The above picture shows a type of igneous rock called obsidian. Why is the top rock so shiny and smoother than the others?

Question

A.
It was formed when lava cooled very quickly.
B.
It is made of sand-sized pieces of other rocks.
C.
It was formed inside a volcano.
D.
Years of weathering has made it smooth.

Answer 1

The correct answer is:

A. It was formed when lava cooled very quickly.

Obsidian is a type of volcanic glass that forms when lava cools rapidly, preventing the formation of crystal structures that are typical in other igneous rocks. This rapid cooling gives obsidian its smooth, shiny surface.