51. volcanic glass (obsidian) is a coarsely crystalline b. noncrystalline; like window glass; typically results from very rapid cooling of a silica rich lava flow c. produced by slow cooling of a lava flow d. produced by escaping gases in a magma

b. noncrystalline

Explanation:

Volcanic glass -

It is a noncrystalline or amorphous substance , which is formed via rapid cooling of the magma . It is matrix or interstitial .

Volcanic rock is also called as the vitreous igneous rocks .

Most commonly called the obsidian , it is a high silica content , rhyotic glass .

Some example of volcanic glass are , Apache tears , Pumice etc .

answer; /// the above statement is ( false);