What is the primary characteristic of pumice that distinguishes it from other volcanic rocks?

Pumice is primarily distinguished from other volcanic rocks by its high porosity. This characteristic arises from the rapid cooling and depressurization of volcanic lava that is rich in gas bubbles. As the lava erupts and cools, these gas bubbles get trapped, forming a light and porous rock. The significant amount of air-filled voids gives pumice its unique texture and low density, making it much lighter than other volcanic rocks. This porosity not only affects its weight but also its ability to float on water, which is a notable feature of pumice. The other characteristics, such as color or crystalline structure, do not define pumice in the same way its porosity does.