Write the mineralogy and texture of basalt. How does basaltic magma form in deep earth?

Mineralogy of Basalt

Basalt is a mafic volcanic rock, meaning it is rich in magnesium and iron. Its mineral composition typically includes:

• Plagioclase Feldspar (typically Labradorite - Bytownite): 40-60% - These are aluminosilicate minerals forming lath-shaped crystals.
• Pyroxene (Augite): 10-30% - Commonly dark green to black, these are important constituents contributing to the rock’s mafic character.
• Olivine: 5-20% - Often present as small, rounded grains, especially in more primitive basalts. Its presence indicates less fractionation.
• Iron-Titanium Oxides (Magnetite, Ilmenite): 1-10% - These contribute to the rock’s magnetic properties and are important indicators of oxygen fugacity during crystallization.
• Minor Minerals: Apatite, and occasionally quartz or amphibole may be present in small amounts.

The exact mineral proportions vary depending on the basalt’s chemical composition and cooling history. More evolved basalts will have less olivine and more plagioclase and pyroxene.

Texture of Basalt

Basalt exhibits a range of textures, reflecting its cooling rate and crystallization conditions:

• Aphanitic: This is the most common texture, characterized by very fine-grained crystals that are not visible to the naked eye due to rapid cooling at the surface.
• Porphyritic: Many basalts are porphyritic, meaning they contain larger crystals (phenocrysts) embedded in a fine-grained groundmass. Phenocrysts typically include plagioclase, pyroxene, or olivine, indicating a two-stage cooling history – slow cooling at depth followed by rapid cooling at the surface.
• Vesicular: Basalt often contains vesicles (gas bubbles) formed as dissolved gases exsolve during eruption. This creates a porous texture. The size and abundance of vesicles can indicate the gas content of the magma.
• Flow Texture: Basaltic lava flows often exhibit flow banding or alignment of crystals and vesicles parallel to the flow direction.
• Columnar Jointing: As thick basalt flows cool and contract, they can develop characteristic columnar joints – hexagonal columns formed due to tensile stress.

Formation of Basaltic Magma in Deep Earth

Basaltic magma primarily originates through partial melting of the Earth’s mantle. The process is complex and occurs in several tectonic settings:

• Mid-Ocean Ridges (MORs): Decompression melting is the dominant mechanism. As mantle rock rises beneath the ridges, the pressure decreases, lowering the solidus temperature and causing partial melting. This produces large volumes of basaltic magma that erupts to form new oceanic crust.
• Hotspots: Mantle plumes, rising from deep within the Earth, cause localized melting in the overlying lithosphere. This generates basaltic magma that can erupt to form volcanic islands (e.g., Hawaii, Iceland) or continental flood basalts.
• Subduction Zones: While primarily associated with andesitic and rhyolitic magmas, basaltic magmas can also form at subduction zones through flux melting. The addition of water from the subducting slab lowers the solidus temperature of the mantle wedge, inducing partial melting.

The composition of the resulting basaltic magma is influenced by several factors:

• Source Rock Composition: The chemical composition of the mantle source region (e.g., depleted mantle, enriched mantle) determines the initial magma composition.
• Degree of Partial Melting: Higher degrees of partial melting produce magmas that are closer in composition to the original source rock.
• Magma Differentiation: As magma ascends through the crust, it undergoes fractional crystallization, assimilation of crustal material, and magma mixing, altering its composition and leading to a range of basaltic magma types (e.g., tholeiitic basalt, alkali basalt).

The process of magma ascent is facilitated by buoyancy differences between the magma and surrounding rocks, as well as by fractures and pathways created by tectonic stresses.