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Engineering Geology CVL 3315 Chapter 3 Igneous Rock Dr. Sari Abusharar University of Palestine Faculty of Applied Engineering and Urban Planning Civil Engineering Department 2nd Semester 2014-2015 1 Outline of Presentation Magma: The Parent Material of Igneous Rock Igneous Textures Classification of Igneous Rocks Igneous Compositions Origin of Magma Evolution of Magmas Intrusive Igneous Activity 2 3 General characteristics of magma Igneous rocks form as molten rock cools and solidifies General Characteristic of magma • Parent material of igneous rocks • Forms from partial melting of rocks inside the Earth • Magma that reaches the surface is called lava Magma vs. Lava 5 General characteristics of magma General Characteristic of magma • Rocks formed from lava at the surface are classified as extrusive سطحية, or volcanic rocks • Rocks formed from magma that crystallizes at depth are termed intrusive تحت سطحية, or plutonic rocks صخور جوفية General 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) General characteristics of magma Crystallization of magma • Cooling of magma results in the systematic arrangement of ions into orderly patterns • Texture in igneous rocks is determined by the size and arrangement of mineral grains • Igneous rocks are typically classified by – Texture – Mineral composition Igneous textures Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals Igneous textures Factors affecting crystal size • Rate of cooling – Slow rate promotes the growth of fewer but larger crystals – Fast rate forms many small crystals – Very fast rate forms glass • Amount of silica (SiO2) present • Amount of dissolved gases Among these, the rate of cooling tends to be the dominant factor. Igneous textures Types of igneous textures • Aphanitic (نسيج دقيقfine-grained) texture – Rapid rate of cooling of lava or magma – Microscopic crystals – May contain vesicles ( فجواتholes from gas bubbles) • Phaneritic (نسيج خشنcoarse-grained) – Slow cooling – Large, visible crystals – Crystals can be identified without a microscope texture Comparison of aphanitic (fine-grained) and phaneritic (coarse-grained) igneous rock textures. Felsic Rock فلسي A term used to describe an igneous rock that has a large percentage of lightcolored minerals such as quartz, feldspar, and muscovite. Also used in reference to the magmas from which these rocks crystallize. Felsic rocks are generally rich in silicon and aluminum and contain only small amounts of magnesium and iron. Granite and rhyolite are examples of felsic rocks. Granite Intermediate Rock An igneous rock that has an intermediate silica content. Examples are syenite and diorite Diorite Mafic Rock A term used to describe an igneous rock that has a large percentage of darkcolored minerals such as amphibole, pyroxene and olivine. Also used in reference to the magmas from which these rocks crystallize. Mafic rocks are generally rich in iron and magnesium Gabbro Igneous textures Types of igneous textures • Porphyritic بورفيريtexture – Minerals form at different temperatures as well as differing rates – Large crystals, called phenocrysts, are embedded in a matrix of smaller crystals, called the groundmass • Glassy texture – Very rapid cooling of molten rock – Resulting rock is called obsidian Porphyritic texture Glassy Texture - Obsidian Igneous textures Types of igneous textures • Pyroclastic فتاتيtexture – Various fragments ejected during a violent volcanic eruption – Textures often appear to more similar to sedimentary rocks • Pegmatitic خشن جداtexture – Exceptionally coarse grained – Form in late stages of crystallization of granitic magmas Mineralogy of common igneous rocks and the magmas from which they form Igneous rock textures نسيج زجاجي Pumice نسيج فتاتي Ignimbrite نسيج أفانيتي ) دقيق/خفي التحبب( Basalt نسيج بورفيري Andesite porphyry نسيج فانيري )خشن/مرئي( Granite Classification of major igneous rocks based on mineral composition and texture Common igneous rocks Igneous Compositions Igneous rocks are composed primarily of silicate minerals • Dark (or ferromagnesian) silicates – Olivine, Pyroxene, Amphibole, Biotite mica • Light (or nonferromagnesian) silicates – Quartz, Muscovite mica, Feldspars Igneous compositions Granitic versus basaltic compositions • Granitic composition – Composed of light-colored silicates – Designated as being felsic ( فلسيfeldspar and silica) in composition – Contains high amounts of silica (SiO2) – Major constituents of continental crust 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 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 Silica content as an indicator of composition • Silica content in crustal rocks exhibits a considerable range – A low of 45 percent in ultramafic rocks – Over 70 percent in felsic rocks Igneous compositions Silica content influences a magma’s behavior • Granitic magma – High silica content – Extremely viscous – Liquid exists at temperatures as low as 700oC • Basaltic magma – Much lower silica content – Fluid-like behavior – Crystallizes at higher temperatures Igneous compositions Naming igneous rocks – granitic (felsic) rocks • Granite – Phaneritic خشن – Over 25 % quartz, about 65 % or more feldspar – May exhibit a porphyritic texture سماقي/نسيج بورفيري – Very abundant as it is often associated with mountain building – The term granite covers a wide range of mineral compositions Granite Igneous compositions Naming igneous rocks – granitic (felsic) rocks • Rhyolite ريوليت – Extrusive equivalent of granite – May contain glass fragments and vesicles – Aphanitic texture – Less common and less voluminous than granite Rhyolite Igneous compositions Naming igneous rocks – granitic (felsic) rocks • Obsidian أوبسيديان – Dark colored – Glassy texture • Pumice بيوميس – الحجر الخفاف – Volcanic – Glassy texture – Frothy appearance with numerous voids Obsidian Pumice Igneous compositions Naming igneous rocks – intermediate rocks • Andesite أنديزيت – Volcanic origin – Aphanitic texture – Often resembles rhyolite Andesite Igneous compositions Naming igneous rocks – intermediate rocks • Diorite ديوريت – Plutonic equivalent of andesite – Coarse grained – Intrusive – Composed mainly of intermediate feldspar and amphibole Diorite Igneous compositions Naming igneous rocks – basaltic (mafic) rocks • Basalt بازلت – Volcanic origin – Aphanitic texture – Composed mainly of pyroxene and calciumrich plagioclase feldspar – Most common extrusive igneous rock Basalt (Scoria )اسكوريا Igneous compositions Naming igneous rocks – basaltic (mafic) rocks • Gabbro جابرو – Intrusive equivalent of basalt – Phaneritic texture consisting of pyroxene and calcium-rich plagioclase – Makes up a significant percentage of the oceanic crust Gabbro Igneous compositions Naming igneous rocks – pyroclastic rocks • Composed of fragments ejected during a volcanic eruption • Varieties – Tuff –طفash-sized fragments (less than 2 mm) – Volcanic breccia –بريشيا بركانيةparticles larger than ash (greater than 2 mm) Ash and pumice layers Origin of Magma Generating magma from solid rock • Produced from partial melting of rocks in the crust and upper mantle Origin of Magma • Role of heat – Temperature increases within Earth’s upper crust (called the geothermal gradient) average 25oC per kilometer – Rocks in the lower crust and upper mantle are near their melting points. – Additional heat may induce melting. A Typical Geothermal Gradient تدرج حرارة األرض Origin of Magma • Role of pressure – An increase in confining pressure causes an increase in a rock’s melting temperature or conversely, reducing the pressure lowers the melting temperature – When confining pressures drop, decompression melting occurs انصھار عند تخفيض الضغط A Typical Geothermal Gradient decompression melting Role of volatiles Evolution of magmas • A single volcano may extrude lavas exhibiting very different compositions • Magma become the source of a variety of igneous rocks. • The crystallization of magma (basaltic magma) occurs over a range of at least 200°C. Evolution of magmas Bowen’s reaction series • N.L. Bowen demonstrated that as a magma cools, minerals crystallize in a systematic fashion based on their melting points • During crystallization, the composition of the liquid portion of the magma continually changes. Evolution of magmas Bowen’s reaction series • At the stage when about a third of the magma has solidified, the melt will be nearly depleted ()استنزفof iron, magnesium and calcium because these elements are major constituents of the earliest-formed minerals. • The removal of these elements causes the melt to become enriched in sodium and potassium. Bowen’s Reaction Series Bowen’s reaction series shows the sequence in which minerals crystallize from a magma Evolution of magmas Processes responsible for changing a magma’s composition • Magmatic differentiation تمايز صھارى • Assimilation تمثل • Magma mixing Evolution of magmas Processes responsible for changing a magma’s composition • Magmatic differentiation تمايز – Separation of a melt from earlier formed crystals to form a different composition of magma – أى عملية تسبب انفصال الصھارة أثناء برودتھا إلى طورين سائلين أو أكثر . لھما تركيب مختلف Magmatic differentiation Evolution of magmas Processes responsible for changing a magma’s composition • Assimilation تمثل – Changing a magma’s composition by the incorporation of foreign matter (surrounding rock bodies) into a magma. مثل أجزاء، – ابتالع الصھارة ألجسام صلبة أو سائلة غريبة وھضمھا وتعرف الصھارة أو الصخر الناتج عنھا بعد. من الصخر المضيف . التمثيل بالصھارة أو الصخر الھجين Evolution of magmas Processes responsible for changing a magma’s composition • Magma mixing – Two chemically distinct magmas may produce a composition quite different from either original magma. Magmatic differentiation, assimilation, and magma mixing Evolution of magmas Partial melting and magma formation • Incomplete melting of rocks is known as partial melting • Formation of basaltic magmas – Most originate from partial melting of ultramafic rock in the mantle – Basaltic magmas form at mid-ocean ridges ( )حيد وسط المحيطby decompression melting or at subduction zones Evolution of magmas Partial melting and magma formation • Formation of basaltic magmas – As basaltic magmas migrate upward, confining pressure decreases which reduces the melting temperature – Large outpourings ( )تدفقاتof basaltic magma are common at Earth’s surface Formation of basaltic magmas Basaltic magmas Evolution of magmas Partial melting and magma formation • Formation of andesitic magmas الماجما األنديزيتية – Interactions between mantle-derived basaltic magmas and more silica-rich rocks in the crust generate magma of andesitic composition – Andesitic magma may also evolve by magmatic differentiation SiO2 وتتميز بأن نسبة السيليكا، – أحد ثالثة أنواع معروفة من الصھارة . من وزنھا%٦٠ بھا تكون حوالى Formation of andesitic magmas andesitic magmas Evolution of magmas Partial melting and magma formation • Formation of granitic magmas – Most likely form as the end product of crystallization of andesitic magma – Granitic magmas are higher in silica and therefore more viscous than other magmas – Because of their viscosity, they lose their mobility before reaching the surface – Tend to produce large plutonic structures Formation of granitic magmas granitic magmas Intrusive Igneous Activity Most magma is emplaced at depth in the Earth. • An underground igneous body, once cooled and solidified, is called a pluton سحيقي. Classification of plutons • Shape – Tabular (sheet-like) صفيحي – Massive كتلي Intrusive Igneous Activity Classification of plutons • Orientation with respect to the host (surrounding) rock – Discordant —غير متفقcuts across sedimentary rock units – Concordant — متطابقparallel to sedimentary rock units Intrusive Igneous Activity Types of intrusive igneous features • Dike( —)قاطعa tabular()مسطح, discordant pluton Dike — a tabular, discordant pluton صخر ناري متداخل متوسط التحبب فريشى الشكل عادة ،له امتداد أفقى واسع ومحدود السمك ويقطع أسطح طباقية الصخور الحاوية له Intrusive Igneous Activity Types of intrusive igneous features • Sill سدود موازية —a tabular, concordant pluton (e.g., Palisades Sill in New York) . حقن موازيا ألسطح الطباقية، • محقون ناري نضيدى Intrusive Igneous Activity Types of intrusive igneous features • Laccolith الكوليث – Similar to a sill – Lens or mushroom-shaped mass – Arches overlying strata upward تشبه القبة أو نبات عيش، • كتلة متداخلة من الصخر النارى ويكون، تسبب تقوس الصخور الرسوبية فوقھا، الغراب . ً مقطعھا دائرى وقاعھا مسطح تقريبا Intrusive Igneous Activity Types of intrusive igneous features باثوليث • Batholith – Largest intrusive body – Surface exposure of 100+ square kilometers )(smaller bodies are termed stocks – Frequently form the cores of mountains • كتلة صخرية ضخمة غير منتظمة الشكل تقطع صخور اإلقليم ،ويظھر منھا على السطح ١٠٠كم ٢على األقل دون مالحظة قاع تلك الكتلة . تتكون عادة من صخور نارية متداخلة )غالبا الصخور الجرانيتية( ،ولكن قد تتكون أحيانا من صخور اإلقليم تحت تأثير حرارة وضغط مرتفع. Granitic batholiths that occur along the western margin of North America. These gigantic, elongated bodies consist of numerous plutons that were emplaced during the last 150 million years of Earth’s history End of Chapter 3