Which statement about lava viscosity and eruptions is correct?

• A. Mafic basaltic lava has high viscosity leading to explosive eruptions
• B. Intermediate or felsic lava tends to have high viscosity and explosive eruptions
• C. Ultramafic lava has the highest viscosity among common lavas
• D. All lava types erupt explosively with equal likelihood

Answer: (B)

Viscosity in lava is largely set by silica content and temperature: silica-rich magmas polymerize into sticky networks, making them high in viscosity, while silica-poor magmas are hotter and flow more easily. When magma is highly viscous, gases get trapped and pressure builds, promoting fragmentation of the melt and explosive eruptions that eject ash, pumice, and other pyroclastic material. In contrast, mafic lavas (basaltic) are generally low in silica, hotter, and flow readily as lava flows rather than explode. Ultramafic lavas, while even richer in Mg and Fe, typically have low silica and thus do not have the highest viscosity among common lavas. Not all eruptions are explosive—low-viscosity magmas can erupt effusively—but the pattern is that intermediate to felsic magmas are associated with more explosive activity due to their higher viscosity.