INSIGHT UPSC QUIZ

ANS. (A) 
Highly viscous lava tends to entrap gas, which form vesicles (bubbles) within the rock as they rise to the surface. Lava with low viscosity tends to easily release bubbling gases as they are formed. Lavas also may contain many other components, sometimes including solid crystals of various minerals, fragments of exotic rocks known as xenoliths and fragments of previously solidified lava. Volcanic eruptions are caused by magma (a mixture of liquid rock, crystals, and dissolved gas) expelled onto the Earth’s surface. At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas forms a separate vapor phase when pressure is decreased as magma rises toward the surface of the Earth. This is similar to carbonated beverages which are bottled at high pressure. The high pressure keeps the gas in solution in the liquid. Gas gives magmas their explosive character, because volume of gas expands as pressure is reduced.