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Igneous Rocks and Magmatic Processes – Vocabulary Flashcards

Formation of Magma

  • Igneous rocks form as magma crystallizes; it cools and solidifies to become solid rock.
  • Magma forms from the partial melting of rocks in the crust and upper mantle.

Mechanisms of Magma Formation (Melting Processes)

  • Melting: solid to liquid transition occurs when the amplitude of thermal vibration of atoms is too high in comparison with interatomic distance.
  • Other mechanisms that enable melting include:
    • Pressure release (decompression melting)
    • Heat transfer
    • Volatile addition (e.g., water, carbon dioxide)

Pressure Release (Decompression Melting)

  • Melting point of minerals increases with increasing pressure.
  • A rock undergoes decompression melting if its temperature stays almost the same while the pressure on it decreases significantly (as it is brought toward shallow depth).

Heat Transfer

  • Rising magma carries mantle heat with it.
  • This heat raises the temperature in nearby crustal rock, which then melts.

Addition of Volatiles

  • Volatiles lower the melting temperature of hot rock.
  • Common volatile components: water (H2O) and carbon dioxide (CO2).
  • Example: Mountain Kilauea eruption, 2018 (illustrative context for volatile addition influencing melting).

Question 1

  • Mountain Kilauea in Hawaii is located above a hotspot and erupted in 2018. What mechanism is responsible for the generation of magma there?
  • Answer: decompression melting (Pressure Release)

Chemical Composition of Magma/Igneous Rocks (Silica-based Classification)

  • Based on silica (SiO2) percentage:
    • Felsic: 66 – 76% SiO2
    • Intermediate: 52 – 66% SiO2
    • Mafic: 45 – 52% SiO2
    • Ultramafic: 38 – 45% SiO2

Physical Properties by Composition

  • Felsic
    • Density: very low
    • Temperature: ~600–850 °C
    • Viscosity: very high
  • Intermediate
    • Density: low
    • Temperature: low
    • Viscosity: high
  • Mafic
    • Density: high
    • Temperature: high
    • Viscosity: low
  • Ultramafic
    • Density: very high
    • Temperature: very high, up to ~1300 °C
    • Viscosity: very low

Magma Variation (Sources and Processes)

  • Source of the melt dictates the initial composition:
    • Mantle source → ultramafic and mafic magmas
    • Crustal source → mafic, intermediate, and felsic magmas
  • Composition of magma can change by:
    • Partial melting
    • Fractional crystallization
    • Assimilation
    • Magma mixing

Partial Melting

  • Melting/crystallization of rocks is rarely complete.
  • Silica-rich (felsic) minerals melt first.
  • Silica-poor (mafic) minerals melt last.
  • Partial melting yields a silica-rich magma (felsic magma) and leaves a mafic residue.

Fractional Crystallization

  • Sequence of crystallization is opposite to sequence of melting.
  • Mafic minerals crystallize first; progressive removal of mafic minerals leaves the melt more felsic.
  • Final crystallizing minerals are felsic.

Question 2

  • During cooling of a magma, the magma will get more with further lowering of temperature. A rock may be melted partially as its temperature increases. The melts will get more with further higher temperature.
  • Answer: A) felsic mafic

Bowen’s Reaction Series

  • Describes the specific temperature at which a specific silicate mineral forms during cooling of magma.
  • Ultra mafic minerals crystallize first; felsic minerals crystallize last.

Question 3

  • Which of the following minerals will crystallize out of magma first? Olivine, pyroxene, amphibole, mica, quartz
  • Answer: Olivine (olivine crystallizes at the highest temperature in Bowen’s series).

Question 4

  • Bowen’s Reaction Series describes the crystallization sequence of silicate minerals in a cooling magma. Which of the following group of minerals is least likely to occur together in igneous rock:
  • A. olivine and amphibole
  • B. amphibole and pyroxene
  • C. pyroxene and mica
  • D. olivine and quartz
  • Answer: D. olivine and quartz

Assimilation

  • Magma melts the country rock that it passes through.
  • Assimilation of these rocks alters magma composition.
  • Diagrammatic sequence: Deep magma rises → partial melting of wall rock produces new magma → mixes with magma from below.
  • Blocks of rock falling into magma dissolve; this is assimilation.

Magma Mixing

  • Different magmas may blend in a magma chamber.
  • Resulting magma has characteristics of the two magmas.

Igneous Rocks: Extrusive and Intrusive

  • Extrusive (volcanic) rocks crystallize at the surface from lava.
  • Intrusive (plutonic) rocks crystallize below the surface from magma.

Extrusive Igneous Rocks

  • Lava flows and cools; typically stacked vertically.
  • Low viscosity, high temperature mafic magma.
  • Pyroclasts – explosive ash eruptions; high viscosity, low temperature felsic magma.

Intrusive Igneous Rocks (Texture and Structures)

  • Tabular forms:
    • Sill – parallel rock fabric (concordant); pushes between rock layers.
    • Dike – crosscuts rock fabric (discordant); cuts across layers.
  • Erosion can remove part of a dike.

Plutonic Relationships

  • Plutonic rocks: large, deep igneous bodies.
  • A large, deep igneous body is called a pluton (discordant).
  • Plutons sometimes coalesce to form a larger batholith.

Question 5

  • Why magma tends to rise upward, but does not stay put?
  • Answer: Magma is less dense than the surrounding rocks and flows upward.

Question 6

  • Burial depth of a magma is one of the key factors controlling the cooling rate of the magma. What other factors may also affect the cooling rate of magma?
  • Answers:
    • Shape: Spherical bodies cool slowly; tabular bodies cool faster.
    • Groundwater: Groundwater removes heat, accelerating cooling.

Texture of Igneous Rocks

  • Texture describes size, shape, and arrangement of minerals.
  • Crystalline textures can be:
    • Crystalline – minerals fit like a jigsaw puzzle pieces.
    • Glassy – made of solid glass or glass shards.
    • Fragmental – pieces of preexisting rocks, often shattered.

Crystalline Igneous Texture Types

  • Phaneritic – coarse crystals; Intrusive; crystals have grown large due to slow cooling.
  • Aphanitic – fine crystals; Extrusive; rapid cooling leads to small crystals.
  • Porphyritic texture – mixture of coarse and fine crystals.
    • Two-stage cooling history: initial slow cooling produces large phenocrysts; subsequent eruption cools remaining magma more rapidly.

Classification of Igneous Rocks

  • Classification is based on two main features:
    • Composition: felsic, intermediate, mafic, ultramafic
    • Texture: fine (aphanitic); coarse (phaneritic)

Igneous Rock Identification Key (Mineral Composition)

  • Dominant minerals:
    • Felsic (Granite): Quartz
    • Intermediate (Andesitic): Plagioclase feldspar, Amphibole
    • Mafic (Basaltic): Plagioclase feldspar, Pyroxene
    • Ultramafic (Peridotite): Olivine, Pyroxene, Plagioclase feldspar
  • Accessory minerals:
    • Plagioclase feldspar, Amphibole, Muscovite, Biotite, Pyroxene, Olivine, etc.
  • Rock types by texture and composition:
    • Coarse-grained: Granite, Diorite, Gabbro, Peridotite
    • Fine-grained: Rhyolite, Andesite, Basalt
    • Porphyritic: Granite porphyry, Andesite porphyry, Basalt porphyry
    • Glassy: Obsidian
    • Vesicular: Pumice (glassy, vesicular, felsic); Scoria (glassy, vesicular, mafic)
    • Pyroclastic (fragmental): Tuff or welded tuff; Volcanic breccia
  • Rock color depends on percentage of dark minerals (composition):
    • 0% to 25% dark minerals: light rocks
    • 25% to 45% dark minerals: intermediate rocks
    • 45% to 85% dark minerals: dark rocks
    • 85% to 100% dark minerals: ultramafic rocks

Tectonic Settings of Igneous Rocks

  • Plate motion is a dominant control on volcanism; igneous types are linked to tectonic boundaries.

Subduction Zone (Andesitic/Intermediate Magmatism)

  • Addition of volatiles triggers melting.
  • Ocean–Ocean: Mafic + Mafic → Mafic magma.
  • Ocean–Continental: Mafic + Felsic → Intermediate magma.

Hot Spots

  • Decompression melting is a key mechanism.
  • Oceanic crust example: Hawaii (mafic magmas).
  • Continental crust example: Yellowstone (more complex with mantle plume and crustal influence).

Continental Rifts

  • Mantle upwelling through continental crust causes decompression melting.
  • Mixing of mantle-derived magma with continental crust can produce Intermediate magmas (Mafic + Felsic → Intermediate).

Mid-Ocean Ridges

  • Mantle upwelling produces new oceanic crust via decompression melting.
  • Mantle + oceanic crust produces Mafic magmas; typically Mafic.

Take Home Message

  • Understand why melting produces different rocks at different places in the Earth.
  • Understand why magma moves into and solidifies in intrusive or extrusive settings.
  • Know that igneous rocks are classified by composition and texture.
  • Recognize where igneous activity takes place, in relation to plate tectonics.