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Unveiling Igneous Rocks: Characteristics, Textures, & Compositions

Dive into the world of igneous rocks with insights on their formation, textures, compositions, and naming conventions. Understand how magma transforms into various rock types and their distinct features.

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Unveiling Igneous Rocks: Characteristics, Textures, & Compositions

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  1. Igneous RocksChapter 4

  2. Igneous Rocks • Igneous rocks form as molten rock (magma) cools and solidifies • General characteristics of magma • Parent material of igneous rocks • Forms from partial melting of rocks • Magma at surface is called lava

  3. General Characteristics of Magma • Rocks formed from lava = extrusive, or volcanic rocks • Rocks formed from magma at depth = intrusive, or plutonic rocks

  4. General Characteristics of Magma, cont’d Magma consists of three components: • Liquid portion = melt • Solids, if any, are silicate minerals • Volatiles = dissolved gases in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)

  5. General Characteristics of Magma, cont’d Crystallization of magma • Cooling of magma results in the systematic arrangement of ions into orderly patterns • Silicate minerals result from crystallization in a predictable order • Texture - size and arrangement of mineral grains

  6. Igneous Textures • Texture - overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals • Several factors affect crystal size

  7. Igneous Textures • Factors affecting crystal size • Rate of cooling • Slow rate = fewer but larger crystals • Fast rate = many small crystals • Very fast rate forms glass • % of silica (SiO2) present • Presence of dissolved gases

  8. Types of Igneous Textures • Aphanitic (fine-grained) texture • Rapid rate of cooling • Microscopic crystals • May contain vesicles (holes from gas bubbles)

  9. Types of Igneous Textures • Phaneritic (coarse-grained) texture • Slow cooling • Large, visible crystals

  10. Types of Igneous Textures • Porphyritic texture • Minerals form at different temperatures • Large crystals (phenocrysts) are embedded in a matrix of smaller crystals (groundmass)

  11. Types of Igneous Textures • Glassy texture • Very rapid cooling of lava • Rock is called obsidian or pumice

  12. Types of Igneous Textures • Pyroclastic texture • Fragmental appearance produced by violent volcanic eruptions • Often appear more similar to sedimentary rocks

  13. Types of Igneous Textures • Pegmatitic texture • Exceptionally coarse grained • Forms in late stages of crystallization of granitic magmas

  14. Igneous Compositions Igneous rocks are composed primarily of silicate minerals • Dark (or ferromagnesian) silicates • Olivine, pyroxene, amphibole, and biotite mica • Light (or nonferromagnesian) silicates • Quartz, muscovite mica, and feldspars

  15. Igneous Compositions, cont’d Granitic versus basaltic compositions • Granitic composition • Light-colored silicates • Termed felsic (feldspar and silica) in composition • High silica (SiO2) content • Major constituent of continental crust

  16. Igneous Compositions, cont’d Granitic versus basaltic compositions • Basaltic composition • Dark silicates and Ca-rich feldspar • Termed mafic (magnesium and ferrum, for iron) in composition • Higher density than granitic rocks • Comprise the ocean floor and many volcanic islands

  17. Igneous Compositions, cont’d Other compositional groups • Intermediate (or andesitic) composition • Contain 25% or more dark silicate minerals • Associated with explosive volcanic activity • Ultramafic composition • Rare composition that is high in magnesium and iron • Composed entirely of ferromagnesian silicates

  18. Igneous Compositions, cont’d • Silica content as an indicator of composition • Crustal rocks exhibit a considerable range--45% to 70% • Silica content influences magma behavior • Granitic magmas = high silica content and viscous • Basaltic magmas = much lower silica content and more fluid-like behavior

  19. Naming Igneous Rocks Granitic rocks • Granite • Phaneritic • Over 25% quartz, about 65% or more feldspar • Very abundant - often associated with mountain building • The term granite includes a wide range of mineral compositions

  20. Naming Igneous Rocks, cont’d Granitic Rocks • Rhyolite • Extrusive equivalent of granite • May contain glass fragments and vesicles • Aphanitic texture • Less common and less voluminous than granite

  21. Naming Igneous Rocks, cont’d • Granitic rocks • Obsidian • Dark colored • Glassy texture • Pumice • Volcanic • Glassy texture • Frothy appearance with numerous voids

  22. Naming Igneous Rocks, cont’d • Intermediate rocks • Andesite • Volcanic origin • Aphanitic texture • Diorite • Plutonic equivalent of andesite • Coarse grained (phaneritic texture)

  23. Naming Igneous Rocks, cont’d • Basaltic rocks • Basalt • Volcanic origin • Aphanitic texture • Composed mainly of pyroxene and calcium-rich plagioclase feldspar • Most common extrusive igneous rock

  24. Naming Igneous Rocks, cont’d • Mafic rocks • Gabbro • Intrusive equivalent of basalt • Phaneritic texture consisting of pyroxene and calcium-rich plagioclase • Significant % of the oceanic crust

  25. Naming Igneous Rocks, cont’d • Pyroclastic rocks • Composed of fragments ejected during a volcanic eruption • Varieties • Tuff = ash-sized fragments • Volcanic breccia = particles larger than ash

  26. Origin of Magma • Generating magma from solid rock • Role of heat • Temperature increases in the upper crust (geothermal gradient) average between 20oC to 30oC per kilometer • Rocks in the lower crust and upper mantle are near their melting points • Additional heat may induce melting

  27. Origin of Magma, cont’d • Role of pressure • Increases in confining pressure increases a rock’s melting temperature • When confining pressures drop, decompression melting occurs • Role of volatiles • Volatiles (primarily water) cause melting at lower temperatures • Important factor where oceanic lithosphere descends into the mantle

  28. Evolution of Magmas • A single volcano may extrude lavas exhibiting very different compositions

  29. Bowen’s Reaction Series • Minerals crystallize in a systematic fashion based on their melting points • During crystallization, the composition of the liquid portion of the magma continually changes

  30. Evolution of Magmas, cont’d • Processes responsible for changing a magma’s composition • Magmatic differentiation • Separation of a melt from earlier formed crystals • Assimilation • Changing a magma’s composition by incorporating surrounding rock bodies into a magma • Magma mixing • Two chemically distinct magmas may produce a composition quite different from either original magma

  31. Partial Melting and Magma Formation • Incomplete melting of rocks is known as partial melting • Formation of basaltic magmas • Most originate from partial melting of mantle rocks at oceanic ridges (decompression melting) or at subduction zones (presence of water) • Large outpourings of basaltic magma are common at Earth’s surface

  32. Partial Melting and Magma Formation, cont’d • Formation of andesitic magmas • Produced by interaction of basaltic magmas and more silica-rich rocks in the crust • May also evolve by magmatic differentiation

  33. Partial Melting and Magma Formation, cont’d • Formation of granitic magmas • Most likely form as the end product of crystallization of andesitic magma • Granitic magmas are more viscous than other magmas – tend to lose their mobility before reaching the surface • Produce large plutonic structures

  34. End of Chapter 4

  35. Lava flows are typically finer grained than intrusive igneous rocks. Why? 04.01 • Intrusive magma is cooler because it is well insulated by the surrounding rock. • Intrusive magma flows onto the Earth's surface and cools very slowly, allowing many small mineral grains to grow. • The extrusive magma cools quickly so the mineral grains do not have time to grow. • The extrusive magma, because it is deep below the surface, cools very slowly, producing very small mineral grains.

  36. 04.02 What does Bowen’s Reaction Series describe? • The pressures that different minerals are formed in metamorphic rocks • Which minerals are recrystallized in a sedimentary rock • The temperatures at which different minerals crystallize out of a melt • All of the above • None of the above

  37. 04.03 Which of the following describes best the difference between magma and lava? • Upon eruption, magma becomes “lava,” the form of the magma that flows out of the volcano • When magma gets to the top of the volcano, it releases gas found within the magma, and the lava that escapes the volcano has therefore lost those gases. • Lava is the name given to the ejected magma from a passive (fluid) volcano, but it is still called “magma” if it is erupted explosively, like at Mt. St. Helens. • If the magma has no crystals or gases within it, it is called “lava.” • Both 1 and 3 • Both 3 and 4 • All of these.

  38. 04.04 Which of the following rocks is likely to have the most quartz within it and why? • Granite; intrusive rock that formed from cooling of relatively high silica magma. • Rhyolite; extrusive rock that formed from cooling of relatively low silica magma. • Diorite; intrusive rock that formed from the cooling of relatively intermediate silica magma. • Granite; intrusive rock that formed from cooling of relatively intermediate silica magma. • Basalt; extrusive rock that formed from cooling of relatively low silica lava. • Basalt; extrusive rock that formed from cooling of relatively high silica lava.

  39. 04.05 What is the most important factor for whether magma cools slowly or quickly? • Pressure of the environment – higher P = slower magma cooling. • The presence or absence of volatiles (gases) – less gases = slower magma cooling. • Temperature of the environment – lower T = slower magma cooling. • The presence or absence of volatiles (gases) – more gases = slower magma cooling. • Temperature of the environment – higher T = slower magma cooling. • Pressure of the environment – lower P = slower magma cooling.

  40. 04.06 Igneous rocks are produced largely by ________. • the changing of a rock from one set of minerals to another • the compaction of metamorphic rocks • the melting of sedimentary rocks • the cooling of magma • Both c and d are correct.

  41. 04.07 Glassy igneous rocks form when the magma _______. • cools so fast that mineral grains cannot crystallize and grow • cools so slowly that only one mineral is formed • is composed of basalt • is a rhyolitic type • cools at an extremely high temperature

  42. 04.08 Quartz is ________. • resistant to weathering and is an important component of sand in river beds and beaches • a main constituent of many igneous rocks • a main constituent of many sedimentary rocks • the most stable of all minerals at Earth’s surface temperatures and pressures • all of the above

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