Answer to Question 1
Gas content and viscosity are both greater in higher-silica magma than in lower-silica magma. Release of dissolved gas from rising magma under decreasing pressure causes expansion of bubbles in the melt that explode to cause explosive eruptions. High viscosity inhibits the ability of bubbles to rise through the magma so that gas pressures tend to be greater in intermediate and felsic magmas than in less viscous (and less gas-rich) mafic magmas. High viscosity also favors the formation of thick but not far traveled lava flows and lava domes, whereas low viscosity favors the formation of thin and extensive lava flows. These roles of gas content and viscosity to determine eruption characteristics explain why intermediate and felsic magmas generally feed relatively explosive eruptions that produce abundant pyroclastic deposits and steep-sided volcanoes, such as composite volcanoes and dome complexes. Mafic magmas, in contrast, tend to erupt less explosively and form low-sloping shield volcanoes with widespread lava flows and very little pyroclastic material.
Answer to Question 2
One obvious reason is that the more lava that is erupted, the larger the area that it will cover. We also know that the area covered by lava relates to the viscosity of the lava flows. Low-viscosity lava flows (mafic) are able to flow more readily and cover large areas, whereas high-viscosity (felsic) lava flows are unable to move beyond the immediate vicinity of the volcanic crater.
On the slopes of Chimborazo, a 6,310 m high volcanic peak in the Andes Mountains of Ecuador, Alexand
A lithograph of the eruption (circa 1888)
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Summary of SOAP Note Content