What IS Used to Classify Igneous Rocks:
* Mineral Composition: This is the most important factor. The types and relative amounts of minerals present determine the rock's name.
* Mafic: Darker rocks rich in magnesium and iron (e.g., basalt, gabbro)
* Felsic: Lighter rocks rich in feldspar and silica (e.g., granite, rhyolite)
* Intermediate: Rocks with a mix of mafic and felsic minerals (e.g., diorite, andesite)
* Texture: The size, shape, and arrangement of mineral crystals within the rock.
* Phaneritic: Large crystals visible to the naked eye (e.g., granite)
* Aphanitic: Fine-grained crystals, too small to see without magnification (e.g., basalt)
* Porphyritic: Larger crystals embedded in a finer-grained matrix (e.g., porphyry)
* Glassy: No visible crystals, the rock cooled too quickly (e.g., obsidian)
* Cooling Rate: This is indirectly related to texture. Slow cooling leads to larger crystals; fast cooling leads to smaller crystals.
* Origin (Extrusive vs. Intrusive):
* Extrusive: Formed from lava that cools on the Earth's surface (e.g., basalt)
* Intrusive: Formed from magma that cools underground (e.g., granite)
What IS NOT Used to Classify Igneous Rocks:
* Color: While color can be a hint, it's not a reliable primary classification tool. Different minerals can have similar colors, and weathering can change a rock's color.
* Location: The specific geographic location where a rock is found doesn't determine its classification. The same type of igneous rock can occur in various locations.
* Age: While age can provide context, it doesn't determine the rock's classification. Two rocks of the same type can have different ages.
Remember: The classification of igneous rocks focuses on their composition, texture, and cooling history, not their color, location, or age.
