When the subducting plate is moving in the same

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same direction, the dip is about 45 degrees.  When the subducting plate is moving in the same  direction at the same speed, the dip is nearly vertical. 11. What is the variation in silica content of rift and subduction volcanoes and what does this variation  affect? (page 83, para 1) The silica content of most rift volcanoes is about 50%.  The silica content of subduction volcanoes  varies from 50-75%.  The silica content largely determines which minerals crystallize from the cooling  melt, and thereby controls the type of volcanic rocks that are formed and the sediments that are  derived from their erosion.  12. What three factors affect the chemical composition of volcanic rocks? (page 85, para 1) Three factors that affect the chemical composition of volcanic rocks are: partial melting, partial  crystallization, and contamination by surrounding rocks. 13. What is "partial melting" and how does it result in lavas in differing chemical compositions? (page  85, para 1) Partial melting is the process by which part of the solid upper mantle or lower crust of the Earth is  turned into magma.  In partial melting, the first fraction to melt will have a larger proportion of the low  melting point compounds than the parent rock 14. What is "partial crystallization" and how does it result in lavas in differing chemical compositions?  (page 85, para 2) Partial crystallization happens as molten rock slowly cools, the minerals with higher melting points  crystallize first, and the residual melt keeps changing in composition. 15. What types of partial melting or partial crystallization create a liquid with a higher silica content?  (page 85, para 3) Both partial melting with small quantities of water present and partial crystallization during the ascent  of magma to the surface produce a liquid with a higher silica content than that of the starting material. 16. Why are subduction volcano lavas more likely to be chemically contaminated than oceanic rift  lavas? (page 85, para 4) The magma that supplies subduction volcanoes often must ascend through ancient continental rocks,  already high in silica, which greatly increases their risk for contamination. 17. What chemical differences result in subduction volcano lavas having more explosive eruptions than  mid-oceanic rift lavas? (page 86, para 2) - (See also the module lecture webpage "Ring of Fire".)
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The greater proportion of water and carbon dioxide dissolved to the magma of subduction volcanoes,  coupled with slightly lower eruption temps that in rift volcanoes, leads to more explosive eruptions. 18. What are the differing products of subduction versus oceanic-rift volancoes? (page 87, para 2) The products of subduction volcanoes are explosive, such as ash, purnice, cinders, blocks, and molten  lava bombs.  The products in rift volcanoes are non explosive, effusive lava flows.
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  • Fall '08
  • Campbell-mccrea,M

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