Which volcanic rock is a type of glass and is light enough to float due to air pockets?

• A. Pumice
• B. Basalt
• C. Obsidian
• D. Granite

Answer: (A)

Pumice is the rock described. It forms when silica-rich lava erupts explosively and cools so fast that it becomes volcanic glass, trapping a lot of gas as countless bubbles. Those air pockets make pumice extremely porous and light, often enough to float on water.

Basalt is dense and fine-grained, not glassy with many bubbles. Granite is coarse-grained and formed deep underground, also not glassy. Obsidian is glassy too, but it usually lacks the numerous vesicles, so it’s dense and sinks.