Chapter 4: Magma, Igneous Rocks, and Intrusive Activity


Magma: Parent Material of Igneous Rock

  • Definition: Magma is defined as completely or partly molten rock located at depth.

  • Difference from Lava: At the surface, this molten material is referred to as lava.

  • Igneous Rocks Formation: Igneous rocks are formed when magma solidifies, comprising mainly silicate minerals.

General Characteristics of Magma

  • Acts as the parent material of igneous rocks.

  • Formed through the process of partial melting in the Earth's crust.

The Nature of Magma

  • Components of Magma: Magma consists of three primary components:

    • Liquid Portion: Referred to as melt.

    • Solids: If present, these consist of crystals of silicate minerals.

    • Volatiles: These are dissolved gases that vaporize under surface pressure. Most common volatiles include:

      • Water vapor (H₂O)

      • Carbon dioxide (CO₂)

      • Sulfur dioxide (SO₂)

From Magma to Crystalline Rock

  • Crystallization: The process involves the cooling of magma, leading to the systematic arrangement of ions into orderly patterns.

  • Formation of Silicon-Oxygen Tetrahedra: Silicon and oxygen atoms initially combine to form silicon−oxygen tetrahedra.

    • As the magma continues to cool, these tetrahedra bond with each other and other ions to develop crystal nuclei.

  • Early Forming Minerals: Minerals that crystallize first tend to have adequate space for growth and exhibit more developed crystal faces compared to those forming later.

Igneous Processes

  • Plutonic or Intrusive Igneous Rocks: Form when magma crystallizes at depth; these rocks become visible at the Earth's surface after the erosion of overlying rocks.

  • Volcanic or Extrusive Igneous Rocks: Result from the solidification of lava or volcanic debris.

Extrusive Versus Intrusive Igneous Rocks

  • Figures illustrate the differences between intrusive and extrusive igneous rocks.

Igneous Compositions

  • Primary Composition: Igneous rocks are mainly composed of silicate minerals categorized into two groups:

    • Dark (Ferromagnesian) Silicates: Rich in iron (Fe) and/or magnesium (Mg); Examples include olivine, pyroxene, amphibole, and biotite mica.

    • Light (Nonferromagnesian) Silicates: Contain more potassium (K), sodium (Na), or calcium (Ca) than iron and magnesium; Examples include quartz, muscovite mica, and various feldspars.

Four Broad Compositional Groups of Igneous Rocks

  1. Felsic

  2. Intermediate

  3. Mafic

  4. Ultramafic

Granitic (Felsic) versus Basaltic (Mafic) Compositions

  • Granitic (Felsic) Composition:

    • Characteristics: Light-colored silicates, composed almost entirely of quartz and potassium feldspar.

    • Silica Content: High in SiO₂ (greater than 70%).

    • Contains approximately 10% dark silicate minerals.

    • Major constituent of the continental crust.

  • Basaltic (Mafic) Composition:

    • Characteristics: Contains at least 45% dark silicates and calcium-rich feldspar.

    • Silica Content: Lower compared to granitic rocks.

    • Usually darker in color, with higher density.

    • Commonly comprises the ocean floor and many volcanic islands, as well as extensive lava flows on continents.

Other Compositional Groups

  • Andesitic (Intermediate) Composition:

    • Composition falls between felsic and mafic groups.

    • Contains 25% or more dark silicate minerals (amphibole, pyroxene, and biotite mica).

    • Often associated with volcanic activity on the seaward margins of continents and volcanic island arcs.

  • Ultramafic Composition:

    • Rare type of igneous rock primarily made up of olivine and pyroxene.

    • Composed almost entirely of ferromagnesian minerals; Peridotite is an example, representing the upper mantle.

Silica Content as an Indicator of Composition

  • Igneous rocks show significant variance in silica content, from 40% in ultramafic rocks to 70% in felsic rocks.

  • Magma Behavior Based on Silica Content:

    • Granitic Magmas: High silica content leads to viscous (thick) behavior and erupts at lower temperatures.

    • Basaltic Magmas: Lower silica content grants more fluid behavior and erupts at higher temperatures.

Igneous Textures: What Can They Tell Us?

  • Definition of Texture: Refers to the overall appearance of a rock based on the size, shape, and arrangement of mineral grains.

  • Influencing Factors:

    • Rate of Cooling:

      • Slow cooling creates fewer but larger crystals.

      • Rapid cooling produces many small crystals.

    • Amount of Silica

    • Amount of Dissolved Gases

Types of Igneous Textures

  • Aphanitic Texture: Fine-grained, indicating rapid cooling with microscopic crystals.

  • Phaneritic Texture: Coarse-grained, demonstrating slow cooling with large, visible crystals.

  • Porphyritic Texture: Characterized by large crystals (phenocrysts) embedded in a matrix of smaller crystals (groundmass);

    • These rocks are called porphyries.

  • Vesicular Texture: Contains voids from gas bubbles in lava, common in extrusive igneous rocks.

  • Glassy Texture: Results from very rapid cooling where ions are frozen before forming an orderly crystalline structure.

  • Pyroclastic (Fragmental) Texture: Forms from consolidated individual rock fragments ejected during explosive eruptions.

  • Pegmatitic Texture: Exceptionally coarse-grained, originating from the late stages of crystallization of magmas (such rocks are called pegmatites).

Naming Igneous Rocks

  • Classification Based on: Texture and mineral composition.

    • Mineralogy: Influenced by the parent magma's chemical composition.

    • Texture: Results from cooling history.

Granitic (Felsic) Igneous Rocks

  • Granite:

    • Coarse-grained (phaneritic), one of the best-known and most abundant igneous rocks with 10−20% quartz and roughly 50% potassium feldspar.

    • Contains small amounts (<10%) of dark silicates.

  • Rhyolite:

    • Extrusive equivalent of granite, exhibiting fine-grained (aphanitic) texture, primarily composed of light-colored silicates, often buff to pink or light gray.

    • Less common than granite.

  • Obsidian:

    • Dark-colored glassy rock formed when silica-rich lava cools quickly at the Earth’s surface, generally black to reddish-brown in color.

  • Pumice:

    • Glassy textured rock featuring vesicular texture formed as gas escapes from lava, characterized by visually noticeable voids resembling fine shards of glass; able to float on water.

Andesitic (Intermediate) Igneous Rocks

  • Andesite:

    • Medium-gray, fine-grained rock with volcanic origin, commonly displaying porphyritic texture.

  • Diorite:

    • Intrusive equivalent of andesite; coarse-grained rock that appears similar to gray granite but lacks visible quartz crystals.

Basaltic (Mafic) Igneous Rocks

  • Basalt:

    • Very dark green to black, fine-grained rock, composed mostly of pyroxene and calcium-rich plagioclase feldspar; often contains small, light-colored feldspar or olivine phenocrysts.

    • Most common extrusive igneous rock forming upper layers of oceanic crust, Hawaiian Islands, and Iceland.

  • Gabbro:

    • Intrusive equivalent of basalt, featuring very dark green to black, phaneritic rock composition, predominantly made up of pyroxene and calcium-rich plagioclase feldspar.

Pyroclastic Rocks

  • Composed of fragments ejected during a volcanic eruption.

  • Tuff: Most common pyroclastic rock made of ash-sized fragments cemented together.

  • Welded Tuff: Formed when ash particles are hot enough to fuse together, can contain larger pieces of pumice and other rock fragments.

  • Volcanic Breccia: Comprises larger particles than ash, including lava blobs and broken blocks of various sizes.

Origin of Magma

  • Generating Magma from Solid Rock: The geothermal gradient typically shows temperature increasing by 25°C per kilometer in the upper crust.

    • Near Melting Points: Rocks in lower crust and upper mantle are often near their melting points. Tectonic processes that induce melting include:

      • Decrease in Pressure

      • Addition of Water

      • Increase in Temperature

Decompression Melting

  • Melting occurs at higher temperatures due to increasing depth and confining pressure, but a reduction in confining pressure lowers the melting temperature (decompression melting).

    • Solid mantle rocks ascend to lower pressure regions facilitating melting, commonly found at:

      • Divergent plate boundaries (spreading centers)

      • Mantle plumes at hot spots.

Addition of Water

  • Water and other volatiles lower the melting temperature of rocks, essential mostly at subduction zones.

    • The process follows the subduction of oceanic crust, where water is released from sediments into the wedge of mantle, triggering partial melting.

Temperature Increase

  • Crustal Rocks Melting: Basaltic magma can melt surrounding crustal rocks during its ascent.

How Magmas Evolve

  • Bowen’s Reaction Series: Minerals crystallizing in a systematic order based on melting points, starting with olivine. As crystallization occurs, the composition of the magma continuously changes.

    • When one-third of the magma crystallizes, the remaining melt loses iron, magnesium, and calcium content.

Magmatic Differentiation and Crystal Settling

  • Crystal Settling: Denser, earlier-formed minerals sink to the bottom of the magma chamber, altering the mineralogy of the solidifying magma.

  • Magmatic differentiation refers to the production of secondary magmas from an original parent magma.

Assimilation

  • Definition: As magma ascends, it can dislodge and melt surrounding rock (country rock), leading to a change in its chemical composition.

Magma Mixing

  • Occurs when two chemically distinct magma bodies merge, leading to a new magma body with a blended composition.

Partial Melting and Magma Composition

  • Partial Melting: Incomplete melting results in most magmas, enriching the melt with ions from minerals that have lower melting points.

    • Examples include:

      • Partial melting of ultramafic rocks yields mafic magmas.

      • Partial melting of mafic rocks produces intermediate magmas.

      • Partial melting of intermediate rocks results in felsic magmas.

Formation of Different Magmas

  • Most basaltic (mafic) magma originates from partial melting at oceanic ridges, termed primary.

  • Andesitic magma can arise through:

    • Magmatic differentiation of basaltic magma.

    • Assimilation of crustal rocks by basaltic magmas.

  • Granitic magmas typically form from melting of lower melting temperature felsic minerals beneath the continental crust or through magmatic differentiation of andesitic magma.

Intrusive Igneous Activity

  • Most magma forms intrusive igneous bodies within the Earth, leading to the classification of plutons by shape and orientation:

    • Tabular: table-shaped, can be discordant (cutting across existing structures) or concordant (parallel to features).

    • Massive: blob-shaped.

Tabular Intrusive Bodies

  • Dikes: Tabular, discordant plutons formed from magma forcibly injected into fractures.

  • Sills: Tabular, concordant plutons that are nearly horizontal, exploiting bedding plane weaknesses.

Massive Intrusive Bodies: Batholiths, Stocks, and Laccoliths

  • Batholiths: Largest intrusive bodies, occurring as linear structures typically a few kilometers thick and comprising mostly intermediate to felsic rock types.

  • Stocks: Smaller than batholiths.

  • Laccoliths: Forcibly injected magma that arches overlying layers upward.