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Igneous Rocks Origin of Magma Highly debated topic Generating magma from solid rock • Produced from partial melting of rocks in the crust and upper mantle • Role of heat – Temperature increases within Earth’s upper crust (called the geothermal gradient) average between 20oC to 30oC per kilometer Approximate temperatures in the crust and mantle Deep Potosi Mine Anecdote Clicker Question 4.1 If geothermal gradient is 25 degrees C per km, and temperature at surface is 10 degrees C, What is temperature at a depth of 30 km? A) B) C) D) E) 660 deg C 760 deg C 860 deg C 960 deg C 1060 deg C Origin of Magma • Role of heat – Rocks in the lower crust and upper mantle are near their melting points – Any additional heat (from rocks descending into the mantle or rising heat from the mantle) may induce melting 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 Decompression melting Origin of Magma • Role of volatiles – Volatiles (primarily water) cause rocks to melt at lower temperatures – This is particularly important where oceanic lithosphere descends into the mantle Evolution of magmas Processes responsible for changing a magma’s composition • Magmatic differentiation – Separation of a melt from earlier formed crystals to form a different composition of magma • Assimilation – Changing a magma’s composition by the incorporation of foreign matter (surrounding rock bodies) into a magma Assimilation and magmatic differentiation Which process is likely to change a magmas chemical composition to a chemistry more similar to the host rock? A. B. C. D. Assimilation Differentiation Crystallization None of above Evolution of magmas Processes responsible for changing a magma’s composition • Magma mixing – Involves two bodies of magma intruding one another – Two chemically distinct magmas may produce a composition quite different from either original magma 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 by 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 of basaltic magma are common at Earth’s surface (Deccan Traps) – The presence of water promotes partial melting of the mantle to create basalt Evolution of magmas Partial melting and magma formation • Formation of andesitic magmas – 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 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 (Pikes Peak, Rocky Mtn Natl Park) 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 Geologic Environments Where Igneous Rocks Form Igneous Rocks Formed in Rift Igneous Rocks Formed Above Sinking Plate Clicker Question 4.2: How do you melt rock? A) Lower pressure B) Raise temperature C) Add H20 D) B & C E) A, B, & C 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 Is this an extrusive, or intrusive igneous rock? A. Extrusive B. Intrusive Intrusive Igneous Rock (Granite) - Sorry no pictures available from 25 kilometers down! 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 Olivene sand on the worlds only “Green Sand” beach on the southern Big Island Hawaii. Olivene is derived from erosion of olivene crystals in volcanic rock. 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 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) Which mineral first? Which mineral do crystallized you think crystallized first? Orthoclase A) A) Feldspar (Plagioclase) Quartz B) B) Quartz Olivene C) C) Hornblende Characteristics of magma Crystallization of magma • Texture in igneous rocks is determined by the size and arrangement of mineral grains • Igneous rocks are typically classified by both: – 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 Factors affecting crystal size • Rate of cooling – Slow rate promotes the growth of fewer but larger crystals Igneous textures Factors affecting crystal size • Rate of cooling – Fast rate forms many small crystals – Very fast rate forms glass • Amount of silica (SiO2) present • Amount of dissolved gases Hint: What are the small grey minerals? Qtz, Plag or Biotite? The big one is Plagioclase Assuming little change in bulk chemistry or dissolved volatiles, what does the texture of this rock tell you about its cooling history? A. B. C. D. E. It cooled evenly deep in the Earth It cooled slowly first (deep), and much faster later (shallow) It first cooled quickly, then cooled slowly It was heated and never cooled None of the above Types of Igneous textures Types of igneous textures • Aphanitic (fine-grained) texture – Rapid rate of cooling of lava or magma – Microscopic crystals – May contain vesicles (holes from gas bubbles) • Phaneritic (coarse-grained) texture – Slow cooling – Crystals can be identified without a microscope Aphanitic texture Phaneritic texture Igneous textures Types of igneous textures • Porphyritic texture – 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 Granite Glassy texture Obsidian Yet more types of Igneous textures Types of igneous textures • Pyroclastic texture – Various fragments ejected during a violent volcanic eruption – Textures often appear to more similar to sedimentary rocks • Pegmatitic texture – Exceptionally coarse grained – Form in late stages of crystallization of granitic magmas Pyroclastic Rock - Superheated Superfast Flows A beautiful pegmatite vein of orthoclase and quartz Igneous Compositions Igneous rocks are composed primarily of silicate minerals that can include: • Dark (or ferromagnesian) silicates – Olivine – Pyroxene – Amphibole – Biotite mica Igneous Compositions Igneous rocks are composed primarily of light silicate minerals that include: – Quartz – Muscovite mica – Feldspars Igneous compositions 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 Igneous compositions Naming igneous rocks – granitic (felsic) rocks • Rhyolite – Extrusive chemical equivalent of granite – 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 (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 – Contains 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 (intrusive) equivalent of andesite – Coarse grained – Composed mainly of intermediate feldspar and amphibole Diorite Clicker Questions 4.3 Name the aphanitic volcanic rock that is probably basalt. C) A) D) B) Name the phaneritic intrusive igneous rock that is granite. A) C) D) B) Name the aphanitic volcanic rock that is probably rhyolite. C) A) D) B) 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 (Yellowstone) Igneous compositions Silica content influences a magma’s behavior • Basaltic magma – Much lower silica content – Fluid-like behavior – Crystallizes at higher temperatures – Gurgles when it erupts (Hawaii) Comparison of Igneous Rocks Summary - Igneous Rock Classification Igneous Textures 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 Frothy appearance with numerous voids (extrusive foam) Guinness at Night Head of gas bubbles on a pint of Guinness is a perfect analog for pumice (and it tastes a lot better!) Clicker Question B) Name rock Name the the intrusive following igneous rock types. List two key characteristics that A) identify each specimen Gabbro Obsidian Pumice C) D) A) and B) Igneous compositions Naming igneous rocks – pyroclastic rocks • Composed of fragments ejected during a volcanic eruption • Varieties – Tuff – ash-sized fragments – Volcanic breccia – particles larger than ash Ash and pumice layers Summary - Igneous Rock Texture and Composition