Extrusive Igneous Rocks (Volcanic)
* Texture:
* Fine-grained: Crystals are small and often invisible to the naked eye. This is because the magma cools quickly at the surface. Examples include basalt and rhyolite.
* Glassy: Magma cools so fast it doesn't have time to form crystals. Examples include obsidian and pumice.
* Vesicular: Contains holes (vesicles) from trapped gas bubbles during cooling. Examples include scoria and pumice.
* Features:
* Often associated with volcanic landforms: Lava flows, volcanic craters, ash deposits.
* May have fragments of other rocks: These are called xenoliths and represent pieces of the surrounding rock that were incorporated into the magma.
Intrusive Igneous Rocks (Plutonic)
* Texture:
* Coarse-grained: Crystals are large and visible to the naked eye. This is because the magma cools slowly underground. Examples include granite and gabbro.
* Features:
* Found in large, often irregular masses: Called plutons, batholiths, or dikes.
* May have a "chilled margin": The edges of the intrusion will have a finer grain size due to faster cooling at the contact with the surrounding rock.
Other Clues:
* Mineral Composition: Some minerals are more common in extrusive rocks (like olivine in basalt) while others are more common in intrusive rocks (like quartz in granite).
* Weathering: Extrusive rocks are often more susceptible to weathering due to their finer grain size and presence of vesicles.
Important Note: There are exceptions to these generalizations. For example, some intrusive rocks can have a fine-grained texture if they cooled quickly near the surface. Always look for multiple clues to make a definitive identification.
