Shield volcanoes are NOT made of viscous (thick) lava; they are formed from highly fluid basaltic lava. Because the lava is thin and flows easily over long distances before cooling, it creates a mountain with a broad, gentle slope resembling a warrior’s shield. Thick, viscous lava is instead characteristic of steep, explosive “Composite” volcanoes. ANSWER: (B) They are composed of highly viscous lava.
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Geomorphology distinguishes shield volcanoes by their “effusive” eruption style. The lava, being low in silica, is very runny (low viscosity). When it erupts, it spreads out in thin layers rather than piling up into a steep cone. Consequently, shield volcanoes, like Mauna Loa in Hawaii, are massive in diameter but have very low-angle slopes. This contrast is a frequent exam point: low viscosity leads to broad shields (constructive), while high viscosity leads to steep, explosive strato-cones (destructive). Understanding lava chemistry is key to identifying these different mountain landforms.