magmatism

• The factors in magma generation are temperature, pressure, composition of magma, and the presence of volatiles
• Magma generation by partial fusion

  The melting process begins with the production of a small amount of liquid that has a composition determined by the shape of the liquidus-solidus curves in phase diagrams
• Only a part of the solid rock undergoes the molten fraction during magma generation
• Magma

  A mobile rock material containing hot liquid which may contain suspended solids and dissolved gases
• Magma is a Greek word meaning “paste”
• Melting of a rock mass is initiated by raising the temperature above the solidus, and additional energy is needed to break the bonds in the crystalline structure to produce liquid
• There is no permanent reservoir of mostly liquid magma; the magmatic liquid must be generated by melting previously solid rock
• Partial melting occurs in the continental crust through the process of anatexis, a high-temperature metamorphic process by which plutonic rocks at deeper levels are dissolved and regenerated as magma
• Generation of Magma

  Magma is a mobile rock material containing hot liquid which may contain suspended solids and dissolved gases. It is normally formed within the solid parts of the earth (e.g. upper mantle) and exhibits temperatures ranging from 600oC to 1,300oC. Magma is generated by the process of partial fusion or partial melting
• Rocks and minerals lose their stability upon encountering very high temperatures leading to partial melting
• Filter pressing is manifested by squeezing a melt of different composition into solid material of different composition
• Flow differentiation occurs as magma moves through a conduit, causing textural differences along a volcanic pipe
• Partial melting

  1. Rocks lose their stability upon encountering very high temperatures leading to partial melting

  2. Geothermal gradient is the rate of temperature increase as depth increases, related to the retention of heat accompanying the earth's formation

  3. Physical parameters such as pressure, presence of volatiles, and composition of materials affect the process of partial melting

  4. High confining pressures allow the earth to maintain a solid interior, exceptions are the asthenosphere and outer core

  5. Decompression leads to rocks beginning to partially melt, usually accomplished by the creation of fractures
• Temperature in the upper mantle below the Philippines is about 1400-1600oC and largely remains solid
• Rocks at depths of about 50-250 km have temperatures of approximately 1200oC, necessary to initiate partial melting
• Fractional crystallization is the separation of crystals from liquid based on specific gravity
• Crystallization of Magma

  1. Magmas solidify or crystallize when the internal energies of atoms are low enough

  2. Lowering of temperatures and pressure conditions initiate crystallization

  3. Formation of mineral species is determined by distinct chemical composition, temperature, and pressure of formation

  4. Bowen’s Reaction Series illustrates the cooling of a basaltic magma producing a sequence of crystallizing minerals

  5. Different types of magmatic differentiation processes include fractional crystallization, liquid immiscibility, filter pressing, flow differentiation, magmatic assimilation, and mixing of magma
• Erupted lavas from the Philippines have temperatures of about 800-1200oC
• Subduction at around 100 km depths releases volatiles which lower the melting temperature of encountered materials, making partial melting possible
• The Philippines is not located in a mid-ocean ridge, yet there should be partial melting due to numerous volcanoes
• Liquid immiscibility is the non-mixing of components at certain temperatures
• Laccoliths exhibit concave shapes and are generally concordant to the intruded layered rocks
• Stocks are smaller than batholiths and are postulated to be batholiths not yet fully exposed
• Special names for igneous bodies based on sizes and shapes

  • Batholiths
  • Stocks
  • Dikes
  • Sills
  • Laccoliths
• Types of lava flows

  • Aa lava
  • Pahoehoe lava
  • Pillow lavas
• Flow differentiation in magma

  Magma moves through a conduit, friction and cooling near the walls reduce velocity and increase viscosity causing suspended particles to migrate towards the center. Textural differences along a volcanic pipe where smaller crystals develop near the wall and larger crystals at the middle. Center of the conduit has high-velocity flows and larger crystals, possibly zones with Na and K. Proximate to the walls where low-velocity flow exists, smaller crystals are formed, more likely of Ca composition. During magmatic assimilation, wall rock may react with or get dissolved by magma, changing its composition. Hot magma temperature must be super-heated, likely above the liquidus curve. Mixing of magmatic liquids of contrasting composition produces an intermediate composition of magma
• Batholiths have widths greater than 100 km and usually form the cores of mountain ranges
• Morphology of volcanoes influences eruptive behavior and is determined by magma properties and eruption type
• Presence of pyroclastic ejecta manifests violent eruptions
• Magma with higher viscosity likely experiences violent eruptions, while lower viscosity magma experiences quiet eruptions
• Dikes occur as discordant tabular or sheet bodies
• Shallow intrusives exemplified by a volcanic pipe, which is a solidified cylindrical conduit
• Magma composition dominantly of silicates affects flow speed, with basaltic magma flowing faster than rhyolitic magma
• Volatiles in magma, such as water vapor and carbon dioxide, inhibit magma flow due to internal pressures and friction
• Factors affecting magma viscosity: temperature, composition, and presence of volatiles
• Magma with higher temperatures has lower viscosity, while lower temperatures result in higher viscosity
• Plutons are magmas that have solidified at subsurface environments
• Volcanoes are passageways or conduits for rising magma, erupted magma can be in the form of lava flows or pyroclastic ejecta
• Eruptive behavior of volcanoes depends on physical properties of magma and morphology of the volcanic edifice
• Example of a Batholith: Agno Batholith in the Central Cordillera Mountain Range, generally made up of dioritic rocks