Chapter 12 Ans

Chapter 12 Ans - Answers for Chapter 12: Layered Mafic...

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Answers for Chapter 12: Layered Mafic Intrusions. 1. Explain the difference between an igneous layer , and igneous layering . An igneous layer is any conformable sheet-like body within an intrusion with a distinctive composition or texture. Igneous layering deals with a series of layers, describing the structure, variation and geometry. 2. What are the two principal types of graded layers? What is the simplest explanation of how each forms? Compositional (mineralogical) and grain-size graded layers. Most simply, compositional graded layers develop when an early-forming mineral crystallizes and settles, followed by settling of increasing amounts of a second phase (when the liquid evolves to a cotectic). Grain-size layering (again simply) occurs when coarse grains settle more quickly than finer grains (see Stoke’s Law in the previous chapter) and are thus more abundant toward the base of a layer. 3. Use Figure 7.2 (below) to explain several different sequences of modal layering that might develop in cooling experimental magmas in the Di-An-Fo system. Be as specific as you can as to the nature and composition of each layer in a sequence. What type or types of layering develop in your model? I simply picked three paths. The blue path results (if continued to the ternary eutectic) in Fo Fo + Di Fo + Di +An. The red path results in An An + Fo An + Fo + Di. The green path results in Di Di + Fo Di + Fo + An. Students should be able to pick six paths with all conceivable sequences (other than special cases that migrate directly away from the first phase to the ternary eutectic). 1
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4. What is rhythmic layering, and how does it differ from intermittent layering? Rhythmic layering is the systematic repetition of layers in their stratigraphic sequence. Intermittent layering is less regular, typically having some rhythmic component that is broken by occasional uniform layers. 5. Regarding your answer to Review Question 3, why is a layer of pure olivine followed by a pure pyroxenite layer so odd? What type of layering would this represent? In general, if pyroxene were to crystallize after olivine had formed to settle and create a pure
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Chapter 12 Ans - Answers for Chapter 12: Layered Mafic...

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