What condition results in magma cooling and forming a “glassy” igneous rock?
a. extremely slow cooling
b. cooling in the presence of oxygen
c. extremely rapid cooling
d. partial melt cooling

The answer to the question stated above is letter c. extremely rapid cooling
Glassy (also termed as vitreous textures) happen during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot take place, resulting to  a natural amorphous glass with few or no crystals.
Examples to "glassy” igneous rock  include obsidian and pumice.

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