Igneous Rocks and Magma lec 3

1. Igneous Rock Classification and Chemistry

1.1 Silicon Saturation

  • Silicon Undersaturated Minerals: These minerals cannot coexist with free (SiO_2) (quartz). If extra silica is present in the system, they react to form silicon-saturated minerals.

  • Incompatibility: Olivine (from the high-temperature end of Bowen's Reaction Series) and Quartz (the low-temperature end) generally do not coexist. If forced, they react to form Pyroxene.

1.2 The TAS (Total Alkali-Silica) Diagram

  • Total Alkaline (TA): Measured as the sum of Sodium (Na) and Potassium (K).

  • Mantle Compatibility: In the mantle, dominant minerals are Olivine and Pyroxene (Mafic minerals rich in Mg and Fe). Because Sodium and Potassium have large ionic radii and a charge of +1 (compared to the smaller +2 charge of Mg and Fe), they do not fit well into the crystal structures of mantle minerals.

  • Classification Factors: The TAS diagram uses (SiO_2) content and TA to categorize volcanic rocks into categories such as ultramafic, mafic (basalt), intermediate (basaltic andesite to andesite), and felsic (dacite and rhyolite).

2. Fundamental Lava Concepts

  • Magma vs. Lava: Molten rock beneath Earth’s surface is called magma (from Greek for ‘moldable’); once erupted, it is called lava.

  • Lava Flows: Often erupt as a crystal-melt slush with exsolving volcanic gases.

    • Pāhoehoe: Smooth, gently undulating, or ropy surface; formed from low-viscosity, gas-rich, hot lava.

    • ‘A‘ā (ah-ah): Characterized by a rough, jagged, and clinker surface.

    • Basaltic Flows: Form thin flows that can move > 20\,km from vents.

    • Rhyolitic Flows: Form thick flows that seldom move beyond 5\,km, often building mound-shaped lava domes.

3. Pyroclastic Rocks

  • Definition: Formed from "fire" (pyro) and "broken" (clastic) volcanic materials blasted into the air during explosive eruptions, then deposited and lithified.

  • Classification by Size:

    1. Volcanic Ash: Smallest fragments (< 2\,mm). Compaction forms Volcanic Tuff.

    2. Lapilli: "Little stones" (2 - 64\,mm). Forms Lapilli Tuff or Lapillistone depending on the ash content.

    3. Blocks and Bombs: Largest fragments (> 64\,mm).

    • Blocks: Solid when ejected.

    • Bombs: Molten or plastic when ejected, often taking on aerodynamic shapes.

4. Chemical Composition and Temperature

  • Felsic: Rich in silica (up to 80\%), low in Mg/Fe (< 10\%). Light-colored (K-feldspar, Quartz). Examples: Granite, Rhyolite. Temperature: 650 - 800^{\circ}C.

  • Intermediate: Gray or "salt and pepper" appearance. Composed of Plagioclase, Quartz, and Amphibole. Common at subduction zones. Examples: Diorite, Andesite.

  • Mafic: Low silica (< 52\%), rich in Mg/Fe. Dark-colored. Examples: Gabbro, Basalt. Temperature: 1000 - 1250^{\circ}C.

  • Ultramafic: Dominated by Olivine and Pyroxene. Examples: Peridotite, Komatiite. Temperature: \approx 1600^{\circ}C (common 3.5 - 1.5\,Gyrs ago when the mantle was hotter; features spinifex texture).

5. Magma Viscosity and Molecular Structure

  • Viscosity: Resistance to flow. Controlled by:

    1. Composition: Higher silica content leads to higher polymerization and higher viscosity.

    2. Temperature: Inversely proportional; atoms vibrate more at higher temperatures, reducing viscosity.

    3. Volatiles: Increasing water content dramatically reduces viscosity.

  • Molecular Scale:

    • Silicate Melt: A 3-D dynamic network of linked [SiO_4] with short-range structure. It lacks the long-range order of crystals like Quartz but is not entirely broken; bonds constantly break and reform.

    • Glass: A quenched snapshot of a melt (amorphous).

5.1 Polymerization

  • Bridging Oxygen (BO): Oxygen bonded to two silicon atoms, linking tetrahedra.

  • Non-Bridging Oxygen (NBO): Oxygen bonded to only one silicon atom.

  • Network Formers: (Si^{4+}, Al^{3+}) build the structural framework. Felsic melts are more polymerized.

  • Network Modifiers: (Na^+, K^+, Ca^{2+}, Mg^{2+}, Fe^{2+}) and H^+ disrupt the framework by breaking bonds to create NBOs.

  • Water's Role: Hydrogen from (H_2O) reacts with a bridging oxygen to form hydroxyl groups (OH), breaking the silicate network and depolymerizing the melt.

6. Magma Ascent and Intrusions

  • Driving Forces: Magma rises due to buoyancy (lower density than surrounding rock) vs. viscosity (resistance to motion).

    • Felsic Magma: High viscosity makes ascent difficult; often requires large volumes to overcome resistance. Frequently gets "stuck" to form intrusive bodies like Granite.

    • Mafic Magma: Lower viscosity allows it to rise in smaller batches through narrow fissures.

  • Intrusive Bodies:

    • Batholiths: Massive irregular bodies (>100\,km^2). Example: Sierra Nevada Batholith.

    • Sills: Tabular intrusions parallel to rock layers (concordant).

    • Dikes: Tabular intrusions that cut across layers (discordant); act as vertical pathways.

7. Volcanic Landforms and Eruption Styles

  • Effusive vs. Explosive:

    • Effusive: Low-viscosity basaltic magma; gases escape easily. Forms shield volcanoes.

    • Explosive: High-viscosity felsic magma; gas pressure builds until violent release. Can also occur if basaltic magma interacts with water (flashing to steam).

  • Eruption Geometry:

    • Point (Central) Vent: Roughly circular; builds volcanic cones.

    • Linear (Fissure): Magma issues from rifts; builds broad lava fields or plateaus.

  • Landforms:

    • Shield Volcanoes: Largest, broad, gentle slopes (e.g., Mauna Loa).

    • Cinder Cones: Smallest, steep-sided; formed from gas-rich pyroclastic debris.

    • Stratovolcanoes (Composite): Alternating layers of lava and pyroclastics; steep-sided (e.g., Mt. Rainier).

    • Calderas: Large collapse depressions (km-scale) formed when a magma chamber is partially emptied.

    • Columnar Jointing: Hexagonal contraction cracks meeting at \approx 120^{\circ}, formed perpendicular to cooling surfaces.

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