Igneous Rocks - Characteristics, Differentiation, and Environments

Common Rock-Forming Mineral Environments

• Continental Crust: Quartz, Potassium Feldspar, Muscovite, Biotite, Hornblende, and Sodium-rich Plagioclase Feldspar.
• Ocean Crust: Calcium-rich Plagioclase Feldspar, Augite, Hornblende (minor), and Olivine (minor).
• Asthenosphere: Olivine (most abundant mineral in the asthenosphere) and Augite.

Magma, Lava, and Igneous Textures

• Magma vs. Lava: Magma is melted rock below the surface consisting of liquid melt and solid crystals ("crystal mush"); lava is melted rock on the surface.
• Intrusive Igneous Rocks: Form as magma crystallizes slowly deep within the crust, producing large, easily visible crystals (phaneritic texture).
• Extrusive Igneous Rocks: Form as lava crystallizes rapidly on the surface, producing small crystals (aphanitic texture) or volcanic glass.
• Porphyritic Texture: Contains a mixture of large crystals (phenocrysts) and small groundmass crystals, formed when magma partially crystallizes in a chamber before erupting and cooling rapidly.
• Volcanic Glass Types:
  • Obsidian: Amorphous glass formed by nearly instantaneous cooling of lava without crystal formation.
  • Pumice: Volcanic glass formed from accumulated tiny, needle-like glass fragments.
  • Tuff: Rock formed from accumulated volcanic ash, rock fragments, and mineral crystals.

Chemical Composition and Classification

• Felsic: Composed primarily of minerals rich in silicon with little iron and magnesium.
• Mafic: Composed of minerals rich in iron and magnesium with smaller amounts of silicon and oxygen.
• Classification: Igneous rocks are classified by combining chemical composition (felsic, intermediate, mafic, or ultramafic) with texture (intrusive vs. extrusive).

Magma Differentiation

• Mantle Origin: All igneous rocks begin as ultramafic peridotite in the upper mantle.
• Four Mechanisms of Differentiation:
  1. Partial Melting: As rock heats, felsic minerals melt first at lower temperatures, while ultramafic minerals melt last at higher temperatures.
  2. Fractional Crystallization: As magma cools, ultramafic minerals crystallize first at higher temperatures, while felsic minerals crystallize last at lower temperatures.
  3. Magma Mixing: Infiltration of mafic magma into continental crust induces crustal partial melting to create felsic magma; the two bodies can mix or remain separate.
  4. Xenolith Incorporation: Foreign wall or roof rock fragments (xenoliths) fall into magma chambers; if melted, they alter the magma composition.

Igneous Environments and Tectonic Settings

• Ocean-Continent Convergent Boundaries: Subducting oceanic plates release water into the mantle, lowering peridotite's melting point to generate mafic magma. Rising mafic magma melts continental crust to generate felsic and intermediate magmas (crystallizing intrusively as granite/diorite or extrusively as rhyolite/andesite).
• Ocean-Ocean Convergent Boundaries: Partial melting of mantle peridotite forms mafic magma that crystallizes intrusively as gabbro or extrusively as basalt (forming island arcs); lacks felsic/intermediate components due to the absence of continental crust.
• Divergent Boundaries & Decompression Melting:
  • Pressure reduction at constant temperature (decompression melting) causes asthenospheric peridotite to melt beneath tectonic rifts.
  • Produces gabbro deep in the crust, vertical sheeted dikes, and pillow basalt on the seafloor.
  • Ophiolites: Ocean crust fragments thrust up onto continental crust at ocean-continent convergent boundaries (e.g., California's Coast Ranges).
• Hotspots & Plate Tectonics:
  • Ocean Hotspots: Mantle plumes cause partial melting of peridotite, forming mafic magma that builds volcanic island chains as tectonic plates move across stationary hotspots.
  • Continental Hotspots: Partial melting of crust and fractional crystallization yield felsic eruptions and vast flood basalt provinces, such as the Columbia River Flood Basalt Province formed $17$\text{--}$6\,\text{million years ago}$ above the Yellowstone hotspot.