Explain why a volcano is likely to explode more violently if it has been dormant for a long time?

A dormant volcano is gradually "recharging" in the sense that pressure is being built up deep within the magmatic chamber by both magma and gases.

To make matters worse, solid lava from earlier eruptions most likely completely clogged the conduit that connected the magmatic chamber to the crater.

An eruption occurs when the pressure created by the accumulated rocks above the magmatic chamber is exceeded by the pressure inside the chamber.

These types of eruptions are called "Plinians" after the historic Plinius, who also perished during the event, and are characterized by dramatic and extremely violent eruptions due to the large amount of stored energy, as exemplified by the 1980 USA eruption of Mount St. Helens or the even more famous 79 A.C. eruption of Vesuvius that destroyed Pompei.

When a volcano has been dormant for a long time, pressure can build up beneath the surface as magma continues to accumulate. This increased pressure can lead to a more explosive eruption when the volcano eventually becomes active again. Additionally, gases dissolved in the magma can become trapped and accumulate over time, further increasing pressure and contributing to a more violent eruption.