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Felsic

Felsic is a geochemical term used in geology to classify certain silicate minerals, magmas, and igneous rocks that are enriched in silica (SiO₂) and alumina (Al₂O₃), with silica contents typically exceeding 66 weight percent, resulting in light-colored compositions dominated by quartz and feldspar minerals.^[1] The name "felsic" derives from "fel" (for feldspar) and "si" (for silica).^[2] Felsic rocks form through the cooling and solidification of felsic magma, which originates primarily from partial melting of the continental crust, often in subduction zone settings where water from the subducting plate lowers the melting point of crustal rocks.^[3]^[4] This magma is highly viscous due to its high silica content, leading to slow crystallization and, in extrusive cases, potentially explosive volcanic eruptions.^[5] Intrusive felsic rocks cool slowly beneath the Earth's surface, developing coarse-grained textures, while extrusive varieties cool rapidly at the surface, forming fine-grained or glassy textures.^[2] Common examples of felsic rocks include granite (intrusive, composed mainly of quartz, potassium feldspar, and plagioclase) and rhyolite (extrusive equivalent), as well as glassy forms like obsidian and vesicular pumice.^[6]^[2] These rocks are characteristically light in color—ranging from white, pink, or light gray—due to their abundance of low-density, ferromagnesian-poor minerals such as quartz, potassium feldspar, and sodium-rich plagioclase, with lesser amounts of biotite or hornblende.^[1] In contrast to denser, darker mafic rocks, felsic varieties are less common in oceanic settings but dominate continental crust, contributing significantly to its overall composition and influencing tectonic processes like crustal growth.^[5]

Terminology and Definition

Core Definition

Felsic denotes a compositional category of igneous rocks and magmas characterized by elevated silica content, typically 65-75 wt% SiO₂, along with enrichment in feldspar minerals that contribute to their characteristically light-colored appearance.^[7] This high silica proportion results in rocks dominated by lighter elements such as silicon, oxygen, aluminum, sodium, and potassium, contrasting with compositions richer in heavier elements like iron and magnesium.^[8] In geochemical classification, felsic materials are distinguished from mafic (45–52 wt% SiO₂), intermediate (52–65 wt% SiO₂), and ultramafic (<45 wt% SiO₂) types using the total alkali-silica (TAS) diagram, which plots total alkalis (Na₂O + K₂O) against silica content to delineate fields such as rhyolite and dacite for felsic volcanic rocks. The TAS scheme, established as a standard for volcanic rock nomenclature, ensures consistent identification based on major element chemistry normalized to 100% on a volatile-free basis. Physically, felsic rocks exhibit low density, generally 2.6–2.8 g/cm³, owing to their silica-rich, low-iron mineralogy, and their melts display high viscosity due to extensive silica polymerization, which impedes flow compared to less siliceous magmas.^[9] These rocks typically appear in light shades such as white, pink, or gray, reflecting the prevalence of pale minerals like quartz and feldspars.^[8] The term "felsic" originated in 1912 as a portmanteau of "feldspar" and "silica," introduced to provide a descriptive, non-chemical alternative to the outdated "acidic" label for high-silica rocks and avoid misconceptions about their pH properties.^[10] This nomenclature gained widespread adoption in the mid-20th century amid evolving geochemical understandings.^[11]

Etymology and Naming

The term "felsic" is a portmanteau derived from "feldspar" and "silica," reflecting the predominance of these components in the rocks it describes.^[12] It was coined in 1912 by American petrologists Whitman Cross, Joseph P. Iddings, Louis V. Pirsson, and Henry S. Washington in their paper modifying an earlier quantitative classification system for igneous rocks, where they proposed "felsic" to denote the collective group of modal feldspars, feldspathoids, and quartz.^[13] This introduction served as a neutral, descriptive alternative to the longstanding but problematic designation "acidic" or "acid rock," which had been applied to high-silica igneous rocks since the 19th century based on analogies to acid-base chemistry in silicate melts—where silica acts as a network former akin to an acid. The "acid" label proved misleading, as it implied a direct relation to pH acidity in aqueous solutions, whereas felsic rocks exhibit high silica content (typically >65 wt% SiO₂) without necessarily producing acidic solutions upon weathering; instead, the term stemmed from melt chemistry, not proton donation.^[14] Following its introduction, the term "felsic" gained traction in petrology literature during the mid-20th century, particularly from the 1950s onward, as quantitative geochemical analyses and modal classifications became more widespread. Its usage was formalized through the efforts of the International Union of Geological Sciences (IUGS) Subcommission on the Systematics of Igneous Rocks, established in 1970 under Albert Streckeisen, which integrated "felsic" into the QAPF (quartz-alkali feldspar-plagioclase-feldspathoid) modal classification scheme for both plutonic and volcanic rocks.^[15] This standardization, detailed in IUGS recommendations from 1973 and refined in subsequent publications (e.g., 1976, 1989), defined felsic rocks as those dominated by >90% QAPF minerals in plutonic equivalents or analogous compositions in volcanics, emphasizing its role in distinguishing light-colored, silica-enriched lithologies.^[15] In parallel with "felsic," the contrasting term "mafic" was introduced earlier by the same group of petrologists in 1903, derived from "magnesium" and "ferric" (iron) to describe iron- and magnesium-rich minerals and rocks, providing a balanced mnemonic pair for compositional spectra in igneous petrology. This duality—felsic for silica- and feldspar-dominated assemblages versus mafic for ferromagnesian ones—facilitated clearer, non-chemical nomenclature, influencing global standards and reducing reliance on outdated terms like "acidic" and "basic."^[15]

Relation to Acid Rocks

The term "acid rock" emerged in 19th-century geology to denote igneous rocks with high silica content, rooted in early chemical theories that viewed silica as occurring primarily in the form of silicic acid within magmas.^[16] This nomenclature, introduced by chemists like Robert Bunsen, categorized rocks as acid (high SiO₂), basic (low SiO₂), or intermediate based on their presumed reaction behaviors analogous to acids and bases.^[1] The terminology gained systematic structure through the 1903 quantitative classification by Whitman Cross, Joseph P. Iddings, Louis V. Pirsson, and Henry S. Washington (CIPW system), which divided igneous rocks into acid, intermediate, and basic series using chemical analyses and normative mineral calculations to reflect silica saturation levels.^[17] However, the "acid" label proved misleading, as it evoked pH connotations irrelevant to solid rock chemistry, prompting a shift toward more neutral descriptors focused on silica content and mineralogy.^[1] The term "felsic" largely supplanted "acid" in the mid-20th century to promote terminological clarity and avoid aqueous solution analogies, emphasizing instead the enrichment in feldspar and silica characteristic of these rocks. This replacement aligned with broader efforts to refine petrographic language, as seen in the etymology of "felsic" itself, derived from feldspar and silica as a direct counter to the limitations of "acid rock." Despite the transition, "acid rock" endures in specific modern applications, notably volcanology, where it describes high-silica (typically >65% SiO₂) lavas and eruptions like those producing rhyolite, highlighting viscous, explosive behaviors.^[18] The International Union of Geological Sciences (IUGS) Subcommission on the Systematics of Igneous Rocks endorses this selective persistence but deems "acid" obsolete for general rock classification, favoring terms like "felsic" in comprehensive schemes such as the total alkali-silica (TAS) diagram.^[19] Terminology shifts are illustrated in geological textbooks: mid-20th-century works, such as those from the 1960s referencing CIPW norms, routinely applied "acid rocks" to high-silica compositions, while post-2000 texts, including practical guides on igneous processes, standardize "felsic" for its precision and avoidance of outdated chemical implications.^[1]^[20]

Composition and Characteristics

Mineralogical Components

Felsic rocks are defined by their mineral assemblage dominated by silica-rich phases, with quartz typically comprising 20-60 vol% of the modal composition, providing structural stability and contributing to the rock's light color and hardness. Alkali feldspar, including varieties such as orthoclase and sanidine, forms the primary component at 35-60 vol%, while plagioclase feldspar, ranging from oligoclase to albite compositions, accounts for 10-30 vol%, influencing the rock's overall alkalinity and sodic character. These proportions are determined through modal analysis, which quantifies volume percentages of visible minerals in thin sections or hand samples.^[21]^[22]^[23] Accessory minerals play a subordinate but texturally significant role, with micas such as muscovite and biotite constituting 5-10 vol%, imparting a flaky or schistose appearance in some varieties, and hornblende appearing as minor prismatic crystals. Trace phases like zircon and apatite occur in less than 1 vol%, often as euhedral inclusions that aid in geochronology and phosphate content, respectively, without substantially altering the dominant felsic framework.^[7]^[24] In plutonic felsic rocks, such as granites, the slow cooling rates promote coarse-grained, phaneritic textures where interlocking crystals of quartz and feldspars are visible to the naked eye, fostering equigranular or porphyritic fabrics. Volcanic equivalents, like rhyolites, exhibit contrasting textures due to rapid surface cooling, resulting in aphanitic groundmasses that may be glassy (as in obsidian) or porphyritic with larger phenocrysts of quartz and feldspar embedded in a fine matrix. These textural differences highlight the control of cooling environment on mineral growth and rock fabric.^[25]^[7] The relative abundances of these minerals in felsic rocks directly reflect fractional crystallization during magma evolution, where progressive removal of denser mafic phases concentrates residual silica and alkalies into quartz and feldspars; modal analysis via the QAPF diagram provides quantitative estimates, placing felsic fields in regions where quartz exceeds 20 vol% of the total Q + A + P components, with alkali feldspar often dominating over plagioclase.^[26]^[27]

Geochemical Properties

Felsic rocks are defined by their high silica content, typically ranging from 68 to 77 wt% SiO₂, which distinguishes them from more mafic compositions. Average major oxide compositions include approximately 70 wt% SiO₂, 14.5 wt% Al₂O₃, 7.6 wt% combined Na₂O and K₂O (with Na₂O around 3.6 wt% and K₂O around 4.0 wt%), and low levels of FeO (about 2.6 wt%), MgO (1.0 wt%), and CaO (2.5 wt%). These proportions reflect the enrichment in light elements and silica typical of felsic magmas, derived from partial melting of crustal materials.^[28]^[29] Geochemical classification of felsic rocks often employs indices such as the silica saturation index, which assesses the degree of silica oversaturation based on the presence of quartz and the stability of silica-deficient minerals, and the aluminum saturation index (ASI), calculated as the molar ratio Al₂O₃ / (CaO + Na₂O + K₂O). An ASI greater than 1 indicates peraluminous compositions with excess alumina, favoring minerals like muscovite or cordierite, while values less than 1 denote metaluminous types balanced by calcium and other cations. These indices provide insights into the magma's differentiation and source characteristics without relying on modal mineralogy.^[1]^[30] Analytical methods for determining these compositions include X-ray fluorescence (XRF) spectrometry for major elements, which offers precise whole-rock analysis through fusion of powdered samples into glass beads, and inductively coupled plasma (ICP) techniques, such as ICP-optical emission spectrometry (OES) or mass spectrometry (MS), for trace elements at parts-per-million levels. These techniques ensure accurate oxide quantification after sample preparation involving acid digestion or fusion to minimize matrix effects.^[31]^[32] Compositional variations exist between continental and oceanic felsic rocks, with continental examples often showing elevated K₂O due to crustal assimilation, particularly in A-type granites that exhibit high K₂O/Na₂O ratios (up to 2.8) and alkaline affinities in anorogenic settings. Oceanic felsics, such as those in island arcs, tend toward lower potassium and more calcic trends influenced by slab-derived fluids.^[33]^[34]

Formation Processes

Magmatic Origins

Felsic magmas primarily originate from partial melting of the continental crust, often involving protoliths such as amphibolites or greywackes at temperatures between 700°C and 900°C under mid- to lower-crustal pressures.^[35] This process is facilitated in collisional orogens where tectonic thickening elevates temperatures sufficiently to induce melting without requiring excessively high heat inputs.^[35] Alternatively, in subduction settings, fluids derived from the dehydrating slab can infiltrate the overlying mantle wedge or crust, lowering the solidus temperature and promoting partial melting to generate felsic compositions.^[36] Hydrous conditions play a critical role in these melting processes, with water contents in the resulting melts typically ranging from 2 to 6 wt%, which depresses the melting point by tens to hundreds of degrees compared to anhydrous systems.^[37] Dehydration reactions of hydrous minerals, such as biotite (around 800–850°C) or hornblende (around 900°C), drive fluid-absent melting in the lower crust, releasing water that further enhances melt production.^[38] These conditions are common in amphibolite-facies rocks, where the breakdown of these minerals generates silica-rich melts.^[39] A key mechanism is crustal anatexis, involving incongruent melting of metasedimentary rocks, which produces peraluminous leucogranitic melts through the partial breakdown of muscovite and biotite without complete equilibration.^[40] This process yields high-silica, alkali-enriched magmas characteristic of felsic compositions.^[37] Isotopic signatures provide strong evidence for crustal derivation, with ⁸⁷Sr/⁸⁶Sr ratios exceeding 0.710—contrasting with mantle values below 0.705—indicating minimal mantle input, as observed in Himalayan leucogranites sourced from thickened metasediments.^[41]

Crystallization and Differentiation

Felsic magmas can evolve through fractional crystallization from a more mafic parent magma, a process where early-formed mafic minerals such as olivine and pyroxene are removed, progressively enriching the residual melt in silica (SiO₂) and alkalis (Na₂O and K₂O). This differentiation occurs as the magma cools, with denser mafic crystals settling to the chamber floor or being filtered out, leaving a more viscous, silica-rich liquid that ultimately solidifies into felsic rocks like granite or rhyolite. The process is well-illustrated by Bowen's reaction series, where early high-temperature phases give way to late-stage felsic minerals like quartz and potassium feldspar, driving the compositional shift toward felsic end-members.^[42]^[43] Assimilation and contamination further modify felsic magma compositions by incorporating surrounding crustal material, particularly during prolonged residence in shallow chambers where heat from the magma melts wall rocks, adding incompatible elements and radiogenic isotopes to the melt. This interaction is quantitatively modeled using energy-constrained assimilation-fractional crystallization (EC-AFC), which accounts for the heat budget required for melting country rock while simultaneous crystallization removes phases, ensuring energy balance in the system. For instance, in felsic systems like the Skye igneous complex, EC-AFC simulations demonstrate how limited assimilation (e.g., 10-20% of the magma mass) can significantly alter trace element ratios without excessive crustal input.^[44]^[45] Liquid immiscibility represents a rarer mechanism in highly evolved felsic systems, where the melt separates into conjugate felsic (silica-rich) and mafic (iron- or phosphorus-enriched) liquids due to thermodynamic instability, potentially contributing to the final stages of differentiation. This process is evidenced in certain granitic intrusions and volcanic glasses, though it is less common than fractionation owing to the narrow temperature-composition window required, often below 800°C.^[46]^[47] Crystallization of felsic magmas typically occurs over a temperature range of 800–650°C, with quartz and alkali feldspars forming as late-crystallizing phases near the solidus. Phase equilibria in the quaternary Qz-Ab-Or-H₂O system, a key model for felsic compositions, reveal a ternary eutectic at approximately 650°C under water-saturated conditions, where the melt fully crystallizes into quartz, albite, and orthoclase assemblages. These diagrams highlight how water content lowers the liquidus, facilitating the sequential precipitation of minerals that define felsic textures.^[42]^[48]^[49]

Types and Classification

Plutonic Felsic Rocks

Plutonic felsic rocks are intrusive igneous rocks that form from the slow crystallization of silica-rich magma at depth, resulting in coarse-grained textures and compositions dominated by quartz, feldspars, and minor mafic minerals. These rocks are classified using the International Union of Geological Sciences (IUGS) QAPF modal diagram, which applies to plutonic rocks where the combined volume percentages of quartz (Q), alkali feldspar (A), plagioclase (P), and feldspathoids (F) exceed 90% of the total rock volume, placing them firmly in the felsic field with high silica content typically above 65 wt%.^[50]^[51] In this system, the felsic plutonic field is defined by quartz contents between 20% and 60%, with the precise rock name determined by the relative proportions of alkali feldspar and plagioclase.^[52] The primary types of plutonic felsic rocks include granite, granodiorite, and pegmatite. Granite is characterized by phaneritic, often equigranular textures with visible interlocking crystals, containing 20–60% quartz and alkali feldspar comprising more than 35% of the total feldspar content.^[50]^[53] Granodiorite, a close relative, shares the 20–60% quartz range but features a higher proportion of plagioclase feldspar, exceeding 65% of total feldspar, often accompanied by biotite and hornblende for a slightly darker appearance.^[52]^[50] Pegmatite represents a late-stage, exceptionally coarse-grained variant, typically of granitic composition, formed from volatile-rich residual melts with crystals larger than 2.5 cm, sometimes reaching meters in size due to enhanced ionic mobility in low-viscosity fluids.^[54]^[50] Diagnostic textures in these rocks reflect their slow cooling history and magmatic processes. Graphic intergrowths, resembling cuneiform writing, occur in granites and pegmatites where quartz and alkali feldspar crystallize interlocked in a symbiotic manner, indicating near-solidus temperatures around 650–700°C.^[50] Xenoliths, angular fragments of assimilated country rock, are common inclusions in granites, evidencing magma-wall rock interaction during emplacement.^[50] These phaneritic textures, with grain sizes from millimeters to centimeters in granite and granodiorite, contrast with the aphanitic varieties of their volcanic equivalents like rhyolite.^[50] Plutonic felsic rocks are emplaced at crustal depths of 5–20 km, where pressures range from 1.5–5 kbar, allowing for the formation of large intrusive bodies such as stocks and batholiths.^[50] Cooling occurs gradually over timescales of 10^5 to 10^6 years, enabling full crystallization and development of their characteristic coarse textures through conductive and convective heat loss.^[55]^[50]

Volcanic Felsic Rocks

Volcanic felsic rocks form through the rapid extrusion and cooling of silica-rich magmas at Earth's surface, resulting in a variety of textures distinct from their intrusive counterparts. These rocks typically exhibit fine-grained or glassy matrices due to quick quenching, often containing phenocrysts of quartz, feldspars such as sanidine or plagioclase, and minor biotite or hornblende.^[56] Unlike slower-cooling plutonic equivalents like granite, volcanic felsic rocks are prone to explosive eruptions owing to their high gas content and viscosity, posing significant hazards through pyroclastic flows, ash falls, and caldera collapses.^[57] The primary types of volcanic felsic rocks include rhyolite, obsidian, pumice, and ignimbrite. Rhyolite, the most common, appears as porphyritic varieties with visible phenocrysts in a finer aphanitic groundmass or as aphanitic flows lacking large crystals; it represents the crystallized extrusive form of felsic magma.^[56] Obsidian is a natural volcanic glass, dark and glossy, formed by extremely rapid cooling that prevents crystallization, often associated with rhyolitic compositions and exhibiting conchoidal fracture. Pumice consists of highly vesicular, frothy material, light-colored and low-density, produced when gas expansion during eruption creates voids in the cooling rhyolitic or dacitic foam; it commonly floats on water due to its porosity exceeding 75%.^[58] Ignimbrite, or welded tuff, arises from hot pyroclastic density currents that deposit and partially fuse ash and pumice fragments, forming layered, welded sheets with fiamme—flattened pumice inclusions—evident in hand specimens.^[57] Classification of volcanic felsic rocks follows the total alkali-silica (TAS) diagram for chemical composition and the QAPF (quartz-alkali feldspar-plagioclase-feldspathoid) scheme adapted for modal mineralogy, but emphasizes textural variations such as vitrophyre (glassy with phenocrysts) or porphyritic forms.^[59] Rhyolite specifically requires silica content greater than 69 wt% SiO₂, distinguishing it from less siliceous dacites, while all felsic volcanics share high alkali (Na₂O + K₂O > 5 wt%) and low iron-magnesium contents.^[56] These systems align with plutonic classifications but highlight extrusive textures like flow banding in obsidian or welding in ignimbrites, aiding identification in the field. Eruption dynamics of felsic magmas are dominated by their high viscosity, typically exceeding 10⁶ Pa·s, which traps volatiles and promotes explosive Plinian-style eruptions characterized by towering ash columns reaching tens of kilometers.^[60] This viscosity, driven by polymerized silica networks and crystal content, hinders gas escape, leading to rapid pressure buildup and fragmentation; such events often culminate in caldera formation as magma chambers evacuate, as seen in supervolcanic systems.^[61] The resulting hazards include widespread tephra dispersal and devastating ignimbrite flows traveling at speeds over 100 km/h, burying landscapes under meters of hot debris.^[57] Cooling rates for volcanic felsic rocks are exceptionally rapid compared to intrusive settings, ranging from hours for glassy obsidian to days for thin flows and up to years for thick rhyolite domes or ignimbrite sheets, preserving delicate textures and phenocrysts.^[62] In rhyolite flows, this allows sanidine phenocrysts to form during late-stage crystallization while the groundmass remains aphanitic or glassy, recording eruption temperatures around 700–800°C before quenching.^[63] Such swift cooling limits devitrification in obsidian and welding in pumice-rich ignimbrites, influencing their durability and use in archaeological contexts.^[64]

Geological Occurrence

Major Formations

Felsic rocks form extensive batholiths and volcanic complexes worldwide, with prominent continental examples including the Sierra Nevada Batholith in the United States, which covers approximately 70,000 km² and consists primarily of granitic intrusions emplaced between 80 and 120 million years ago during the Cretaceous period.^[65]^[66] Another key continental formation is the Caledonian granites of the Scottish Highlands, dating to around 400 million years ago in the Silurian-Devonian period and representing a major component of the exposed Precambrian and Paleozoic crust in the region.^[67]^[68] In oceanic and arc settings, the Coastal Batholith of Peru exemplifies large-scale felsic magmatism, stretching over 1,600 km along the Andean margin with significant rhyodacite components intruded between approximately 100 and 40 million years ago in the Late Cretaceous to Eocene.^[69]^[70] Similarly, the Taupo Volcanic Zone in New Zealand hosts multiple rhyolitic calderas, such as the 35-km-wide Taupo Caldera, with activity spanning the last 2 million years and cumulative rhyolitic eruptions exceeding 10,000 km³ since 1 million years ago.^[71]^[72] Super eruptions highlight the explosive potential of felsic magmatism, as seen in the Yellowstone Caldera system in the United States, where the Lava Creek Tuff eruption approximately 640,000 years ago produced about 1,000 km³ of rhyolitic tuffs.^[73]^[74] The Fish Canyon Tuff in Colorado represents an even larger event, with an estimated volume of 5,000 km³ of crystal-rich rhyolitic ignimbrite erupted around 28 million years ago from the La Garita Caldera.^[75] The age distribution of major felsic formations shows peaks in the Precambrian, particularly in the Superior Province of Canada where Archean granites and gneisses formed around 2.7 billion years ago, and in Phanerozoic continental arcs driven by subduction processes.^[76]^[77]

Tectonic Settings

Felsic magmatism predominantly occurs in convergent plate margins, where subduction processes drive the generation of calc-alkaline felsic rocks through slab dehydration and subsequent crustal melting.^[37] In these settings, hydrous fluids released from the dehydrating subducted oceanic slab flux the overlying mantle wedge, inducing partial melting that produces basaltic magmas; these then interact with the crust, leading to differentiation and the formation of felsic melts.^[78] The majority of Phanerozoic granites, including I-type and S-type varieties, form in such convergent environments, reflecting the dominant role of subduction in continental crustal growth.^[79] Explosive volcanism is common in these arc settings due to the volatile-rich nature of the magmas.^[80] Intraplate tectonic settings, such as continental rifts and hotspots, are associated with A-type (alkaline) felsic rocks, which arise from partial melting of the lower crust or mantle in extensional environments lacking significant subduction influence.^[81] These magmas often exhibit peralkaline compositions and are linked to lithospheric thinning and upwelling of asthenospheric material. A representative example is the peralkaline granites of the Oslo Rift, formed during Permo-Carboniferous extension within the stable Fennoscandian Shield.^[82] In collisional orogens, felsic magmatism occurs syn- to post-collisionally, driven by melting of thickened continental crust due to radiogenic heating and tectonic decompression.^[83] S-type leucogranites, derived primarily from metasedimentary sources, exemplify this process; the Miocene Himalayan leucogranites formed through partial melting of the overthickened (>50 km) crust in the Greater Himalayan Sequence following India-Asia collision.^[83] Tectonic discrimination of felsic rocks relies on geochemical diagrams, such as those developed by Pearce et al., which use immobile trace elements like Rb, Y, and Nb to differentiate settings.^[84] For instance, the Rb vs. (Y + Nb) plot distinguishes volcanic arc granites (VAG), characterized by low Y + Nb and moderate Rb from subduction-related origins, from within-plate granites (WPG), which show higher Y + Nb due to intraplate enrichment.^[84] These diagrams aid in assigning ancient felsic suites to specific regimes without relying on field relations alone.^[85]

References

[1]
General Classification of Igneous Rocks - Tulane University
Jan 11, 2011 · The silica saturation concept can thus be used to divide rocks in silica undersaturated, silica saturated, and silica oversaturated rocks.
[2]
None
### Summary of 'Felsic' from Igneous Rocks.pdf
[3]
https://web.crc.losrios.edu/~jacksoh/lectures/magmapt2.html
[4]
Characteristics of Igneous Rocks – Geology 101 for Lehman ...
Felsic magma usually originates in the crust or by the shedding of mafic minerals as magma rises through the crust. The igneous texture tells us how the magma ...
[5]
[PDF] Magmas and Igneous Rocks - Tulane University
Sep 3, 2015 · So, melting a mafic source thus yields a felsic or intermediate magma. Melting of ultramafic (peridotite source) yields a basaltic magma.
[6]
Igneous Rocks Classified by Composition
Felsic rocks are mostly feldspar (especially K-feldspar), at least 10% quartz, and less than 15% mafic minerals (biotite, hornblende).
[7]
felsic – An Introduction to Geology - OpenGeology
Can refer to a volcanic rock with higher silica composition, or the minerals that make up those rocks, namely quartz, feldspar (both potassium feldspar and ...
[8]
Volcanoes - Geography 101 Online
Property. Mafic Magma. Felsic Magma ; Composition. iron, magnesium. silica, aluminum ; Color. darker. lighter ; Density. higher (3 g/cm3). lower (2.7 g/cm3).
[9]
Felsic - Etymology, Origin & Meaning
Originating in 1912 from feldspar, silica, and the suffix -ic, the word describes a compound related to these mineral components.
[10]
Definition of felsic - Mindat
A mnemonic adjective derived from (fe) for feldspar, (l) for lenad or feldspathoid, and (si) for silica, and applied to light-colored rocks.<|separator|>
[11]
felsic, adj. meanings, etymology and more - Oxford English Dictionary
The earliest known use of the adjective felsic is in the 1910s. OED's earliest evidence for felsic is from 1912, in a text by W. Cross et al. felsic is formed ...Missing: JP Buwalda 1947
[12]
classification of igneous rocks - jstor
WHITMAN CROSS, J. P. IDDINGS, L. V. PIRSSON, H. S. WASHINGTON. During the ... we suggest the term felsic for the group of modal feldspars, feld-.Missing: coined | Show results with:coined
[13]
Felsic Rock - an overview | ScienceDirect Topics
Felsic rocks are defined as light-colored igneous rocks that contain high-silica minerals, typically comprising around 63% SiO2, with common examples including ...
[14]
[PDF] igneous rocks: a classification and glossary of terms
See also felsic and salic. (Cross et al., 1912, p.561; Johannsen v.1, p.180; Tomkeieff p.336). MAFITITE. A term originally proposed for rocks with a colour ...
[15]
The Origin of Igneous Rocks (Chapter 4) - A Brief History of Geology
Apr 6, 2018 · ... rocks rich in Si and Al. The old terms acid and basic, still used today, were introduced by the German chemist Robert Bunsen (1811–1899) for ...
[16]
A Quantitative Chemico-Mineralogical Classification and ...
A Quantitative Chemico-Mineralogical Classification and Nomenclature of Igneous Rocks ; Whitman Cross, ; Joseph P. Iddings, ; Louis V. Pirsson and ; Henry S.
[17]
Acid Volcanics - an overview | ScienceDirect Topics
Acid volcanic refers to a type of volcanic rock characterized by a silica (SiO2) content greater than 63%. This classification is based on chemical composition ...
[18]
[PDF] IGNEOUS ROCKS: A Classification and Glossary of Terms
See also felsic and salic. (Cross et al., 1912, p.561; Johannsen v.1, p.180; Tomkeieff p.336). MAFITITE. A term originally proposed for rocks with a colour ...
[19]
[PDF] Igneous Rocks and Processes: A Practical Guide
This book covers magmas, magmatic rocks, basalts, magma differentiation, gabbroic, ultramafic, ultrabasic, andesite, dacite, rhyolite, pyroclastic processes, ...
[20]
ALEX STREKEISEN-Granite-
Granite is a medium-tocoarse-grained acid igneous rock with essential quartz (>20%) and feldspar, where alkali feldspar constitutes between 100 and 35% of the ...Missing: 25-35% 30-40%
[21]
Identifying & Interpreting Plutonic Rocks
> Granitic rock composed almost entirely of reddish to peach-colored alkali feldspar (40-80%) and quartz (20-60%) with very little plagioclase feldspar. > ...<|separator|>
[22]
[PDF] The Sandia granite--Single or multiple plutons?
The average modal analysis for the granite is. 35 percent quartz, 15 percent microcline, 35 percent plagioclase, 10 percent biotite, and 5 percent ...
[23]
https://nmgs.nmt.edu/publications/guidebooks/downloads/33/33_p0221_p0223.pdf
[24]
2 Igneous Rocks – Open Petrology - OpenGeology
Typical characterizing accessories include biotite, muscovite, amphibole, pyroxene, olivine, and glass.
[25]
Igneous Rocks - Geology (U.S. National Park Service)
Nov 8, 2023 · Igneous rocks are “fire-born,” meaning that they are formed from the cooling and solidification of molten (melted) rock.
[26]
(PDF) The IUGS systematics of igneous rocks - ResearchGate
Aug 6, 2025 · The principles are: (1) use descriptive attributes; (2) use actual properties; (3) ensure suitability for all geologists; (4) use current ...
[27]
Fractional Crystallization - an overview | ScienceDirect Topics
If the minerals once formed are separated rapidly then this process is described as fractional crystallization and is illustrated in Figure 4.5. It is one of ...
[28]
Average Rock Compositions
A compilation of average whole rock chemical analyses estimated for major rock types.
[29]
[PDF] Igneous Rocks - SOEST Hawaii
Typical basalt Typical granite. SiO2. 50%. 70%. Al2O3. 15%. 12%. FeO+MgO. 15%. 3%. CaO. 8%. 2%. K2O+Na2O. 5%. 8%. Composition of melts affects behavior while ...
[30]
[PDF] Aluminum in zircon as evidence for peraluminous and ... - UCLA SIMS
Shand [1927] classified rocks as peraluminous if the whole rock molar ratio of Al2O3/(CaO 1 Na2O 1 K2O). (ASI; Aluminum Saturation Index) is greater than 1.
[31]
[PDF] Report (pdf)
Feb 13, 2024 · Chemical analyses have been determined for 134 samples of granitic rocks, ranging in composition from granite to quartz diorite, from the ...
[32]
[PDF] Analysis of Major and Trace Elements - Ocean Drilling Program
This Technical Note is designed to offer principles and guidelines for geochemical analysis by inductively coupled plasma–atomic emission spectrometry (ICP-AES) ...
[33]
[PDF] Geochemical, modal, and geochronologic data for 1.4 Ga A-type ...
Relative to other granitic rocks, A-type granites have high. FeO*/(FeO*+MgO), high K2O and K2O/Na2O, are metalumi- nous to weakly peraluminous, and are ...
[34]
[PDF] CHAPTER E: GEOCHEMICAL INTERPRETATION OF SAMPLES OF ...
Total alkali versus silica diagram with fields (after LeBas and others, 1986) showing the compositional range of the volcanic rocks. C. Modified alkali- lime ...Missing: distinction | Show results with:distinction
[35]
A new concept for the genesis of felsic magma - Nature
May 26, 2020 · Supercritical liquid may ascend in the mantle wedge more effectively than silicate melt, because of the reduced viscosity. The large 230Th ...Missing: density | Show results with:density
[36]
When the Continental Crust Melts | Elements | GeoScienceWorld
Aug 1, 2011 · Partial melting in such settings can lead to intense, local reworking of the continental crust, but such thermal reworking is not generally ...Introduction · Types Of Melting In The... · Tectonic And Geodynamic...
[37]
Subduction zone fluids and arc magmas conducted by lithospheric ...
Nov 29, 2021 · The presence of water reduces the solidus temperature (wet-solidus curve) allowing partial melting generation at lower temperatures and lower ...
[38]
Dehydration Melting - an overview | ScienceDirect Topics
Biotite dehydration melting can produce over 30% melt in rocks and is probably responsible for the formation of some schlieren migmatites and S-type granites.
[39]
Melting of Biotite + Plagioclase + Quartz Gneisses: the Role of H2O ...
At 10 kbar when 4 wt % of water was added to the system the biotite melting reaction occurred below 800°C and produced garnet + amphibole + melt. At 15 kbar the ...
[40]
Himalayan Leucogranites: A Geochemical Perspective | Elements
Dec 1, 2024 · Himalayan leucogranites are characterized by strongly peraluminous compositions that are comparable to melts derived from anatexis of sedimentary rocks.
[41]
Himalayan-like Crustal Melting and Differentiation in the Southern ...
Leucogranite 87Sr/86Sri values range from 0.709 to 0.717 (average 0.713), the two-mica granite 87Sr/86Sri values are 0.711 and 0.710, and 87Sr/86Sri = 0.706 ...
[42]
Magmas and Igneous Rocks - Tulane University
Sep 3, 2015 · Fractional Crystallization - When magma crystallizes it does so over a range of temperature. Each mineral begins to crystallize at a ...
[43]
7. 4.2 Crystallization of Magma - Maricopa Open Digital Press
Felsic magmas tend to be cooler than mafic magmas when crystallization ... This process is known as fractional crystallization. The crystals that settle ...
[44]
[PDF] Energy-Constrained Recharge, Assimilation, and Fractional ...
Constrained Assimilation-Fractional Crystallization (EC-. AFC) model to magmatic systemsJ. Petrol., 42, 1019 – 1041. Bohrson, W. A., and F. J. Spera (2003) ...Missing: felsic | Show results with:felsic
[45]
[PDF] Geochemical Modeling of Assimilation in Magmatic Systems
Even though EC‐AFC models include heat budget and partial melting of the wall rock in the trace element and isotope calculations, the phase changes governing ...
[46]
Silicate liquid immiscibility in magmas and in the system K2O-FeO ...
A review of the compositions and general behavior of systems involving immiscibility, both stable and metastable, and of the evidence for natural immiscibility.
[47]
Magmatic Differentiation - Tulane University
Jan 30, 2012 · Since basaltic magmas are generally much hotter than rhyolitic magmas, liquid immiscibility is not looked upon favorably as an explanation ...
[48]
Timescales of Quartz Crystallization and the Longevity of the Bishop ...
May 30, 2012 · In this work, we present four lines of evidence to constrain the timescales of crystallization of the Bishop magma body: (1) quartz residence ...
[49]
Ternary Phase Diagrams - Tulane University
Jan 12, 2011 · Crystallization begins at a temperature of about 850° with separation of crystals of A. This continues as the liquid composition changes from S ...Missing: felsic Qz-
[50]
7 Plutons and Plutonic Rocks – Open Petrology - OpenGeology
The basic QAPF classification system for plutonic rocks, depicted in Figure 7.30, works well for silicic and intermediate quartzofeldspathic rocks but not ...Missing: felsic | Show results with:felsic
[51]
IUGS Chart - Plutonic QAP
Names are based on the relative modal proportions of quartz (Q), alkali feldspar (A), and plagioclase feldspar (P). Names for plagioclase-rich fields depend on ...
[52]
[PDF] DRAFT Geologic Map Unit Classification, ver. 6.1
Feb 8, 2002 · A plutonic rock defined in the QAPF diagram as having Q < 5% or. F/(F+A+P) < 10%, and P/(A+P) between 10 and 35%[]. 4.4.8. Quartz monzonite. A ...
[53]
Classification of plutonic rocks - Geology is the Way
Plutonic rocks are classified primarily based on their mineral content (or mode). According to the International Union of Geological Sciences (IUGS),
[54]
Granitic pegmatite - ALEX STREKEISEN
Granitic pegmatite. A pegmatite is a holocrystalline intrusive igneous rock composed of interlocking phaneritic crystals usually larger than 2.5cm in size.
[55]
[PDF] How Fast Do Rocks Form? - DigitalCommons@Cedarville
As a result, only a month and a half of cooling is required to produce a granite. This is true, however, only if the rate of cooling is as high as this. The ...
[56]
Glossary - Volcanoes of the United States [USGS]
Rhyolite, A volcanic rock containing more than 68% silica with a very high viscosity when in a molten state. ; Shield volcano, A volcano shaped like an inverted ...
[57]
Glossary of Volcanic Terms - Volcanoes, Craters & Lava Flows (U.S. ...
May 22, 2024 · ... types of rocks. fiamme, A flattened structure ignimbrite made of a pumice fragment that has collapsed during welding. ... obsidian. perlitic ...
[58]
Definitions | Volcano World | Oregon State University
Light-colored, frothy volcanic rock, usually of dacite or rhyolite composition, formed by the expansion of gas in erupting lava. Commonly seen as lumps or ...
[59]
USGS: Volcano Hazards Program Glossary - Basalt
Apr 8, 2015 · Basalt is a hard, black volcanic rock with less than about 52 weight percent silica (SiO2). Because of basalt's low silica content, it has a low viscosity.
[60]
[PDF] Shear viscosity of rhyolite-vapor emulsions at magmatic ...
6 Pa were generated, and viscosity ranged from 105.9 to 108.6 Pa s. Porosity was determined after each experiment from immersion densitom- etry of glass ...
[61]
Magma Melts and Eruption Types - Grand Canyon-Parashant ...
Apr 5, 2024 · Felsic magma has an unbelievable viscosity of 1,000,000,000 - 100,000,000,000 Pa s. Interestingly, felsic magmas are much less dense than ...
[62]
[PDF] Field-Based Description of Rhyolite Lava Flows of the Calico Hills ...
Lithostratigraphic variation in rhyolite flows is governed by several processes, including cooling rate and response of the lava to a temperature gradient ...
[63]
Post-eruptive mobility of lithium in volcanic rocks - PMC
Aug 13, 2018 · The central, microcrystalline parts of the deposit cool slowly and may stay hot for months to years after deposition, allowing for efficient ...
[64]
Hotter Side of Obsidian - Volcano World - Oregon State University
The viscosity of obsidian must be lower than rhyolite so it can flow; difference in eruption temperature is the greatest control over this difference. Initially ...
[65]
[PDF] Disruption of regional primary structure of the Sierra Nevada ...
The Sierra Nevada batholith is a composite batholith that formed largely between 130 and 80 Ma by sequential large- volume magma pulses into an accretionary ...<|separator|>
[66]
[PDF] AGE DETERMINATIONS OF TEE ROCKS OF THE BATHOLITHS OF ...
Twenty-five age determinations on rocks from the batholith of southern California ranging from tonalite to granite give an average age of 110 + 13 million years ...
[67]
Scotland's Geology
As the Caledonian Orogeny drew to a close 400 million years ago, melting of rock beneath the mountains created magma that rose upwards to form granite, and ...Scotland’s Fossils · Geology of Scotland Map · Geological Timescale
[68]
Geochronology and geodynamics of Scottish granitoids from the late ...
Nov 22, 2017 · The aim of this paper is to report new U–Pb zircon ages of c. 600 to c. 400 Ma granites in Scotland and use these together with published data ...
[69]
The Coastal Batholith of central Peru - GeoScienceWorld
Mar 3, 2017 · A sector of the Peruvian Coastal Batholith 120 km long has been mapped in detail. Little altered volcanics of Cretaceous and Lower Tertiary age ...Missing: rhyodacites size<|separator|>
[70]
The Peruvian Coastal Batholith is located in the western Cordillera....
Detrital zircon U-Pb ages define significant clusters that are tentatively interpreted as intense arc magmatism at ca. 72 Ma, ca. 60 Ma, and ca. 43 Ma. A major ...
[71]
Caldera volcanoes of the Taupo Volcanic Zone, New Zealand - Wilson
Sep 30, 1984 · The Taupo volcanic zone (TVZ) has been active since 2 Ma and has erupted >104 km3 of dominantly rhyolitic magma during the last 1 m.y. Most ...
[72]
Taupo - Global Volcanism Program
Lake Taupo fills a roughly 35-km-wide caldera that is the site of the most prolific rhyolitic volcano of the Taupo volcanic zone. The caldera was formed ...
[73]
Age of the Lava Creek supereruption and magma chamber ...
Jul 2, 2015 · The last supereruption from the Yellowstone Plateau formed Yellowstone caldera and ejected the >1000 km3 of rhyolite that composes the Lava ...
[74]
Fish Canyon Magma Body, San Juan Volcanic Field, Colorado
The Fish Canyon Tuff, with a total volume of ∼5000 km3, was erupted during the collapse of the ∼100 km × 35 km La Garita caldera as a highly fragmented crystal- ...
[75]
Comparative orotomy of the Archean Superior and Phanerozoic ...
We compare and contrast the materials and mechanisms of continental crustal growth in the largest preserved regions of Phanerozoic and Archean juvenile ...
[76]
Three billion years of crustal evolution in eastern Canada ...
Jan 25, 2016 · The geological record of SE Canada spans more than 2.5 Ga, making it a natural laboratory for the study of crustal formation and evolution ...2 Tectonic Setting · 6 Discussion · 6.6 Precambrian...
[77]
Origins of felsic magmas in Japanese subduction zone ...
Jun 5, 2015 · A large heat source is required for forming large volumes of felsic crustal melts and is usually supplied by the mantle via basalt. Hydrous arc ...
[78]
I-type and S-type granites in the Earth's earliest continental crust
Mar 6, 2023 · Alternatively, according to modern-day environments, the I- and S-type granites mainly occur in convergent plate margin settings. This is ...
[79]
[PDF] Subduction Factory: How it operates in the evolving earth
Jul 4, 2005 · (B) An andesitic magma is produced by mixing between a basalt magma produced by slab dehydration induced by mantle melting and a felsic magma ...
[80]
A-type granites in space and time - ScienceDirect.com
May 15, 2023 · In this study we focus on the secular distribution of A-type granites and their tectonic settings. We examine A-type frequency in terms of ...
[81]
The Oslo Rift: a review - ResearchGate
Aug 6, 2025 · The tectonomagmatic history of the Oslo Rift may be subdivided into 5 main periods. Stage 1 (> 300 Ma): development of a shallow depression.
[82]
Himalayan Leucogranites: Field Relationships and Tectonics
Dec 1, 2024 · Himalayan leucogranites are crustal melts, derived from in-situ melting processes within the upper part of the GHS from a pelitic–migmatite source.
[83]
Trace Element Discrimination Diagrams for the Tectonic ...
Granites may be subdivided according to their intrusive settings into four main groups—ocean ridge granites (ORG), volcanic arc granites (VAG), within plate ...
[84]
An evaluation of the Rb vs. (Y + Nb) discrimination diagram to infer ...
The most commonly used tectonic discrimination diagrams for granites were introduced by Pearce et al. ... VAG to WPG boundary. One can minimize this problem by ...