Lecture 7: Igneous Rocks

Igneous Rocks – Quick Notes

• Igneous rocks form by melting and crystallization of magma. They are a key part of the rock cycle alongside sedimentary and metamorphic rocks.

• Magma formation settings:

  • Divergent plate boundaries (rifting, mid-ocean ridges)

  • Convergent plate boundaries (subduction zones)

  • Hot spots (mantle plumes; e.g., Hawaii, Yellowstone)

• Magma emplacement processes:

  • Intrusion (crystallizes below surface; plutonic rocks)

  • Volcanism (eruption at surface; volcanic rocks)

Texture and Grain Size

• Texture = crystal (grain) interlocking pattern.

• Interlocking texture requires time and space for crystals to grow; no gaps unless vesicles

• Crystal size categories:

  • Fine-grained (aphanitic): crystals too small to see with naked eye

  • Coarse-grained (phaneritic): crystals visible with naked eye

• Volcanic glass forms when lava cools too quickly for crystals to grow (quenched melt).

• Vesicles form when dissolved gases escape from cooling lava.

Cooling Rate and Grain Size

• Cooling rate controls grain size:

  • Fine-grained = fast cooling

  • Coarse-grained = slow cooling

• Speed ranges:

  • Fast cooling: surface eruptions or rapid quenching

  • Slow cooling: underground crystallization over long times

• Porphyritic texture: some crystals (phenocrysts) crystallize underground before eruption; groundmass is fine-grained.

Intrusive vs Extrusive Rocks

• Intrusive (plutonic): crystallize below surface; typically coarse-grained

  • Common examples:

  • Granite (felsic)

  • Diorite (intermediate)

  • Gabbro (mafic)

• Extrusive (volcanic): erupt at surface; can be fine-grained or glassy

  • Common examples:

  • Rhyolite (felsic)

  • Andesite (intermediate)

  • Basalt (mafic)

  • Obsidian (glassy felsic to intermediate)

  • Pumice, Vesicular basalt (gas-rich)

Volcanic Glass and Vesicular Rocks

• Volcanic glass: rapid cooling yields glassy rocks (no crystals)

  • Examples: obsidian (Si-rich glass), basaltic glass

• Vesicular rocks: gas escape creates vesicles (empty bubbles) in rock.

Silicate Minerals – Key Players (nine primary minerals)

• Olivine

• Calcium-plagioclase feldspar (Ca-plagioclase)

• Pyroxene

• Amphibole

• Sodium-plagioclase feldspar (Na-plagioclase)

• Orthoclase feldspar (K-feldspar)

• Biotite

• Muscovite

• Quartz

• Crystallization depends on magma composition and temperature; quartz and olivine are rarely found together in the same rock.

• Silicate minerals can be grouped into felsic vs mafic minerals:

  • Light minerals (felsic): quartz, muscovite, orthoclase, Na-plagioclase (to some extent)

  • Dark minerals (mafic): olivine, pyroxene, amphibole, Ca-plagioclase, Biotite

  • The exact mineralogy of a rock reflects its source magma composition and cooling history.

Magma Flavors: Composition and Silicate Content

• Magmas have varying silica and metal content, giving them a "flavor":

  • Felsic: high ext{SiO}_2, low Fe (Fe) and Mg; abundant light minerals (quartz, K-feldspar)

  • Intermediate: moderate ext{SiO}_2 and Fe/Mg

  • Mafic: lower ext{SiO}_2, higher Fe and Mg; abundance of dark minerals (olivine, pyroxene)

  • Ultramafic: very low ext{SiO}_2, high Fe and Mg; dominated by olivine and pyroxene

• Common rock names by composition (intrusive vs extrusive):

  • Felsic: Granite (intrusive), Rhyolite (extrusive)

  • Intermediate: Diorite (intrusive), Andesite (extrusive)

  • Mafic: Gabbro (intrusive), Basalt (extrusive)

  • Ultramafic: Peridotite (intrusive), Komatiite (extrusive)

• Composition relationships (simplified):

  • ext{Felsic}
    ightarrow ext{high SiO}_2, ext{ light minerals}

  • ext{Mafic}
    ightarrow ext{lower SiO}_2, ext{ high Fe/Mg}

  • Ultramafic rocks resemble upper mantle compositions.

• Mineralogical implications: the relative % of light vs dark minerals helps identify rock type even if hand sample looks featureless.

Quick Reference: Common Rock Associations

• Granite (felsic, intrusive) – coarse-grained

• Diorite (intermediate, intrusive) – medium-grained

• Gabbro (mafic, intrusive) – coarse-grained

• Peridotite (ultramafic, intrusive) – very coarse/granular

• Rhyolite (felsic, extrusive) – fine-grained or glassy

• Andesite (intermediate, extrusive) – fine-grained

• Basalt (mafic, extrusive) – fine-grained

• Komatiite (ultramafic, extrusive) – ultrafine to glassy

Additional Quick Concepts

• Gas content in magma leads to vesicles in the resulting rock; gas escape happens as pressure drops at surface.

• The rock cycle connects igneous, sedimentary, and metamorphic processes via weathering, erosion, burial, metamorphism, and melting.

• Crystallization textures depend on cooling history; interlocking grains indicate crystallization in a melt that cooled slowly enough for crystals to grow, while glass indicates instantaneous quenching.

Note: All color-coded mineral names and rock types above reflect common igneous rock classifications and are intended for quick recall and exam review.