Chapter 19 Ans

Chapter 19 Ans - Answers for Chapter 19: Continental...

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Answers for Chapter 19: Continental Alkaline Rocks 1. How do active and passive rifting differ from one another? What is actually active or passive in these situations? Active rifting occurs when extension is driven by mantle rising, and passive rifting occurs when extension results in mantle ascension. Interestingly, it’s not the rifting (extension) that’s the focus here, but the upwelling mantle. In active rifting the upwelling is actively driving the overall upwelling and rifting process, and in passive rifting the mantle upwelling is a passive response. 2. Sketch a three-rift triple junction and indicate the motion of the three plates, with mutually consistent vector arrows, one on each plate, such that one rift arm spreads much more slowly than the other two. The vectors on the diagram at right are compatible with the southern spreading leg being slower than the other two. 3. Cratons are the oldest part of the continents. What are the two major younger parts, and how do they relate to the cratons? Stable interior platforms and orogenic belts. Stable platforms are probably eroded and depressed cratonal areas with a thin veneer of later sedimentary strata covering the lowlands and orogenic belts are typically areas that have been subjected to later subduction-related activity, including island arc accretion or continental collision. 4. What mineralogical characteristics do nephelinites, leucitites, and melilitites have in common? What would they be called if plotted on Figure 2.4 (see Table 19.2)? What is melilite? 1
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They are all highly silica-undersaturated volcanics that contain abundant (clinopyroxene plus the mineral from which they derive their name). Melilite is a Ca-Na-rich sorosilicate: (Ca,Na) 2 (Al,Mg,Fe 2+ )(Si,Al) 2 O 7 that occurs in silica-undersaturated rocks. 5. How do tephrites and basanites differ from the three rock types in the previous question, and how do they differ from each other? Tephrites and basanites are more like silica-undersaturated basalts (plagioclase is abundant) that contain a feldspathoid. Basanites typically have olivine and tephrites do not. 6. Briefly describe the stages of African rifting and volcanism. Extension began with lithospheric thinning and diffuse faulting (the “prerift stage”), followed by listric faulting and the development of half-grabens. Faulting in the opposite sense to the initial listric faults usually occurs slightly later, resulting in the formation of an axial graben system (the “rift stage”). Prerift volcanism began to the north in Africa with the extrusion of extensive flood basalts covering the thinning lithosphere. Rhyolites and rhyolitic ignimbrites accompanied the later flood basalt activity. The rift stage began in the Eastern Branch with a half graben about 15 Ma ago, and a full graben at about 4 to 3 Ma. Activity in the narrower and less active Western Branch began with transitional basalts giving way to alkaline basalts. About 3 Ma ago, volcanism in the Western Branch became more alkaline, more silica-undersaturated, and notably more
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Chapter 19 Ans - Answers for Chapter 19: Continental...

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