Why are silicic lavas more viscous than basic lavas?

Reader

It caused by the geometry of the first crystals that form as the magma begins to cool. Basaltic magmas have a relatively low amount of silica. The first crystal (or mineral) to form as a basaltic magma cools slowly is OLIVINE. The crystal structure of olivine consists of isolated components of SiO4 (the silica tetrahedron). A silicic magma, such as dacite or rhyolite, has a relatively high amount of silica. And so the first crystals to form as a silicic magma cools slowly are PYROXENE, AMPHIBOLES, and FELDSPARS. In pyroxene, silica tetrahedra form long chains. In amphiboles, silica tetrahedra form double chains. As a result, the chains fo pyroxene and amphibole crystals that are beginning to form become entangled . . . . hence, the magma is viscous. The isolated silica tetrahedra of olivine do not become entangled . . . . . hence, basaltic magma is fluid.

Sincerely,
John Dvorak

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