CH. 4,5&6 Igneous Rocks: A Comprehensive Study Guide

🔥 Igneous Rock Formation: Magma and its Transformations

Magma: Molten rock material generated by partial melting of Earth's mantle and crust. It contains liquids, crystals, gases, and rock fragments in varying proportions depending on temperature, pressure, and chemical conditions. At temperatures above 1200°C, magma is mostly liquid and dissolved gases. As it cools, crystals form.

Lava: Magma that reaches the Earth's surface. Its eruption creates various volcanic features and deposits.

Intrusive (Plutonic) Rocks: Igneous rocks formed from magma that cools slowly below the Earth's surface. Slow cooling allows for the growth of large, visible crystals. Examples include granite, gabbro, diorite. Advanced Concept: Recent research suggests magma is often stored in "mush reservoirs," where it resides in pore spaces between crystals and interacts chemically with surrounding rock.

Extrusive (Volcanic) Rocks: Igneous rocks formed from lava that cools rapidly on the Earth's surface. Rapid cooling results in small crystals or glassy textures. Examples include basalt, andesite, rhyolite. Advanced Concept: The rate of cooling significantly impacts crystal size and texture. Extremely rapid cooling can produce entirely glassy rocks (obsidian).

Partial Melting: The process by which only a portion of a solid rock melts, creating magma. The composition of the resulting magma depends on the original rock and the conditions of melting. Advanced Concept: Different minerals have different melting points; thus, partial melting produces magmas with compositions different from the original rock.

Decompression Melting: Melting that occurs due to a decrease in pressure. This is a significant mechanism for magma generation at mid-ocean ridges. Advanced Concept: The relationship between pressure and melting point is crucial in understanding magma generation in various tectonic settings.

Volatile Content: The amount of dissolved gases (primarily water vapor, carbon dioxide, and sulfur dioxide) in magma. Volatiles significantly influence magma viscosity and eruptive style. Advanced Concept: The solubility of volatiles in magma is pressure-dependent; decompression leads to exsolution (separation) of gases, often driving explosive eruptions.

Crystallization: The process by which minerals form from a cooling magma. The order of crystallization is governed by Bowen's Reaction Series. Advanced Concept: The sequence of mineral crystallization is influenced by factors beyond temperature, including pressure, volatile content, and magma composition.

Magmatic Differentiation: Processes that cause the chemical composition of magma to change. These include fractional crystallization, assimilation, and magma mixing. Advanced Concept: Magmatic differentiation is a complex interplay of physical and chemical processes that lead to the diversity of igneous rock compositions.

💎 Igneous Rock Classification: Composition and Texture

Chemical Composition: The relative proportions of elements and minerals in a rock. This is a primary basis for classifying igneous rocks.

Silica (SiO2) Content: The percentage of silica in a rock, a crucial factor in classification. Rocks are classified as ultrabasic (<45%), basic (45-52%), intermediate (52-66%), and acidic (>66%). Advanced Concept: Silica content strongly influences magma viscosity and the types of minerals that crystallize.

Mafic: Describes rocks rich in magnesium and iron (Mg, Fe). These rocks are typically dark-colored. Advanced Concept: The term "mafic" refers to the minerals present, while "basic" refers to the chemical composition (low silica).

Felsic: Describes rocks rich in silicon and aluminum (Si, Al). These rocks are typically light-colored. Advanced Concept: Felsic rocks are often associated with continental crust, while mafic rocks are more common in oceanic crust.

Color Index (CI): The percentage of dark-colored minerals in a rock. This is used to classify rocks based on their mafic content. Advanced Concept: The color index is a field-based estimation and can be less precise than laboratory analyses.

Mineral Composition: The types and relative abundances of minerals in a rock. This, along with texture, is used to classify igneous rocks. Advanced Concept: The mineral assemblage reflects the magma's chemical composition and the conditions under which it crystallized.

Modal Mineralogy: The actual mineral composition of a rock, determined by visual identification or microscopic analysis. Advanced Concept: Modal analysis requires careful identification and quantification of minerals, which can be challenging in fine-grained rocks.

Normative Mineralogy: A calculated mineral composition based on the rock's chemical analysis. This is used for rocks where minerals are difficult to identify. Advanced Concept: Normative mineralogy relies on assumptions about crystallization conditions and may not perfectly reflect the actual mineral assemblage.

IUGS Classification: A widely used system for classifying igneous rocks based on their modal or normative mineral composition and texture. Advanced Concept: The IUGS system uses QAPF diagrams (for felsic rocks) and other diagrams to classify rocks based on the relative proportions of quartz (Q), alkali feldspar (A), plagioclase (P), and feldspathoids (F).

🔬 Igneous Rock Textures: A Window into Cooling History

Texture: The size, shape, arrangement, and degree of crystallinity of a rock's constituents. Texture provides crucial information about the cooling history of the magma.

Crystalline Texture: A texture characterized by the presence of visible crystals. This is common in intrusive rocks.

Phaneritic Texture: A crystalline texture with crystals large enough to be seen with the naked eye (1-30 mm). This indicates slow cooling. Subdivisions include fine-grained, medium-grained, and coarse-grained.

Aphanitic Texture: A crystalline texture with crystals too small to be seen without magnification (<1 mm). This indicates rapid cooling. Subdivisions include microcrystalline and cryptocrystalline.

Porphyritic Texture: A texture with two distinct crystal sizes: large phenocrysts embedded in a finer-grained groundmass. This suggests two stages of cooling.

Pegmatitic Texture: A texture with exceptionally large crystals (>30 mm). This is often associated with high volatile content and late-stage crystallization.

Glassy Texture: A texture lacking visible crystals, formed by extremely rapid cooling. Obsidian is a common example.

Vesicular Texture: A texture with numerous gas bubbles (vesicles). This is characteristic of volcanic rocks that cooled rapidly near the surface.

Amygdaloidal Texture: A vesicular texture where the vesicles have been filled with secondary minerals.

Pyroclastic Texture: A texture formed from the accumulation and consolidation of volcanic fragments (pyroclasts). Tuff and breccia are examples.

Euhedral: Crystals with well-formed crystal faces. These typically form early in crystallization when there is ample space.

Subhedral: Crystals with partially formed crystal faces. These often form later in crystallization when space is limited.

Anhedral: Crystals lacking well-formed crystal faces. These typically form in the final stages of crystallization.

🌋 Mafic Igneous Rocks: Oceanic Crust and More

Basalt: A fine-grained, mafic extrusive rock. It is the most abundant volcanic rock on Earth, forming the oceanic crust.

Gabbro: A coarse-grained, mafic intrusive rock. It is the intrusive equivalent of basalt.

Dolerite: A medium-grained, mafic intrusive rock. It forms in dikes and sills.

Pyroxenite: A mafic intrusive rock dominated by pyroxene minerals.

Komatiite: A rare, ultramafic volcanic rock with very high MgO content. It is associated with very high-temperature magmas.

Ophiolite: A sequence of rocks representing oceanic crust and upper mantle that has been obducted (thrust) onto continental crust. Ophiolites provide valuable insights into the formation of oceanic crust. Advanced Concept: Ophiolites typically consist of layered gabbros, sheeted dikes, pillow basalts, and ultramafic rocks.

Shield Volcanoes: Large, gently sloping volcanoes formed by the accumulation of fluid basaltic lava flows. These are characteristic of mafic volcanism.

Flood Basalts (Large Igneous Provinces - LIPs): Extensive outpourings of basaltic lava that cover vast areas. These are associated with massive volcanic events.

✨ Felsic Igneous Rocks: Continental Crust and Explosive Eruptions

Rhyolite: A fine-grained, felsic extrusive rock. It is the extrusive equivalent of granite.

Granite: A coarse-grained, felsic intrusive rock. It is a major component of continental crust.

Granodiorite: An intermediate intrusive rock between granite and diorite.

Tuff: A pyroclastic rock formed from the consolidation of volcanic ash.

Pumice: A very light, porous, felsic volcanic rock formed from frothy lava.

Obsidian: A glassy, felsic volcanic rock formed by rapid cooling.

Composite Volcanoes (Stratovolcanoes): Steep-sided volcanoes built up from alternating layers of lava flows and pyroclastic deposits. These are often associated with felsic volcanism and explosive eruptions.

Calderas: Large, basin-shaped volcanic depressions formed by the collapse of a volcano's summit. These are often associated with large, explosive eruptions.

⚖ Magma Viscosity and Eruptive Style: The Three Vs

Viscosity: A measure of a fluid's resistance to flow. Magma viscosity is a critical factor in determining eruptive style.

Volatiles: Dissolved gases in magma. These reduce viscosity and can drive explosive eruptions.

Volume: The amount of magma erupted. Large volumes can lead to more extensive volcanic features.

Effusive Eruptions: Relatively gentle eruptions characterized by the outpouring of lava flows. These are common with low-viscosity magmas.

Explosive Eruptions: Violent eruptions characterized by the ejection of pyroclastic material. These are common with high-viscosity magmas.

🌍 Tectonic Settings and Igneous Rock Formation

Mid-Ocean Ridges: Divergent plate boundaries where new oceanic crust is formed. Mafic magmas are generated by decompression melting.

Hot Spots: Plumes of magma rising from deep within the mantle. These can produce both mafic and felsic volcanism.

Subduction Zones: Convergent plate boundaries where one plate is forced beneath another. These can produce a range of magma compositions, from mafic to felsic.

Continental Rifts: Stretching and thinning of continental crust. This can lead to the formation of mafic magmas.

📚 IUGS Classification Schemes: A Detailed Look

This section will delve into the specifics of the IUGS classification system, including the use of QAPF diagrams for felsic and intermediate rocks, and the triangular diagrams for mafic and ultramafic rocks. It will also cover the TAS classification (Total Alkali Silica) used for volcanic rocks where mineral identification is difficult. The focus will be on understanding how to use these diagrams to classify rocks based on their mineral or chemical composition. Advanced Concept: The IUGS system is not without limitations; it doesn't encompass all igneous rock types and can be challenging to apply to fine-grained or glassy rocks.

Table: Summary of Igneous Rock Types

Rock Type Texture Composition Silica Content (%) Eruptive Style Tectonic Setting(s) Basalt Aphanitic Mafic 45-52 Effusive Mid-ocean ridges, hot spots Gabbro Phaneritic Mafic 45-52 Intrusive Oceanic crust Andesite Aphanitic Intermediate 52-63 Effusive/Explosive Subduction zones Diorite Phaneritic Intermediate 52-63 Intrusive Subduction zones Rhyolite Aphanitic Felsic >66 Explosive Continental crust Granite Phaneritic Felsic >66 Intrusive Continental crust Komatiite Aphanitic Ultramafic <45 Effusive Ancient Archean Peridotite Phaneritic Ultramafic <45 Intrusive Mantle Pumice Vesicular Felsic >66 Explosive Continental crust Obsidian Glassy Felsic >66 Explosive Continental crust Tuff Pyroclastic Variable Variable Explosive Various

Facts to Memorize:

1. Magma is molten rock beneath the Earth's surface; lava is magma erupted onto the surface.

2. Igneous rocks are classified based on texture (size and arrangement of crystals) and composition (mineral content).

3. Intrusive rocks cool slowly underground, forming large crystals; extrusive rocks cool rapidly on the surface, forming small crystals or glass.

4. Silica (SiO2) content is a key factor in classifying igneous rocks: ultrabasic (<45%), basic (45-52%), intermediate (52-66%), acidic (>66%).

5. Mafic rocks are dark-colored and rich in magnesium and iron; felsic rocks are light-colored and rich in silicon and aluminum.

6. Bowen's Reaction Series describes the order of mineral crystallization from magma.

7. Phaneritic textures have visible crystals; aphanitic textures have microscopic crystals; porphyritic textures have both large and small crystals.

8. Pegmatitic textures have exceptionally large crystals; glassy textures lack crystals.

9. Vesicular textures contain gas bubbles; amygdaloidal textures have filled vesicles.

10. Pyroclastic textures are formed from volcanic fragments.

11. Viscosity, volatiles, and volume are the three Vs that control eruptive style.

12. Mid-ocean ridges produce mafic rocks; hot spots can produce both mafic and felsic rocks; subduction zones produce a range of compositions.

13. Ophiolites are sections of oceanic crust and mantle exposed on land.

14. Granite and rhyolite are felsic; gabbro and basalt are mafic.

15. The IUGS classification uses diagrams to classify igneous rocks based on mineral or chemical composition.

16. Partial melting produces magmas with compositions different from the source rock.

17. Decompression melting is a major mechanism for magma generation at mid-ocean ridges.

18. Magmatic differentiation leads to the diversity of igneous rock compositions.

19. Modal mineralogy is the actual mineral composition; normative mineralogy is a calculated composition.

20. Textural equilibration involves changes in grain boundaries after initial crystal formation.