A volcanologist extracts a piece of rock near a volcano site and observes no visible crystals. After studying the rock for a while, he assumes it is obsidian. Why does he do so?
A) Obsidian is always hot.
B) Obsidian forms underground.
C) Obsidian is green.
D) Obsidian forms in lava flows.

Question

Quizplus · Accepted Answer

Obsidian is an igneous rock that forms from quickly cooled lava flows, resulting in a glass-like texture with no visible crystals.

A Volcanologist Extracts a Piece of Rock near a Volcano