GEOL212 Due 10/16/17 Homework VII

Transcription

1 GEOL212 Due 10/16/17 Homework VII General instructions: Although you are allowed to discuss homework questions with your classmates, your work must be uniquely your own. Thus, please answer all questions in your own intelligible words. (When in doubt, use complete sentences.) If calculations are involved, show all work (so that I will have some basis for giving partial credit.) Be sure to use appropriate units and significant figures, where appropriate. Plate tectonics: Merck s experience is that well-meaning people often think they grasp plate tectonics when they actually don t. Because we understand so much more about Earth than we do any other planet, it is the foundation of our understanding of all planetary bodies, so let s make sure you understand this significant feature of its geology. For fun, let s use fictional examples. Middle Earth: The release of the Lord of the Rings trilogy of films, along with the Hobbit films of the last three years has sparked a cultural resurgence for J. R. R. Tolkien s classic novels. Despite this, no one has really looked at the geology of Middle Earth, the region where the stories take place. That now falls to you. Let s assume that Middle Earth has a lithosphere and asthenosphere like the real Earth s. Examine the map in figure 1 and answer the following questions about it. 1.) Tolkien aficionados have little to say about the Ered Luin, the orogeny of region A in the northwest because no action of the Lord of the Rings trilogy occurs there. As our map reveals, they are actually rather interesting because their northern half (north-northeast of the A) contains several simultaneously active volcanoes. Bathymetry of the ocean nearby reveals a deep trench (B) that parallels these active volcanoes. Seismology reveals that the region is earthquake prone. The ones with the shallowest foci are near the trench, with deeper ones occurring at greater distances southeast of it. a.) Do these mountains represent a peripheral or an interior orogeny? b.) Provide an example of a similar orogeny from our Earth. 2.) The plate boundary. a.) Do the features described above represent a plate boundary? If so, what kind?

2 b.) Draw arrows on the map indicating the relative direction of plate motion here. 3.) Although Erebor, the Lonely Mountain (C) gives off fire and heat, we ignore it here because these phenomena are caused by a dragon a better topic for a biology course. Instead consider the single large active volcano (D) at the east end of Ered Mithrin, in the north. Examination reveals that the other mountains of this range are extinct volcanoes and that they get progressively older as one traces them from east to west. a.) What kind of volcanic region does this mountain represent? b.) Assuming that the magma source for this mountain is stationary, in what direction is the lithospheric plate on which the mountain sits moving? 4.) Also in the eastern half of the map are linear ranges of mountains or hills at H, I, and J. For each, assume that it originates from the same process as Ered Mithrin and indicate whether its orientation (and the lithospheric direction of motion it implies) is consistent or inconsistent with your interpretation above for Ered Mithrin. 5.) The Misty Mountains run from north to south, bisecting Middle Earth. Although they don t contain active volcanoes, they do show the remnants of very ancient extinct ones. These are concentrated on their western flanks. a.) Do these represent a peripheral or an interior orogeny? b.) Provide an example of a similar orogeny on our Earth. c.) Do you see any evidence that the Misty Mountains are anywhere near a plate boundary that is currently active, or does it represent an ancient boundary? 6.) A cross-section of the Misty Mountains at the location F-G is given in the inset box. Other cross-sections are similar. We see continental crust on both sides of the mountains, however the crust is different on the east and west sides.

3 a.) In the west, we see ancient remnants of the rise of magma through the crust, including large bodies of granite (magma that solidified underground), frozen bodies of basalt, and the remnants of ancient volcanoes. What kind of ancient plate boundary might the western flanks of the Misty Mountains represent? b.) In the east, we see none of these volcanic features. Instead, we see that large blocks of crust have been broken and pushed on top of one another. What event in in the history of the continental crust making up the eastern flank might be responsible for this? c.) Note that between the continental crust of the eastern and western flanks are remnants of oceanic crust and sea-floor sediment that have somehow gotten themselves stranded in the middle of a mountain range. How might they have found their way to the middle of a continent? d.) Your answers above should be consistent with your answer to question 3b. Are they? Tau Ceti: 7.) Again it is 2675 and you are back on an Earth-like exoplanet orbiting Tau Ceti. Fig. 2 provides a schematic map of the geology of a small continent under survey and the islands and oceans near it. Geologists have determined that the exoplanet has an Earth-like lithosphere and asthenosphere. The locations of active volcanoes, young mountains, ancient eroded mountains, oceanic trenches, and mid-ocean ridges are marked. Draw lines on the figure indicating the locations of all plate boundaries for which there is clear evidence. 8.) For each boundary, indicate whether it is convergent, divergent, or transform. 9.) Orogenies: a.) What kind of orogeny is represented by the mountains at location C? b.) What kind of feature is A?

4 10.) Indicate with arrows the directions of relative motion of the lithospheric plates you have identified. Extra Credit (5 pts): Return to the map of Middle Earth and reconsider the crosssection of the Misty Mountains. If a subduction zone had ever been active there, was the subducting slab diving toward the west or toward the east?

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