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Introduction Igneous rocks make up large parts of the continents and all the oceanic crust - indicate plate boundaries and hot spot activity Chapter 4 Igneous Rocks Granite Mostly quartz and feldspar Used for kitchen counters, facings on buildings, tombstones, etc and Intrusive Igneous Activity How do igneous rocks differ from one another? How do igneous rocks differ from one another? Texture – size of crystals CoarseCoarse-grained rocks FineFine-grained rocks Mixed texture rocks How do igneous rocks differ from one another? Texture is related to rate of cooling. Intrusive igneous rocks – slow cooling – coarse texture The Properties and Behavior of Magma and Lava Molten rock material below the Earth's surface is called magma. If the magma reaches the surface it is called lava. Magma is erupted as either Lava flows or Pyroclastic materials. Extrusive igneous rocks – rapid cooling – fine texture 1 IGNEOUS ROCKS 1. Texture Igneous textures Pyroclasts Volcanic ash Bomb Pumice Pyroclasts Extrusive rocks Extrusive rocks Porphyry Porphyry Intrusive rocks Intrusive rocks IGNEOUS ROCKS 1a. Texture 1. Extrusive pyroclasts form in violent eruptions from lava in the air. Bomb IGNEOUS ROCKS 1a. Texture Pyroclasts Volcanic ash Extrusive rocks Bomb Mafic Basalt Pumice 1. Extrusive pyroclasts form in violent eruptions from lava in the air. Felsic Rhyolite Porphyry Pumice Intrusive rocks Volcanic ash Pyroclasts Extrusive rocks IGNEOUS ROCKS 1a. Texture Pyroclasts Volcanic ash Extrusive rocks Porphyry Intrusive rocks Bomb Mafic Basalt Pumice Felsic Rhyolite IGNEOUS ROCKS 1a. Texture Pyroclasts 1. Extrusive pyroclasts form in violent eruptions from lava in the air. Volcanic ash Extrusive rocks 2. Extrusive igneous rocks cool rapidly and are fine-grained. Bomb Mafic Pumice 1. Extrusive pyroclasts form in violent eruptions from lava in the air. Felsic Basalt Rhyolite Gabbro Granite Porphyry 2. Extrusive igneous rocks cool rapidly and are fine-grained. Intrusive rocks 2 IGNEOUS ROCKS 1a. Texture Pyroclasts IGNEOUS ROCKS 1a. Texture Mafic Volcanic ash Extrusive rocks Bomb Pumice Mafic Basalt Felsic Rhyolite Gabbro Granite Porphyry Felsic 1. Extrusive pyroclasts form in violent eruptions from lava in the air. 2. Extrusive igneous rocks cool rapidly and are fine-grained. 2. Extrusive igneous rocks cool rapidly and are fine-grained. Basalt 3. Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form. Rhyolite 3. Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form. Intrusive rocks Gabbro IGNEOUS ROCKS 1a. Texture Pyroclasts Volcanic ash Extrusive rocks Bomb Pumice Mafic Basalt Felsic Rhyolite Gabbro Granite Porphyry Intrusive rocks IGNEOUS ROCKS 1a. Texture Pyroclasts 1. Extrusive pyroclasts form in violent eruptions from lava in the air. Volcanic ash Extrusive rocks 2. Extrusive igneous rocks cool rapidly and are fine-grained. Mafic Pumice Intrusive rocks 1. Extrusive pyroclasts form in violent eruptions from lava in the air. Felsic Rhyolite 2. Extrusive igneous rocks cool rapidly and are fine-grained. Gabbro Granite 3. Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form. Porphyry 3. Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form. Phenocrysts Bomb Basalt Phenocrysts Porphyry Porphyry 4. Some intrusive porphyritic crystals grow large, but the remaining melt cools faster, forming smaller crystals. The Properties and Behavior of Magma and Lava IGNEOUS ROCKS 1a. Texture Composition of Magma Phenocrysts Porphyry Granite 4. Some intrusive porphyritic crystals grow large, but the remaining melt cools faster, forming smaller crystals. Silica is the primary constituent of magmas Magmas are differentiated based on the relative proportions of silica, iron, and magnesium. Felsic – silica rich, iron and magnesium poor Intermediate – intermediate between felsic and mafic Mafic – silica poor, iron and magnesium rich Parent magma composition largely determines the composition of igneous rocks. 3 Igneous Rocks 1c. Chemical & Minerals IGNEOUS ROCKS 1c. Chemical & Minerals Chemical and mineral composition of igneous rocks Four compositional groups: Felsic igneous rocks Intermediate igneous rocks Mafic igneous rocks Ultramafic igneous rocks Igneous Rocks 1c. Chemical & Minerals The Properties and Behavior of Magma and Lava How Hot Are Magma and Lava? Measured temperatures of lavas suggest that typical mafic magma is somewhat hotter than 1200ºC felsic magma is somewhat hotter than 900ºC. Fig. 4.2, p. 87 The Properties and Behavior of Magma and Lava The Properties and Behavior of Magma and Lava Viscosity – Resistance to Flow Viscosity – Resistance to Flow Viscosity measures a substance’s resistance to flow. Examples: Water, syrup, mafic magmas Mafic magmas are hotter and have less silica. This makes them flow better. Viscosity measures a substance’s resistance to flow. High viscosity – thick, stiff flow Examples: Tar, glacial ice, felsic magma Fig. 4.3 b, p. 88 Fig. 4.3 a, p. 88 4 The Properties and Behavior of Magma and Lava How do magmas form? Why do rocks melt? Viscosity – Resistance to Flow The viscosity of magma is controlled primarily by temperature and composition (silica and iron content). Other factors include loss of volatiles, crystallinity, bubble content, and shear stress during movement. How Does Magma Originate and Change? How do magmas form? Magma accumulates in reservoirs in the lower crust or upper mantle in magma chambers What is a magma chamber? A rising blob of magma that pushes aside crustal rocks as it rises through the crust. How Does Magma Originate and Change? Molten rock is less dense than solid rock, and so it is buoyant and begins to rise thru the lithosphere. Magmatic differentiation Later the magma may stall out and crystallize at a level of neutral buoyancy. Magmatic Differentiation – • A process by which rocks of varying composition can arise from a uniform parent magma. Stoping - A process where a large body of magma can push aside the overlying crust and assimilate blocks of crust on its way toward the surface. • The first minerals to crystallize from a cooling magma are the ones that are the last to melt. Fig. 4.7, p. 92 5 Magmatic differentiation Magmatic differentiation Fractional crystallization – • the process by which the crystals are formed in a cooling magma and are segregated from the remaining liquid. THE PALISADES INTRUSION • Example: basaltic intrusion like Palisades, New Jersey How Does Magma Originate and Change? Magmatic differentiation Bowen’s reaction series THE PALISADES INTRUSION Basaltic intrusion 245–275 m (800–900 ft) Sandstone Basalt Mostly sodium-rich plagioclase feldspar; no olivine Bowen’s reaction series describes the sequence of mineral crystallization in a cooling magma. Calcium-rich plagioclase feldspar and pyroxene; no olivine Olivine Basalt There are two branches in the reaction series. Sandstone Discontinuous series Continuous series Fig. 4.4, p. 89 How Does Magma Originate and Change? How Does Magma Originate and Change? Bowen’s reaction series Bowen’s reaction series The discontinuous series produces ferromagnesian minerals. Given ideal cooling conditions, a mafic magma will yield a sequence of different minerals, each of which is stable within specific temperature ranges. As each mineral forms, it removes elements from the melt, leaving the remaining melt to form the next mineral in the sequence. Fig. 4.4, p. 89 The continuous series produces a variety of plagioclase feldspars Substitution occurs in the crystal structure of plagioclase feldspar. Calcium is replaced with sodium during cooling. These minerals react continuously with the magma to adjust their compositions during cooling. . Fig. 4.4, p. 89 6 How Does Magma Originate and Change? How Does Magma Originate and Change? Bowen’s reaction series Bowen’s reaction series The reverse of Bowen’s reaction series describes the melting of rock Bowen’s reaction series predicts that olivine will form first at high temperatures and quartz (SiO²) is the last mineral to crystallize from a cooling magma. The first mineral to melt would be quartz. • followed by muscovite • followed by potassium feldspar • followed by biotite and Na-rich plagioclase feldspar . Fig. 4.4, p. 89 Decreasing Reaction Amphibole (hornblende) Co Plag ntinuo iocla us b se fe ranc ldsp h Con ars tinu ous reac tion seri es ch bran ous tinu Pyroxene (augite) on Disc Reaction temperature Types of magma Calcium-rich plagioclase Olivine Fig. 4.4, p. 89 Igneous activity and plate tectonics Magmatic geosystems: geosystems: Mafic (45–52% silica) Island arc plate subduction Intermediate (53–65% silica) Plate divergence Reaction Biotite mica Sodium-rich plagioclase Potassium feldspar HotHot-spot volcanism Felsic (>65% silica) Muscovite mica Continental plate subduction Quartz Stepped Art Fig. 4-4, p. 89 Igneous Rocks: Igneous activity vs plate tectonics Geomagmatic systems of Earth How Does Magma Originate and Change? The Origin of Magma at Spreading Ridges. Magmas that forms beneath spreading ridges as a result of mantle plumes are mafic. Fig 4.5, p. 92 7 Plate divergence boundary Mid-Atlantic Ridge, Iceland PLATE DIVERGENCE Mid-ocean ridge Basaltic extrusives Basaltic intrusives Basaltic extrusives & intrusives Partial melting Of upper mantle Rising magma Igneous Rocks: Igneous activity vs plate tectonics, Plate Divergence Boundary How Does Magma Originate and Change? Subduction Zones and the Origin of Magma Along subduction zones at convergent boundaries, partial melting of the subducting plate produces intermediate to felsic magmas. Fig 4.5, p. 92 How Does Magma Originate and Change? Island arc plate subduction The Origin of Magma at Spreading Ridges and Subduction Zones ISLAND ARC PLATE SUBDUCTION Island arc volcano Mafic to intermediate intrusives (plutonism) Mafic to intermediate extrusives (volcanism) Mafic to intermediate intrusives & extrusives Magmas accumulate in magma chambers Subduction zone Oceanic lithosphere Igneous Rocks: Igneous activity vs plate tectonics, A few kilometers beneath divergent boundaries Tens of kilometers beneath convergent boundaries. Fig 4.5, p. 92 8 Continental plate subduction Continental margin volcano, Mt. Rainier, Washington CONTINENTAL PLATE SUBDUCTION Mafic to felsic intrusives Mafic to felsic extrusives Mafic to felsic extrusives & intrusives Continental margin volcano Subduction zone Co nt Co inen nt in tal lit ent crus ho al sp ma t he re ntle Oceanic crust ic ere an h ce sp O tho li Igneous Rocks: Igneous activity vs plate tectonics How Does Magma Originate and Change? Continental plate subduction Hot Spots and the Origin of Magma. CONTINENTAL PLATE SUBDUCTION Mafic to felsic intrusives Mafic to felsic extrusives Mafic to felsic extrusives & intrusives Continental margin volcano Subduction zone Oceanic crust ic ere an h ce sp O tho li Co nt Co inen ta nt lc in ru lit ent st ho al sp ma he re ntle Igneous Rocks: Igneous activity vs plate tectonics Hot Spot volcano, Hawaii Magmas can originate from hot spot activity Rising plumes in the ocean create volcanic islands far from plate boundaries. Flood basalts, huge thick flat-lying layers of basalt, originate this way. HotHot-spot volcanism HOT-SPOT VOLCANISM Hot-spot volcano Basaltic extrusives Basaltic intrusives Basaltic extrusives & intrusives Mantle plume (hot spot) Mantle Igneous Rocks: Igneous activity vs plate tectonics 9 How Does Magma Originate and Change? Igneous Rocks: Igneous activity vs plate tectonics Geomagmatic systems of Earth Processes That Bring About Compositional Changes in Magma 1. Crystal settling - the first formed ferromagnesian minerals can settle to the base of a magma chamber or body, withdrawing some of the iron and magnesium and enriching the remaining magma in other elements. During crystallization, the remaining melt becomes progressively more silicaenriched. Fig. 4.7a, p. 92 Igneous Rocks 4 Magmatic differentiation, example THE PALISADES INTRUSION How Does Magma Originate and Change? Processes That Bring About Compositional Changes in Magma 2. Assimilation of country rock, ~1200°C ~600°C Plagioclase feldspar Plagioclase feldspar Olivine crystals Pyroxene Pyroxene Olivine Magma with composition A Magma with composition B Olivine crystallizes first. Pyroxene and plagioclase feldspar crystallize. Olivine Plagioclase feldspar Magma with composition C Magma with composition C A gradient of pyroxene and feldspar is established. Plagioclase feldspar continues to crystallize. Igneous Rocks 4 Magmatic differentiation Partial melting creates a magma of a particular composition. Cooling causes minerals to crystallize and settle. Crystallizing minerals Magma chamber A Magma chamber A Magma chamber B Partial melting of country rock A basaltic magma chamber breaks through. Mixing results in andesitic magma. Basaltic magma Crystals may accumulate on the sides and roof of the chamber due to turbulence. sometimes by stoping, can change a magma's composition, but usually only to a limited extent. 3. Magma mixing can also result in compositional change. Two different magmas can blend together (mix) if their viscosities are very similar, or they can mingle without blending if their viscosities are very different. Fig. 4.8, p. 93 Igneous Rocks Their Characteristics and Classification Igneous Rock Textures Minerals begin to crystallize from magma and lava after small crystal nuclei form and grow. Two broad groups based on texture are: volcanic (extrusive) rocks which have an aphanitic texture plutonic (intrusive) rocks which have a phaneritic texture. Rocks with more complex cooling histories are characterized by porphyritic textures. 10 Igneous Rocks: Magmatic intrusions Igneous Rocks: Magmatic intrusions Rising magma wedges open overlying country rock. Country rock Country rock Batholith intrusions Igneous Rocks: Magmatic intrusions Rising magma wedges open overlying country rock. The magma melts the surrounding rock... Country rock The magma melts the surrounding rock... …which changes the magma’s composition. Country rock Igneous Rocks: Magmatic intrusions Rising magma wedges open overlying country rock. Igneous Rocks: Magmatic intrusions Rising magma wedges open overlying country rock. The magma melts the surrounding rock... …which changes the magma’s composition. Igneous Rocks Their Characteristics and Classification Igneous Rock Textures Rapid cooling typifies volcanic rock and produces aphanitic textures. Country rock Slow cooling of plutonic magmas produces phaneritic textures with mineral grains that are easily visible without magnification. The magma also breaks off xenoliths that sink into the magma. Porphyritic textures are characteristic of rocks with complex cooling histories and contain mineral grains of different sizes. Other igneous rock textures include vesicular, glassy, and pyroclastic. 11 Igneous Rocks Their Characteristics and Classification Composition of Igneous Rocks Phenocrysts Mg-Fe-rich magma is called “mafic,” Gabbros and basalts are products of mafic magmas Na, K, Al and H²O rich magma is “felsic.” Granites and rhyolites are products of felsic magmas Most magma, like most minerals, consists of silicon and oxygen with lesser amounts of other elements, such as magnesium (Mg), iron (Fe), sodium (Na), potassium (K), and aluminum (Al). Stepped Art Fig. 4.10, p. 93 Fig. 4-9, p. 93 Igneous Rocks Their Characteristics and Classification Igneous Rocks Their Characteristics and Classification Composition of Igneous Rocks Classifying Igneous Rocks Depending on whether magma erupts, it can produce plutonic (deep-seated) or volcanic (eruptive) rocks. It is best to learn the different kinds of igneous rocks as pairs of equivalent plutonic and volcanic compositions; that is, each plutonic rock has its volcanic compositional equivalent, and visa versa. For example, gabbro is chemically equivalent to basalt. Gabbro is a plutonic rock. Basalt is a volcanic rock. After geologists identify a rock as plutonic or volcanic, the name it is given depends on the relative percentages of certain key minerals, such as quartz, orthoclase, plagioclase, olivine, and pyroxene. . Igneous Rocks Their Characteristics and Classification Igneous Rocks Their Characteristics and Classification Classifying Igneous Rocks Classifying Igneous Rocks Ultramafic Rocks Peridotite is an ultramafic rock, meaning that it contains more iron and magnesium than basalt and gabbro which are mafic. Its composition is close to that of the upper mantle. Fig. 4.10, p. 93 Peridotite Gabbro/basalt, and granite/rhyolite are pairs of common rock types They represent two ends of the compositional range of most igneous rocks at Earth’s surface. Gabbro/basalt are mafic Granite/rhyolite are felsic Fig. 4.12b, p. 95 Fig. 4.14b, p. 96 Figure 4.11, p. 95 12 Igneous Rocks Their Characteristics and Classification Igneous Rocks Their Characteristics and Classification Classifying Igneous Rocks Classifying Igneous Rocks Basalt-Gabbro – Mafic composition Rhyolite-Granite – Felsic composition Gabbro is chemically equivalent to basalt. Gabbro is a plutonic rock. Basalt is a volcanic rock. Granite is chemically equivalent to rhyolite. Granite is a plutonic rock. Rhyolite is a volcanic rock. Fig. 4.12, p. 95 Fig. 4.14, p. 96 Igneous Rocks Their Characteristics and Classification Classifying Igneous Rocks Igneous Rocks Their Characteristics and Classification It is best to learn the different kinds of igneous rocks as pairs of equivalent plutonic and volcanic compositions Classifying Igneous Rocks Andesite-Diorite – Intermediate composition Diorite is chemically equivalent to andesite. Diorite is a plutonic rock. Andesite is a volcanic rock. Rhyolite porphyry Andesite porphyry Basalt Diorite Gabbro Granite Fig. 4.13, p. 96 Igneous Rocks Their Characteristics and Classification Igneous Rocks Their Characteristics and Classification Classifying Igneous Rocks Other Igneous Rocks Classifying Igneous Rocks Pegmatite Pegmatite is a rock type closely related to granite that contains many minerals not ordinarily found in other igneous rocks. By definition, pegmatites contain crystals that measure at least 1 cm across. Typically they form in waterrich magmas Fig. 4.12, p. 95 Fig. 4.13, p. 96 Fig. 4.14, p. 96 Tuff Composed of volcanic ash. Obsidian Composed of volcanic glass. Pumice & Scoria Pegmatite Composed of volcanic glass Vesicular 4.15b, p. 97 13 Intrusive Igneous Bodies - Plutons Forms of magmatic intrusion Plutons are bodies of igneous rock which have been intruded in country rock or have formed in place far beneath the surface. Concordant plutons include sills and laccoliths. Discordant plutons include dikes, volcanic necks, batholiths and stocks. Each of these plutons is defined by volume and geometry. Geo-inSight 1., p. 100 Igneous Rocks: Magmatic intrusions Volcanic neck with radiating dikes Stock Ash falls and pyroclasts Volcano Volcanic neck with radiating dikes Stock Dike Sill Sill Sill Sill to n Plu to n Plu lith tho Ba lith tho Ba Igneous Rocks: Magmatic intrusions Lava flow Sill Volcanic neck with radiating dikes Sill Dikes cut across layers of country rock… Dik e Sill Sill Ash falls and pyroclasts Volcano Stock Dik e Stock Country rock Dik e Volcanic neck with radiating dikes Sill to n Plu lith tho Ba Dikes cut across layers of country rock… Sill Dik e Volcano Dike Country rock Igneous Rocks: Magmatic intrusions Ash falls and pyroclasts Dike Lava flow Dikes cut across layers of country rock… Sill Dik e Dike Sill Country rock Dik e Volcano Lava flow Dik e Country rock Igneous Rocks: Magmatic intrusions Ash falls and pyroclasts Dik e Lava flow to n Plu …but sills run parallel to them. lith tho Ba …but sills run parallel to them. Batholiths are the largest forms of plutons, covering at least 100 km2. 14 Intrusive Igneous Bodies - Plutons Dikes and Sills Dikes and sills are the most common sheet-like igneous intrusions. Dikes are discordant features (meaning they cut across layering in the country rock) Sills are concordant (parallel to the rock layers). Geo-inSight 5., p. 101 Igneous Rocks: Magmatic intrusions A sill runs parallel to country rock layers. Sill Igneous Rocks: Magmatic intrusions A sill runs parallel to country rock layers. Sill Sill Igneous Rocks: Magmatic intrusions A sill runs parallel to country rock layers. Sill Dike Igneous Rocks: Magmatic intrusions Igneous Rocks: Magmatic intrusions 15 A sill runs parallel to country rock layers. Sill Sill A dike cuts across layers. A dike cuts across layers. Dike Dike Dike Igneous Rocks: Magmatic intrusions Igneous Rocks: Magmatic intrusions Intrusive Igneous Bodies - Plutons Intrusive Igneous Bodies - Plutons Laccoliths, Volcanic pipes and necks Laccoliths, Volcanic pipes and necks Laccoliths are sill-like bodies with inflated cores. Volcanic pipes are magma-filled, cylindrical feeder channels beneath volcanoes. Pipes can become volcanic necks with deep erosion. Geo-inSight 6. and 7., p. 101 Intrusive Igneous Bodies - Plutons Laccoliths, Volcanic pipes and necks Geo-inSight 3., p. 100 Intrusive Igneous Bodies - Plutons Batholiths and Stocks Dikes, sills, and laccoliths radiate from many volcanic pipes and necks. Geo-inSight 1 ., p. 100, Geo-Focus Fig. 2, p. 102 Batholiths are plutons that have more than 100 km² in area of exposure. Stocks are somewhat smaller plutonic bodies. Geo-inSight 2. & 4. , p. 100 16 How Are Batholiths Intruded into Earth’s Crust? Most are composed of granite. How did we get so much granite in the Earth’s crust? End Why are there so many batholiths? Why are they felsic (silica rich) if the source is mafic or ultramafic? Ideas 1. Assimilation – they melt their way into the crust Problem: insufficient heat to create this much country rock 2. Forceful injection– magma moves the country rock aside and slowly rises Preferred idea 3. Stoping – at shallower depths, the crust is fractured as the magma moves up. Magma replaces the country rock as it moves up. Fig. 4.18, p. 99 17