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Chapter 4: Igneous Rocks and Intrusive Activity - Vocabulary Flashcards

4.1 Magma: Parent Material of Igneous Rock

  • Magma is molten rock that forms igneous rocks when it cools and solidifies. It consists of three major components:
    • Liquid portion: the melt.
    • Solids: crystals of silicate minerals.
    • Volatiles: dissolved gases in the melt that vaporize at surface pressure.
    • Most common volatiles in magma: ext{H}2 ext{O}, ext{CO}2, ext{SO}_2.
  • At the surface, magma is called lava.
  • Magma forms by partial melting deep in the Earth’s crust; the original rock melts to produce a silicate melt.
  • From magma to crystalline rock:
    • Crystallization is the cooling of magma, causing ions to organize into orderly patterns.
    • Silicon–oxygen tetrahedra form first; as cooling continues, tetrahedra link with other ions to form crystal nuclei.
    • Minerals that crystallize early have space to grow and develop well-formed crystal faces; later-form minerals have less space.
  • Igneous processes:
    • Intrusive (plutonic) igneous rocks form when magma crystallizes at depth.
    • Extrusive igneous rocks form when lava or volcanic debris solidifies at the surface after eruption.
    • Uplift and erosion bring intrusive rocks to the surface.
  • Key terms:
    • Plutons: cooled, emplaced magma within preexisting rocks.
    • Intrusive bodies are classified by shape and orientation relative to surrounding rock: tabular vs massive; discordant vs concordant.

4.2 Igneous Compositions

  • Four major igneous compositions (based on silicate minerals): felsic, intermediate, mafic, ultramafic.
  • Silicate-mineral groups:
    • Ferromagnesian (dark) silicates: ext{Fe}, ext{Mg}; examples: olivine, pyroxene (augite), amphibole (hornblende), biotite.
    • Nonferromagnesian (light) silicates: ext{K}, ext{Na}, ext{Ca}; examples: quartz, muscovite, feldspars.
  • Felsic (granitic) composition:
    • Light-colored silicates; composed almost entirely of quartz and potassium feldspar.
    • High silica content; about 10\% dark silicates.
    • Major constituent of continental crust.
  • Mafic (basaltic) composition:
    • Contain at least 45\% dark silicates and calcium-rich feldspar; contain no quartz.
    • Higher density than granitic rocks; constitutes oceanic crust and many volcanic islands; forms extensive lava flows on continents.
  • Andesitic (intermediate) composition:
    • Contain \ge 25\% dark silicate minerals.
    • Common at subduction-zone volcanic arcs (seaward margins of continents and volcanic island arcs).
  • Ultramafic composition:
    • Mostly olivine and pyroxene; composed almost entirely of ferromagnesian minerals.
    • Peridotite is a typical ultramafic rock; main constituent of the upper mantle.
  • Silica content as an indicator of composition:
    • Crustal rocks range from about 40\% to 70\% SiO₂ across ultramafic to felsic.
    • Granitic magmas have high silica content and are viscous; basaltic magmas have lower silica and are more fluid.
    • Eruptions: felsic magmas tend to erupt at lower temperatures (1200^\circ\text{F}) and are more viscous; basaltic magmas erupt at higher temperatures (1920-2280^\circ\text{F}) and are more fluid.
  • Granitic vs basaltic rock names (and their extrusive/intrusive counterparts):
    • Granitic (felsic): Granite (intrusive, phaneritic coarse-grained) and Rhyolite (extrusive, aphanitic fine-grained).
    • Obsidian: dark, glassy rock with silica-rich composition; forms from rapid cooling; usually similar chemical composition to granite.
    • Pumice: glassy rock with a vesicular texture; forms when large amounts of gas escape from silica-rich lava; floats in water.
    • Andesitic (intermediate): Andesite (extrusive, fine-grained) and Diorite (intrusive, coarse-grained).
    • Basaltic (mafic): Basalt (extrusive, fine-grained) and Gabbro (intrusive, coarse-grained).
    • Pyroclastic rocks: Tuff (ash-sized fragments cemented together), Welded tuff (hot ash fused), Volcanic breccia (fragments larger than ash).
  • Quick reference rock relationships:
    • Granite ↔ Rhyolite (felsic, high SiO₂)
    • Diorite ↔ Andesite (intermediate)
    • Gabbro ↔ Basalt (mafic)
    • Obsidian ↔ Granite (same overall composition, different texture due to rapid cooling)

4.3 Igneous Textures: What Can They Tell Us?

  • Texture describes size, shape, and arrangement of mineral grains; reveals environment of rock formation.
  • Factors influencing texture:
    • Rate of cooling: slow cooling yields fewer but larger crystals; fast cooling yields many small crystals.
    • Amount of silica and amount of dissolved gases in the melt.
  • Six major textures:
    • Aphanitic (fine-grained): rapid cooling; microscopic crystals.
    • Phaneritic (coarse-grained): slow cooling; visible crystals.
    • Porphyritic: large crystals (phenocrysts) embedded in a matrix of smaller crystals (groundmass).
    • Vesicular: rocks with voids left by gas bubbles in the lava; common in extrusive rocks.
    • Glassy: very rapid cooling; ions are frozen before forming an orderly crystalline structure.
    • Pyroclastic (fragmental): forms from consolidation of rock fragments ejected during explosive eruptions.
    • Pegmatitic: exceptionally coarse-grained; forms in late stages of crystallization of magmas (pegmatites).
  • rock classification is based on texture and mineral composition; same composition can have different textures depending on cooling history.

4.4 Naming Igneous Rocks

  • Granitic (felsic) igneous rocks:
    • Granite: coarse-grained (phaneritic); ~10-20\% quartz; ~50\% potassium feldspar; <10\% dark silicates; may be porphyritic.
    • Rhyolite: extrusive equivalent of granite; light-colored silicates; typically buff to pink or light gray; less volumetric than granite.
    • Obsidian: dark, glassy; silica-rich lava cools quickly at the surface; color due to trace metallic ions; composition similar to granite.
    • Pumice: glassy with vesicular texture; forms from volatile-rich silica melts; floats in water when in fresh condition.
  • Andesitic (intermediate) igneous rocks:
    • Andesite: medium-gray, fine-grained; volcanic origin; commonly porphyritic.
    • Diorite: intrusive equivalent of andesite; coarse-grained; looks gray like granite but lacks visible quartz (salt-and-pepper appearance).
  • Basaltic (mafic) igneous rocks:
    • Basalt: very dark, fine-grained; mainly pyroxene and calcium-rich plagioclase; may have light-colored phenocrysts in porphyritic basalt.
    • Gabbro: intrusive equivalent of basalt; very dark, coarse-grained; pyroxene and calcium-rich plagioclase; common in oceanic crust but less common on continental crust.
  • Pyroclastic rocks (fragmental):
    • Rock names do not imply mineral composition; named with a modifier (e.g., rhyolitic tuff).
    • Tuff: most common pyroclastic rock; composed of ash-sized fragments cemented together.
    • Welded tuff: ash particles fused by heat.
    • Volcanic breccia: composed of larger particles (blocks, pumice fragments, glass fragments).

4.5 Origin of Magma

  • Major processes generating magma from solid rock:
    • Geothermal gradient: temperatures in the upper crust rise about 25^ extcircled{C} ext{ per kilometer} (i.e., dT/dz \approx 25^ extcircled{C} / ext{km}).
    • Rocks in the lower crust and upper mantle approach their melting points.
    • Tectonic processes trigger melting by lowering the melting point or changing pressure/temperature conditions.
  • Three main melting mechanisms:
    • Decompression melting: melting occurs at higher temperatures with increasing depth but decreasing confining pressure; rising hot mantle rocks melt as they ascend to regions of lower pressure. Divergent plate boundaries and mantle plumes (hot spots) are key settings.
    • Addition of water (flux melting): water and other volatiles lower the melting temperature; mainly occurs at subduction zones as seawater or fluids from crustal rocks migrate into the overlying mantle wedge.
    • Melting of crustal rocks: mantle-derived basaltic magma can melt surrounding crustal rocks; continental collisions can generate heat that melts crust to form granitoids.

4.6 How Magmas Evolve

  • Magmatic differentiation and crystal settling:
    • Crystal settling: earlier-formed minerals are denser than the remaining melt and sink to the base of the magma chamber.
    • As the remaining magma solidifies, the mineralogy and chemistry of the solid product differ from the parent magma.
  • Assimilation and magma mixing:
    • Assimilation: as magma moves through crust, it incorporates surrounding rock, melting and changing its chemical composition.
    • Magma mixing: two chemically different magmas may merge; convective mixing blends melts to form a mixed composition.

4.7 Partial Melting and Magma Composition

  • Partial melting is incomplete melting of rock; the melt is enriched in ions from minerals with the lowest melting temperatures.
  • Basic outcomes:
    • Ultramafic rocks yield mafic magmas (basalts).
    • Mafic rocks yield intermediate magmas.
    • Intermediate rocks yield felsic magmas.
  • Formation of basaltic magmas:
    • Most erupted magmas are basaltic (mafic) and originate from partial melting of mantle rocks at oceanic ridges; these melts are primitive (primary) magmas that have not yet evolved.
  • Formation of andesitic and granitic magmas:
    • Andesitic magmas can form by magmatic differentiation of mantle-derived basaltic magma or by basaltic magma assimilating crustal rocks.
    • Granitic magmas commonly form when basaltic magma ponds beneath continental crust, heating and melting lower-milting-temperature felsic minerals; granite can also form via differentiation of andesitic magma.

4.8 Intrusive Igneous Activity

  • Intrusive igneous structures (plutons): dikes, sills, batholiths, stocks, laccoliths.
  • Plutons and emplacement:
    • A pluton is cooled, emplaced magma intruded into preexisting rocks (country rock).
    • Country rock: the preexisting rocks surrounding a pluton.
  • Tabular intrusive bodies:
    • Dike: a tabular, discordant pluton; magma is forcibly injected into fractures cutting across bedding planes; transport magma upward.
    • Dike swarms: parallel groups of dikes; radiate from a volcanic neck like spokes on a wheel.
    • Sill: a tabular, concordant pluton; nearly horizontal; magma exploits weaknesses along bedding planes; tends to accumulate magma and increase in thickness; resembles buried lava flows; may show columnar jointing.
  • Massive intrusive bodies:
    • Batholiths: the largest intrusive bodies; linear structures hundreds of kilometers long; surface exposure >10^2 ext{ km}^2; usually less than 10 km thick; typically composed of felsic to intermediate rocks.
    • Stocks: smaller equivalents of batholiths (smaller surface exposure).
  • Emplacement processes and features:
    • Buoyancy of magma drives ascent through a process called shouldering.
    • Stoping: blocks of country rock are dislodged and sink into the magma, later encasing xenoliths in the pluton.
    • Laccoliths: forcible injection between sedimentary strata that arches the overlying strata upward; considered overinflated sills.
  • Key terms:
    • Xenoliths: fragments of country rock enclosed in plutons; evidence of emplacement.
    • Columnar jointing: cooling fractures that yield six-sided columns in some rocks.

Concept Checks (End of Chapter 4)

  • 4.1 Magma: Parent Material of Igneous Rock
    • What is magma? How does magma differ from lava?
    • List and describe the three components of magma.
    • Compare and contrast extrusive and intrusive igneous rocks.
  • 4.2 Igneous Compositions
    • Igneous rocks are composed mainly of which group of minerals?
    • How do light-colored igneous rocks differ in composition from dark-colored rocks?
    • List the four basic compositional groups of igneous rocks, in order from highest silica content to lowest.
  • 4.3 Igneous Textures: What Can They Tell Us?
    • How does the rate of cooling influence crystal size? What other factors influence texture?
    • List the six major igneous rock textures.
    • What does a porphyritic texture indicate about cooling history?
  • 4.4 Naming Igneous Rocks
    • List the two criteria by which igneous rocks are classified.
    • How are granite and rhyolite different? In what way are they similar?
    • Describe each of the following in terms of composition and texture: diorite, rhyolite, and basalt porphyry.
  • 4.5 Origin of Magma
    • Explain the process of decompression melting.
    • What role does water play in magma formation?
    • Briefly explain one way basaltic magma can generate felsic magma.
  • 4.6 How Magmas Evolve
    • Define magmatic differentiation.
    • How does crystallization and settling alter the remaining magma’s composition?
    • Describe assimilation and magma mixing.
  • 4.7 Partial Melting and Magma Composition
    • Why does partial melting yield magma with a different composition from the source rock?
    • What process is thought to generate most basaltic magmas? Most granitic magmas?
  • 4.8 Intrusive Igneous Activity
    • What is meant by the term country rock?
    • Describe dikes and sills using the terms massive, discordant, tabular, and concordant.
    • Distinguish among batholiths, stocks, and laccoliths in terms of size and shape.