Igneous Rocks

1. Igneous Rocks

2. What are Igneous Rocks? from the Latin word for “fire” - ignis Thus, rocks that are “fire-formed” Molten rock (magma) cools to form a solid rock

3. What is magma? Magma is molten rock, and contains: Melt Liquid, composed of mobile ions Solids Silicate minerals that have already crystallized from the melt Volatiles gases dissolved in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)

4. Identifying Igneous Rocks

5. How to classify igneous rocks • Composition • Texture

6. Igneous Rocks Rocks formed on the surface Volcanic (extrusive) rocks Rocks formed inside Earth Plutonic (intrusive) rocks

7. Igneous Rocks (two types) Extrusive Lavaand Pyroclastic Debris Extruded at surface or at very shallow levels. Basalt is Extrusive Intrusive Magma crystallized slowly within the crust. No exposure to the cool atmosphere. Plutonic– intrusive igneous rock at great depth within crust or mantle. Granite is Intrusive

8. Dark iron and/or magnesium Olivine Pyroxene Amphibole Biotite mica Light potassium, sodium, & calcium Quartz Muscovite mica Feldspars Magma Compositions Magma is composed mainly of silicate minerals

10. Bowen’s Reaction Series Magma crystallizes over a temperature range of several hundred degrees Minerals crystallize in a predictable order Last minerals to crystallize are very different in composition from the earlier formed minerals

11. Bowen’s Reaction Series Bowen’s Reaction Series Animation

12. Bowen’s Reaction Series Al Na Olivine Na Ca Ca Mg Si Si K Pyroxene K Al Si Fe Mg Fe Amphibole Al Mg Na Ca Na Si Ca Plagioclase Si Fe K Fe Mg K Biotite Mg Si Si Si Mg Si Fe Al Fe Muscovite Al Al Si Al Mg Mg Fe Na Na Ca Ca Orthoclase Fe Al Si Al K K Quartz

13. Partial Melting Silica-rich compounds melt before other compounds. Magma Differentiation Four types: • Magma Mixing • Crystal Settling • Magma Assimilation • Magma Migration

14. Changing a Magma’s Composition Crystal settling Earlier-formed minerals are denser than the liquid portion and sink to the bottom of the magma chamber

15. Changing a Magma’s Composition Assimilation Changing a magma’s composition by the incorporation of foreign matter (surrounding rock bodies) into a magma

16. 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

17. Texture is estimated using visual grain size(depends on crystallization history)

18. Composition is estimated using visual colorCan be misleading

19. Mafic minerals crystallize early and felsic minerals crystallize late in magma Minerals at the top of Bowen’s Reaction Series: • Dark in color • Mafic to ultramafic • Iron and magnesium • Minerals at the bottom of Bowen’s Reaction Series: • Light in color • Felsic • Sodium plagioclase, potassium plagioclase, and quartz are light.

20. Igneous rocks are named on the basis of their texture and composition

21. Igneous Textures The size, shape, and arrangement of the minerals. • Glassy – Made of solid glass or glass shards. • Interlocking crystals – Minerals that fit like jigsaw pieces. • Fragmental – Pieces of pre-existing rocks.

22. Aphanitic Phaneritic Porphyritic Glassy Pyroclastic Pegmatitic Igneous Textures • Rapid rate of cooling of lava or magma • Microscopic crystals • Typically occurs in extrusive / volcanic rocks

23. Basalt: Aphanitic Texture

24. Aphanitic Phaneritic Porphyritic Glassy Pyroclastic Pegmatitic Igneous Textures • Slow cooling • Crystals can be identified without a microscope • Typically occurs in intrusive / plutonic rocks

25. Granite: Phaneritic Texture

26. Thin section showing APHANITIC TEXTURE

27. Thin section showing PHANERITIC TEXTURE

28. Aphanitic Phaneritic Porphyritic Glassy Pyroclastic Pegmatitic Igneous Textures • Minerals form at different temperatures & rates • Large crystals (phenocrysts) are embedded in a matrix of smaller crystals

29. Granodiorite: Porphyritic Texture Potassium Feldspar Phenocryst

30. Aphanitic Phaneritic Porphyritic Glassy Pyroclastic Pegmatitic Igneous Textures • Very rapid cooling of molten rock • Resulting rock is called obsidian

31. Obsidian: Glassy Texture

32. Aphanitic Phaneritic Porphyritic Glassy Pyroclastic Pegmatitic Igneous Textures • Various fragments ejected during a violent volcanic eruption • Textures often appear similar to sedimentary rocks

33. Aphanitic Phaneritic Porphyritic Glassy Pyroclastic Pegmatitic Igneous Textures • Exceptionally coarse grained • Form in late stages of crystallization of granitic magmas

34. Quartz Vein: Pegmatitic Texture

36. Crystal Shapes • Euhedral - grains completely bounded by crystal faces • Subhedral - grains only partly bounded by crystal faces • Anhedral - grains completely devoid of crystal boundaries Source: 1, 2

37. Igneous Textures Arrangement of the grains Equigranular Slightly inequigranular inequigranular

38. Igneous Textures

39. Magma Compositions Igneous rocks are often classified by mineral (chemical) composition Ultramafic Mafic (or basaltic) Intermediate (or andesitic) Felsic (or granitic)

40. Ultramafic Compositions Magma Composition: Rare high in magnesium and iron composed entirely of ferromagnesian silicates Fe Fe Olivine Mg Mg Fe Mg Pyroxene Mg Fe

41. Mafic (or basaltic) Compositions Magma Composition: Mafic: magnesium and ferrum (iron) Contains substantial dark silicate minerals and calcium-rich plagioclase feldspar More dense than granitic rocks Al Olivine Fe Fe Na Mg Ca Mg Si Pyroxene Fe Si Ca Ca Mg Na Mg Plagioclase Fe Na Al Si

42. Extrusive / Volcanic Rock: Basalt Aphanitic texture Composed mainly of pyroxene and calcium-rich plagioclase Most common extrusive igneous rock Intrusive / Plutonic Rock: Gabbro Phaneritic texture Composed of pyroxene and calcium-rich plagioclase Makes up a significant portion of the oceanic crust Mafic (or basaltic) Compositions

43. Intermediate (or andesitic) Compositions Magma Composition Contains at least 25% dark silicate minerals Associated with explosive volcanic activity Al Amphibole Fe Fe Na Mg Ca Mg Si Plagioclase Si Ca Ca Mg Na Fe Na Al Si

44. Extrusive / Volcanic Rock: Andesite Aphanitic texture Dark grey color May have plagioclase phenocrysts Intrusive / Plutonic Rock: Diorite Phaneritic texture Composed of intermediate feldspar and hornblende “Black and white makes DIORITE” Intermediate (or andesitic) Compositions

45. Felsic (or granitic) Compositions Magma Composition Felsic: feldspar and silica Composed almost entirely of light-colored silicates Amphibole Si Al Plagioclase Fe Al Fe K Na Biotite Mg Ca Mg Si K Si Al Muscovite Ca Ca Mg Na Orthoclase K Fe K Na Al Si Quartz

46. Extrusive / Volcanic Rock: Rhyolite Aphanitic texture Buff or pink color May have glass phenocrysts Intrusive / Plutonic Rock: Granite Phaneritic texture Over 25 % quartz, about 65 % or more feldspar The term “granite” covers a wide range of mineral compositions Felsic (or granitic) Compositions

47. Obsidian Volcanic Dark colored Glassy texture Pumice Volcanic Glassy texture Frothy appearance with numerous voids Varied Compositions

48. Varied Compositions Pyroclastic rocks Composed of fragments ejected during a volcanic eruption Tuff – ash-sized fragments Volcanic breccia – particles larger than ash lvo.wr.usgs.gov/gallery/BishopTuff_1.html www.gc.maricopa.edu

49. Igneous Rock Classification

50. All rocks on Earth have evolved from the first igneous rocks All Igneous Rocks Result from Magma Differentiation.