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Igneous Rocks Igneous Rocks Characteristics of magma Igneous rocks form as molten rock cools and solidifies Characteristics of magma (molten rock) • Parent material of igneous rocks • Forms from partial melting of rocks inside the Earth • Magma that reaches the surface is lava through a volcano or crack (fissure) Characteristics of magma Molten rock (magma) originates in the mantle and crust as a result of partial melting of solid rock Characteristics of magma General Characteristic of magma • Rocks formed from lava at the surface are classified as extrusive, or volcanic rocks • Rocks formed from magma that crystallizes at depth are termed intrusive, or plutonic rocks Extrusive Igneous Rock - Lava Extrusive Igneous Rock - Lava (Hawaii) Intrusive Igneous Rock Most magma never reaches the surface. Rather it crystallizes at depth to produce intrusive or plutonic igneous rocks Intrusive Igneous Rock (Granite) Intrusive Igneous Rock Although intrusive igneous rocks form at depth, uplifting and erosion often expose them at the surface Characteristics of magma The nature of magma • Consists of three components: – A liquid portion, called melt, that is composed of mobile ions – Solids, if any, are silicate minerals that have already crystallized from the melt – Volatiles, which are gases dissolved in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2) Characteristics of magma Crystallization of magma • Cooling of magma results in the systematic arrangement of ions into orderly patterns • The silicate minerals resulting from crystallization form in a predictable order Crystallization of minerals in magma bodies Bowen’s reaction series and the composition of igneous rocks • N.L. Bowen demonstrated that as a magma cools, minerals crystallize in a systematic fashion based on their melting points • A listing of the most common silicate minerals in igneous rocks • Different minerals crystallize from magmas at different temperatures Bowen’s reaction series Bowen’s reaction series also lists the minerals in the order in which they melt with increasing temperatures Minerals higher on the reaction series crystallize before minerals lower on the reaction series Bowen’s reaction series has two branches. They are: Discontinuous reaction series, from olivine to biotite Continuous reaction series, from Ca plagioclase to Na plagioclase Bowen’s reaction series The discontinuous reaction series involves the dark-colored ferromagnesian minerals: Olivine Pyroxene Amphibole Biotite Each successive mineral, from olivine to biotite, has a different composition and a different silicate crystal structure Bowen’s reaction series The continuous reaction series involves the plagioclase feldspars Plagioclase feldspars consists of two end members: Albite or Na plagioclase, the sodium end member Anorthite or Ca plagioclase, the calcium end member The series of plagioclase minerals is called continuous because all of the plagioclase minerals have the same crystal structure Bowen’s reaction series Bowen’s reaction series help us to understand why certain minerals tend to occur together in igneous rocks For example, basalt and gabbro tend to contain olivine, pyroxene, and calcium-rich plagioclase feldspar These are all minerals which crystallize at high temperatures Bowen’s reaction series Bowen’s reaction series help us to understand why certain minerals do Not occur together in igneous rocks For example, olivine and quartz are unlikely to occur in the same igneous rock, because olivine is a high temperature, and quartz is a low temperature mineral Bowen’s Reaction Series Granite Hand Sample Order of Crystallization Granite Thin Section Zoned feldspar (plagioclase) showing change in composition with time in magma chamber (calcic in core to sodic at rim) Processes responsible for changing a magma’s )العمليات المسؤولة عن تغير تركيب الماجما ( composition )ترسيب البلورات( Crystal Settling )ادخال مواد خارجية في الحشوة النارية( Assimilation )اختالط الماجما( Magma Mixing Magmatic Differentiation ()تكوين عدة انواع من الصخور The formation of several different rock types from one initial magma, through separation of earlier-formed crystals, causing the magma to evolve to become more silica-rich Magmatic Differentiation Crystal Settling: As a magma cools, the early-formed crystals may settle to the bottom of the magma chamber. This would produce a rock type at the bottom of the magma chamber that is dominated by early-formed minerals such as olivine, pyroxene, and calcic plagioclase Crystallized minerals have a density greater than the magma and settle to the bottom due to gravity Because Fe and Mg are first removed, melt becomes rich in SiO2, Na, and K Magmatic Differentiation Assimilation: Magma reacts with the “country rock” ( )صخر مضيفwhich is adjacent to the magma chamber ()غرفة الصهارة Magma composition is altered according to the composition of the assimilated country rock Inclusions are rocks incompletely melted chunks of country rock Magmatic Differentiation Magma Mixing: Magmas of different compositions are mixed together Resulting magma is of a composition intermediate between the parents Magma Mixing Magma Mixing Characteristics of magma ()خصائص الماجما Crystallization of magma ()تبلور الماجما • Texture in igneous rocks is determined by the size and arrangement of mineral grains • Igneous rocks are typically classified by both ()تصنيف الصخور النارية يعتمد على: – Texture ( )النسيج – Mineral composition ()التركيب المعدني Igneous textures () انسجة الصخور النارية Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals ()معادن متداخلة Factors affecting crystal size ( العوامل التي تؤثر )على حجم البلورات • Rate of cooling ()معدل التبريد – Slow rate promotes the growth of fewer but larger crystals (thousands to millions of years) Igneous textures () انسجة الصخور النارية Factors affecting crystal size • Rate of cooling ()معدل التبريد – Fast rate forms many small crystals (hours, days, weeks) – Very fast rate forms glass (seconds, minutes) • Amount of silica (SiO2) present ()نسبة السليكا • Amount of dissolved gases ()كمية الغازات Types of Igneous textures ()انواع انسجة الصخور النارية Types of igneous textures • Aphanitic (fine-grained) texture ()صخور نارية دقيقة النسيج – Rapid rate of cooling of lava or magma – Microscopic crystals – May contain vesicles (( )الفتحات الناتجة عن تسرب الغازاتholes from gas bubbles in the lava or magma) – Very porous; may resemble a sponge; low density; may float on water • Phaneritic (coarse-grained) texture ()صخور نارية خشنة النسيج – Slow cooling – Crystals can be identified without a microscope Aphanitic texture Phaneritic texture Igneous textures Types of igneous textures • Porphyritic texture ()صخور نارية بورفيرية النسيج – Minerals form at different temperatures as well as differing cooling rates – Large crystals, called phenocrysts ()حبيبات كبيرة, are embedded in a matrix of smaller crystals, called the groundmass ()حبيبات ناعمة من الصخور النارية • Glassy texture ()صخور نارية زجاجية النسيج – Very rapid cooling of molten rock – Resulting rock is called obsidian Porphyritic texture Porphyritic texture Granite Glassy texture Obsidian Types of Igneous textures Types of igneous textures • Pyroclastic texture ()صخور نارية فتاتيه نارية النسيج – Various fragments ejected during a violent volcanic eruption – Textures often appear to more similar to sedimentary rocks • Pegmatitic texture ()صخور نارية متباينة النسيج – Exceptionally coarse grained (many over 2 cm) – Form in late stages of crystallization of granitic magmas – Granite pegmatite or pegmatitic granite Pyroclastic Rock Pegmatitic texture Igneous Compositions ()التركيب المدني Igneous rocks are composed primarily of silicate minerals that include: • Dark (or ferromagnesian) silicates ()معادن سليكاتية داكنة اللون – Olivine – Pyroxene – Amphibole – Biotite mica Igneous Compositions ()التركيب المدني Igneous rocks are composed primarily of light silicate minerals that include: ()معادن سليكاتية ناصعة اللون – Quartz – Muscovite mica – Feldspars Igneous Compositions ()التركيب المدني Igneous compositions ()التركيب المدني Granitic versus basaltic compositions • Granitic composition – Composed of light-colored silicates – Designated as being felsic (feldspar and silica) in composition – Contains high amounts of silica (SiO2) – Major constituents of continental crust Igneous compositions Naming igneous rocks – granitic (felsic) rocks • Granite – Phaneritic – Over 65 percent quartz, about 25 percent or more feldspar – Abundant and often associated with mountain building – The term granite covers a wide range of mineral compositions Granite Granite rocks contain 70% silica and are abundant in the continental crust Granite Igneous compositions Naming igneous rocks – granitic (felsic) rocks • Rhyolite – May contain glass fragments and vesicles – Aphanitic texture – Less common and less voluminous than granite – Phenocrysts can include quartz and feldspar Rhyolite Igneous compositions Granitic versus basaltic compositions • Basaltic composition – Composed of dark silicates and calcium-rich feldspar – Designated as being mafic (magnesium and ferrum, for iron) in composition – More dense than granitic rocks – Comprise the ocean floor as well as many volcanic islands Igneous compositions Naming igneous rocks – basaltic (mafic) rocks • Basalt – Volcanic origin – Aphanitic texture – Composed mainly of pyroxene and calciumrich plagioclase feldspar – Most common extrusive igneous rock Basalt Basalt Basaltic rocks are rich in dark minerals and contain about 50% silica. Basalt makes up the ocean floor as well as lava flows and volcanic structures. Basalt (Scoria with Gas Bubbles) Basalt (Scoria with Gas Bubbles) Igneous compositions Naming igneous rocks – basaltic (mafic) rocks • Gabbro – Intrusive equivalent of basalt – Phaneritic texture consisting of pyroxene and calcium-rich plagioclase – Makes up a significant percentage of the oceanic crust Gabbro - a mafic Igneous rock Igneous compositions Other compositional groups • Intermediate (or andesitic) composition – Contain at least 25 percent dark silicate minerals – Associated with explosive volcanic activity • Ultramafic composition – Rare composition that is high in magnesium and iron – Composed entirely of ferromagnesian silicates Igneous compositions Naming igneous rocks – intermediate rocks • Andesite – Volcanic origin – Aphanitic texture – Often resembles rhyolite Andesite Igneous compositions Naming igneous rocks – intermediate rocks • Diorite – Plutonic equivalent of andesite – Coarse grained – Intrusive – Composed mainly of intermediate feldspar and amphibole Diorite Igneous compositions Silica content influences a magma’s behavior • Granitic magma – High silica content – Extremely viscous – Liquid exists at temperatures as low as 700oC – Goes Boom when it erupts Igneous compositions Silica content influences a magma’s behavior • Basaltic magma – Much lower silica content – Fluid-like behavior – Crystallizes at higher temperatures Summary - Igneous Rock Classification Igneous compositions Naming igneous rocks – granitic (felsic) rocks. Extrusive products can include: • Obsidian – Dark colored – Glassy texture • Pumice – Volcanic – Glassy texture – Frothy appearance with numerous voids (extrusive foam) Obsidian Pumice Igneous compositions Naming igneous rocks – pyroclastic rocks • Composed of fragments ejected during a volcanic eruption • Varieties – Tuff – ash-sized fragments (less than 2 mm) – Volcanic breccia – particles larger than ash (greater than 2 mm) Ash and pumice layers Summary - Igneous Rock Texture and Composition FORMS OF INTRUSIVE IGNEOUS ROCKS ()اشكال الصخور النارية • Commonly observed forms of Plutonic (Subsurface igneous bodies) rocks observed in the field are: dykes, sills, laccoliths, batholiths and Stock. • Based on the attitudes of the associated country rocks the forms are called either as Concordant or Discordant. • Concordant: Oriented parallel to surrounding layered rocks. Formed from magma that is injected between the layers of the surrounding rock. • Discordant:Oriented at an angle to surrounding layered rocks. Magmas cut through the existing layering FORMS OF INTRUSIVE IGNEOUS ROCKS Dykes • They are discordant • Cut across the bedding of the rocks in which they intrude • Vertical to steeply inclined and sheetlike body • Thickness vary widely from less than 1 cm to 1 km • Injected through fractures, and joints Dykes Mafic dyke SILLS Sills are relatively thin tabular sheetlike body that penetrates parallel to the bedding planes. Laterally it may extends for 100s of km and up to 10 km in width. Mostly sills are made up of gabbros, dolerites and basalts Sills limestone sandstone mudstone LACCOLITHS: It is a concordant body, with flat bottom and convex upward. It is dome shaped. When viscous magma is injected rapidly along the bedding, as it cannot spreads it pushes up the overlying layers and keep on piling up. It causes folding of the overlying rock layers. BATHOLITHS: are the largest kind of plutons, irregular in shape and occupies large area. Their occurrence is commonly associated with the mountain-building process More than 100 km2 These are either granites or granodiorites in composition Stocks: Are smaller irregular bodies with 10 km in maximum dimension, and are associated with batholiths.