Chapter 1-8: Igneous Rocks and Melting Concepts (Vocabulary)
Module 4 Notes: Igneous Rocks and Volcanoes (Key Concepts from Lecture)
Overview
Focus on igneous rocks and the processes that form them, with two related topics: igneous rocks and volcanoes.
Schedule context: module 4 precedes lectures on volcanoes; next two lectures cover volcanoes and volcano hazards.
Course calendar reminders: module 3 activity and quiz deadlines, lab activities, and the first exam after completing module 4 (on a Friday after fall break).
Goal for today: differentiate igneous rocks by texture and composition, name six basic igneous rocks (plus ultramafic case), understand magma generation, and link rocks to plate boundaries.
Learning outcomes (what you should be able to do by the end)
Classify igneous rocks by composition and texture.
Use terminology: felsic, intermediate, mafic; and texture terms: fine grain vs coarse grain.
Explain the difference between intrusive (plutonic) and extrusive (volcanic) rocks.
Describe the three ways magma can be generated (partial melting, decompression melting, flux/ water-assisted melting) and how these relate to rock types.
Relate igneous rock types to plate boundaries and magmatic processes.
Texture: how to distinguish rocks by crystal size and cooling environment
Texture relates to crystal size and how the rock cooled.
Coarse-grained (phaneritic): large crystals visible to the naked eye; forms when cooling is slow underground (intrusive).
Fine-grained (aphanitic): tiny crystals, often not visible; forms when cooling is rapid at or near the surface (extrusive).
Glassy texture: no crystals (obsidian); extremely rapid cooling so no minerals crystallize.
Vesicular texture: vesicles (bubble holes) trapped gas; common in rocks formed from lava that contained dissolved gases.
Porphyritic texture: two-step cooling; large crystals (phenocrysts) in a fine-grained groundmass; rock type = porphyry/porphyritic.
Welded vs non-welded textures (pyroclastic): welded textures form when hot volcanic ash/pieces fuse together in a hot eruption cloud; non-welded is just loose ash.
Visual cues and terminology used in the class:
Coarse-grained rock: visible minerals; examples include granite (felsic) and diorite (intermediate) and gabbro (mafic).
Fine-grained rock: minerals too small to see without a microscope; examples include rhyolite (felsic) and andesite (intermediate) and basalt (mafic).
Obsidian as a prime example of glassy texture.
Vesicular rocks indicate gas escape during eruption (e.g., vesicular rhyolite/basalt).
Composition (SiOâ‚‚-based classification) and mineral color proxies
Silica and color relationship (proxy for composition):
Felsic rocks: high silica content; light in color; typically pink/white minerals.
Mafic rocks: lower silica content; dark in color; many dark minerals.
Intermediate rocks: middle range of silica; gray to gray-black colors.
- Silica content thresholds used in the lecture (approximate):
ext{felsic}: ext{SiO}_2 ext{ content} \ge 70\%
ext{intermediate}: 60\% \le \text{SiO}_2 < 70\%
ext{mafic}: 50\% \le \text{SiO}_2 < 60\%
ext{ultramafic}: \text{SiO}_2 < 50\%
Common mineral associations by composition (colors and minerals):
Felsic minerals: light-colored minerals such as quartz, orthoclase feldspar, plagioclase (sodium-rich), muscovite; often pink or white in color.
Mafic minerals: dark minerals such as olivine, pyroxene, amphibole, biotite; calcium-rich plagioclase common.
Ultramafic minerals: olivine and pyroxene predominate; mantle rocks.
Mantle vs crust color proxies:
Felsic rocks: light-colored overall (quartz, feldspars).
Mafic rocks: dark-colored (olivine, pyroxene, amphibole, biotite).
Ultramafic rocks: very dark, often green/black (peridotite).
Ultramafic rock example: peridotite (mantle rock), characterized by green + black minerals; no light minerals.
Rock names by combining texture and composition (six plus ultramafic = seven core rock names)
Felsic rocks
Intrusive: granite (coarse-grained, pink minerals visible; light-colored overall).
Extrusive: rhyolite (fine-grained, light-colored; pink minerals often not visible).
Intermediate rocks
Intrusive: diorite (coarse-grained, salt-and-pepper appearance; gray with light and dark minerals).
Extrusive: andesite (fine-grained; gray to gray-black).
Mafic rocks
Intrusive: gabbro (coarse-grained, dark): typically black/green crystals.
Extrusive: basalt (fine-grained, dark): very dark overall.
Ultramafic rocks
Intrusive: peridotite (coarse-grained; green + black; mantle rock).
Note: There is no practical extrusive counterpart for mantle peridotite in the lecture context.
Special texture term: porphyry/porphyritic (two textures; large crystals in a finer matrix; common in rocks that experienced two-stage cooling, often with large phenocrysts in a fine groundmass).
How to identify an igneous rock (two-variable approach)
Step 1: Determine mineral size (texture) to decide if intrusive vs extrusive.
Step 2: Determine composition (color proxy and mineral content) to decide felsic, intermediate, mafic, or ultramafic.
Step 3: Match texture + composition to rock name using the seven core options:
Fine-grained + felsic → rhyolite
Coarse-grained + felsic → granite
Fine-grained + intermediate → andesite
Coarse-grained + intermediate → diorite
Fine-grained + mafic → basalt
Coarse-grained + mafic → gabbro
Coarse-grained + ultramafic → peridotite
Note on lab labeling: countertop-style labels (e.g., granite) may be misleading; rely on mineral content and texture for correct geologic naming (e.g., some “granite” looking pieces may actually be diorite in disguise).
Example synthesis exercise described in lecture:
A rock with no visible minerals under naked eye → fine-grained extrusive → likely rhyolite or basalt (use color to distinguish felsic vs mafic).
A rock with visible minerals and pink components → felsic, coarse-grained → granite.
The magma-formation process: three primary ways to melt rock (Bowen’s reaction series context)
Partial melting via temperature increase (increase in heat to the solid):
Minerals melt at characteristic temperatures depending on their chemistry; the rock melts in a sequence according to mineral melting points.
Analogy used in class: chocolate chip ice cream melting; the felsic components (e.g., quartz) melt first in the melt, while some mafic components (e.g., chocolate chips) melt later.
Key concept: partial melting yields melts whose composition is enriched in the minerals that melted first; the remaining solid is depleted in those minerals.
Practical implication: a rock that starts mafic can partially melt to yield felsic magma if felsic minerals melt first; the final melt composition can be felsic even if the original rock was mafic.
Decompression melting (pressure decrease, temperature held constant or rising locally):
Occurs where mantle rocks rise toward the surface and pressure drops faster than cooling, allowing melting.
Primary settings: divergent plate boundaries and mantle plumes/hot spots.
Outcome: melts are mafic, derived from ultramafic mantle rocks; results in basalt/gabbro when cooled and crystallized; ocean crust forms at divergent boundaries via decompression melting.
Flux (water) melting (adding volatiles to rocks to lower solidus):
Water and other volatiles reduce the melting temperature of rocks, enabling melting at lower temperatures than dry rocks.
Primary setting: subduction zones where oceanic crust carries water-rich sediments and hydrates into the mantle.
Outcome: melt tends to be intermediate to felsic depending on crustal involvement; explains generation of andesitic to rhyolitic magmas at subduction zones.
Bowen’s reaction series (crystallization order during cooling; melting order during partial melting as taught in lecture):
In crystallization (from melt): olivine and calcium-rich plagioclase crystallize first; then pyroxene and amphibole; then biotite; finally quartz, muscovite, and orthoclase feldspar crystallize last.
In melting (as taught in class via the ice-cream analogy): quartz melts first, potassium feldspar melts next, amphibole last (note: this is the course-specific melting-order analogy; standard Bowen’s series emphasizes crystallization order from melt).
The practical takeaway: melting and crystallization are temperature-dependent sequences tied to mineral chemistry; high-temperature minerals melt or crystallize first, and low-temperature minerals melt or crystallize last.
Plate tectonics context: linking melt mechanisms to plate boundaries and rock types
Divergent boundaries (oceanic ridges, continental rifts) and hot spots (mantle plumes):
Primary melt process: decompression melting (pressure drops as mantle rises).
Common products: mafic magmas (basalt at ocean ridges, gabbro in crustal intrusions).
Oceanic crust typically forms basalt at the surface (extrusive) and gabbro underground (intrusive).
Oceanic crust thickness and distribution: about 7–10 km thick; majority of oceanic crust is gabbro (underground cooling), with basalt representing the erupted surface layer.
Hot spots (e.g., Hawaii): mantle-derived mafic magmas rise; surface rocks are typically mafic (dark) initially; weathering may alter color on older rocks.
Subduction zones (ocean-ocean, ocean-continent):
Primary melt process: fluid-induced melting due to water released from subducted oceanic crust into the mantle wedge; lowers solidus and enables melting at lower temperatures.
Output magmas depend on input materials: predominantly mafic inputs (basalt) plus significant water- and sediment-derived components; productivity leads to intermediate to felsic magmas at various depths (andesites to rhyolites).
Ocean-ocean subduction tends to produce predominantly intermediate magmas (andesite/diorite) with mantle-derived basalts modified by sediments; island arc volcanism commonly yields andesite-diorite compositions.
Ocean-continent subduction introduces more felsic material due to crustal involvement; continents host continental crust that is felsic on average, so melts tend toward intermediate to felsic compositions (andesite to rhyolite).
Continental rifts and continental hot spots:
A combination of decompression melting in the mantle and partial melting of surrounding crust when magma resides in thick continental crust.
Long residence in crust can produce felsic melts (rhyolite) in continental hot spots (e.g., Yellowstone’s rhyolitic volcanism).
The magma chemistry can range from mafic to felsic depending on residence time and crustal contamination.
Practical takeaway: the input material and the depth at which melting occurs, plus time for crustal interaction, determine the final magma composition and resulting volcanic rock type.
Practical exercise and caveats
A common classroom exercise involves identifying rock samples by texture and composition and then naming the rock (seven primary names): rhyolite, granite, andesite, diorite, basalt, gabbro, peridotite.
Lab notes emphasize that some countertop rock samples labeled as granite may not be geologically granite (due to mislabeling or compositional differences); use mineral content and texture to verify.
Example clarifications from the lecture:
A rock that is dark and has no visible minerals under naked-eye viewing is an extrusive basalt (fine-grained mafic).
A rock with pink minerals and coarse grains is granite (felsic intrusive).
A two-texture rock with large crystals in a fine matrix is porphyry (porphyritic).
A rock with dark minerals and salt-and-pepper appearance is diorite (intermediate intrusive).
The fractionation concept: as minerals crystallize out of melt, the remaining melt changes composition (chemistry of the magma chamber evolves); this can drive the melt toward more felsic or more mafic compositions depending on which minerals are removed.
A quick memory aid: the crystallization and melting sequences depend on temperature, pressure, and composition; partial melting and fractional crystallization link to plate boundary settings and resulting rock types.
Quick reference: plate-boundary–magma–rock-type mapping (one-stop overview)
Ocean-ocean convergence with water addition (subduction): fluid-induced melting; outputs range from mafic to intermediate to felsic depending on sediments and crust; rocks likely include diorite and andesite; typically no pure mantle ultramafic melts at surface.
Divergent boundary (ocean): decompression melting of mantle; outputs mafic; rocks: basalt (extrusive) and gabbro (intrusive).
Ocean hotspot: mantle-derived decompression melting in a column; melts are mafic; rocks: basalt/gabbro in a plume setting.
Ocean-continent convergence: subduction + crustal melting; outputs intermediate to felsic; rocks: andesite to rhyolite; crustal contamination increases felsic tendency.
Continental hotspot or continental rift: combination of mantle decompression melting plus crustal melting; rocks range from mafic to felsic depending on crustal residence time; Yellowstone rhyolites are an example where crustal melting dominates.
Summary takeaways for exam prep
Two main rock-characteristics to memorize: texture (crystal size) and composition (SiOâ‚‚-related categories).
Rock names align with a two-axis framework: texture (intrusive vs extrusive) and composition (felsic, intermediate, mafic, ultramafic).
The seven core rocks to know (with typical textures):
Granite (felsic, coarse-grained, intrusive)
Rhyolite (felsic, fine-grained, extrusive)
Diorite (intermediate, coarse-grained, intrusive)
Andesite (intermediate, fine-grained, extrusive)
Gabbro (mafic, coarse-grained, intrusive)
Basalt (mafic, fine-grained, extrusive)
Peridotite (ultramafic, coarse-grained, mantle rock)
Three magma generation pathways to memorize:
Partial melting by increasing temperature (top-down melting of minerals with different melting points; partial melt composition reflects minerals that melt first).
Decompression melting by lowering pressure (divergent boundaries, hotspots).
Flux/melt melting by adding water/volatiles (subduction zones; lowers solidus).
Bowen’s reaction series conceptually links crystallization order (and the related melting sequence in class) to mineralogy and rock composition.
Always relate rock type back to its plate boundary context to predict likely magma and rock outcomes.
Quick glossary (to help memorize terms)
Intrusive (plutonic): rocks cooling underground; coarse-grained.
Extrusive (volcanic): rocks cooling at/near surface; usually fine-grained or glassy.
Phaneritic: coarse-grained texture visible to the naked eye.
Aphanitic: fine-grained texture requiring a microscope to see minerals.
Porphyritic: two textures present (phenocrysts in a finer groundmass).
Vesicular: rock with vesicles (gas bubbles).
Obsidian: natural glass; no crystals.
Peridotite: ultramafic mantle rock (green-black); primary constituent of the mantle.
Mafic vs felsic vs intermediate: based on silica content and color proxies; mafic = dark, felsic = light, intermediate = gray.