Final answer:
Viscous lava leads to more violent eruptions due to its ability to trap gases which build up pressure until released explosively. Less viscous lava allows for easier gas escape and tends to be associated with less explosive eruptions.
Step-by-step explanation:
The more viscous the lava, and the greater the amount of gas trying to escape, the more violent the eruption.
Lava viscosity is a key factor in determining the explosivity of volcanic eruptions. Viscous lavas, like rhyolite, are thick and resist flowing, trapping gasses more effectively. As a consequence, pressure builds up until it is released explosively, leading to more violent eruptions. On the contrary, less viscous lavas, such as basalt, allow gases to escape more easily and tend to produce less explosive eruptions.
Volcanic rocks like vesicular basalt that have vesicles (holes from trapped gas) evidence the gas content in volcanic materials. Similarly, volcanic glasses like obsidian, which are formed by rapid cooling of viscous lava, also indicate a more explosive eruption due to their high silica content which increases viscosity.
Considering the differences in the rate of flow as well, basaltic lava can flow quickly and over longer distances compared to the slower and shorter flows of more viscous lavas.