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Geology 101 Name: Homework assignment 3 Due in class on by e-­‐mail Monday, October 21, 5 p.m. 1. There seems to be a contradiction! On page 73 of the text, it is claimed that “Lava with a high content of dissolved gases flows more readily (less viscous) than one with a lesser amount of gases…” But on page 90, it is claimed that “In highly viscous, felsic magma, expansion is inhibited and gas pressure increases. Eventually, the pressure may become great enough to cause an explosion…” Explain the seeming contradiction between the two passages, and explain why it’s not really a contradiction. Hint: It hinges on one word. 2. What is the source of the magma that will eventually generate volcanoes in the volcanic arc at a subduction zone (like those shown in figure 4.4, page 76)? 3. Figure 4.6 (page 78) shows the process “crystal settling” (also known as “fractional crystallization”). The figure omits a critical factor of the original magma that changes as the crystal settling occurs. What is this factor, and why is it critical for this set of pictures (and for the continuation of crystal settling in this chamber)? 4. Pegmatite dikes are found throughout the Cascades, and are the principal source of gold and silver deposits found in the Cascades. Read over pages 83, 84 and 86 of the text and explain why pegmatite dikes would be associated with gold. 5. What is the most accepted explanation for where batholiths come from? (In other words, what occupied the volume that the batholith would eventually occupy?)