Earth Science And Geology Practice Exam

Which example represents an igneous rock formed by rapid cooling at the surface?

Obsidian Limestone Slate Granite

Correct Answer:

Obsidian

Explanation:

Rapid cooling of molten rock at the surface creates a glassy texture, because crystals don’t have time to form. Obsidian is the natural volcanic glass that forms when lava erupts and cools rapidly at the surface or just beneath it. That quick cooling gives obsidian its smooth, glassy appearance and conchoidal fracture. The other options don’t fit this scenario: limestone forms from compacted shells and minerals in sediment; slate forms from metamorphism of shale under directed pressure; granite forms underground and cools slowly to develop visible mineral grains. So obsidian is the example of igneous rock formed by rapid surface cooling.

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