CH.6.5 Magma, Rocks, and Earth's Structures Explained

Understanding Magma and Intrusive Structures

🌋 Introduction to Magma and Intrusive Structures

Magma is a molten rock material found beneath the Earth's surface. When it rises to the surface and erupts, it forms volcanic rocks. Understanding magma and its behavior is crucial for comprehending volcanic activity and the formation of various geological structures.

  • Magma: Molten rock beneath the Earth's surface.

  • Lava: Magma that reaches the Earth's surface.

  • Intrusive Structures: Geological formations created when magma cools and solidifies beneath the surface.

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🔥 Rock Melting Processes

Anatexis

Anatexis refers to the partial melting of a source rock, which produces two components:

  • A liquid melt fraction enriched in low melting temperature, incompatible constituents.

  • A residual rock component enriched in higher temperature, compatible elements.

Equilibrium Melting

  • Equilibrium Melting: Occurs in a closed system where the composition remains constant while melts and solids evolve.

    • The melt produced is enriched in low melting temperature constituents.

    • As melting progresses, the solid residue becomes more refractory.

Fractional Melting

  • Fractional Melting: Involves the separation of solids and melt into isolated fractions.

    • Produces a melt that is more evolved than the parent rock.

    • Early melts are enriched in low melting temperature constituents, while later melts are less enriched.

🌍 Factors Influencing Anatexis and Initial Melt Composition

  1. Increasing Temperature: The geothermal gradient varies with depth, influencing melting.

  2. Decreasing Pressure: Decompression melting occurs when pressure decreases, allowing melting without temperature increase.

  3. Volatile-Induced Melting: The presence of volatiles, especially water vapor, lowers melting temperatures significantly.

📊 Magma Diversification Processes

Magma diversification refers to the changes in magma composition after its initial generation. This can occur through:

  • Closed System Differentiation: Changes occur without external material exchange.

  • Open System Processes: Involves interaction with surrounding rocks or other magmas.

Process Type Description Key Characteristics Closed System Differentiation Changes in magma composition without external influence Fractional crystallization, crystal-liquid separation Open System Processes Interaction with surrounding materials Assimilation, magma mixing, magma mingling

🏔 Intrusive Structures

Intrusive structures form when magma cools and solidifies beneath the Earth's surface. Key types include:

  • Batholiths: Large, irregularly shaped plutons with surface exposures greater than 100 km².

  • Stocks: Smaller plutons with surface exposures less than 100 km².

  • Sills: Tabular, concordant intrusions that parallel the layering of surrounding rocks.

  • Dikes: Discordant intrusions that cut across existing rock layers.

🌌 Magma Series

Magma series refer to groups of genetically related magmas that have changed composition from a common original magma. The main types include:

  1. Calcalkaline Series: Characterized by moderate to high Na2O, K2O, and SiO2 concentrations. Common rocks include andesite and rhyolite.

  2. Tholeiitic Series: Low Na2O and K2O but high FeO and CaO concentrations, producing basalt.

  3. Alkaline Series: High Na2O and K2O concentrations, producing rocks with feldspathoid minerals.

📚 Important Concepts and Definitions

  • Anatexis: Partial melting of a source rock.

  • Equilibrium Melting: Closed system melting where composition remains constant.

  • Fractional Melting: Isolated melting processes that produce evolved melts.

  • Magma Diversification: Changes in magma composition after initial generation.

  • Intrusive Structures: Geological formations created by solidified magma.

🧪 Facts to Memorize

  1. Anatexis produces both a liquid melt and a residual rock component.

  2. Equilibrium melting results in a melt composition similar to the parent rock if fully melted.

  3. Fractional melting can lead to significant differences in the composition of the resulting magma.

  4. Increasing temperature, decreasing pressure, and volatile content are key factors in rock melting.

  5. Batholiths and stocks are types of intrusive structures formed from solidified magma.

  6. The calcalkaline magma series is associated with convergent plate boundaries.

  7. Tholeiitic magmas are typically found at ocean spreading centers.

  8. Alkaline magmas are characterized by high concentrations of alkali elements.

  9. Magma diversification processes can significantly alter the composition of the resulting igneous rocks.

  10. Understanding magma and intrusive structures is crucial for predicting volcanic activity and its impacts.