A finely crystalline or glassy igneous texture indicates that
the rock cooled quickly.
the rock cooled slowly.
the rock broke apart as it flowed.
there was a lot of gas in the magma.
the ash and pumice were hot and became compacted.
The correct answer and explanation is :
Correct Answer: The rock cooled quickly.
Explanation (300 words):
Igneous rocks are formed from the cooling and solidification of molten rock, either magma beneath the Earth’s surface or lava on the surface. The texture of an igneous rock—meaning the size, shape, and arrangement of its mineral crystals—depends largely on the rate at which the magma or lava cools.
A finely crystalline or glassy texture indicates rapid cooling. When lava erupts onto the surface (extrusive igneous activity), it loses heat very quickly to the atmosphere or ocean. This rapid cooling doesn’t give the mineral crystals enough time to grow large, resulting in a fine-grained texture, where the crystals are so small they may be difficult to see without a microscope. Common rocks with this texture include basalt and andesite.
If the cooling is extremely fast—as in the case of lava that comes into contact with water or air instantly—the rock may cool so quickly that no crystals can form at all, resulting in a glassy texture. An example of this is obsidian, which looks like black volcanic glass.
In contrast, slow cooling allows large crystals to form and results in coarse-grained textures, typical of intrusive igneous rocks like granite.
The other options in the question describe different processes:
- “The rock broke apart as it flowed” refers more to mechanical processes, not crystallization.
- “There was a lot of gas in the magma” results in vesicular textures, like in pumice or scoria, not glassy or fine-grained ones.
- “The ash and pumice were hot and became compacted” describes pyroclastic processes, forming rocks like tuff—not finely crystalline or glassy textures.
In summary, a finely crystalline or glassy texture in igneous rocks is a clear indication of quick cooling, often associated with volcanic (extrusive) environments.
