Question

1. mainidea infer why obsidian, which is black or red in color, usually has a granitic composition.

Explanation:

Obsidian forms from rapid cooling of lava. A granitic - composition lava can cool quickly on the Earth's surface to form obsidian. The black or red color can be due to impurities or trace elements. Rapid cooling prevents large crystals from forming, resulting in a glassy texture typical of obsidian.

Answer:

Obsidian forms from rapid cooling of granitic - composition lava, and its color is due to impurities or trace elements.