Chapter 04 Ans

Chapter 04 Ans - Answers for Chapter 4: Igneous Structures...

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Igneous Structures and Field Relationships. 1. How does magma viscosity vary with the concentration of a) SiO 2 and b) H 2 O? Explain. Magma viscosity increases with increasing concentration of SiO 2 because higher SiO 2 generally results in more framework silicate formation so that melts, at least at lower temperatures, resemble in structure the minerals that would crystallize from it (in this case higher levels of -Si-O-Si-O- polymerization). The more linked polymers, the greater the resistance to flow. H 2 O breaks up the Si-O-Si polymers and therefore reduces viscosity. 2. Basaltic eruptions are rarely explosive. What evidence is there, however, that basaltic magma, when erupted, contains significant dissolved volatiles? Considerable gas emission is monitored at volcanic places such as Hawaii. Lava fountaining is also observed, which must be propelled by gas pressure. Vesicles are also evidence of gas escape as are large bubbles bursting at the surface of lava lakes. 3. If both basaltic and rhyolitic magmas have considerable volatile content, why are rhyolite eruptions typically far more explosive? The much higher viscosity of SiO 2 -rich rhyolites resists gas escape until the gas pressure becomes large enough to overcome that resistance. There is a general relationship between the SiO 2 content of a magma and the violence of an eruption. 4. Contrast a shield and a composite/strato-volcano. Shield volcanoes are typically basaltic, they are dominated by flows, they have gentle slopes and may be quite large. Composite/strato volcanoes are more silicic (andesitic to rhyolitic), are composed of both flows and pyroclastic layers, they have steeper slopes than shields and tend to be smaller. 5. What type of plutonic body would be associated with a fissure eruption? A dike: a tabular/planar body that filled a fracture that cross-cuts the external layering. 6. How does a maar differ from a scoria cone? A scoria cone results from the collection of airborne ash, lapilli, and blocks as they fall around a central vent in association with weak explosive activity. They build up on top of the land surface. A maar results from phreatic activity when magma interacts explosively with groundwater. A maar is primarily a negative feature, in that the phreatic eruption excavates a crater into the original substrate (but it is surrounded by a pyroclastic blanket). 7. How does an endogenous dome differ from an exogenous one? Endogenous domes are inflated from the inside, whereas magma breaks through the outer crust in exogenous domes. 8. How does pahoehoe differ from aa? Pahoehoe forms a smoother surface that may appear corrugated (ropy). Aa has a chunky blocky surface. The common theory is that aa flows as a cooler mass which forms a thicker crust that breaks up as the flow advances. 1
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Chapter 04 Ans - Answers for Chapter 4: Igneous Structures...

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