Rock IdentifierIgneous Rocks: Born of Fire
Key takeaways
- Slow, deep cooling makes coarse crystals; fast, surface cooling makes fine or glassy rock.
- Felsic rocks are light-colored and silica-rich; mafic rocks are dark and iron/magnesium-rich.
- Obsidian is volcanic glass — it cooled too fast to form crystals at all.
Intrusive vs extrusive
Igneous rocks split by where they cooled. Intrusive (plutonic) rocks crystallize slowly deep underground, giving atoms time to build large, visible crystals — granite is the poster child. Extrusive (volcanic) rocks erupt as lava and cool in hours to days, so crystals are microscopic or absent, as in basalt.
Reading the grain size
Grain size is a thermometer of cooling speed. Coarse, interlocking crystals you can see with the naked eye mean slow, deep cooling. A fine, sugary or smooth texture means rapid surface cooling. A porphyritic texture — big crystals set in a fine background — records a two-stage history: slow cooling underground, then a fast finish after eruption.
Felsic vs mafic — the color clue
Chemistry sets the color. Felsic rocks are rich in silica and light minerals like quartz and feldspar, so they look pale (granite, rhyolite). Mafic rocks are rich in iron and magnesium, so they look dark (basalt, gabbro). A pale, quartz-bearing igneous rock and a dark, dense one tell you very different stories about their source magma.
The glassy outliers
Some lavas quench so fast that no crystals form at all. Obsidian is the result: a natural glass with smooth conchoidal fracture and razor edges (early humans knapped it into blades). Pumice is frothy volcanic glass so full of gas bubbles it floats on water. Both are igneous, yet neither has true crystals.
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