Question

What extrusive igneous rock type cooled so quickly that visible crystal grains never grew, and the rock is composed entirely of black silicate glass that easily shatters into sharp, knife-like shards?

(A) Obsidian
(B) Pumice
(C) Scoria
(D) Tuff

Answer 1

I think the answer is A

Answer 2

A. Obsidian

Step-by-step explanation:

It fits the property described in the question.

It cools so fast that no visible crystal or grain could form. It has high silica Composition, black and glassy. It is also used to make tools with sharp edges