Which statement about lava viscosity and eruptions is correct?

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 of lava is mainly set by silica content and temperature, with higher silica making lava more viscous. When lava is very viscous, volcanic gases can’t escape easily, causing pressure to build up and magma to fragment explosively, producing ash plumes and pyroclastic flows. In contrast, low-viscosity magma, like basaltic magma, flows easily and tends to erupt as lava flows rather than explode, which is why basaltic eruptions are often effusive rather than explosive. Ultramafic lavas are extremely hot and runny, not highly viscous, so claiming they have the highest viscosity isn’t accurate. Intermediate and felsic lavas (such as andesite and rhyolite) have higher silica content and viscosity and are associated with more explosive eruptions. That connection makes the statement the best fit: higher viscosity in intermediate or felsic lava leads to explosive eruptions.