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Igneous Rocks Lecture 3

Types of Rock include Igneous, Sedimentary and Metamorphic. Igneous Rocks Lecture 3. Marble demo, rock specimens, Petrographic Microscope, Olivine Porphyry or Gabbro. Characteristics of magma. Igneous rocks form as molten rock cools and solidifies

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Igneous Rocks Lecture 3

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  1. Types of Rock include Igneous, Sedimentary and Metamorphic Igneous RocksLecture 3 Marble demo, rock specimens, Petrographic Microscope, Olivine Porphyry or Gabbro

  2. Characteristics of magma • Igneous rocks form as molten rock cools and solidifies • Characteristics of magmas (molten rock) depend on parent material and where they crystallize • Where determines speed of crystallization • At surface, fast cooling makes small crystals

  3. Geothermal Gradient Cool Silica-rich rocks (with Quartz) melt at cooler temperatures. Melts are viscous Silica-poor rocks (with Olivine, Pyroxene) melt at higher temperatures Melts are very fluid Hot

  4. http://www.casdn.neu.edu/~geology/department/staff/colgan/iceland/usgs/http://www.casdn.neu.edu/~geology/department/staff/colgan/iceland/usgs/ Characteristics of magma • General Characteristics of molten rock • Forms from partial melting of rocks inside the Earth • 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

  5. Two Geologic Environments Where Igneous Rocks Form Igneous Rocks Formed in Rift Igneous Rocks Formed Above Sinking Plate

  6. Extrusive Igneous Rock - Lava (Hawaii)

  7. Intrusive Igneous Rock (Granite) – This granite cooled 30 kilometers under the surface

  8. 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)

  9. 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 Rock-forming minerals crystallize with increasing complexity as the magma cools. The most complex 3-D minerals crystallize last. The hottest magmas can only crystallize Olivine (Independent Tetrahedra), but as the magma cools, more complex minerals can form.

  10. Crystallization of minerals in magma bodies • Bowen’s reaction series and the composition of igneous rock • Norman L. Bowen demonstrated that as a magma cools, minerals crystallize in a systematic fashion based on their melting points http://vgp.agu.org/bowen_paper/bowen_paper.html

  11. Bowen’s Reaction Series Molten- VERY Hot Molten- Not so hot 100% Solid

  12. Fine crystals Need a microscope Intermediate Low silica, HOT, fluid High silica, warm, viscous Course crystals Easily seen

  13. Bowens reactrion series says: as a granitic melt cools, Biotite Mica and Plagioclase Feldsparcrystallize out before Quartz Granite Hand Sample Order of Crystallization We can see the order of crystallization under the microscope Granite Thin Section

  14. Crystals can react with the melt if they touch it If the first formed crystals of Calcium-rich (Ca) Plagioclase are allowed to react with the melt, they will become more sodium-rich on their outer rims Zoned feldspar (plagioclase) showing change in composition with time in magma chamber (calcium-rich in core to sodium-rich at rim)

  15. However, if early crystals are removed the melt becomes richer in Silica Remove Fe, Mg, Ca Some Si Left with K and Al Most of Si You can start with a Mafic (silica-poor) magma and end up with some Felsic (silica-rich) Granites. Marble Demo A rising melt may leave its mafic components behind, and the upper part may be granitic

  16. 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

  17. Igneous textures • Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals • Most important is crystal size • Factors affecting crystal size • Rate of cooling • Slow rate promotes the growth of fewer but larger crystals

  18. Igneous textures • Factors affecting crystal size • Rate of cooling (continued) • Fast rate forms many small crystals • Very fast rate forms glass • Amount of silica (SiO2) present • Amount of dissolved gases

  19. 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

  20. Aphanitic texture Fine grained because it cooled quickly at the surface

  21. Phaneritic texture Coarse crystals cooled slowly at great depth

  22. 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

  23. Porphyritic texture Granite

  24. Glassy texture Obsidian

  25. 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

  26. Pyroclastic Rock - Superheated Flows

  27. Naming igneous rocks – pyroclastic rocksComposed of fragments ejected during a volcanic eruptionVarietiesTuff – ash-sized fragmentsVolcanic breccia – particles larger than ash

  28. Ash and pumice layers

  29. Still more types of Igneous textures • Types of igneous textures • Pegmatitic texture • Exceptionally coarse grained crystals • Form in late stages of fractionation of magmas • This is often what prospectors are looking for

  30. A Pegmatite with Feldspar and Zircon Zircon is very good for obtaining radiometric ages

  31. Show tray of Mafic Minerals Igneous Compositions • Igneous rocks are composed primarily of silicate minerals that include: • dark (or ferromagnesian) colored silicates • Olivine • Pyroxene • Amphibole • Biotite mica • And … “MAFIC” Magnesium and Iron

  32. Igneous Compositions Show tray of Felsic Minerals • Igneous rocks also contain light colored silicate minerals that include: • Quartz • Muscovite mica • Feldspars “FELSIC” Feldspar and Silica

  33. Igneous Rock Classification- Bowen’s Reaction Series on its side Note Minerals in Note Minerals in Felsic rocks crystallize from warm melts Mafic from hot melts

  34. Igneous 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

  35. Igneous compositions • Naming igneous rocks – granitic (felsic) rocks • Granite • Phaneritic • Over 20 percent quartz, about 25 percent or more feldspar (usually much more feldspars). • Plagioclase is Sodium-rich • Abundant and often associated with mountain building • The term granite covers a wide range of mineral compositions

  36. Igneous compositions Figure 3-7 continued • Naming igneous rocks – granitic (felsic) rocks • Rhyolite • Extrusive equivalent of granite • May contain glass fragments and vesicles • Aphanitic texture (means fine grained minerals) • Less common and less voluminous than granite • Phenocrysts can include quartz and feldspar

  37. Rhyolitefine grained because extruded, so crystallized quickly

  38. Igneous compositions • Basaltic composition can be fine or coarse • Composed of dark Olivine and Pyroxene and grey calcium-rich plagioclase feldspar • No Potassium-rich feldspar (no K-spar ‘Microcline’) • Designated as being mafic (magnesium and ferrum, for iron) in composition • Much denser than granitic rocks - sinks • Comprises the ocean floor as well as many volcanic islands such as Hawaii. Also rift valley lavas

  39. Igneous compositions • Naming igneous rocks – basaltic (mafic) rocks: Fine-grained • Basalt • Volcanic origin • Aphanitic texture • Composed mainly of pyroxene, some olivine and also calcium-rich plagioclase feldspar • Most common extrusive igneous rock

  40. Scoria type Basalt note Gas Bubbles

  41. Igneous compositions • Naming igneous rocks – basaltic (mafic) rocks: Coarse Grained • Gabbro • Intrusive equivalent of basalt • Phaneritic texture consisting of pyroxene and calcium-rich plagioclase (med. grain “diabase”) • Makes up a significant percentage of the oceanic crust, beneath the basalt pillow lavas.

  42. Gabbro - a mafic igneous rock. A large- grain version of Basalt

  43. Igneous compositions • Other compositional groups • Intermediate (or andesitic) composition • Contain at least 25 percent dark silicate minerals • Associated with explosive volcanic activity • Often gray

  44. Igneous compositions • Intermediate rocks • Andesite • Volcanic origin • Aphanitic texture • Often resembles rhyolite • Intermediate silica content • Frequent composition in volcanoes above subduction zones, e.g. in Andes Mountains

  45. Igneous compositions • Extrusive products can include: • Pumice • Volcanic • Glassy texture, very light weight, mostly air • Frothy appearance with numerous voids (extrusive foam) • Forms when lavas have a lot of water and other volatiles Common with intermediate compositions

  46. Igneous compositions • Intermediate rocks • Diorite • Plutonic equivalent of andesite • Coarse grained • Intrusive • Composed mainly of intermediate feldspar and amphibole

  47. Igneous compositions • Silica content influences a magma’s behavior • Granitic magma • High silica content • Extremely viscous • Liquid exists at temperatures as low as 700oC • Huge explosion if it erupts (Yellowstone, Toba) Plutonic “Granite” Volcanic “Rhyolite”

  48. 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)

  49. Origin of Magma • 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 with increasing depth. The geothermal gradient averages between 20oC to 30oC per kilometer

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