Which rock forms when magma cools slowly beneath the Earth's surface?

Which rock forms when magma cools slowly beneath the Earth's surface?

• A. Basalt
• B. Obsidian
• C. Granite
• D. Pumice

Answer: (C)

Slow cooling of magma beneath the surface lets crystals grow larger, producing a coarse-grained, intrusive igneous rock. Granite forms this way when magma crystallizes slowly underground, giving big interlocking crystals and a light, silica-rich mineral makeup (quartz and feldspars). Other options come from different cooling histories: basalt is produced by magma that erupts and cools quickly at or near the surface, giving a fine-grained rock; obsidian forms from very rapid cooling, making a natural glass with few or no crystals; pumice forms from explosive eruptions with rapid cooling and gas loss, resulting in a light, vesicular texture.

Slow cooling of magma beneath the surface lets crystals grow larger, producing a coarse-grained, intrusive igneous rock. Granite forms this way when magma crystallizes slowly underground, giving big interlocking crystals and a light, silica-rich mineral makeup (quartz and feldspars).

Other options come from different cooling histories: basalt is produced by magma that erupts and cools quickly at or near the surface, giving a fine-grained rock; obsidian forms from very rapid cooling, making a natural glass with few or no crystals; pumice forms from explosive eruptions with rapid cooling and gas loss, resulting in a light, vesicular texture.