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Ch 4-Igneous Rocks.pptBBBBBBBBBBBBBBBBBB
- 1. Igneous Rocks Igneous Rocks Chapter4 Chapter 4
- 2. General characteristics General characteristics ofmagma of magma • Igneous rocks form as molten rock Igneous rocks form as molten rock cools and solidifies cools and solidifies • General characteristics of General characteristics of magma: magma: • Parent material of igneous rocks Parent material of igneous rocks • Forms from partial melting of rocks Forms from partial melting of rocks • Magma at surface is called Magma at surface is called lava lava
- 3. General characteristics General characteristics ofmagma of magma • General characteristic of magma General characteristic of magma • Rocks formed from lava = Rocks formed from lava = extrusive extrusive, or , or volcanic rocks volcanic rocks • Rocks formed from magma at depth = Rocks formed from magma at depth = intrusive intrusive, or , or plutonic rocks plutonic rocks
- 4. General characteristics General characteristics ofmagma of magma • The nature of magma The nature of magma • Consists of three components: Consists of three components: – Liquid portion Liquid portion = = melt melt – Solids Solids, if any, are silicate minerals , if any, are silicate minerals – Volatiles Volatiles = dissolved gases in the melt, = dissolved gases in the melt, including water vapor (H including water vapor (H2 2O), carbon dioxide O), carbon dioxide (CO (CO2 2), and sulfur dioxide (SO ), and sulfur dioxide (SO2 2) )
- 5. General characteristics General characteristics ofmagma of magma • Crystallization of magma Crystallization of magma • Cooling of magma results in the Cooling of magma results in the systematic arrangement of ions into systematic arrangement of ions into orderly patterns orderly patterns • The silicate minerals resulting from The silicate minerals resulting from crystallization form in a predictable crystallization form in a predictable order order • Texture - size and arrangement of Texture - size and arrangement of mineral grains mineral grains
- 6. Igneous textures Igneous textures •Texture Texture is used to describe the is used to describe the overall appearance of a rock based overall appearance of a rock based on the size, shape, and arrangement on the size, shape, and arrangement of interlocking minerals of interlocking minerals • Factors affecting crystal size Factors affecting crystal size • Rate of cooling Rate of cooling – Slow rate = fewer but larger crystals Slow rate = fewer but larger crystals – Fast rate = many small crystals Fast rate = many small crystals – Very fast rate forms glass Very fast rate forms glass
- 7. Igneous textures Igneous textures •Factors affecting crystal size Factors affecting crystal size • % of % of silica silica (SiO (SiO2 2) present ) present • Dissolved gases Dissolved gases
- 8. Igneous textures Igneous textures •Types of igneous textures Types of igneous textures • Aphanitic Aphanitic (fine-grained) texture (fine-grained) texture – Rapid rate of cooling Rapid rate of cooling – Microscopic crystals Microscopic crystals – May contain May contain vesicles vesicles (holes from gas (holes from gas bubbles) bubbles) • Phaneritic Phaneritic (coarse-grained) texture (coarse-grained) texture – Slow cooling Slow cooling – Large, visible crystals Large, visible crystals
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- 11. Igneous textures Igneous textures •Types of igneous textures Types of igneous textures • Porphyritic Porphyritic texture texture – Minerals form at different temperatures Minerals form at different temperatures – Large crystals ( Large crystals (phenocrysts phenocrysts) are embedded ) are embedded in a matrix of smaller crystals ( in a matrix of smaller crystals (groundmass groundmass) ) • Glassy Glassy texture texture – Very rapid cooling of lava Very rapid cooling of lava – Resulting rock is called Resulting rock is called obsidian obsidian
- 12. Igneous textures Igneous textures •Types of igneous textures Types of igneous textures • Pyroclastic Pyroclastic texture texture – Fragmental appearance produced by violent Fragmental appearance produced by violent volcanic eruptions volcanic eruptions – Often appear more similar to sedimentary Often appear more similar to sedimentary rocks rocks • Pegmatitic Pegmatitic texture texture – Exceptionally coarse grained Exceptionally coarse grained – Form in late stages of crystallization of Form in late stages of crystallization of granitic magmas granitic magmas
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- 15. Igneous compositions Igneous compositions •Igneous rocks are composed primarily Igneous rocks are composed primarily of silicate minerals of silicate minerals • Dark (or Dark (or ferromagnesian ferromagnesian) silicates ) silicates – Olivine, pyroxene, amphibole, and biotite Olivine, pyroxene, amphibole, and biotite mica mica • Light (or nonferromagnesian) silicates Light (or nonferromagnesian) silicates – Quartz, muscovite mica, and feldspars Quartz, muscovite mica, and feldspars
- 17. Igneous compositions Igneous compositions •Granitic Granitic versus versus basaltic basaltic compositions compositions • Granitic Granitic composition composition – Light-colored silicates Light-colored silicates – Termed Termed felsic felsic ( (fe feldspar and ldspar and si silica) in lica) in composition composition – High amounts of silica (SiO High amounts of silica (SiO2 2) ) – Major constituent of continental crust Major constituent of continental crust
- 18. Igneous compositions Igneous compositions •Granitic Granitic versus basaltic compositions versus basaltic compositions • Basaltic composition Basaltic composition – Dark silicates and calcium-rich feldspar Dark silicates and calcium-rich feldspar – Termed Termed mafic mafic ( (ma magnesium and gnesium and fe ferrum, for rrum, for iron) in composition iron) in composition – Higher dense than granitic rocks Higher dense than granitic rocks – Comprise the ocean floor and many Comprise the ocean floor and many volcanic islands volcanic islands
- 19. Basaltic lava droppinginto the ocean along Kilauea Volcano along Basaltic lava dropping into the ocean along Kilauea Volcano along the southeastern coast of the big island of Hawaii the southeastern coast of the big island of Hawaii
- 20. Igneous compositions Igneous compositions •Other compositional groups Other compositional groups • Intermediate Intermediate (or (or andesitic andesitic) composition ) composition – Contain 25% or more dark silicate minerals Contain 25% or more dark silicate minerals – Associated with explosive volcanic activity Associated with explosive volcanic activity • Ultramafic Ultramafic composition composition – Rare composition that is high in magnesium Rare composition that is high in magnesium and iron and iron – Composed entirely of ferromagnesian Composed entirely of ferromagnesian silicates silicates
- 21. Igneous compositions Igneous compositions •Silica content Silica content as an indicator of as an indicator of composition composition • Exhibits a considerable range in the crust Exhibits a considerable range in the crust – 45% to 70% 45% to 70% • Silica content Silica content influences magma influences magma behavior behavior • Granitic magmas = high silica content Granitic magmas = high silica content and viscous and viscous • Basaltic magmas = much lower silica Basaltic magmas = much lower silica content and more fluid-like behavior content and more fluid-like behavior
- 23. Igneous compositions Igneous compositions •Naming igneous rocks – granitic rocks Naming igneous rocks – granitic rocks • Granite Granite – Phaneritic Phaneritic – Over 25% quartz, about 65% or more Over 25% quartz, about 65% or more feldspar feldspar – Very abundant - often associated with Very abundant - often associated with mountain building mountain building – The term granite includes a wide range of The term granite includes a wide range of mineral compositions mineral compositions
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- 25. Igneous compositions Igneous compositions •Naming igneous rocks – granitic rocks Naming igneous rocks – granitic rocks • Rhyolite Rhyolite – Extrusive equivalent of granite Extrusive equivalent of granite – May contain glass fragments and vesicles May contain glass fragments and vesicles – Aphanitic texture Aphanitic texture – Less common and less voluminous than Less common and less voluminous than granite granite
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- 27. Igneous compositions Igneous compositions •Naming igneous rocks – granitic rocks Naming igneous rocks – granitic rocks • Obsidian Obsidian – Dark colored Dark colored – Glassy texture Glassy texture • Pumice Pumice – Volcanic Volcanic – Glassy texture Glassy texture – Frothy appearance with numerous voids Frothy appearance with numerous voids
- 28. Pumice is veryglassy and sharp, with countless vesicles.
- 29. Igneous compositions Igneous compositions •Naming igneous rocks – intermediate Naming igneous rocks – intermediate rocks rocks • Andesite Andesite – Volcanic origin Volcanic origin – Aphanitic texture Aphanitic texture • Diorite Diorite – Plutonic equivalent of andesite Plutonic equivalent of andesite – Coarse grained Coarse grained
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- 32. Igneous compositions Igneous compositions •Naming igneous rocks – basaltic rocks Naming igneous rocks – basaltic rocks • Basalt Basalt – Volcanic origin Volcanic origin – Aphanitic texture Aphanitic texture – Composed mainly of pyroxene and calcium- Composed mainly of pyroxene and calcium- rich plagioclase feldspar rich plagioclase feldspar – Most common extrusive igneous rock Most common extrusive igneous rock
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- 34. Igneous compositions Igneous compositions •Naming igneous rocks – mafic rocks Naming igneous rocks – mafic rocks • Gabbro Gabbro – Intrusive equivalent of basalt Intrusive equivalent of basalt – Phaneritic texture consisting of pyroxene Phaneritic texture consisting of pyroxene and calcium-rich plagioclase and calcium-rich plagioclase – Significant % of the oceanic crust Significant % of the oceanic crust
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- 36. Igneous compositions Igneous compositions •Naming igneous rocks – Naming igneous rocks – pyroclastic pyroclastic rocks rocks • Composed of fragments ejected during Composed of fragments ejected during a volcanic eruption a volcanic eruption • Varieties Varieties – Tuff Tuff = ash-sized fragments = ash-sized fragments – Volcanic breccia Volcanic breccia = particles larger than ash = particles larger than ash
- 38. Origin of magma Originof magma • Highly debated topic Highly debated topic • Generating magma from solid rock Generating magma from solid rock • Role of heat Role of heat – Temperature increases in the upper crust Temperature increases in the upper crust ( (geothermal gradient geothermal gradient) average between ) average between 20 20o o C C to 30 to 30o o C per kilometer C per kilometer of depth of depth – Rocks in the lower crust and upper mantle Rocks in the lower crust and upper mantle are near their melting points are near their melting points – Any additional heat may induce melting Any additional heat may induce melting
- 39. Origin of magma Originof magma • Role of pressure Role of pressure – Increases in confining pressure cause an Increases in confining pressure cause an increase in a rock’s melting temperature increase in a rock’s melting temperature – When confining pressures drop, When confining pressures drop, decompression melting occurs decompression melting occurs • Role of volatiles Role of volatiles – Volatiles (primarily Volatiles (primarily water water) cause rocks to ) cause rocks to melt at lower temperatures melt at lower temperatures – Important factor where oceanic lithosphere Important factor where oceanic lithosphere descends into the mantle descends into the mantle
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- 41. Evolution of magmas Evolutionof magmas • A single volcano may extrude lavas exhibiting A single volcano may extrude lavas exhibiting very different compositions very different compositions • Each volcanic eruption tends to exhibit a Each volcanic eruption tends to exhibit a unique geochemical fingerprint, defined by unique geochemical fingerprint, defined by trace element percentages trace element percentages • Bowen’s reaction series Bowen’s reaction series • Minerals crystallize in a systematic fashion Minerals crystallize in a systematic fashion based on their melting points based on their melting points • During crystallization, the composition of the During crystallization, the composition of the liquid portion of the magma continually changes liquid portion of the magma continually changes
- 42. Bowen’s reaction series Bowen’sreaction series
- 43. Evolution of magmas Evolutionof magmas • Processes responsible for changing a Processes responsible for changing a magma’s composition magma’s composition • Magmatic differentiation Magmatic differentiation – Separation of a melt from earlier formed Separation of a melt from earlier formed crystals crystals • Assimilation Assimilation – Changing a magma’s composition by the Changing a magma’s composition by the incorporation of surrounding rock bodies incorporation of surrounding rock bodies into a magma into a magma
- 44. Magma evolves asthe Magma evolves as the hotter minerals crystallize hotter minerals crystallize and settle to the bottom of and settle to the bottom of the magma chamber the magma chamber
- 45. Evolution of magmas Evolutionof magmas • Processes responsible for changing a Processes responsible for changing a magma’s composition magma’s composition • Magma mixing Magma mixing – Two chemically distinct magmas may Two chemically distinct magmas may produce a composition quite different from produce a composition quite different from either original magma either original magma
- 46. Assimilation, magma mixing, Assimilation,magma mixing, and magmatic differentiation and magmatic differentiation
- 47. Evolution of magmas Evolutionof magmas • Partial melting and magma formation Partial melting and magma formation • Incomplete melting of rocks is known as Incomplete melting of rocks is known as partial melting partial melting • Formation of basaltic magmas Formation of basaltic magmas – Most originate from partial melting of Most originate from partial melting of ultramafic rock in the mantle at oceanic ultramafic rock in the mantle at oceanic ridges ridges – Large outpourings of basaltic magma are Large outpourings of basaltic magma are common at Earth’s surface common at Earth’s surface
- 48. Evolution of magmas Evolutionof magmas • Partial melting and magma formation Partial melting and magma formation • Formation of andesitic magmas Formation of andesitic magmas – Produced by interaction of basaltic magmas Produced by interaction of basaltic magmas and more silica-rich rocks in the crust and more silica-rich rocks in the crust – May also evolve by magmatic differentiation May also evolve by magmatic differentiation
- 49. Evolution of magmas Evolutionof magmas • Partial melting and magma formation Partial melting and magma formation • Formation of granitic magmas Formation of granitic magmas – Most likely form as the end product of Most likely form as the end product of crystallization of andesitic magma crystallization of andesitic magma – Granitic magmas are more viscous than Granitic magmas are more viscous than other magmas so they tend to lose their other magmas so they tend to lose their mobility before reaching the surface mobility before reaching the surface – Tend to produce large plutonic structures Tend to produce large plutonic structures