are organic materials (wood and cloth) that have been dated using the radiocarbon (C-14) method. The rest of the chart lists different types of rocks that have been dated. Which types of rocks are listed in this chart?
[Make a link to this external site; set to bring up a second window if that doesn’t happen automatically]
A)Igneous
B)Metamorphic
C)Sedimentary
D)All of the above
Question 8 [multiple choice; set so that the correct answer is displayed when students receive their scores; ONE POINT]
Radiometric dating works because certain isotopes decay over long periods of time to other isotopes. For example, Potassium-40 (K) decays over time to Argon-40 (Ar). Fortunately, this decay occurs at a predictable rate, and so if we can measure the ratio of the parent isotope (in this case, K) to the daughter isotope (in this case, Ar), we can obtain a numerical age for the rock. The half-life is the rate of radioactive decay for a particular system. For example, the K-Ar system has a half-life of 1.25 billion years. This means that after 1.25 billion years, 50% of the K in a mineral or rock will have changed into Ar.
The link below is to a web site called Virtual Dating (of rocks, that is). On the first page, click on “Virtual Dating Isochron for rocks and minerals”. Read the information provided, then click on the “Next” button to go to the next page, which explains the Half Life concept. Experiment with the interactive diagram by sliding the bar at the bottom to see how the parent (red dots) / daughter (blue dots) ratio changes with time (given in half-life increments). What fraction of the parent isotopes remain after 2 half lives?
(Note: The Virtual Dating web site uses JavaScripts—see link on first page about System and Technical Requirements. If you have problems with the Virtual Dating site, you may use the following site, where you can estimate the parent fraction using a non-interactive diagram—a graph that shows the number of parent and daughter atoms versus number of half lifes:
[Make a link to this external site; set to bring up a second window if that doesn’t happen automatically]
A)50%
B)25%
C)12.5%
D)6.25%
Question 9 [multiple choice; set so that the correct answer is displayed when students receive their scores; ONE POINT]
Not all atomic clocks have half lives of billion of years. Radiocarbon (C-14) is a system with a much shorter half life that is used to date young geologic and archeological events. Clink on the link below to return to the USGS web site about radiometric dating. The half life of C-14 is how many years? (Note that this information is provided in the text below the chart with other half-life values)
[Make a link to this external site; set to bring up a second window if that doesn’t happen automatically]
A)2,050 years
B)5,730 years
C)10,130 years
D)1,750,000 years
Question 10 [multiple choice; set so that the correct answer is displayed when students receive their scores; ONE POINT]
We have looked at how radiometric dating can be used to obtain numerical ages for rocks (sometimes referred to as absolute methods). However, because only certain minerals and rocks can be used for radiometric dating, we must also use other methods for calibrating the geologic time scale. Go to the link below and click on the third bulleted time: A Theoretical Example. What method for establishing a relative time scale (based on the temporal ordering of geologic events) is presented in this example?
[Make a link to this external site; set to bring up a second window if that doesn’t happen automatically]
A)First and last occurrence of fossils
B)Magnetic polarity reversals
C)Global sea-level changes
D)Layer with high iridium concentration