Part 2 (10 pts) A basalt consists of what minerals? (1 pts) A. B. What is the ma

Question

Part 2 (10 pts) A basalt consists of what minerals? (1 pts) A. B. What is the major element composition of a typical basalt expressed as oxide percents? (2 pts) C. D. E. F. G. How would you distinguish an alkali from a subalkaline basalt? (1 pts) Which type primaritly occurs at mid-oceanic spreading ridges? (1 pt) Which type occurs in oceanic hot-spot islands such as Hawaii? (1 pt) Which type occurs in convergent plate boundary volcanic arcs? (1 pt) What are three factors that determine whether mantle melting will yield a subalkaline or an alkaline basalt? (3 pts) Part 3 (14 pts) A. What is the fundamental cause of mantle melting and the generation of basaltic magmas below mid-oceanic spreading ridges? (3 pts) B. What is the fundamental cause of mantle melting below convergent plate boundary magmatic arcs? (3 pts) C. What type of rock melts in both of these different tectonic environments? (1 pt) D. In which of these two tectonic environments do volatiles play a more important role, and why? (3 pts) E. Why do volcanic arcs only occu r over regions of the subducted oceanic lithosphere where the subducted plate is approximately 110 km deep or deeper? (2 pts) res than are erupted along F. Why are more andesites and rhyolites erupted along convergent plate boundary volcanic a mid-oceanic spreading centers? (2 pts)

Explanation / Answer

Part 2:

A) Basalt mainly consists of calcic plagioclase, clinopyroxene and iron ore. It may also contain olivine, quartz, hornblende, nepheline, orthopyroxene, etc.

B) Basalt generally has a composition of 45-55 wt percent SiO2, 2-6 wt percent total alkalis, 0.5-2.0 wt percent TiO2, 5-14 wt percent FeO and 14 wt percent or more Al2O3. Contents of CaO are commonly near 10 wt percent, those of MgO commonly in the range 5 to 12 wt percent.

C)

D) Mid Oceanic Ridge Basalt (MORB) are found at spreading mid oceanic ridges.

At chegg we are allowed to answer only 4 subparts. Kindly, repost the remaining questions.

Sub Alkali Basalts Alkali Basalts Mid?ocean ridges, oceanic islands Island arcs, collision/subduction zones Fine-grained Coarse Grained No olivine Olivine common No alkali feldspar Interstitial alkali feldspar or feldspathoid Shallower melting (25% melting at <30 km) Deeper melting (25% melting at 60 km = alkaline basalt) Greater % partial melting ( 30% melting at 60 km) Lesser % partial melting (20% melting at 60 km = alkaline basalt)

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