Igneous Rocks - Study Notes (Unit 4, Lesson 4.2)

Introduction

  • Igneous rocks originate from the solidification of molten material (magma or lava).
  • The term igneous comes from the Latin word ignis, meaning fire.
  • Formation temperature is not lower than 700∘C700^{\circ}C.
  • Rocks surrounding a campfire do not melt easily; igneous rocks require very high rock-melting heat, akin to conditions on the early Earth.
  • Geologists differentiate igneous rocks from other rock types by origin, texture, and mineral composition.

Learning Competency

  • Classify rocks into igneous, sedimentary, and metamorphic (S11/12ES-Ib-10).

Learning Objectives

  • Identify and differentiate the different types of igneous rocks.
  • Describe the origin and environment of formation of igneous rocks.

Warm-Up: Crystallize!

  • Materials: Colored tags for each student (colored papers on folders) with yarn to wear as a tag.
  • Procedure (summary):
    • Each student wears a tag and spreads out as far apart as possible.
    • A person yells “crystallize!” and students search for others with the same tag.
    • After ~5 seconds, the group stops; count members in each tag color group.
    • Repeat with different time intervals (10, 30, 60 seconds).
  • Guiding questions:
    • How many members formed at different intervals?
    • How does time interval relate to the number of members formed during crystallization?
    • What can you infer about crystal formation in rocks from this activity?

Learn about It

  • Common minerals in igneous rocks: quartz, feldspar, pyroxene, olivine, amphibole, and mica.
  • Igneous rocks derive from cooled magma; temperature and pressure influence formation.
  • Two main types based on solidification conditions: extrusive (on/near the surface) and intrusive (within the crust).
  • Crystal size indicates cooling rate: larger crystals imply slower cooling (deeper formation); smaller crystals imply rapid cooling (near-surface).
  • Time also affects crystal formation; longer cooling yields larger crystals.

Formation of Igneous Rocks

  • Igneous rocks show intergrowth of crystals and/or volcanic glass.
  • Crystal size is directly proportional to cooling rate: larger crystals = slow cooling; smaller crystals = fast cooling.
  • Examples:
    • Rock with large crystals: diorite.
    • Rock with small crystals: obsidian (glassy).
  • Model: cooling rate vs crystal size (Fig. shows slower cooling -> larger crystals; faster cooling near surface -> smaller crystals).

Bowen’s Reaction Series

  • Crystallization of silicate minerals occurs as magma cools; a general model shows mineral crystallization at specific temperatures.
  • Norman L. Bowen developed the reaction series in the early 1900s by melting igneous rocks at 1200∘C1200^{\circ}C and cooling them to observe crystal formation.
  • Two branches emerge:
    • Discontinuous branch (left): olivine → pyroxene → amphibole → biotite mica as temperature falls and minerals react/transform.
    • Continuous branch (right): development of calcium-rich plagioclase feldspar to its sodium-rich counterpart; as cooling proceeds, the branches merge and the magma crystallizes into potassium feldspar, muscovite mica, and quartz.
  • Mnemonic to remember the order in the discontinuous branch: “Old People Are Bright” (Olivine, Pyroxene, Amphibole, Biotite).
  • The continuous branch involves plagioclase feldspar: Ca-rich to Na-rich; the final consolidation yields K-feldspar, muscovite, and quartz.
  • Key takeaway: Bowen’s reaction series explains the sequence of mineral crystallization with decreasing temperature in silicate magmas.

Types of Igneous Rocks

  • Classification based on solidification conditions:
    • Extrusive (volcanic): magma erupts onto the surface and cools rapidly, producing fine-grained textures; often glassy or with small crystals.
    • Intrusive (plutonic): magma crystallizes below the surface, slow cooling, producing coarse-grained textures.

Extrusive Igneous Rocks

  • Form on Earth’s surface; textures are typically fine-grained or glassy due to rapid cooling.
  • Common textures: smooth, vesicular, or rocks with large crystals embedded in a fine groundmass (porphyritic).
  • Examples: obsidian (glassy texture), andesite, pumice.
  • Porphyritic texture: larger crystals (phenocrysts) embedded in a finer-grained groundmass.
    • Example: hornblende andesite (phenocrysts in a glassy or fine groundmass).
  • Analogy: hot cocoa with marshmallows can illustrate porphyritic texture (large crystals form first underground, then rest cool rapidly after eruption).

Intrusive Igneous Rocks

  • Form when magma crystallizes within the crust; coarse-grained textures due to slow cooling.
  • Also called plutonic rocks.
  • Intrusive bodies can be classified as:
    • Concordant (sill): runs parallel to existing rock beds.
    • Discordant (dike): cuts across beds.
    • Laccoliths: mushroom-shaped intrusions that are typically concordant but can be discordant depending on geometry.
  • Batholiths: large intrusive bodies formed deep within Earth.
  • Examples of textures: coarse-grained (phaneritic).

Other Ways of Classifying Igneous Rocks

  • Textures: aphanitic, phaneritic, pegmatitic, vesicular, porphyritic, amygdaloidal, pyroclastic.
  • Mineral composition categories: felsic, intermediate, mafic, ultramafic.

Types of Igneous Rocks Based on Texture

  • Aphanitic: crystals too fine-grained to be seen with naked eye or hand lens.
  • Phaneritic: grains coarse enough to be visible to the naked eye.
  • Pegmatitic: crystals larger than 3 cm due to very slow cooling.
  • Vesicular: contains small holes (vesicles) from gas bubbles in lava/magma.
  • Porphyritic: large crystals embedded in smaller crystals.
  • Amygdaloidal: vesicles filled by low-temperature minerals after solidification.
  • Pyroclastic: formed from consolidation of rock fragments ejected during a volcanic eruption.

Types of Igneous Rocks Based on Mineral Composition

  • Felsic (granitic): silica-rich; rich in quartz, potassium feldspar, and plagioclase.
  • Intermediate: medium gray/green; minerals include amphibole, plagioclase, and feldspar.
  • Mafic (basaltic): dark gray to black; abundant plagioclase and amphibole; small amounts of olivine.
  • Ultramafic: very dark green to black; dominated by olivine and pyroxene; very low silica (SiO2).

Naming Igneous Rocks

  • Binary system: classification based on texture and composition.
  • Table (texture vs composition):
    • Extrusive (volcanic): felsic → rhyolite; intermediate → andesite; mafic → basalt.
    • Intrusive (plutonic): felsic → granite; intermediate → diorite; mafic → gabbro.
  • Rocks can be named as either extrusive or intrusive based on their cooling history.
  • Felsic rocks are silica-rich; mafic rocks are silica-poor; intermediate rocks have intermediate silica content.
  • Quick observational rule: a rock with few crystals (volcanic) and dark minerals is likely mafic (e.g., basalt).
  • Common pyroclastic rocks from eruptions: rhyolitic tuff, andesitic tuff, basaltic tuff.

Key Points

  • Igneous rocks come from the Latin ignis (fire); formed by solidification of magma or lava emanating from depth.
  • Igneous rocks can be classified by: temperature-pressure conditions during solidification (extrusive vs intrusive), texture, and mineral composition.
  • Bowen’s reaction series describes the crystallization sequence of silicate minerals at specific temperatures.
  • Igneous rocks can be named based on texture and composition.
  • About the Earth’s crust: it is composed of approximately 95%95\% igneous rocks.

Check Your Understanding

A. Bowen’s reaction series diagram questions

  1. Which mineral in the Bowen’s reaction series has the highest silica content? → Quartz (SiO2)
  2. Which mineral in the Bowen’s reaction series has high amounts of calcium and sodium? → Plagioclase feldspar (Ca- and Na-rich end members; e.g., anorthite to albite)
  3. Which mineral is the lowest-temperature iron-bearing silicate mineral that can form a melt? → Biotite
  4. Which minerals contain iron and magnesium? → Olivine, Pyroxene, Amphibole, and Biotite

B. Name the igneous rock based on description

  1. High silica content, coarse-grained texture → Granite
  2. Low silica content, fine-grained texture → Basalt
  3. High silica content, fine-grained texture → Rhyolite
  4. Equal amounts of dark and light colored minerals, fine-grained texture → Andesite
  5. Low silica content, coarse-grained texture → Gabbro
  6. Equal amounts of dark and light colored minerals, coarse-grained texture → Diorite
  7. Peridotite → Ultramafic, intrusive
  8. Scoria → Mafic, extrusive (vesicular)
  9. Tuff → Pyroclastic, extrusive
  10. Dacite → Intermediate, extrusive (fine-grained)

C. Identify whether the following igneous rocks are intrusive or extrusive

  1. Gabbro → Intrusive
  2. Rhyolite → Extrusive
  3. Basalt → Extrusive
  4. Andesite → Extrusive
  5. Diorite → Intrusive
  6. Granite → Intrusive
  7. Peridotite → Intrusive
  8. Scoria → Extrusive
  9. Tuff → Extrusive
  10. Dacite → Extrusive

Photo Credits

  • Fig. 2. Diorite by Rojinegro81 is licensed under CC BY-SA 3.0 via Wikimedia Commons.
  • Obsidian by Anonymous via Wikimedia Commons.
  • Fig. 4. Pumice by Lassen is licensed under CC BY-SA 2.0 via Wikimedia Commons.
  • Sierra Grande Andesite by James St. John is licensed under CC BY-SA 2.0 via Wikimedia Commons.

Bibliography

  • Amethyst Galleries, Inc. “Mineral Gallery.” Accessed April 5, 2018. http://www.galleries.com/default.html
  • Coenraads, Robert R. 2005. Rocks & Fossils: A Visual Guide. Australia: Weldon Owen Inc.
  • Tarbuck, Edward J. and Frederick J. Lutgens. 2012. Earth Science, 13th Edition. New Jersey: Pearson Prentice Hall.
  • Thompson, Graham R. and Jonathan Turk. 1997. Introduction to Physical Geology. United States: Brooks Cole Publishing.
  • Williams, Linda. 2004. Earth Science Demystified. United States: The McGraw-Hill Companies, Inc.