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Rock Hound Lab Identify Major Characteristics of Different Rock Textures.

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Presentation on theme: "Rock Hound Lab Identify Major Characteristics of Different Rock Textures."— Presentation transcript:

1 Rock Hound Lab Identify Major Characteristics of Different Rock Textures

2 Igneous rocks can be identified by the determination of the composition and texture of the rock. Once these two characteristics have been identified, the Igneous Rock Identification chart is used to identify the rock name.

3 Igneous Rock Identification Chart COMPOSITION TEXTURE FelsicIntermediateMaficUltramafic Pegmatitic Granite Pegmatite Diorite Pegmatite Gabbro Pegmatite PhaneriticGraniteDioriteGabbroDunite AphaniticRhyoliteAndesiteBasalt PorphyriticRhyoliteAndesiteBasalt GlassyObsidianBasaltic Glass VesicularPumiceScoria PyroclasticVolcanic Tuff

4 Composition Color is often an indicator of the composition of most igneous rocks Felsic composition - Light colors, including white, light gray, tan and pink Felsic compositions are rich in silica (SiO2) (sand, quartz, diatoms) Mafic or Ultramafic composition - Dark colors, such as black and dark brown Mafic compositions are poor in silica, but rich in iron (Fe) and magnesium (Mg) Intermediate compositions - often gray or consisting of equal parts of dark and light mineral Beware that even though an igneous rock may have a felsic composition (light color), the rock can contain dark colored minerals Mafic rocks may contain light colored minerals as well As mentioned above, the composition of most igneous rocks can be identified using this system, formally known as the Color Index

5 Two Most Notable Exceptions! Obsidian and Dunite Obsidian is volcanic glass which erupts as a lava flow Most obsidian is felsic in composition, yet typically it will have a very dark color (dark brown to black) Dunite has an ultramafic composition yet is apple green to yellowish green in color Dunite is composed almost entirely of the mineral olivine which usually contains both iron and magnesium

6 Color Index FelsicIntermediateMafic Igneous Rock Composition Chart FelsicIntermediateMafic EARTH SCIENCE LAB Composition of Igneous Rocks

7 Texture The texture of an igneous rock does not refer to the roughness or smoothness of the surface Textures are based primarily on crystal size

8 Texture Pegmatitic texture is composed of very large crystals (larger than 2-3 cm) Phaneritic texture is composed of crystals which are large enough to see but smaller than pegmatitic texture, and the entire rock is composed of crystals Aphanitic texture is a fine grained texture but the crystals are too small to see (ie. Basalt) Porphyritic texture is composed of crystals of two different sizes (larger crystals embedded in finer grain matrix) Large crystals (phenocrysts) are visible while the smaller crystal are not (referred to as groundmass)

9 Phaneritic Texture Phaneritic ("FAN-a-RIT- ic") rocks Have mineral grains that are large enough to be seen with the naked eye or a hand lens like this granite

10 Equigranular Texture Rocks with equigranular ("EC-wi-GRAN-ular") texture Have mineral grains that are generally the same size This example is a granite

11 Aphanitic Texture Aphanitic ("AY-fa-NIT- ic") rocks have mineral grains that are mostly too small to be seen with the naked eye or a hand lens like this rhyolite

12 Porphyry Porphyry ("PORE-fer- ee") Is a name used for any igneous rock with conspicuous larger grains— phenocrysts—floating in a fine-grained groundmass

13 Porphyritic Texture Rocks with porphyritic ("POR-fi-RIT-ic") texture Like this andesite Have larger mineral grains, or phenocrysts ("FEEN-o-crists"), in a matrix of smaller grains Andesite is an extrusive or intrusive igneous rock that is higher in silica than basalt and lower than rhyolite or felsite

14 Texture Glassy texture is the most readily recognized. The rock is composed entirely of glass. Few, if any, crystals will be visible Vesicular texture is formed when lava solidifies before gases are able to escape ("bubbly" appearance) Pyroclastic texture is composed of volcanic fragments. These fragments or clasts [fine (ash) or coarse (lapilli) or very coarse (bombs and blocks)]

15 Glassy Texture Glassy (or hyaline or vitreous) rocks Have no or almost no grains at all, as in this quickly chilled pahoehoe basalt or in obsidian Obsidian is an extrusive rock: lava that cooled without forming crystals, giving it a glassy texture Basalt most common lava rock type Pahoehoe and Aa Basalt Pronounced – “Pa-ho-ho” "ah-ah." Both of these basalt flows have the same composition, but while they were molten, the smooth pahoehoe lava was hotter than the jagged aa lava Pahoehoe Basalt Obsidian Basalt

16 Obsidian Flow in Oregon Obsidian Blocks Obsidian flows develop a blocky surface as their outer shell quickly solidifies Big Obsidian Flow in Newberry Caldera, Oregon Obsidian Flow Texture Obsidian may display complex folding and segregation of minerals in bands and round masses consisting of feldspar or cristobalite (high-temperature quartz). Weathering of Obsidian Water attacks obsidian readily because none of its material is locked up in crystals, making it prone to alteration into clays and related minerals. Obsidian Tools Obsidian is the best material for making stone tools. The stone doesn't need to be perfect to make useful implements

17 Vesicular Texture Rocks with vesicular ("ve-SIC-ular") texture Are full of bubbles It always indicates a volcanic rock, like this scoria Pumice is basically lava froth An extrusive rock frozen as its dissolved gases come out of solution It looks solid but often floats on water Scoria (Volcanic Cinders) Like pumice, is a lightweight extrusive rock, but scoria has large, distinct gas bubbles and a darker color (landscape product and running tracks) Unlike pumice, scoria usually has broken, connected bubbles and does not float in water Pumice Used for abrasive grit or soil amendments

18 Pyroclastic Texture Rocks with pyroclastic ("PY-ro- CLAS-tic") texture Are made of pieces of volcanic material Created in an explosive eruption Like this welded tuff Tuff is technically a sedimentary rock formed by the accumulation of volcanic ash plus pumice or scoria

19 Test Your Igneous Rock Identification IQ Click on the link above You will see your 1 st rock to identify. Click on all the images provided for different views of this unknown rock Identify the Composition by identifying the color and determine the Texture by examining the crystal sizes that are present. Use the information provided in class about igneous rock Composition and Textures. Now, click the appropriate Composition and Texture for this rock, then click the right Rock Type. Once this is done, click “Grade Identification” for your answer. Write all your answers for each of the categories on the answer sheet provided to help you identify each rock type Do all 12 samples.

20 #1 - Composition Texture

21 #2 - Composition Texture

22 #3 - Composition Texture

23 #4 - Composition Texture

24 #5 - Composition Texture

25 #6 – Composition Texture

26 #7 - Composition Texture

27 #8 - Composition Texture

28 #9 - Rhyolite Composition Felsic Texture Porphyritic

29 #10 - Composition Texture

30 #11 - Composition Texture

31 #12 – Composition Texture

32 Metamorphic Rocks These are what happens when sedimentary and igneous rocks become changed, or metamorphosed, by conditions underground

33 The 4 Main Agents That Metamorphose Rocks Heat Pressure Fluids Strain These agents can act and interact in an infinite variety of ways. As a result, most of the thousands of rare minerals known to science occur in metamorphic ("shape- changed") rocks.

34 Heat and pressure The clay minerals of sedimentary rocks are a good example Clays are surface minerals, which form as feldspar and mica break down in the conditions at the Earth's surface. With heat and pressure they slowly return to mica and feldspar Feldspar & Mica  Clays  + Heat & Pressure  Feldspar & Mica

35 Fluids Important agent of metamorphism Every rock contains some water Sedimentary rocks hold the most First there is the water that was trapped in the sediment as it became rock Second is the water that is liberated by clay minerals as they change back to feldspar and mica This fluid may be acidic or alkaline or full of sulfides or carbonates or metals

36 Strain Refers to any change in the shape of rocks due to the force of stress Movement on a fault zone is one example In shallow rocks, shear forces simply grind and crush the mineral grains

37 Foliation Mineral layers, observed when identifying a metamorphic rock Increased strain, the foliation becomes more intense, the mineral layers become thicker The foliated rock types that form under these conditions are called schist or gneiss, depending on their texture Schist is finely foliated Whereas gneiss is organized in wide bands of minerals

38 The Basic Metamorphic Rock Types

39 Sedimentary rock Shale  slate  phyllite  mica-rich schist The mineral quartz does not change under high temperature and pressure, although it becomes more strongly cemented Sandstone  quartzite Mixture of sand and clay  mudstones  schists or gneisses Limestone  marble

40 Metamorphic Rocks Rocks that have undergone a change from their original form due to Temperature, pressure or chemical alteration Their classification is based on the minerals that are present And the temperature and pressure at which these minerals form

41 Texture Texture is divided into two groups

42 Foliated and Unfoliated Rocks First steps towards identifying metamorphic rocks is to determine if it has foliation or not Foliation means banding or alignment of the minerals resulting from pressure Foliated textures can be flat like a piece of slate or folded. Non-foliated textures have minerals that are not aligned. Essentially, the minerals are randomly oriented. A good example of that is the banding present in a metamorphic rock called gneiss (pronounced nice)

43 Foliation Four types of foliation Slaty cleavage - composed of platy minerals that are too small to see. Typically, these rocks split along parallel, planar surfaces Phyllitic foliation - composed of platy minerals that are slightly larger than those found in slaty cleavage, but generally are still too small to see with the unaided eye The larger size gives the foliation a slightly shiny appearance

44 Foliation Four types of foliation Schistose foliation - is composed of larger minerals which are visible to the unaided eye Platy minerals tend to dominate Gneissic banding - is the easiest of the foliations to recognize It is composed of alternating bands of dark and light minerals

45 Non-Foliation Identified by their lack of planar character Identification of non-foliated rocks is dependent on the composition of the minerals or components in the rock Anthracite coal is similar to bituminous coal Both are black in color, and is composed of carbon Anthracite coal is generally shiny in appearance and breaks with a conchoidal fracture (broken glass also shows this type of fracture)

46 Non-Foliation Metaconglomerate is composed of pebbles and gravel that have been flattened due to directed pressure Quartzite is composed of quartz sand grains Quartz has a hardness of 7, which makes it difficult to scratch Marble is composed of calcite and will readily react to a small drop of HCl

47 Metamorphic Rock Identification Chart TEXTUREFOLIATIONCOMPOSITIONTYPEPARENT ROCKROCK NAME Foliated slatymicaRegionalMudstoneSlate phyllitic quartz, mica, chlorite RegionalMudstonePhyllite schistosemica, quartzRegionalSlateSchist schistose amphibole, plagioclase RegionalBasalt or GabbroAmphibolite gneissic banding feldspar, mica, quartz RegionalSchistGneiss Non-Foliated carbonContact or RegionalBituminous CoalAnthracite Coal quartz, rock fragments Contact or RegionalConglomerateMetaconglomerate calciteContact or RegionalLimestoneMarble quartzContact or RegionalSandstoneQuartzite

48 Test Your Metamorphic Rock Identification IQ Click on the link above You will see your 1 st rock to identify. Click on all the images provided for different views of this unknown rock Identify the correct Texture (Foliated or Non-Foliated) by using the information provided in class about metamorphic rocks Now, click the appropriate Foliation, Composition, and Parent Rock for this rock type, then click the right Rock Type. Once this is done, click “Grade Identification” for your answer. Write all your answers for each of the categories on the answer sheet provided to help you identify each rock type Do all 8 samples.

49 #1 - Texture Foliation Composition Parent Rock

50 #2 - Texture Foliation Composition Parent Rock

51 #3 - Texture Foliation Composition Parent Rock

52 #4 - Texture Foliation Composition Parent Rock

53 #5 - Texture Foliation Composition Parent Rock

54 #6 - Texture Foliation Composition Parent Rock

55 #7 – Texture Foliation Composition Parent Rock

56 #8 – Texture Foliation Composition Parent Rock

57 Sedimentary Rocks

58 Composed of sediment. Sediment is deposited in a number of environments of deposition, by both moving air and moving water Identification is primarily based on composition

59 Texture Three types of "texture" are considered – Clastic, Chemical, Biologic 1.Clastic – Contain clasts Fragments or pieces of rock or minerals The composition of clastic sedimentary rocks is divided into three types – Clay/silt, sand, and gravel Clay and silt are less than 1/16 mm Not visible to the unaided eye Sand is clasts between 1/16 and 2 mm in size Gravel is greater than 2 mm

60 Texture 2.Chemical – Mineral from which they are composed. Quartz, halite, gypsum and calcite Quartz has a hardness of 7 and is very difficult to scratch Gypsum is relatively soft (Hardness =2) and can be scratched easily with a fingernail Halite is common table salt and is most easily identified by taste (not recommended) Halite has a hardness of 2.5 and cannot be scratched by a fingernail (unpolished fingernail) Calcite readily reacts with a small drop of HCl

61 Texture 3.Biologic – Form as the result of the accumulation of organic material or biologic activity Coal is usually obvious to most people The dark brown to black color is the most obvious charateristic Coquina and limestone are both composed of calcite Coquina is composed almost entirely of shell or fossil fragments Limestone may or may not contain fossils fragments Both will react to HCl Limestone containing fossils is referred to as fossiliferous limestone

62 Sedimentary Rock Identification Chart TEXTUREGRAIN SIZECOMPOSITIONROCK NAME Clastic >2 mm rounded quartz, feldspar and rock fragments Conglomerate >2 mm angular quartz, feldspar and rock fragments Breccia 1/ mmquartz, feldsparSandstone >1/16 mmfeldspar, quartzArkose <1/16 mmquartz, clay minerals Siltstone (Mudstone, Shale) <1/256 mmquartz, clay mineralsClaystone Chemical silica (quartz)Chert dolomiteDolostone calciteLimestone haliteRock Salt gypsumRock Gypsum Biologic silica (quartz)Chert loosely compacted organic material and plant fragments Peat densely compacted organic material and plant fragments Bituminous Coal calciteLimestone calcite, micro-skeletal fragmentsChalk calcite, almost entirely shell and skeletal fragments Coquina calcite with some shell and skeletal fragments Fossiliferous Limestone dolomite with some shell and skeletal fragments Fossiliferous Dolostone

63 Test Your Sedimentary Rock Identification IQ Click on the link above You will see your 1 st rock to identify. Click on all the images provided for different views of this unknown rock Identify the Texture, Grain Size, Composition and Rock Type Identify the correct Texture (Clastic, Chemical, or Biologic) by using the information provided in class about sedimentary rocks Now, click the appropriate Grain Size and Composition Then click the right Rock Name Once this is done, click “Grade Identification” for your answer. Write all your answers for each of the categories on the answer sheet provided to help you identify each rock type Do all 12 samples.

64 #1 - Texture Grain Size Composition

65 #2 - Texture Grain Size Composition

66 #3 – Texture Grain Size Composition

67 #4 – Texture Grain Size Composition

68 #5 – Texture Grain Size Composition

69 #6 - Texture Grain Size Composition

70 #7 – Texture Grain Size Composition

71 #8 - Texture Grain Size Composition

72 #9 - Texture Grain Size Composition

73 #10 - Texture Grain Size Composition

74 #11 - Texture Grain Size Composition

75 #12 – Texture Grain Size Composition

76 Now that you are done…… Study your identifications for each Rock Type from each Rock Class. Know the difference and how each class was formed. We will go over your answers tomorrow and review for the quiz on Friday Turn in your answer sheet at the end of the period.


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