Igneous Rocks Lecture 3

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

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, K-feldspar) melt at cooler temperatures. Melts are viscous Silica-poor rocks (with Olivine, Pyroxene, Ca-feldspar) melt at higher temperatures Melts are very fluid Hot

4. 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 Both melts are "Basaltic" i.e. Olivine, Pyroxene and Ca-Feldspars

6. Extrusive Igneous Rock - Lava (Hawaii)

7. Intrusive Igneous Rock (Granite) – This granite cooled 30 kilometers under the surface Plagioclase Feldspar Quartz Amphibole K Feldspar

8. Characteristics of magma • Three parts: • Liquid portion, called melt, that is mobile ions • Solids, if any, are silicate minerals 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. Bowen’s Reaction Series Molten- VERY Hot No solids First mineral to crystallize out Molten- Not so hot 100% Solid

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

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

13. Crystals can react with the melt if they touch it If the first formed crystals of Calcium-rich (Ca) Plagioclase touch the melt they will react with it, and 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)

14. 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 melt will crystallize its mafic components first, and the remaining melt may be granitic

15. Characteristics of magma • Igneous rocks are typically classified by both: • Texture • Mineral composition • Texture in igneous rocks is determined by the size and arrangement of mineral grains

16. Igneous textures • Most important is crystal size • 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

17. 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

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

19. Phaneritic texture Coarse crystals cooled slowly at great depth

20. 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

21. Porphyritic texture Granite Two-stage cooling?

22. Glassy texture Obsidian Fast cooling

23. 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

24. Pyroclastic Rock - Superheated Flows

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

26. Ash and pumice layers

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

28. 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 • versus … “MAFIC” Magnesium and Iron

29. 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

30. 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

31. 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

32. Igneous compositions • 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 fine grained because extruded, so crystallized quickly

33. 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

34. 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

35. Scoria type Basalt: note Gas Bubble Pits

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

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

38. 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

39. 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

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

41. Silica Content • 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” When Yellowstone explodes, half of Wyoming will perish Volcanic “Rhyolite”

42. Silica Content • 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)

43. Origin of Magma • Role of Pressure • Reducing the pressure lowers the melting temperature – the rock probably melts • RIDGE: When confining pressures drop, decompression melting occurs

44. Origin of Magma • Role of volatiles - WATER • Volatiles (primarily water) cause rocks to melt at lower temperatures • This is particularly important where oceanic lithosphere descends into the mantle in a subduction zone

45. Assimilation and magmatic differentiation Show Samples

46. Basalts forming in rifts and MORs Decompression Melting: Magma under lithosphere heats and cracks it. Mantle rock is exposed to low pressures – itpartially melts

47. Origin of Andesite & Diorite:intermediate silica content Basaltic here Good diagram for the Andes Mountains Small blobs, not much heat in them Assimilate some crust, fractionate

48. PlateTectonics- Andesite Line Andes Andesites form above the deep portions of a subduction zone

49. Origin of Granitic Rocks Huge blobs under thick part of continent w/ low temps but lots of magma, fractionation & assimilation => Granite Batholiths Can also get small amounts of granites from deep felsic rock passed by ascending magma

50. Some intrusive igneous structures