Chapter 01 Ans

Chapter 01 Ans - Answers for Chapter 1: Some Fundamental...

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1. In the early 19 th century the origin of igneous rocks was hotly debated between the “ Plutonists ,” who believed in an igneous origin, and the “ Neptunists ,” who believed that the crystalline nature originated as a submarine precipitate. What field and textural criteria can you imagine that would indicate the former rather than the latter? Field criteria are probably of greatest importance here, including cross-cutting relationships, chill zones, and contact metamorphic effects. The shape of many plutons would also clearly eliminate a layered sedimentary origin. Textures are less impelling, as the interlocking texture may result from some form of precipitation from an aqueous solution. Cumulate texture, if present, may be more indicative of crystallization from a melt. 2. What distinguishes: a. Volcanic from plutonic rocks? Volcanic rocks result when magma reaches the surface of the Earth. They tend to form lava flows or pyroclastic deposits. They are distinguished by their fine grain size and/or fragmental nature. Plutonic rocks crystallize underground. They generally exhibit coarser grain size and form dikes, sills, stocks, batholiths, etc. b. Pyroclastic volcanics? Pyroclastic volcanics result from explosive eruptions and have fragmental textures, c. Magma from lava ? Both magma and lava are molten rock material. Magma is the general term, but lava is restricted to magma that reaches the Earth’s surface. 3. What are the four principal compositional subdivisions of the Earth? What material or rock type broadly represents each and what is the approximate thickness of each? Continental crust: broadly granitic Oceanic crust: broadly basaltic Mantle: broadly ultramafic (peridotite and high-pressure equivalents) Core: Fe-Ni alloy (metallic) 4. An alternative way to consider the makeup of the Earth is to categorize it on the basis of mechanical properties. What are the five principal rheological subdivisions of the Earth and how do their rheological properties differ? Lithosphere: the more competent (rigid) upper ~100 km of the mantle and the crust, Asthenosphere: the more ductile portion of the mantle beneath the lithosphere. It is perhaps 150 km thick (extending to ~250 km depth). Mesosphere: the apparently stronger mantle beneath the asthenosphere. Outer core: liquid Fe-Ni alloy.
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Chapter 01 Ans - Answers for Chapter 1: Some Fundamental...

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