Answer
The lava associated with shield volcanoes is typically characterized by its low viscosity and basaltic composition. Here are some key points about the composition and viscosity of shield volcano lava:
Composition:
- Shield volcanoes are formed primarily by the eruption of basaltic lava, which is rich in iron, magnesium, and calcium, and has a relatively low silica content.
- Basaltic lava is derived from the upper mantle or the oceanic crust and is often associated with hotspots or divergent plate boundaries.
- The lava is mafic in composition, meaning it is rich in dark-colored minerals like pyroxene and olivine.
Viscosity:
- Shield volcano lava has a low viscosity, which means it flows more easily compared to lava with higher viscosities.
- The low viscosity is attributed to the basaltic composition, which has a lower silica content and a higher temperature compared to more silica-rich lavas like andesite or rhyolite.
- The low silica content reduces the amount of polymerization (bonding) between the lava's silicate minerals, resulting in a more fluid and less sticky lava flow.
- Due to its low viscosity, shield volcano lava can travel long distances in broad, gentle slopes, creating expansive lava flows.
The combination of basaltic composition and low viscosity lava contributes to the characteristic shape of shield volcanoes. The lava flows easily from the central vent and spreads out in broad, gentle slopes, building up the shield-like structure over time. This lava behavior allows for the formation of large, relatively flat volcanic cones with a low height-to-width ratio.
It's worth noting that the exact composition and viscosity of shield volcano lava can vary depending on specific factors such as the local geology, the presence of volatile gases, and the eruption dynamics. However, overall, shield volcanoes are typically associated with basaltic lava flows that exhibit low viscosity and can travel significant distances.
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