igneous rocks

Introduction to Igneous Rocks Study Guide

Intrusive vs. Extrusive

• Intrusive (Plutonic) Rocks:

  • Definition: Form when magma cools slowly beneath Earth’s surface.

  • Characteristics:

  • Allows large, visible crystals to grow.

  • Examples: Granite, Gabbro.

• Extrusive (Volcanic) Rocks:

  • Definition: Form when lava cools quickly at or near the surface.

  • Characteristics:

  • Rapid cooling produces fine-grained or glassy textures.

  • Examples: Basalt, Obsidian.

Magma vs. Lava

• Magma:

  • Definition: Molten rock found underground.

• Lava:

  • Definition: Molten rock that has erupted at the surface.

Factors that Melt Rock

• Geothermal Gradient:

  • Definition: Temperature increases with depth; rocks reach melting point at certain depths.

• Rock Chemistry:

  • Note: Different minerals melt at varying temperatures.

• Heat Transfer:

  • Definition: Contact with other magma bodies can elevate temperature enough to melt surrounding rock.

• Pressure Changes:

  • Definition: Decompression melting occurs when pressure decreases, common at divergent boundaries.

• Volatiles:

  • Definition: Water and gases that lower melting temperature, promoting melting especially at subduction zones.

Volatile Components

• Definition: Dissolved gases (H₂O, CO₂, SO₂) in magma.

• Significance:

  • Reduce melting temperature.

  • Drive explosive volcanic eruptions when released.

Three Components of Magma

• Melt:

  • Definition: The liquid portion of magma.

• Crystals:

  • Definition: Solid minerals forming as magma cools.

• Volatiles:

  • Definition: Dissolved gases contained within the magma.

Igneous Rock Textures

• Texture Types:

  • Intrusive:

  • Coarse-grained with large crystals.

  • Extrusive:

  • Fine-grained with small crystals.

• Texture Meaning:

  • Refers to crystal size, shape, and arrangement, controlled by the cooling rate of the magma.

• Pegmatitic Texture:

  • Definition: Extremely large crystals, form from very slow cooling in water-rich magma (intrusive).

• Phaneritic Texture:

  • Definition: Visible crystals formed from slow cooling underground (intrusive).

• Aphanitic Texture:

  • Definition: Fine-grained texture resulting from rapid cooling at the surface (extrusive).

• Crystal Size vs. Cooling Rate:

  • Larger crystals correspond to slower cooling while smaller crystals result from faster cooling.

• Glassy Texture:

  • Definition: Characterizes extrusive rocks with no crystals due to instantaneous cooling (e.g., obsidian).

  • Note: Different from crystalline textures.

• Vesicles:

  • Definition: Gas bubbles trapped in lava, forming pumice or scoria.

• Order of Textures:

  • Largest to none: Pegmatitic → Phaneritic → Aphanitic → Glassy.

• Pyroclastic Texture:

  • Definition: Fragmental texture formed from explosive eruptions (includes ash, pumice, volcanic breccia).

• Porphyritic Texture:

  • Definition: Results from two-stage cooling; features large crystals (phenocrysts) embedded in a fine-grained groundmass.

Igneous Rock Compositions

• Basis of Classification:

  • Based on silica content and mineral proportions.

• Composition Types:

  • Felsic:

  • Definition: Rocks high in silica and light-colored minerals (e.g., quartz, feldspar).

  • Mineral percentage: ~70% light minerals.

  • Intermediate:

  • Definition: Rocks with a mix of light and dark minerals (e.g., andesite, diorite).

  • Mineral percentage: ~50/50.

  • Mafic:

  • Definition: Low silica content and mainly dark minerals (e.g., pyroxene, olivine, basalt).

  • Mineral percentage: ~70% dark minerals.

  • Ultramafic:

  • Definition: Very low silica, predominantly olivine/pyroxene (e.g., peridotite).

  • Mineral percentage: ~90% dark minerals.

Bowen’s Reaction Series

• Purpose:

  • Explains the sequence of mineral crystallization as magma cools and predicts mineral assemblages.

• Order of Mineral Crystallization (High to Low Temperature):

  • Olivine → Pyroxene → Amphibole → Biotite → K-feldspar → Muscovite → Quartz.

• High-Temperature Rocks:

  • Mafic rocks (e.g., basalt, gabbro) contain olivine and pyroxene.

• Discontinuous Branch:

  • Definition: Sequence in which mafic minerals crystallize; silicate structures evolve from isolated tetrahedra (olivine) to single chains (pyroxene), to double chains (amphibole), to sheets (biotite).

• Felsic Minerals:

  • Crystallize at lower temperatures (e.g., quartz, muscovite, K-feldspar).

• First Mineral to Crystallize:

  • Olivine at the highest temperature.

Generation of Magma Types

• Primary Magma:

  • Definition: Magma that forms directly from the mantle; usually basaltic (mafic).

• Crystal Settling:

  • Definition: Process where early formed dense minerals sink, enriching the remaining magma in silica.

• Intermediate Magma:

  • Definition: Formed by mixing mafic and felsic magmas or by fractional crystallization.

• Assimilation:

  • Definition: Process where magma melts surrounding rock, changing its composition.

• Xenolith:

  • Definition: A piece of surrounding rock that is trapped in magma and preserved in the final rock formation.

Classification of Igneous Rocks

• Based on Texture:

  • Coarse-grained (plutonic/intrusive).

  • Fine-grained/glassy (volcanic/extrusive).

• Based on Composition:

  • Felsic:

  • Associated with continental crust.

  • Mafic:

  • Associated with oceanic crust.

  • Intermediate:

  • Found in volcanic arcs.

  • Ultramafic:

  • Associated with the mantle.

• Identifying Minerals:

  • Quartz, feldspar, olivine, pyroxene, amphibole, biotite.

• Naming Rocks:

  • Combine texture with composition:

  • Granite = felsic + phaneritic.

  • Rhyolite = felsic + aphanitic.

  • Diorite = intermediate + phaneritic.

  • Andesite = intermediate + aphanitic.

  • Gabbro = mafic + phaneritic.

  • Basalt = mafic + aphanitic.

  • Peridotite = ultramafic + phaneritic.

  • Obsidian = felsic + glassy.