WELCOME BACK: Procedure continued from Investigations Manual, Investigation 2A.
In the first part of this investigation, we found that the ocean floor exhibits considerable topographic relief and some of the features of the ocean floor provide information on the geologic history of the ocean basins. Indeed, the most prominent features of the ocean floor, trenches and the oceanic ridge system, are the products of enormous stresses arising from the movement of tectonic plates. In this part of the investigation, we take a closer look at the large-scale geological structure of the ocean bottom focusing on plate boundaries and associated earthquake activity.
Recall that Earth's lithosphere is broken into a dozen massive plates (and many smaller ones) that are slowly moving across the face of the globe. All plates, except the Pacific, include parts of continents as well as ocean basins. In some places, plates are spreading apart forming divergent boundaries whereas in other places, plates are coming together forming convergent boundaries. Also, some plates are sliding past one another; these are transform plate boundaries. Important geological processes are associated with plate boundaries including earthquakes, volcanic eruptions, and mountain building.
Go to the DS Ocean Website and under "Geological," click on "Bathymetry." The topography of the ocean floor and continents are displayed on this global map produced by NOAA's National Geophysical Data Center (NGDC). Elevations are color-coded in meters above (+) or below (-) mean sea level. (See the scale to the left of the map.) The map divides the globe into 45 degree by 45 degree squares.
Click on the map square that covers most of the South Atlantic Ocean basin. The topographic feature that trends roughly north/south on the ocean floor is a segment of the Mid-Atlantic ridge and marks a divergent plate boundary. (For an enlarged view, click on the image again. Then use the scroll bars to bring the ridge into view.) The plate to the west of the ridge is moving generally toward the [(west)(east)] and the plate to the east of the ridge is moving generally toward the [(west)(east)].
Lava wells up along divergent plate boundaries from Earth's interior, cools and solidifies, adding new oceanic crust to a plate's edge. Within the abutting plates, the crust migrates away from the ridge as new material is added. (Imagine two treadmills end-to-end with their treads moving outward in opposite directions.) From the Mid-Atlantic ridge westward toward the east coast of South America, the age of the oceanic crust [(increases)(does not change)(decreases)].
Return to the NGDC global bathymetry map and click on the map square that includes most of South America and the adjacent Pacific Ocean. Click on the resulting image for enlargement. Just to the west of South America and paralleling the coast is a narrow zone where the ocean floor is exceptionally [(deep)(shallow)]. This [(continental shelf)(trench)] marks a convergent plate boundary where a [(more dense)(less dense)] oceanic plate subducts under a [(more dense)(less dense)] continental plate. The nearby continental topography in the image indicates the possibility that mountain building [(is)(is not)] associated with this plate boundary.
Geologically, coasts are either tectonically active (near a plate boundary) or passive. The west coast of South America is [(tectonically active)(passive)].
Now return to the "Geological" section of the homepage and click on "Current Earthquake Activity" for a global map of the locations of earthquake activity. The map is color-coded for earthquakes that occurred during the past week, or, if chosen, for the last 30 days. The diameter of the location circle increases with the magnitude of the earthquake. (Earthquake magnitude or intensity is a measure of the amplitude of seismic waves and is specified by the Richter scale. The most intense earthquakes rank greater than 8 on the Richter scale.)
Click on "Show Last 30 Days of Earthquakes." The map shows earthquakes are most frequent along the margins of the [(Atlantic)(Pacific)(Indian)] Ocean basin. Earthquakes are [(more)(less)] frequent along the Mid-Atlantic ridge than along the Pacific plate margins.
The boundaries of tectonic plates are shown faintly on the map in yellow. The majority of earthquakes occur [(along plate boundaries)(in continental interiors)].
Go to the "Earth System" section on the course homepage and click on "NASA Earth Observatory." Click on "Natural Hazards" and then on "Unique Imagery" in the list to the right. Near the bottom of the page, click on "Unique Imagery Archive." Then scroll to bottom of the page and click on page 3. On the page that appears, scroll down to and click on "Earthquake Spawns Tsunamis." Finally, scroll down under "Other Images for this Event" and click on "Posted: Dec 28, 2004." The regional map shows the location of the earthquake (marked by a red star) that caused the devastating tsunami of 26 December 2004 which killed more than 220,000 people. Also shown on the map are the tectonic features of the area.
The Sunda (Java) Trench shows the boundary between the India and Australia Plates (referred to as the Indo-Australian Plate in Figure 2 of this investigation) and the Burma Plate. According to information provided on the map and accompanying text, this is a [(divergent) (convergent)] plate boundary along which the [(India and Australia) (Burma)] plate(s) is (are) undergoing subduction.
Image 1 was a global map of the locations of volcanic activity on 13 Sept. 2005. It is an example of maps available from "Geological" section on the course website under the link of "Current Volcanic Activity". (This map may be slow in downloading and the global map display is not currently available.) When available, yellow triangles on the onscreen map identify ongoing activity and red triangles, if any, pinpoint new activity unrest.
Based on this map and the other information you have concerning tectonic plates, it appears that the majority of ongoing volcanic activity is occurring [(along plate boundaries)(in continental interiors)].
Return to the "Geological" section and click on "Bathymetry" to return to the NGDC global bathymetry map. Click on the map square west of the U.S. that includes only Pacific Ocean. Click again to enlarge the image. Note that the "Big Island" of Hawaii is located at the southeast end of a chain of many volcanoes on the ocean floor trending toward the northwest. The tops of [(all)(some)] of these volcanoes are above sea level. This volcanic activity is believed to be unlike most volcanic activity in that it is caused by a hot spot (in Earth's mantle) over which a tectonic plate (the Pacific Ocean plate) is sliding. (See pages 50-51 of your textbook for a description of hot spots.)
Self-Assessment Revisited: Respond to the statements below. Compare your responses now with those you made at the beginning of the investigation. Has learning taken place?
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True, False |
Confidence |
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The ocean floor is relatively flat compared to the great topographical relief of the continents. |
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Processes operating at the mid-oceanic ridges and trenches provide evidence of Earth's tectonic plate movements. |
Professional Practice Transfer:
Driving Question: State an additional Driving Question appropriate to your classroom that you have conceived as the result of conducting this Investigation.
Implementation: Describe how you would use what you have learned in this investigation to implement inquiry-based learning experiences for your students.
Place the answers to Investigation 2A and the above questions on the Investigations Response Form which will be available Thursday and linked from the DataStreme Ocean website. Directions for delivery of your weekly investigation will appear at the end of the Thursday online portion of Investigation 2B.
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