1. Shape and Size:
* Batholith: A massive, irregularly shaped intrusion of igneous rock, usually covering hundreds of square kilometers. They often form the cores of mountain ranges.
* Stock: A smaller, more localized version of a batholith.
* Laccolith: A dome-shaped intrusion that pushes overlying rock layers upward.
* Sill: A tabular intrusion that is concordant, meaning it runs parallel to the existing rock layers.
* Dike: A tabular intrusion that is discordant, meaning it cuts across existing rock layers.
* Plug: A cylindrical intrusion that often fills the conduit of a volcano.
2. Composition:
* Mafic: Intrusions composed of dark-colored, magnesium- and iron-rich minerals like pyroxene and olivine. They typically form from basaltic magma.
* Felsic: Intrusions composed of light-colored, silica-rich minerals like quartz and feldspar. They typically form from rhyolitic magma.
* Intermediate: Intrusions with a mix of mafic and felsic minerals, often formed from andesitic magma.
3. Depth of Intrusion:
* Plutonic: Intrusions that formed deep within the Earth's crust. These rocks are often coarsely-grained due to slow cooling rates.
* Hypabyssal: Intrusions that formed at shallower depths, closer to the surface. These rocks tend to have finer-grained textures.
4. Contact Metamorphism:
* Aureole: The zone surrounding an intrusion where heat from the magma has altered the existing rocks. The type of metamorphism depends on the composition of the intrusion and the original rock.
5. Structural Relationship to Surrounding Rocks:
* Conformable: Intrusions that follow the layering of the existing rocks (like sills).
* Discordant: Intrusions that cut across the layering of the existing rocks (like dikes).
6. Other Factors:
* Age: The relative age of the intrusion compared to surrounding rocks can help determine its emplacement history.
* Texture: The grain size and shape of the minerals within the intrusive rock can provide clues about cooling rates and magma composition.
* Mineralogy: The specific minerals present can help determine the chemical composition of the magma and the conditions under which it crystallized.
By combining these factors, scientists can accurately classify intrusive igneous features and understand their formation, evolution, and relationship to the surrounding geological environment.
