Igneous Processes and Structures

1. Igneous Processes and Structures GLY 2010 – Summer 2012 Lecture 7

2. Definition of Igneous • Igneous - Said of a rock or mineral that solidified from molten or partly molten material, i.e. from a magma • Etymology: Latin ignis, ''fire”

3. Magma • Magma is naturally occurring mobile rock material “molten rock” • Capable of intrusion and extrusion • Igneous rocks are derived from magma through solidification and related processes

4. Viscosity • Viscosity is the property of a substance to offer internal resistance to flow; its internal friction

5. Flow Viscosity Initial Position After flow starts Viscosity increases from left to right

6. Viscosity in Magma • Video shows a rod being poked in hot, viscous magma on Kilauea, Hawaii

7. Factors Influencing the Viscosity of Magma • Temperature • Chemical composition • Gas content

8. Temperature • The higher the temperature, the lower the viscosity • Basaltic magmas at 1200°C or higher, are much more fluid (less viscous) than granitic magmas at 800°C

9. Low Viscosity Flow Animation Typical of basaltic magma

10. High Viscosity Flow Animation • Typical of Andesitic or Rhyolitic Magmas

11. Chemical Composition • The higher the silica content of magma, the higher the viscosity

12. Gas Content • As gas content increases, the viscosity decreases • Gases inhibit silica chain formation, and lower overall viscosity

13. Effects of Increasing Viscosity • Volcanic violence related to viscosity (magma type) • Magma type related to geologic, and often, plate tectonic setting

14. Partial Melting • Different minerals melt at different temperatures • As temperature increases, this leads to partial melting

15. Igneous Structures • Intrusive – magma freezes below the surface • Extrusive – magma erupts onto or above the surface

16. Intrusive Structures • Plutons are large bodies of magma that solidified well below the surface • Magma may be injected under pressure into cracks in the rock

17. Intrusive Structures, Cont. • Igneous sill – Parallel to existing layers

18. Igneous Sill • Salt River Canyon, Arizona – the dark band is basalt intruded into horizontal layers of igneous rock

19. Intrusive Structures, Cont. • Igneous Dike - intrusion cuts across the rock layers

20. Igneous Dike • Thin, pink aplite dikes cut the black basaltic dikes and the gray granite • Photo C.A. Giovanella • Location Pender Harbor, Southwest British Columbia

21. Igneous Structure Diagram

22. Aerial View of Intersecting Dikes • Intersecting dikes (Tertiary) at Spanish Peaks, Las Animas and Huerfano Counties, CO. View to east. (10Apr66)

23. Spanish Peak Dikes • West Spanish Peak (13,623 ft) and dikes (Tertiary), Las Animas and Huerfano Counties, CO. View to the south. (10Apr66)

24. Intrusive Structures, Cont. • Laccolith

25. Igneous Laccolith

26. Shiprock, New Mexico • The neck of an ancient volcano, which has eroded • Structure in the background is a dike

27. Igneous Vein • Extensional veins in a thick carbonate turbidite from the Liguride Complex in the Northern Apennines, Italy • Photo David Bice, Carleton College

28. Extrusive Structures • Lava Flows • Obsidian

29. Lava Flow From the Air

30. Lava Flow From the Ground • Lava flows from Nyiragongo volcano • Eruption January 18, 2002 • City is Gomo, Congo

31. Newberry Caldera, Oregon • Obsidian flow from a vent along the south wall of the caldera • Road gives scale

32. Obsidian Domes Photo M.L. Bevier • Holocene obsidian domes, Long Valley Caldera California - obsidian is formed from very viscous magma, which is unable to flow long distances

33. Obsidian • Upper left: Thin piece of obsidian, showing flow banding • Lower left: Thicker piece of obsidian showing conchoidal fracture

34. Classification of Igneous Rocks • Mineral content • Chemical analysis • Texture • Geologic Association

35. Mineral Content • Igneous rocks may be classified on the basis of what minerals they contain • Essential Minerals • Accessory Minerals

36. Chemical Content • Rocks may be analyzed to see what elements they contain • Results are reported as weight percent oxides

37. Texture and Fabric • Texture is the general physical appearance or character of a rock • Fabric refers to the orientation (or lack of it) in space of the elements of which a rock is composed

38. Aphanitic Texture • A very fine grain texture, with crystals invisible to the naked eye • Photo M.L. Bevier

39. Pheneritic Texture • Grains are visible and identifiable using the naked eye • Photo M.L. Bevier

40. Porphyritic Texture • Large crystals in a fine-grained or aphanitic groundmass - Field of view 2 cm • Photo: E.J. Tarbuck

41. Pegmatitic – Black Hills, South Dakota • Coarse texture, resulting from rapid crystal growth, due to presence of water in melt at time of crystallization • Arrows point to the location of spodumenne crystals, removed during mining • Spodumene is a source of lithium

42. Relation of Texture to Cooling History • Aphanetic - Rapid cooling leads to very fine crystals or to glass - typical of extrusive rocks • Phaneritic - Slower cooling leads to medium or coarse grained rocks, typical of intrusive igneous rocks • Pegmatitic - Very slow cooling in a water-rich magma leads to the coarse crystals • Porphyritic - Slow cooling while the magma ascends through the earth, followed by rapid cooling after the magma erupts on the surface • Crystals are often medium to coarse • Groundmass texture will be fine-grained, or glassy, depending on the rate of cooling

43. Geologic Association • Rocks associated by age, position, and by characteristically being found together • Constant or regular variation of features within the rock body

44. Bowen's Reaction Series

45. Bowen – Tuttle Experiments

46. Magmatic Differentiation • Fractional Crystallization (Crystal settling) • Partial Melting

47. Fractional Crystallization

48. Evolution of Magma • Over time, fractional crystallization changes the composition of magma from “A” to “B” to “C”

49. Intrusive Medium to coarse grained 100% Crystalline May contain inclusions of rock which fell into the magma (xenoliths) Extrusive Aphanitic to fine grain size Often glassy Often contain gas bubbles Intrusive Vs. Extrusive Rocks

50. Granite, Rhyolite • Granite has clearly visible crystals • Rhyolite has few visible crystals, and patches of glass • Both rocks are felsic