Lecture 5 - Igneous Petrology.pdf

Igneous Petrology Petrology “The science concerned with rocks , including their mode of occurrence, composition, classification , origin, and their relations to geological processes and history.” Related disciplines: 1. Petrogenesis – interpretations of the origin of rocks 2. Petrography - pla...

Igneous Petrology Petrology “The science concerned with rocks , including their mode of occurrence, composition, classification , origin, and their relations to geological processes and history.” Related disciplines: 1. Petrogenesis – interpretations of the origin of rocks 2. Petrography - places emphasis on the purely descriptive part of rock science from textural, mineralogical and chemical points of view. Introduction: The rock cycle The Earth’s Interior Crust: Oceanic crust Thin: 10 km Relatively uniform stratigraphy = ophiolite suite: • Sediments • pillow basalt • sheeted dikes • more massive gabbro • ultramafic (mantle) Continental Crust Thicker: 20-90 km average ~35 km Highly variable composition  Average ~ granodiorite The Earth’s Interior Mantle: Peridotite (ultramafic) Silicate Perovskite (Bridgmanite) (Mg,Fe)SiO3 Ferropericlase (Fe,Mg)O Upper to 410 km  Low Velocity Layer 60-220 km Transition Zone as velocity increases ~ rapidly Lower Mantle has more gradual velocity increase Figure 1-2. Major subdivisions of the Earth. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. The Earth’s Interior Core: Fe-Ni metallic alloy Outer Core is liquid  No S-waves Inner Core is solid Figure 1-2. Major subdivisions of the Earth. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. PHYSICAL GEOLOGY Layers of Earth The Core Inner and Outer Core H.D.A.Reyes Correlations 1 Compositional layering vs. Change in physical properties with depth Variation in P and S wave velocities with depth. Compositional subdivisions of the Earth are on the left, rheological subdivisions on the right. After Kearey and Vine (1990), Global Tectonics. © Blackwell Scientific. Oxford. Figure 1-5. Relative atomic abundances of the seven most common elements that comprise 97% of the Earth's mass. An Introduction to Igneous and Metamorphic Petrology, by John Winter , Prentice Hall. Mineral Property, Crystallography and Crystal Chemistry CHEMISTRY ELEMENT AND ISOTOPES H.D.A. Reyes | Correlation 1 Types of plate boundaries: 1. • Divergent boundaries -- where new crust is generated as the plates pull away from each other. 2. • Convergent boundaries -- where crust is destroyed as one plate dives under another. 3. • Transform boundaries -- where crust is neither produced nor destroyed as the plates slide horizontally past each other. Plate Boundaries Plate Tectonics • Earth’s Major Plates  Lithospheric plates or Tectonic plates  A massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere Source: https://getech.co m/platetectonics-50/ HDA. Reyes | Principles of Geology Plate Tectonics • Earth’s Major Plates  Lithospheric plates or Tectonic plates  Major Plates: 1. North American Plate 2. South American Plate 3. Pacific Plate 4. African Plate 5. Eurasian Plate 6. Australian-Indian Plate 7. Antarctic plates.  Minor Plates: Caribbean, Nazca, Philippine, Arabian, Cocos, Scotia, and Juan de Fuca plates HDA. Reyes | Principles of Geology Plate Tectonics • Plate Boundaries  Divergent boundaries  Also known as constructive margins where two plates move apart, resulting in upwelling of hot material from the mantle to create new seafloor Divergent in Iceland Source: https://kidsgeo.com/geology-for-kids/divergent-boundaries/ HDA. Reyes | Principles of Geology Plate Tectonics • Plate Boundaries  Divergent boundaries  Located along the crests of oceanic ridges and can be thought of as constructive plate margins because this is where new ocean floor is generated.  Divergent boundaries are also called spreading centers, because seafloor spreading occurs at these boundaries. HDA. Reyes | Principles of Geology Plate Tectonics • Plate Boundaries  Divergent boundaries  Oceanic Ridges  elevated areas of the seafloor that are characterized by high heat flow and volcanism.  The global ridge system is the longest topographic feature on Earth’s surface, exceeding 70,000 kilometers in length.  Mid-Atlantic Ridge, East Pacific Rise, and Mid-Indian Ridge. Source: http://www.rci.rutgers.edu/~schlisch/103web/Pangeabreakup/fractzon es.html HDA. Reyes | Principles of Geology Plate Tectonics • Plate Boundaries  Divergent boundaries  Oceanic Ridges Source: https://www.thoughtco.com/map-ofthe-mid-ocean-ridges-1441097 HDA. Reyes | Principles of Geology Plate Tectonics • Plate Boundaries  Divergent boundaries  Oceanic Ridges  The primary reason for the elevated position of the oceanic ridge is that newly created oceanic crust is hot, making it less dense than cooler rocks found away from the ridge axis.  As soon as new lithosphere forms, it is slowly yet continually displaced away from the zone of upwelling.  Thus, it begins to cool and contract, thereby increasing in density. This thermal contraction accounts for the increase in ocean depths away from the ridge crest.  It takes about 80 million years for the temperature of the crust to stabilize and contraction to cease.  By this time, rock that was once part of the elevated oceanic ridge system is located in the deep-ocean basin, where it may be buried by substantial accumulations of sediment. HDA. Reyes | Principles of Geology Plate Tectonics • Plate Boundaries  Divergent boundaries  Rift Valley  A deep down faulted structure  This structure is evidence that tensional forces are actively pulling the ocean crust apart at the ridge crest. The Thingvellir fracture zone at Thingvellir National Park in southwestern Iceland is an example of a rift valley. The Thingvellir fracture lies in the MidAtlantic Ridge, which extends through the centre of Iceland. Source: https://www.britannica.com/science/rift-valley HDA. Reyes | Principles of Geology Plate Tectonics • Plate Boundaries  Divergent boundaries  Seafloor Spreading  The mechanism that operates along the oceanic ridge system to create new seafloor  Typical rates of spreading average around 5 cm (2 inches) per year. Source: https://vgd.no/samfunn/religion-og-livssyn/tema/1768914/tittel/tror-noenher-at-jorden-er-6-000-aar-gammel/side/15 HDA. Reyes | Principles of Geology Plate Tectonics • Plate Boundaries  Divergent boundaries  Continental Rifting  Occurs where opposing tectonic forces act to pull the lithosphere apart.  The initial stage of rifting tends to include mantle upwelling that is associated with broad upwarping of the overlying lithosphere  As a result, the lithosphere is stretched, causing the brittle crustal rocks to break into large slabs.  As the tectonic forces continue to pull the crust apart, these crustal fragments sink, generating an HDA. Reyes | Principles of Geology Source: https://www.pinterest.ph/pin/157485318204210178/ Plate Tectonics • Plate Boundaries  Divergent boundaries  Fact!  The remains of some of the earliest humans, Homo habilis and Homo erectus, were discovered by Louis and Mary Leakey in the ast African Rift. Scientists consider this region to be the “birthplace” of the human race. Source: https://www.pinterest.ph/pin/157485318204210178/ HDA. Reyes | Principles of Geology Plate Tectonics • Plate Boundaries  Convergent boundaries  Also known as destructive margins where two plates move together, resulting in oceanic lithosphere descending beneath an overriding plate, eventually to be reabsorbed into the mantle or possibly in the collision of two continental blocks to create a mountain system. Source: https://www.slideshare.net/esteeseetoh/convergent-boundaries HDA. Reyes | Principles of Geology Plate Tectonics • Plate Boundaries  Convergent boundaries  Also called subduction zones, because they are sites where lithosphere is descending (being subducted) into the mantle.  Subduction occurs because the density of the Source: https://www.earthobservatory.sg/resources/images-graphics/subductiondescending tectonic plate zone-beneath-philippines is greater than the density of the underlying asthenosphere.HDA. Reyes | Principles of Geology Plate Tectonics • Plate Boundaries  Convergent boundaries  Deep-ocean Trenches  The surface manifestations produced as oceanic lithosphere descends into the mantle.  These large linear depressions are remarkably long and deep. HDA. Reyes | Principles of Geology Plate Tectonics • Plate Boundaries  Convergent boundaries  Oceanic-Continental Convergence  When a descending oceanic slab reaches a depth of about 100 km, melting is triggered within the wedge of hot asthenosphere that lies above it.  This process, called partial melting, is thought to generate about 10% molten material, which is intermixed with unmelted mantle rock.  Why?  “Wet” rock in a high-pressure environment melts at substantially lower temperatures than does “dry” rock of the same composition. HDA. Reyes | Principles of Geology Plate Tectonics • Plate Boundaries  Convergent boundaries  Oceanic-Continental Convergence  Result  Continental volcanic arcs HDA. Reyes | Principles of Geology Plate Tectonics • Plate Boundaries  Convergent boundaries  Oceanic-Oceanic Convergence Subduction - the sideways Volcanic Island Arc and downward movement Philippines Islands. of the edge of a plate of the earth's crust into the mantle beneath another plate Obduction the overthrusting of oceanic lithosphere onto continental lithosphere at a convergent plate boundary where continental lithosphere is being subducted beneath oceanic lithosphere. HDA. Reyes | Principles of Geology Good example would be the Plate Tectonics • Plate Boundaries  Convergent boundaries  Continental-Continental Convergence HDA. Reyes | Principles of Geology Plate Tectonics • Plate Boundaries  Transform boundaries  Also known as conservative margins where two plates grind past each other without the production or destruction of lithosphere Source: https://polarpedia.eu/en/transformplate-boundary/ HDA. Reyes | Principles of Geology Plate Tectonics • Plate Boundaries  Transform boundaries  Fracture Zone prominent linear breaks in the seafloor  Include both the active transform faults as well as their inactive extensions into the plate interior HDA. Reyes | Principles of Geology Plate Tectonic - Igneous Genesis 1. Mid-ocean Ridges 2. Intracontinental Rifts 3. Island Arcs 4. Active Continental Margins 5. Back-arc Basins 6. Ocean Island Basalts 7. Miscellaneous IntraContinental Activity  kimberlites, carbonatites, anorthosites... Selected References Lutgens, F., Tarbuck, E. and Tasa, D. (2012). Essentials of Geology Eleventh Edition. Upper Saddle River, New Jersey: Pearson Prentice Hall Gill, R. (2010). Igneous Rocks and Processes A Practical Guide. 9600 Garsington Road, Oxford: Wiley-Blackwell A John Wiley & Sons, Ltd., Publication. B.D. Payot (2018). Geology 250 Igneous Petrology: Introduction [Power Point Slides]. NIGS-UP Dilliman, Quezon City. Disclaimer: Photos and illustrations credit to the owners H.D.A.Reyes Correlations 1 PETROLOGY  “PETRA”= rock  “LOGOS”= disclosure or explanation Petrology= the branch of geology dealing with the origin, occurrence, structure, and history of rocks. ROCKS IGNEOUS -primary rocks -source is magma or lava SEDIMENTARY -thin veener above the Sial and Sima in Oceanic and Continental Crusts -secondary rocks METAMORPHIC -change of forms of Ig. And Sed. Due to Temperature, Pressure and Chemical Fluids • Igneous rocks: “ignis”= fire • Rocks that directly solidify from molten or partially molten material, i.e. magma. • Two (2) basic types: intrusive igneous rocks and extrusive igneous rocks. • Sedimentary rocks: “sedimentum”= settling/sinking down • rocks that result from the consolidation of loose particles (sediments) or the chemicals precipitating from solution at or near the Earth’s surface; or organic rock consisting of the secretions or remains of plants or animals. • Can be divided into (1) detrital (clastic) sedimentary rocks and (2) chemical sedimentary rocks. • Metamorphic rocks: “metamorphosis”= change in form • Rocks derived from pre-existing rocks by mineralogical, chemical or structural changes (especially in the solid state). • Can be divided into two groups: foliated metamorphic rocks and nonfoliated metamorphic rocks. • NOTE: borderline situations and rock types exist. • Volcanic tuffs (igneous or sedimentary) • Serpentinite (igneous or metamorphic) How do we classify rocks? • Outcrop characteristics • General texture • Mineral assemblages present • Abundance of different rock types: • (1) The abundance of the 3 rock groups on the continents sedimentary ~66% Igneous + Metamorphic ~34% (bulk is igneous) • (2) If we consider the ocean sediments then some of the schemes have sedimentary rocks as high as 80%. Igneous Petrology • Igneous rock: any crystalline or glassy rock that forms from the cooling of a magma. • Magma: consists mostly of liquid rock matter, but may contain crystals of various minerals, and may contain a gas phase that may be dissolved in the liquid or may be present as a separate gas phase. • Magma can cool to form an igneous rock either on the surface of the Earth to produce an extrusive (volcanic) igneous rock, or beneath the surface of the Earth to produce an intrusive (plutonic) igneous rock. Types of Magma • Determined by the chemical composition. Three general types are recognized: • Basaltic magma: 45-55% SiO2, high in Fe,Mg,Ca; low in K, Na; 1000-1200°C • Andesitic magma: 55-65% SiO2, intermediate in Fe,Mg,Ca,K,Na; 800-1000°C • Rhyolitic magma: 65-75% SiO2, low in Fe,Mg, Ca; high in K, Na; 650-800°C. • Note: nearly all magmas contain dissolved gas at depth. Magma Type Solidified Rock Chemical Composition Temperature Range (°C) Viscosity Gas Content Basaltic Basalt 4555%SiO2;high Fe,Mg,Ca; low K,Na 1000-1200 Low Low Andesitic Andesite 5565%SiO2;inter mediate Fe,Mg,Ca,K,N a 800-1000 Intermediate Intermediate Rhyolitic Rhyolite 6575%SiO2;low Fe,Mg,Ca;high K,NA 650-800 High High Origin of Magmas • Where does magma come from? • the core is not likely to be the source of magmas because it does not have the right chemical composition. • The outer core is mostly Iron, but magmas are silicate liquids. Thus magmas DO NOT COME FROM THE MOLTEN OUTER CORE OF THE EARTH. • Magma is generated from the Earth’s Mantle. But how? PARTIAL MELTING. Heat Sources in the Earth 1. Heat from the early accretion and differentiation of the Earth  still slowly reaching surface 2. Heat released by the radioactive breakdown of unstable nuclides Heat Transfer 1. Radiation 2. Conduction 3. Convection Geothermal Gradient and Partial Melting • Temperature increases as depth increases. • Under normal conditions, the geothermal gradient is not high enough to melt rocks, and thus with the exception of the outer core, most of the Earth is solid. • Since rocks mixtures of minerals, they behave somewhat differently. Unlike minerals, rocks do not melt at a single temperature, but instead melt over a range of temperatures. Thus, it is possible to have partial melts from which the liquid portion might be extracted to form magma. • Two general cases: melting of “dry” rocks and melting of “wet” rocks. Melting of dry rocks is similar to melting of dry minerals, melting temperatures increase with increasing pressure, except there is a range of temperature over which there exists a partial melt. The degree of partial melting can range from 0 to 100% Melting of rocks containing water or carbon dioxide is similar to melting of wet minerals, melting temperatures initially decrease with increasing pressure, except there is a range of temperature over which there exists a partial melt. Ways to Generate Magma • In order to generate a magma in the solid part of the earth either the geothermal gradient must be raised in some way or the melting temperature of the rocks must be lowered in some way. • The geothermal gradient can be raised by upwelling of hot material from below either by uprising solid material (decompression melting) or by intrusion of magma (heat transfer). Lowering the melting temperature can be achieved by adding water or Carbon Dioxide (flux melting). Decompression Melting Lithospheric plates are continuously being stretched along divergent plate boundaries, resulting to their thinning. The decrease in overburden will cause the underlying mantle to rise and to experience lower confining pressure. Lower pressure will cause the melting point of mantle materials to drop which will eventually to partial melting. Transfer of Heat Under normal conditions the temperature in the Earth, shown by the geothermal gradient, is lower than the beginning of melting of the mantle. Thus in order for the mantle to melt there has to be a mechanism to raise the geothermal gradient. One such mechanism is convection, wherein hot mantle material rises to lower pressure or depth, carrying its heat with it. • If the raised geothermal gradient becomes higher than the initial melting temperature at any pressure, then a partial melt will form. Liquid from this partial melt can be separated from the remaining crystals because, in general, liquids have a lower density than solids. Basaltic magmas appear to originate in this way. • Upwelling mantle appears to occur beneath oceanic ridges, at hot spots, and beneath continental rift valleys. Thus, generation of magma in these three environments is likely caused by decompression melting. Transfer of Heat When magmas that were generated by some other mechanism intrude into cold crust, they bring with them heat. Upon solidification they lose this heat and transfer it to the surrounding crust. Repeated intrusions can transfer enough heat to increase the local geothermal gradient and cause melting of the surrounding rock to generate new magmas. Flux Melting • Addition of water or carbon dioxide lowers melting temperature. Addition of such “fluxes” deep in the Earth where the temperature is already high causes decrease in melting temperature which results in partial melting of rocks. • Common mechanism in subduction zones, where hydrous minerals (hornblende) in rocks are melted and contributes to the addition of water, aside from the water present in pore spaces of the seafloor. Composition of Magma • The initial composition of the magma is dictated by the composition of the source rock and the degree of partial melting. In general, melting of a mantle source (garnet peridotite) results in mafic/basaltic magmas. Melting of crustal sources yields more siliceous magmas. • In general more siliceous magmas form by low degrees of partial melting. As the degree of partial melting increases, less siliceous compositions can be generated. So, melting a mafic source thus yields a felsic or intermediate magma. Melting of ultramafic (peridotite source) yields a basaltic magma. Magmatic Differentiation I. Definition • Magmatic differentiation refers to the process whereby an originally homogeneous magma changes it composition or becomes heterogeneous . • Mechanisms: 1. Fractional Crystallization 2. Magma mixing 3. Magmatic assimilation • Crystal fractionation is likely to be the most important in controlling magmatic differentiation Useful terms: A primitive magma (as opposed to parental magma) is one which is close to its original composition and has therefore in theory not undergone crystal fractionation. An evolved magma is one in which crystal fractionation has taken place such the magma composition is different from the starting composition. Fractional Crystallization When magma crystallizes it does so over a range of temperature. Each mineral begins to crystallize at a different temperature, and if these minerals are somehow removed from the liquid, the liquid composition will change. The processes is called magmatic differentiation by Fractional Crystallization. Because mafic minerals like olivine and pyroxene crystallize first, the process results in removing Mg, Fe, and Ca, and enriching the liquid in silica. Thus crystal fractionation can change a mafic magma into a felsic magma. Magmatic Mixing If two magmas with different compositions happen to come in contact with one another, they could mix together. The mixed magma will have a composition somewhere between that of the original two magma compositions. Magma Mixing Magmatic Assimilation Assimilation: magma reacts with the “country rock” which is adjacent to the magma chamber. Magma composition is altered according to the composition of the assimilated country rock Inclusions are incompletely melted chunks of country rock. In the course of reaction , the magma becomes contaminated by incorporation of material originally present in the wall rock. Certain minerals in the wall rock become partially or completely melted and in this way incorporated into the liquid fraction of the magma. Magmatic assimilation Reaction between the magma and the wall rock/ host rock. In the course of reaction , the magma becomes contaminated by incorporation of material originally present in the wall rock. Certain minerals in the wall rock become partially or completely melted and in this way incorporated into the liquid fraction of the magma. Enclaves or xenoliths Bowen’s Reaction Series N.L. Bowen found by experiment that the order in which minerals crystallize from a basaltic magma depends on temperature. As a basaltic magma is cooled Olivine and Ca-rich plagioclase crystallize first. Upon further cooling, Olivine reacts with the liquid to produce pyroxene and Ca-rich plagioclase react with the liquid to produce less Ca-rich plagioclase. But, if the olivine and Ca-rich plagioclase are removed from the liquid by crystal fractionation, then the remaining liquid will be more SiO2 rich. If the process continues, an original basaltic magma can change to first an andesite magma then a rhyolite magma with falling temperature. Igneous Petrology Magmatic Differentiation Any process that cause magma composition to change 1. 2. 3. 4. 5. 6. Distinct melting events from distinct sources Various degree of partial melting from the same source Crystal Fractionation / Fractional Crystalization Mixing of 2 or more magma (Magma mixing) Assimilation/ Contamination of magma by crustal rocks Liquid Immiscibility Magmatic Differentiation 1. Distinct melting events from distinct sources • Each magma might represent melting at a different source rock at different times during the heating event 2. Various degree of partial melting from the same source • In multicomponent rock systems, each component has its own melting range temperature. Magmatic Differentiation 3. Crystal Fractionation / Fractional Crystalization • In phase diagram, it is clear that liquid compositions can change as a result of removing crystal from the liquid as they form. If the crystal is removed, then different magma compositions can be generated from the initial parent liquid. Two mechanism for fractional crystallization 1. Crystal settling/ Floating Crystals • Higher density crystal sinks and lower density crystal floats 2. Inward Crystallization • Magma body is hot and the country rock that surrounds it is expected to be cooler, heat will move outward, away from the magma; walls of magma body will be cooled, crystallization expected to take place in cooler portion; magma expected to crystallize from wall inward. Magmatic Differentiation 4. Mixing of 2 or more magma (Magma mixing) • Two or more magma with different compositions mixes Factors that can inhibit Mixing: a. Temperature contrast b. Density Contrast c. Viscosity Contrast Evidence of mixing: a. Marble cake b. Disequilibrium mineral assemblages c. Reversed zoning in Minerals d. Glass Inclusions e. Chemical Evidences Magmatic Differentiation 5. Assimilation/ Contamination of magma by crustal rocks 6. Liquid Immiscibility Liquid immiscibility is a state in which two liquids with different compositions coexist in equilibrium with each other. Immiscible liquids do not mix and form an emulsion of droplets or networks of one liquid within the other. How do we classify Igneous Rocks? I. Based on composition II. Based on Fabric and Texture III. Based on Field Relation Classification Based on Composition Norm VS Mode Modal Composition or Mode - The most straightforward approach to determining rock mineralogy involves visually identifying the minerals and determining their percentages by volume. Normative mineralogy (Cross et al., 1902 ; Kelsey, 1965 ) is an indirect scheme using data derived from chemical analysis of a rock sample. The first norm classification was devised by Cross, Iddings, Pirsson and Washington (Cross et al., 1902 ), and is referred to as the CIPW norm classification in their honor. Normative classification systems are commonly used in aphanitic or glassy volcanic rocks, in which a rock ’s modal mineral composition can not be determined. Classification Based on Composition Percent by weight of Silica NOTE: Basic and Acidic do not have the same meaning with what chemistry taught us. For short, does not have something to do with pH. Classification Based on Composition Percentage of dark - colored or light - colored minerals  Dark -colored minerals are generally enriched in the elements iron and magnesium and are referred to as ferromagnesian or mafic minerals.  Light -colored felsic minerals are depleted in ferromagnesian elements and are generally enriched in elements such as silicon, oxygen, potassium and sodium. Color Index Classification Based on Composition Percentage of dark - colored or light - colored minerals Classification Based on Composition Classification of igneous rocks III. Classification based on mineralogical and modal composition Igneous Rock Classification by Color index (% of Mafic Minerals) Shand classification % Dark Minerals Type <30 Leucocratic 30-60 Mesocratic 60-90 Melanocratic >90 Hypermelanic Classification of igneous rocks III. Classification based on mineralogical and modal composition Igneous Rock Classification by Color index (% of Mafic Minerals) Ellis classification % Dark Minerals <10 Type Example Holofelsic 10-40 Felsic Granite 40-70 Mafelsic Diorite 70-90 Mafic Gabbro Ultramafic Peridotite >90 Classification Based on Composition Classification of igneous rocks Classification based on mineralogical and modal composition (cont’n) Based on mineral composition: Classification Based on Composition • Peridotite is a very dark colored (ultramafic) rock, depleted in SiO 2 (ultrabasic) and commonly enriched in the minerals pyroxene, olivine, amphibole and plagioclase. Ultramafic plutonic rocks occur in Earth ’ s Mantle • Basalt and gabbro are dark - colored (mafic), SiO 2- poor (basic) rocks rich in plagioclase, pyroxene and olivine. Classification Based on Composition • Andesite and diorite are gray - colored (intermediate) to salt and pepper - colored rocks rich in hornblende, pyroxene and plagioclase. Andesite and diorite contain more than half to almost two - thirds SiO 2. • Dacite and granodiorite are light - colored (felsic) rocks, containing approximately two thirds SiO 2 , rich in plagioclase, alkali feldspar and quartz and also containing small amounts of hornblende and biotite. andesite – dacite volcanoes. Classification Based on Composition • Rhyolite and granite are light colored (felsic) rocks containing more than two -thirds SiO 2 (silicic or acidic) and rich in quartz, alkali feldspar with small percentages of plagioclase and biotite. • Non - crystalline rocks, those characterized by the absence of crystals, include frothy, vesicular rocks such as pumice (light colored) and scoria (dark colored). Other non - crystalline rocks include those with glassy textures such as obsidian or those enriched in rock fragments. Fragmental, also known as pyroclastic, volcanic rocks include tuff (volcanic ash to gravel size) and breccia (larger than gravel size). II. Classification based on fabric and texture • Fabric – encompasses non-compositional properties of a rock that comprise textures and generally large-scale structures • Texture (also called microstructures) – based on the proportions of glass relative to mineral grains and their sizes, shapes, and mutual arrangements that are observable on the scale of a hand specimen or thin section under the microscope • Structures - larger-size features generally seen in an outcrop, such as bedding in a pyroclastic rocks or pillows in a submarine lava flow. II. Classification based on fabric and texture 1. Aphanitic – very fine-grained as a result of rapid cooling at the surface. - minerals too small to be seen by the naked eye. 2. Phaneritic – coarse-grained mineral sizes due to magma cooling at depth. Fine grained – 1mm to 3mm Medium grained – 3mm to 1cm Coarse grained – 1cm to 3cm 3. Porphyritic – very large crystals (phenocrysts) embedded in smaller crystals (groundmass) II. Classification based on fabric and texture 4. Glassy or vitric - contain variable proportion of glass; molten rock quenched quickly as it was ejected into the atmosphere. Glass is basically a highly viscous liquid, disordered on atomic scale, formed from polymerized silicate melt. Note: vitrophyre – a porphyritic rock that contains scattered phenocrysts in a glassy matrix. 5. Volcaniclastic/Pyroclastic – produced by fragmenting processes that creates broken pieces of volcanic rock and/or mineral grains. Classification of volcaniclasts parallels that of sedimentary clasts according to their particle size II. Classification based on fabric and texture DEGREE OF CRYSTALLINITY Holocrystalline - wholly crystalline texture Hypocrystalline - partially crystalline/partially glass texture Holohyaline - wholly glassy textures CRYSTAL FORM 1.Euhedral /idiomorphic– crystal is bounded by faces; developed under circumstances such as slow cooling of magma in a deep-seated condition. 2.Subhedral - intermediate stage of development 3.Anhedral/xenomorphic –crystal faces are absent; developed as the growth of crystals has been hindered by such factors as disturbing environment, reaction with magma and juxtaposition of other growing crystals. As a result, they have had to take the shapes of whatever open spaces were available between the already crystallized minerals. 4.Hypidiomorphic-granular texture-there is a mix of euhedral, subhedral and anhedral grains. Additional textural and structural features of igneous rocks A. Crystallinity and grain size 1. Glassy texture Notes: a. Obsidian-massive, high silica glass appearing in hand samples to have zero crystallinity. Under the microscope, high magnification reveals that obsidian contains abundantly nucleated submicrometer-size crystallites that experienced limited growth in the highly viscous glass. b. All glass is metastable and therefore susceptible to secondary hydration, devitrification (delayed crystallization of silicic glasses) and other types of alteration that progress over time to achieve a more stable state. Important alteration product of devitrification - palagonite Spherulitic texture – product of devitrification; spherulites are spherical to ellipsoidal clusters of radiating fibrous alkali feldspars and a polymorph of SiO2 Perlitic texture – develops by hydration of obsidian on fracture surfaces that are exposed to moisture in the atmosphere or to meteoric water (groundwater). As the outer rind hydrates, it expands and separates along a crack from the nonhydrated substrate. Inward repetition of this process creates a sequence of concentric perlitic cracks that reflect light, creating the characterisitic pearl gray color. Note: Pitchstone – some massive glass having a waxy luster and dark color in hand sample into which 6-16 wt% water has been absorbed Pegmatitic Texture Pegmatitic texture characterized by large crystals averaging more than 30 mm in diameter. Pegmatites display large, early formed euhedral crystals surrounded by later formed subhedral crystals. In contrast, APLITIC TEXTURE refers to extremely fine grained minerals Pegmatitic Granite Vesicular Texture Vesicular textures contain spherical to ellipsoidal void spaces called vesicles , which are analogous to holes in a household sponge. Vesicular textures develop due to exsolution and entrapment of gas bubbles in lava as it cools and solidifies. Scoria and Pumice Vesicular Basalt Rocks that contain smaller amounts (5 – 30%) of vesicles are named using a modifier such as vesicular basalt or vesicular andesite, while those rocks with just a few vesicles ( < 5%) are given names such as vesicle - bearing basalt and andesite. III. Classification based on field relations The location where magma was emplaced provides a basis for rock classification 1. Extrusive (volcanic) – magma emplaced onto the surface of the Earth as coherent lava flows or as fragmental deposits. These rocks are typically aphanitic and glass. Many are porphyritic. Some have fragmental (volcaniclastic) fabric). 2. Intrusive (plutonic) – igneous rocks formed at depth; typically phanieritic Note: hypabyssal rocks – formed at intermediate depth not clearly distinct from those of volcanic and plutonic rocks. They can have fabric similar to that of plutonic and volcanic rocks. Intrusive Igneous Bodies • Stock – small discordant pluton • Batholith – more than 100 sq. km. in outcrop area • Dike – tabular body cutting across bedding • Sill – concordant tabular body • Laccolith – blister-shaped sill Intrusive igneous Bodies Note: lopolith Vulcanology What is volcano • A volcano is a naturally occurring landform produced where lava erupts onto Earth ’s surface. Volcanic activity vividly displays the dynamic nature of our hot, turbulent planet, and profoundly impacts Earth in many ways. Why do volcanoes erupt? Due to decompression Magma is lighter than the solid rock around it Types of volcanoes 1. Shield – slopes are gentle (15o or less); shape resembles a Roman shield lying on the ground; made up of successive lava flows 2. Cinder cone – relatively small (<300 m high); steep slopes (30 – 40o); made up of pyroclastic material 3. Composite or strato-volcano – layered structure (tephra and lava flows) Distribution of volcanoes • • • • Pacific Ring of Fire Hot spots Spreading centers Subduction Zones How big are volcanic eruptions? Volcanic Explosivity Index or VEI - is based on a number of things (e.g. plume height, volume, etc.) that can be observed during an eruption. Volcano explosivity index Desc. Plume Height 0 nonexplosive < 100 m 1000s m3 Hawaiian daily Kilauea 1 gentle 100-1000 m 10,000s m3 Haw/Strombolian daily Stromboli 2 explosive 1-5 km 1,000,000s m3 Strom/Vulcanian weekly Galeras, 1992 3 severe 3-15 km 10,000,000s m3 Vulcanian yearly Ruiz, 1985 4 cataclysmic 10-25 km 100,000,000s m3 Vulc/Plinian 10's of years Galunggung, 1982 5 paroxysmal >25 km 1 km3 Plinian 100's of years St. Helens, 1981 6 colossal >25 km 10s km3 Plin/Ultra-Plinian 100's of years Krakatau, 1883 7 supercolossal >25 km 100s km3 Ultra-Plinian 1000's of years Tambora, 1815 8 megacolossal >25 km 1,000s km3 Ultra-Plinian 10,000's of years Yellowstone, 2 Ma VEI Volume Class. How often Example Volcanic eruptions Volcano Year Cubic Kilometers "Large" Eruptions Kilauea, Hawaii 1983 0.1 Mauna Loa, Hawaii 1976 0.375 Mauna Loa, Hawaii 1984 0.22 Mt. Pelee, Martinique 1902 0.5 Mount St. Helens 1980 0.7 Askja, Iceland 1875 2 Vesuvius, Italy 79 3 "Major" Eruptions Pinatubo, Philippines 1991 10 Krakatoa, Indonesia 1883 18 Ilopango, El Salvador 300 40 Santorini, Greece 1450BC 60 Mazama, Oregon 4000BC 75 1815 150 Tambora, Indonesia Volcano eruptions Strombolian - short-lived, explosive outbursts of pasty lava ejected a few tens or hundreds of meters into the air - no sustained eruption column - episodic explosions with booming blasts Volcano eruptions Hawaiian - calmest eruption types - characterized by the effusive emission of highly fluid basalt lavas with low gas contents - steady lava fountaining and the production of thin lava flows Curtain of fire Source: https://meggietailor.wordpress.com/2010/01/09/10unbelievable-natural-phenomena/ fire fountains Volcano eruptions Icelandic Eruption - characterized by effusions of molten basaltic lava that flow from long, parallel fissures. Such outpourings often build lava plateaus. Source: https://www.whatson.is/event/volcano-house-free-exhibit-daily-screenings/ Tryggvagata 11 101 Reykjavík Iceland Volcano eruptions Vulcanian - occur as a series of discrete, canon-like explosions that are short-lived, lasting for only minutes to a few hours, often with high-velocity ejections of bombs and blocks. Once the volcano "clears its throat," however, the subsequent eruptions can be relatively quiet and sustained. - more explosive than Strombolian eruptions with eruptive columns commonly between 5 and 10 km high. Volcano eruptions Plinian - generate sustained eruptive columns, with some reaching heights of ~45 km. These eruptive columns produce widespread dispersals of tephra which cover large areas with an even thickness of pumice and ash. Volcano eruptions Surtseyan or Phreatomagmatic generated by the intereaction of magma with either groundwater or surface water. - much more explosive; as the water is heated, it flashes to steam and expands explosively, thus fragmenting the magma into exceptionally fine-grained ash. Ukinrek, Alaska (1977) Volcano eruptions Phreatic - a steam eruption without lava ejection. - Phreatic eruptions are a common precursor of volcanic activity. - The eruptions are caused by groundwater flashing to steam as it is heated by magma. http://www.vulkaner.no/v/volcan/island/ Mount Pinatubo eruption Magmatic explosive eruption on 12 June 1991 forms enormous eruption column of gas and ash above the volcano. Volcanoes in the Philippines www.phivolcs.dost.gov.ph Volcanoes and Volcanic Hazards Volcano • 3 types of volcanoes according to activity and historical records 1. Active Volcano - An active volcano is a volcano that has had at least one eruption during the past 10,000 years. An active volcano might be erupting or dormant. 2. Potentially-active Volcano / Inactive Volcano - is an active volcano that is not erupting, but supposed to erupt again. Hazards posed by active volcanoes 3. Dormant/ Extinct volcano - has not had an eruption for at least 10,000 years and is not expected to erupt again in a comparable time scale of the future. HDA. Reyes | Petrology Volcanoes and Volcanic Hazards Volcano • Active Volcanoes in the Philippines (24) NAME OF VOLCANO LATITUDE LONGITUDE LOCATION/ PROVINCE Babuyan Claro 19.52408 121.95005 Banahaw 14.06038 121.48803 Biliran (Anas) Bud Dajo 11.63268 6.01295 124.47162 121.05772 Bulusan 12.76853 124.05445 Cabalian 10.27986 125.21598 Southern Leyte in Visayas Cagua 18.22116 122.11163 Cagayan in Luzon Camiguin de Babuyanes 18.83037 121.86280 Babuyan Island Group, Cagayan in Luzon HDA. Reyes | Petrology Babuyan Island Group, Cagayan in Luzon Boundaries of Laguna and Quezon in Luzon Leyte in Visayas Sulu Sorsogon, Bicol Region in Luzon Volcanoes and Volcanic Hazards Volcano • Active Volcanoes in the Philippines (24) NAME OF VOLCANO LATITUDE LONGITUD LOCATION/ PROVINCE E Didicas 19.07533 122.20147 Babuyan Island Group, Cagayan in Luzon Hibok-Hibok 9.20427 124.67115 Camiguin in Mindanao Iraya 20.46669 122.01078 Batan Island, Batanes in Luzon Iriga 13.45606 123.45479 Camarines Sur in Luzon Isarog 13.65685 123.38087 Camarines Sur in Luzon Kanlaon 10.41129 123.13243 Leonard Kniaseff 7.39359HDA. Reyes | Petrology 126.06418 Boundaries of Negros Oriental and Negros Occidental in Visayas Davao del Norte in Volcanoes and Volcanic Hazards Volcano • Active Volcanoes in the Philippines (24) NAME OF VOLCANO Makaturing Matutum LATITUDE 7.64371 6.36111 Mayon 13.25519 Musuan (Calayo) 7.87680 Parker 6.10274 LONGITUD LOCATION/ PROVINCE E 124.31718 Lanao del Sur in Mindanao 125.07603 Cotobato in Mindanao Albay, Bicol Region in 123.68615 Luzon 125.06985 Bukidnon in Mindanao South Cotobato/General Santos/North 124.88879 Cotabato/Sarangani Provinces in Mindanao HDA. Reyes | Petrology Volcanoes and Volcanic Hazards Volcano • Active Volcanoes in the Philippines (24) NAME OF VOLCANO LATITUDE Pinatubo 15.14162 Ragang 7.69066 Smith 19.53915 Taal 14.01024 LONGITUD LOCATION/ PROVINCE E Boundaries of Pampanga, 120.35084 Tarlac and Zambales in Luzon Lanao del Sur and 124.50639 Cotobato in Mindanao Babuyan Island Group, 121.91367 Cagayan in Luzon 120.99812 Batangas in Luzon Source: https://www.phivolcs.dost.gov.ph/index.php?option=com_content&view=article&id=8235:active-volcanoes&catid=55:volcanoes-of-the-philippines HDA. Reyes | Petrology Volcanoes and Volcanic Hazards Volcano • Most active volcanoes in the Philippines 1. Mayon Volcano 2. Taal Volcano 3. Mt. Hibok-hibok 4. Mt. Pinatubo 5. Mt. Kanlaon 6. Mt. Bulusan 7. Mt. Musuan • Potentially-active Volcanoes in Philippines (27) Source: https://news.sky.com/story/warning-inphilippines-to-evacuate-or-face-death-penalty-asmayon-volcano-threatens-deadly-eruption11219540 Source: http://philippineslifestyle.com/kanlaonvolcano-negros-island-erupts-unexpectedly/ the Source: https://ph.ambafrance.org/Understanding-TaalVolcano-for-a-better-forecast-of-its-next-eruptions HDA. Reyes | Petrology Source: http://noypicollections.blogspot.com/2011/07/mtpinatubo-dilemma-turns-to-tourist.html Volcanoes and Volcanic Hazards • Volcanic Activity Volcanic eruption - the sudden occurrence of a violent discharge of steam and volcanic material.  • Factors Affecting the nature of volcanic eruption (Viscosity). a. Magma Composition b. Temperature c. Volatile/Dissolved Gasses Viscosity • The state of being thick, sticky, and semifluid in consistency, due to internal friction. • The more viscous the material, the greater its resistance to flow Mauna Loa Eruption Source: https://www.huffingtonpost.com/2014/05/20/mauna-loa-volcanoeruption_n_5354376.html HDA. Reyes | Petrology Volcanic hazards Volcanic hazards Pyroclastic Volcanic eruptions eject broken rock particles of varying sizes, known as pyroclasts (which means fiery fragment). Pyroclasts may be ejected into the atmosphere as airborne tephra or transported along Earth ’ s surface as pyroclastic flows. Following accumulation, these particles are cemented or welded together to produce volcanic rocks with fragmental or pyroclastic textures . Pyroclasts are classified according to their composition, size and shape Pyroclasts consist of several different types of materials: • Lithic pyroclasts contain fragments such as basalt, andesite or other rocks. • Vitric pyroclasts are composed of glassy fragments, most commonly pumice or scoria shards. • Crystal pyroclasts contain minerals. Pyroclastics  Pyroclast – an individual particle ejected during volcanic eruption; usually classified according to size.  Pyroclastic Rock – any rock consist of unreworked solid material of whatever size explosively or serially ejected from a volcanic vent.  Pyroclastic material – any volcanic material that is ejected into the air (ejecta). Composition can range from basaltic to rhyolitic, but higher viscous magmas (andesitic/ryholitic with higher SiO2 contents are usually more commonly erupted as ejecta). Accumulations of ejecta are called pyroclastic rocks or tephra. Classified according to size Pyroclastics  Pyroclastic material o Blocks and Bombs – Blocks are solid when ejected, bombs are liquid when ejected • Breadcrust Bombs – Formed if the outside of the pyroclastic bombs solidify during their flight. They can develop cracked outer surface as the interiors continue to expend o Lapilli – rock fragments formed from ejected droplet of magma o Ash usually glass, but sometimes contain mineral fragments; consolidated ash (welded) is called tuff. Breadcrust Bomb Pyroclastics  Principal components o Juvenile magmatic clasts – ra nge in vesicularity from highly vesicular pumice and scoria to variably vesicular lava bombs and blocks o Glass Shards o Free Crystals and Crystal Fragments o Lithic Fragments – Primary or from wall rocks, etc o Accretionary Lapilli – Small spherical balls of volcanic ash; small spherical balls of volcanic ash that form from a wet nucleus falling through a volcanic ash cloud. They can flatten on hitting the ground or may roll on loose ash and grow like a snowball. Heritage of volcaniclasts a. Juvenile or cognate clasts- derived directly from magma involved in the volcanic activity and consequently always consist in large part of glass formed by rapid quenching of the extruded melt. a. Accidental clasts – derived from older rock torm from the vent walls or swept up from the ground surface by lava or pyroclastic walls • Note: xenoliths/xenocrysts Pyroclastics  Explosive eruption and Pyroclastic Deposits o Explosive Eruptions – Involves rapid release and decompression of gas which results simultaneously in fragmentation and ejection of magma and/or wall rocks 3 types/style based on differences in the source of the gas and extend of direct involvement of magma: a. Explosive magmatic b. Phreatomagmatic c. Phreatic (The last 2, both hydrovolcanic phenomena, involving steam generated from external water); all three styles are capable of generating abundant pyroclastic ranging from fine ash to blocks a few meter across. Pyroclastics  Pyroclastic disperesed by: o Injection into the atmosphere followed by FALLOUT from suspension o Ground hugging, relatively high particle concentration PYROCLASTIC FLOW o Relatively low particle concentration PRYROCLASTIC SURGES Pyroclastics  Pyroclastic disperesed by: o Pyroclastic Fall – mantle bedding with plane parallel and no internal erosion, good sorting, juvenile clasts with angular to ragged shape (also known as tephra fall) o Pyroclastic surge – Nonmantling beds, thickening into low-lying areas, with cross-stratification, pinch and swell bedding and scoured contacts, moderate sorting, juvenile clasts with some degree or rounding o Pyroclastic Flow – Landscape filling units generally poorly bedded to non-bedded, poor sorting, rounded juvenile clasts. Process of fragmentation • 1. Pyroclastic processes- explosive ejection and aerial dispersal of pyroclasts (also called ejecta/tephra) of rock and magma from a volcanic vent; essential attribute is the presence of vitroclasts • 2. Autoclastic processes- formed as a result of the breaking up of the cooler, crusted, more rigid margin as the hotter more mobile interior continuous to move. Extrusions of all but the least viscuous magma create bloc-size autoclasts. • 3. Epiclastic processes – epiclasts of a wide range of sizes are created by weathering and disintegration of volcanic rock-the same processes that produce sedimentary clasts. Pyroclastics  Transport and deposition • Mass flow transport – group of clasts, or clasts plus interstitial fluid (air, water, volcanic gas) move together and interact; mass flows vary widely in rheology and particle concentration • Traction transport – Clasts are entertained in moving interstitial fluids and are free to behave independently • Suspension transport – Clasts are fully suspended in interstitial fluid. Pyroclastics  Important textures and structures • Pyroclastic texture – found in lavas, syn-volcanic intrusion, lavalike ignimbrites and clasts derived from volcanic type. • Pyroclastic flows – Hot high-concentrated, ground hugging highly mobile, gas-particle dispersions generated by volanic eruptions.  Particles formed from explosive disintegration of erupting magma • • • Block and ash flow or Nuees Ardentes – hot avalanches in association with extrusion of lava dome and lava flows; Scoria and ash flows - by collapse of vertical explosive column, collapse may follow immediately after single explosion or a series of closely spaced explosions, as occurs in some vulcanian eruption Pumiceous and scoria pyroclastic flows - From upwelling and overflow direct from vents Nuees ardentes Types of volcano • Shield Volcano o Shield volcanoes are broad, sloping edifices that cover hundreds to thousands of square kilometers with shapes that resemble the defensive shields of ancient warriors. Of the conical volcanic landforms, shield volcanoes encompass the greatest volume. Shield volcanoes are produced by hot, low viscosity, basaltic lava that fl ows great distances from the vent. The slopes of shield volcanoes are not steep, generally 2 – 10 ° . Shield volcanoes occur over hotspots that emit large volumes of basaltic magma from central vents, fissure rifts and flank eruptions. Types of volcano • Pahoehoe vs Aa lava • Pahoehoe lava consists of low viscosity, “ runny ” basaltic lava which produces thin flows with a billowing, rippled and/or ropey surface • More viscous aa lava tends to produce thicker, slower moving lava flows with angular, jagged, fractured surfaces. Types of volcano • Pillow Lava As hot (1100 – 1300 ° C) basaltic magma rises upward and reacts with cold seawater, spheroidal pillow lavas develop. The outer shell of the pillow lava is quenched instantaneously producing a glassy rind. The interior of the pillow cools more slowly. After the outer glassy rind forms, radial cooling joints develop within the interior of the pillow. Types of volcano • Composite volcanoes o are majestic cone - shaped mountains encompassing tens to hundreds of square kilometers in area with slopes ranging from 10 ° to 30 °. Composite volcanoes consist of alternating layers of pyroclastic debris and lava flows that build volcanic cones. In a sense, these volcanoes are a composite of many different rock types, generating stratified layers (hence the alternative name stratovolcanoes ). Types of volcano • Pyroclastic cones are steep - sided ( ∼30 – 35 ° ) conical features composed of tephra. Tephra consists of volcanic rock fragments of various sizes and compositions emitted during explosive eruptions. Common ash - to bomb – sized rock fragments include basalt and andesite, with lesser amounts of dacite and rhyolite. Pyroclastic cones develop from tephra emitted from central vents. Pyroclastic cones occur in a wide variety of settings including continental rifts, convergent plate boundaries, divergent plate boundaries and over hotspots. Pyroclastic cones include: • Scoria cones composed predominantly of vesicular basaltic material. • Cinder cones consisting of ash, lapilli and bomb - sized particles of various compositions that accumulate as circular to oval -shaped conical volcanoes References Kevin Hefferan and John O’Brien (2010). Earth Materials. 9600 Garsington Road, Oxford: Wiley-Blackwell A John Wiley & Sons, Ltd., Publication. Lutgens, F., Tarbuck, E. and Tasa, D. (2012). Essentials of Geology Eleventh Edition. Upper Saddle River, New Jersey: Pearson Prentice Hall. Villegas, M (2015-2016). Igneous, Metamorphic, and Sedimentary Petrology [Powerpoint Presentation]. Adamson University, Manila, Philippines. Gemal, G. (2013). Elementary Petrology [Power Point Presentation]. Adamson University, Manila, Philippines. https://www.thefreedictionary.com/Volcano+(geological+landform) https://www.phivolcs.dost.gov.ph/index.php?option=com_content&view=a rticle&id=8235:active-volcanoes&catid=55:volcanoes-of-the-philippines https://www.britannica.com/science/volcano/Six-types-oferuptions#ref388825 www.volcanolive.com/phreatic.html BASALT History o THE DIFFERENT TECTONIC ENVIRONMENTS IN WHICH THE MAJOR MAGMA SERIES COMMONLY OCCUR History o Late latin “basaltes” misspelling of L. basanites – very hard stone, imported from Ancient Greek (basanites), from basanos, “touchtone”, and originated in Egyptian bauhun “slate”. Basalts • A fine grained mafic igneous rock consisting essentially of augite and calcic plagioclase o Augite – a variety of high-Ca pyroxene o Calcic Plagioclace – pl with more anorthite (CaAl2Si2O8) than albite (NaAlSi3O8) • Basic rock, with SiO2 range 45% - 52% • Restricted to rocks with total alkali content (Na2O + K2O mass %) less than 5%. • Texture: aphyric-devoid of phenocrysts; more commonly contain phenocrysts or microphenocrysts of olivine, and of pyroxene and/or plagioclase Basalts Basalts Basalts Basalts Classification of Basalts Classification of Basalts Classification of Basalts Classification of Basalts • Alkali basalt – used in place of nepheline olivine basalt Basalts Other types of Basalts • Olivine phyric basalt – Basalt containing olivine • Picrite – a basaltic rock visibly enriched in olivine crystals, often as phenocrysts • Ankaremite – basaltic rock rich in olivine and augite phenocrysts; on some the abundance of mafic phenocrysts may have been enhanced by gravitational accumulation (olivine, augite crystals being denser than basaltic melt). Basalts • • • • Other types of Basalts Picrobasalts – having lower SiO2 content than basalt (ultrabasic in composition), more olivine-rich and contain little plagioclase Basaltic Andesite – have mafic mineral similar to basalt but contain plagioclase of more sodic composition (typically andesite) Trachybasalts, basanites, and tephrites – usually contain recognizable alkali feldspar or feldpathoids Boninite – a high-Mg form of basalt that is erupted generally in back-arc basins, distinguished by its low Titanium content and trace element composition. Where basalts occur Erupted in a wide variety of tectonic environments: 1. Mid-oceanic ridge basalt (MORBs) 2. Ocean Island basalts (OIB) 3. Large igneous provine (LIPs); oceanic plateaus and continental flood basalts (CFBs) 4. Intra-continental rift basalts 5. Subduction-related basalts a. b. c. d. Low K or island arc tholeiite (IAT) arc basalt High K arc basalts Basalt from back-arc basin Basalts from active continental margins Where basalts occur 1. Mid-oceanic ridge basalt (MORBs) o MOR system - >60,000 km total length o Erupts basaltic lava at average rate 3 km3/yr o Basalts erupted at MOR are olivine tholeiitic basalts, may be aphyric but more commonly contain phenocrysts of ol±chromite±pl±augite; pl most abundant o Most distinctive aspect is chemical composition: low K2O and other incompatible elements Where basalts occur 1. Mid-oceanic ridge basalt (MORBs) o Mid-ocean ridge basalts can be subdivided: o Normal MORB (N-MORB) - are strongly depleted in highly incompatible elements. o Enriched MORB (EMORB) magma represent 20 – 30% partial melting of a well-mixed, depleted mantle. Where basalts occur 2. Ocean Island Basalt (OIBs) o Intraplate volcanic islands and sea mounts (eg. Hawaii chain of islands) o Age of volcanism correlates with island’s position in the chain o The elevation,composition, age and thicker crust of the large intraplate ocean islands are quite distinct from abyssal plains from which most of them stand o Hot Spot – localized source of magma supply whose output thickens the basaltic crust in its immediate neighbourhoods: by construction of a volcanic edifice on the seafloor, by intrusion of magma within the crust and by “underplating” magma at the base of the crust (note average oceanic crustal crust is 6-7 km) o Majority are of alkali character Where basalts occur 2. Ocean Island Basalt (OIBs) o Development: starts with • Early phase of voluminous growth lasting up to a million years, constructs large gently sloping, subaerial Shield Volcano; • Hiatus during which erosion of the shield occurs; • Renewed, low volume, more alkaline and more explosive volcanism, during which evolved alkali magmas may be more prominent. Where basalts occur 3. Large igneous provine (LIPs) oceanic plateaus and continental flood basalts (CFBs) o Ocean floor bathymetry studies revealed enormous submarine basaltic plateaus- represents enormous intraplate volcanic outpourings (eg. Ontong java Plateau, western Pacific; Shatsky Rise, Pacific) (LIP) o Basalts dominantly ol-pl-phyric low–K tholeiites o CFBs- continental counterpart; consist of great thicknesses of subalkali, usually ol-pl-phyric basaltic lavas. o Evidence of extensive domal uplift associated with early stages of CFB evolution o Eg. North Atlantic Volcanic Province and Deccan “trap” in India. o Estimated melt production – 5 km3/yr; greatly exceeding present-day global hot spot output of 0.5km3/yr o Most CFBs fond in passive margins where continental fragments have since separated. Where basalts occur Where basalts occur Where basalts occur 4. Intra-continental rift basalts o Basaltic volcanism associated with intra-continental rifting • Extension caused by doming above a subcontinental mantle hot spot (Kenya-Ethiopia) rift system in E. Africa from 25 Ma BP to present • In regions of post-subduction extension such as Basinand-Range Province in the western USA • Each characterized by elevated heat flow, broad gravity low consistent with thinned lithosphere o Alkali and transitional basalts predominate basalt from a continuum with more alakali and strongly undersaturated mafic rock such as nepheline and melilitite, accompanied by relatively large volume of more evolved volcanics such as trachyte and phonolite Where basalts occur 5. Subduction-related basalts o Oceanic lithosphere formed from MOR interred back into the Earth’s mantle o More evolved magmas erupted in many island arcs (andesite) and active continental margins (dacites and rhyolites) • Low-K island tholeiite (IAT) arc basalts – immature oceanic island arcs; ol-pl and augite, sparse opx, and/or magnetite • Medium-K arc basalts – more characteristic product of mature island arc volcanism; high alumina basalts, often notably porphyritic containing phenocrysts of plol-augite and magnetite occasionally accompanied by hb • High-K arc basalts – high-K calc-alkali association relatively scarce; execeeded in volume by high-K andesites more evolved member contain phenocrysts of ol, augite accompanied by hb, mg/pl Where basalts occur Where basalts occur 5. Subduction-related basalts Where basalts occur 5. Subduction-related basalts Typical ophiolite emplacement Ophiolite Sequence 5. Subduction-related Ophiolite Sequence 5. Subduction-related Rock types in the mantle Peridotite is the dominant rock type of the Earth’s upper mantle • Lherzolite: fertile unaltered mantle; mostly composed of olivine, orthopyroxene (commonly enstatite), and clinopyroxene (diopside), and have relatively high proportions of basaltic ingredients (garnet and clinopyroxene). • Dunite (mostly olivine) and Harzburgite (olivine + orthopyroxene) are refractory residuum after basalt has been extracted by partial melting • Wehrlite: mostly composed of olivine plus clinopyroxene. wehrlite lherzolite Selected References: Gill, R. (2010). Igneous Rocks and Processe