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Common Rock Types

Common Rock Types

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Common Rock Types

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  1. Common Rock Types Colby Geology

  2. Igneous Rocks Classification based on composition and texture. COMPOSITION Principally the rock-forming minerals Olivine Pyroxenes Amphiboles Micas Feldspars Quartz Constitute 95% of minerals in ALL igneous rocks.

  3. COMPOSITION You can determine basic igneous rock composition just by the COLOR of the rock. DARK COLORED igneous rocks = MAFIC LIGHT COLORED igneous rocks = FELSIC GRAY COLORED igneous rocks = INTERMEDIATE Color allows an approximation of igneous rock composition Related to Bowen’s Reaction Series (red vs. green)

  4. 1200°C Gabbro-Basalt 900°C Diorite-Andesite Granite-Rhyolite 600°C


  6. TEXTURE Types of Textures Phaneritic Large crystals > 2 mm Slow cooling rate = Plutonic Granite, Diorite, Gabbro Light Gray Dark Felsic Intermediate Mafic

  7. TEXTURE Types of Textures Aphanitic Small crystals < 2 mm Fast cooling rate = Volcanic Rhyolite, Andesite, Massive Basalt Felsite Scoria Light Gray Dark Felsic Intermediate Mafic

  8. Rhyolite Felsite

  9. Andesite

  10. Massive Basalt Scoria

  11. TEXTURE Types of Textures Glassy No crystals, lava cools too fast Extremely fast cooling rate = Volcanic Obsidian, Pumice Light Felsic Pumice Obsidian

  12. TEXTURE Types of Textures Porphyritic Two distinct sizes of crystals Starts cooling slowly, remainder cools quickly Phenocrysts Groundmass

  13. TEXTURE Types of Textures TEXTURE is related to origin of magma or lava. Phaneritic = Coarse-grained = Intrusive Plutonic Aphanitic = Fine-grained = Extrusive Glassy or smaller Volcanic Identification/Classification of Igneous Rocks Key on page 75-77 in lab manual Chart on page 91 in lab manual


  15. Sedimentary Rocks Clastic Sedimentary Rocks COMPOSITION Derived from pre-existing rocks. Five main constituents: Quartz Feldspar Rock fragments Clay (and clay minerals) Calcite

  16. Sediment Size (mm) Particle Name Rock Name >256 Boulder 64-256 Cobble Conglomerate Breccia Gravel 4-64 Pebble Granule 2-4 0.063-2 Sand Sandstone 0.004-0.063 Silt Siltstone Mudstone Mud <0.004 Clay Shale TEXTURE CLASTIC – fragments of rock debris Classified by analyzing grain-size

  17. Particles >2mm = coarse-grained CONGLOMERATE (rounded clasts) BRECCIA (angular clasts) 1/16 - 2mm = medium-grainedSANDSTONE <2mm = fine-grainedSHALE Sandstones further classified on the basis of their mineralogical composition: Mostly quartz = QUARTZ SS. >25% feldspar = ARKOSE 1/3 silt, clay and rock fragments = GREYWACKE

  18. Coarse-grained Clastic Sedimentary Rocks Conglomerate Breccia

  19. Medium-grained Clastic Sedimentary Rocks Sandstones Arkose Greywacke Qtz. Sandstone

  20. Fine-grained Clastic Sedimentary Rocks Shale

  21. Sedimentary Rocks Chemical Sedimentary Rocks CHEMICAL SEDIMENTARY ROCKS precipitated from sea water (evaporites) initially dissolved in water very similar in appearance to igneous rocks exceptMONOMINERALIC. texture is crystalline if visible microcrystalline if not OOLITIC LIMESTONE CHERT DOLOMITE

  22. Oolitic Limestone

  23. Chert

  24. Dolostone

  25. CHEMICAL SEDIMENTARY ROCKS ROCK SALT or HALITE composed of halite (NaCl) microcrystalline to crystalline precipitates from sea water forms economic deposits

  26. Sedimentary Rocks Organic Sedimentary Rocks ORGANIC or BIOCLASTIC SEDIMENTARY ROCKS can be crystalline or clastic (bioclastic) all particles are derived from organisms typically composed of calcite or aragonite (CaCO3)

  27. Lithographic Limestone or Micrite Very fine-grained limestone Formed from calcareous algae Fizzes in HCl

  28. Fossiliferous Limestone

  29. NONCARBONATE ORGANIC SED. ROCKS CHERT microcrystalline composed of skeletons, microscopic in size made of SiO2 hardness = 7 may exhibit conchoidal fracture

  30. NONCARBONATE ORGANIC SED. ROCKS PEAT composed of compressed plant debris basic building block of coal used as a fuel source in some areas

  31. Identification/Classification of Sedimentary Rocks Key on page 79-80 in lab manual Chart on page 93 in lab manual

  32. Metamorphic Rocks Rocks are fundamentally changed by HEAT and/or PRESSURE May be derived from ANY pre-existing rock type CLASSIFICATION based on the presence or absence of FOLIATION FOLIATION is the parallel alignment of the tabular minerals (micas and clay minerals) and by varying degrees of banding.

  33. Metamorphic Rocks Types of Foliation Slatey cleavage very fine-grained rocks Usually found in slate and phyllite

  34. Metamorphic Rocks Types of Foliation Schistosity platy minerals visible Usually found in schist

  35. Metamorphic Rocks Types of Foliation Gneissic banding minerals segregated into bands Usually found in gneiss

  36. Metamorphic Rocks Foliated Rocks SLATE Have slatey cleavage PHYLLITE Rock splits along parallel planes SCHIST Has schistosity GNEISS Has gneissic banding

  37. Metamorphic Rocks Nonfoliated Rocks NO structure is apparent in the rock Rock is typically MASSIVE (monomineralic)

  38. Metamorphic Rocks Nonfoliated Rocks MARBLE Is composed of calcite (CaCO3), soft H = 3 Will react to dilute hydrochloric acid

  39. Metamorphic Rocks Nonfoliated Rocks QUARTZITE Metamorphosed quartz sandstone Hardness = 7

  40. Metamorphic Rocks Nonfoliated Rocks ANTHRACITE COAL Formed from organic material (plant debris) Contains no minerals

  41. Metamorphic Rocks Nonfoliated Rocks GREENSTONE Composed of metamorphosed mafic igneous rocks

  42. Identification/Classification of Metamorphic Rocks Key on page 82 in lab manual Chart on page 95 in lab manual