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Common Rock Types Colby Geology. Igneous Rocks Classification based on composition and texture. COMPOSITION Principally the rock-forming minerals.

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Presentation on theme: "Common Rock Types Colby Geology. Igneous Rocks Classification based on composition and texture. COMPOSITION Principally the rock-forming minerals."— Presentation transcript:

1 Common Rock Types Colby Geology



4 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.

5 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)

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


8 TEXTURE Types of Textures Phaneritic Large crystals > 2 mm Slow cooling rate=Plutonic Granite, Diorite, Gabbro LightGrayDark FelsicIntermediateMafic

9 TEXTURE Types of Textures Aphanitic Small crystals < 2 mm Fast cooling rate=Volcanic Rhyolite, Andesite, Massive Basalt FelsiteScoria LightGrayDark FelsicIntermediateMafic

10 RhyoliteFelsite

11 Andesite

12 Massive BasaltScoria

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

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

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



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


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

21 Particles >2mm= coarse-grained CONGLOMERATE (rounded clasts) BRECCIA (angular clasts) 1/16 - 2mm= medium-grained SANDSTONE <2mm= fine-grained SHALE 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

22 Coarse-grained Clastic Sedimentary Rocks ConglomerateBreccia

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

24 Fine-grained Clastic Sedimentary Rocks Shale

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

26 Oolitic Limestone

27 Chert

28 Dolostone

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

30 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 (CaCO 3 )

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

32 Fossiliferous Limestone

33 NONCARBONATE ORGANIC SED. ROCKS CHERT microcrystalline composed of skeletons, microscopic in size made of SiO 2 hardness = 7 may exhibit conchoidal fracture

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

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

36 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.




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

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

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

43 Metamorphic Rocks Foliated Rocks SLATEHave slatey cleavage PHYLLITERock splits along parallel planes SCHISTHas schistosity GNEISSHas gneissic banding

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

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

46 QUARTZITE Metamorphosed quartz sandstone Hardness = 7 Metamorphic Rocks Nonfoliated Rocks

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

48 GREENSTONE Composed of metamorphosed mafic igneous rocks Metamorphic Rocks Nonfoliated Rocks

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


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