The process that leads directly to the formation of pumice rock is volcanic eruption.

Here's why:

* Volcanic Eruptions: Pumice is a type of volcanic rock formed when lava containing a large amount of dissolved gases erupts explosively. The rapid expansion of these gases creates a frothy, bubbly texture.

* Cooling and Solidification: As the lava cools and solidifies, the gas bubbles become trapped, creating the characteristic porous and lightweight structure of pumice.

So, the correct answer is volcanic eruption.