Obsidians glassy texture indicates that it formed

on the surface rapidly

Explanation:

Obsidian glassy texture indicates that the rock formed on the surface rapidly.

Like every other magma, the condition of the environment determines how the magma cools and solidifies.

When magma cools rapidly, nucleation centers will be difficult to come- by .. At nucleation centers, minerals aggregates and grows to form different grain sizes. When rapid cooling occurs, not enough time is given for the aggregation around the nucleus. This leaves a rock mass that is finely grained.

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