Answer to Question 1
The unexpectedly dark color of this felsic rock results from the presence of tiny crystals of black magnetite scattered throughout transparent glass. Thin slivers of obsidian are colorless, rather than black, which shows that even a low concentration of magnetite crystals strongly affects the color of thick samples.
Answer to Question 2
Partial melting occurs when a rock does not melt completely to form magma. Only some minerals in the rock completely melt, so the resulting magma does not have the same composition as the original rock. Partial melting typically creates magma that is relatively richer in silica than the original rock. For example, partial melting of an ultramafic rock (peridotite) forms a mafic (basaltic) magma; partial melting of a mafic rock (basalt) forms to intermediate or felsic magmas.
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