The type of volcanic rock that is extremely vesicular and glassy is pumice.

Here's why:

* Vesicular: Pumice is characterized by numerous gas bubbles (vesicles) trapped within the rock as it cooled rapidly. This gives it a very porous and lightweight structure.

* Glassy: Pumice forms when lava cools so quickly that mineral crystals don't have time to form. This results in a glassy texture.

Let me know if you'd like to know more about pumice or other volcanic rocks!