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Unveiling Igneous Rocks: Formation and Classification

Learn about how igneous rocks are formed by the crystallization of molten rock and their classification based on mineral composition and texture. Discover the various types such as felsic, mafic, and silica composition, as well as differentiate between extrusive and intrusive rocks. Understand the different textures and structures of igneous rocks through detailed examples.

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Unveiling Igneous Rocks: Formation and Classification

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  1. Igneous Rocks

  2. Igneous rocks are formed by the crystallization of molten rock (lava or magma).

  3. Classification • Mineral Composition • Texture of Grain Size

  4. Mineral Composition

  5. Felsic: high amounts of silica & light in color

  6. Felsic Rocks and Minerals

  7. Mafic: low amounts of silica & dark in color

  8. Mafic Rocks

  9. Silica Composition Felsic rocks >65% silica content Intermediate rocks (also called andesitic) 50-65% silica content Mafic rocks <40-50%

  10. Pure Silica

  11. Texture of Grain Size

  12. Igneous rocks formed above ground from lava are called “Extrusive” or “Volcanic” rocks. Obsidian Felsite Rhyolite

  13. When Lavacools rapidly above ground, crystals have a limited time to form. Therefore, Igneous rocks with small or no crystals are most likely Volcanic.

  14. Resulting Textures: • Glassy • Porous/Vesicular • Fine-Grained

  15. Obsidian-Glassy Felsic

  16. Basalt- Fine-grained Mafic

  17. Andesite- Fine-grained Intermediate

  18. Rhyolite- Fine-grained Felsic

  19. Pumice- Vesicular Felsic

  20. Vesicular Basalt- Mafic

  21. Scoria-Vesicular Mafic

  22. Igneous rocks formed underground from Magma are called Plutonic or Intrusive rocks. Notice the large crystals

  23. When Magma cools slowly underground, large crystals have time to form. Therefore, Igneous rocks with large crystals are most likely Plutonic.

  24. Resulting Textures 1. Coarse Grained

  25. Granite- Coarse-grained Felsic

  26. Diorite- Coarse-grained Intermediate

  27. Gabbro- Coarse-grained Mafic

  28. Igneous rocks that begin to cool underground and then are erupted to the surface have both fine and coarse grains. These rocks have undergone two stages of cooling, therefore they have two sizes of crystals. These rocks are called porphyry or said to have porphyritictexture.

  29. Basalt Porphyry- Mafic

  30. Rhyolite Porphyry-Felsic

  31. Andesite Porphyry- Intermediate

  32. Igneous Rock Structures

  33. Intrusive Igneous Rock forms structures such as…

  34. Dikes Dike: Sheet-like Igneous rock that cuts across the structure of pre-existing rock.

  35. Dike

  36. Volcanic Neck Volcanic Dike

  37. This would form a Dike.

  38. Sills Sills: Sheet-like Igneous rock that lies parallel to the layering of pre-existing rock.

  39. Sill

  40. Sill

  41. Laccoliths Laccoliths: A Laccolith is a vertically developed sill.

  42. Exposed/Eroded Laccoliths

  43. Batholiths Batholiths: A Batholith is a large Plutonic mass of Igneous Rock (old Magma “bubble”).

  44. Halfdome- Yosemite, California

  45. Enchanted Rock-Texas

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