Presentation on theme: "Identify Major Characteristics of Different Rock Textures"— Presentation transcript:
1 Identify Major Characteristics of Different Rock Textures Rock Hound LabIdentify 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.
4 CompositionColor is often an indicator of the composition of most igneous rocksFelsic composition - Light colors, including white, light gray, tan and pinkFelsic compositions are rich in silica (SiO2) (sand, quartz, diatoms)Mafic or Ultramafic composition - Dark colors, such as black and dark brownMafic 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 mineralBeware that even though an igneous rock may have a felsic composition (light color), the rock can contain dark colored mineralsMafic rocks may contain light colored minerals as wellAs 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 DuniteObsidian is volcanic glass which erupts as a lava flowMost 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 colorDunite is composed almost entirely of the mineral olivine which usually contains both iron and magnesium
6 Felsic Intermediate Mafic EARTH SCIENCE LAB Composition of Igneous RocksColor IndexFelsicIntermediateMaficIgneous Rock Composition ChartFelsicIntermediateMafic
7 Textures are based primarily on crystal size The texture of an igneous rock does not refer to the roughness or smoothness of the surfaceTextures are based primarily on crystal size
8 TexturePegmatitic 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 crystalsAphanitic 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 lenslike this granite
10 Equigranular TextureRocks with equigranular ("EC-wi-GRAN-ular") textureHave mineral grains that are generally the same sizeThis 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 lenslike 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 andesiteHave larger mineral grains, or phenocrysts ("FEEN-o-crists"), in a matrix of smaller grainsAndesite is an extrusive or intrusive igneous rock that is higher in silica than basalt and lower than rhyolite or felsite
14 TextureGlassy texture is the most readily recognized. The rock is composed entirely of glass. Few, if any, crystals will be visibleVesicular 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 Pronounced – “Pa-ho-ho” "ah-ah." Glassy TextureGlassy (or hyaline or vitreous) rocksHave no or almost no grains at all, as in this quickly chilled pahoehoe basalt or in obsidianObsidian is an extrusive rock:lava that cooled without forming crystals, giving it a glassy textureBasalt most common lava rock typePahoehoe and Aa BasaltPronounced – “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 lavaPahoehoe BasaltBasaltObsidian
16 Obsidian Flow in Oregon Obsidian Flow TextureObsidian may display complex folding and segregation of minerals in bands and round masses consisting of feldspar or cristobalite (high-temperature quartz).Obsidian BlocksObsidian flows develop a blocky surface as their outer shell quickly solidifiesBig Obsidian Flow in Newberry Caldera, OregonObsidian ToolsObsidian is the best material for making stone tools. The stone doesn't need to be perfect to make useful implementsWeathering of ObsidianWater attacks obsidian readily because none of its material is locked up in crystals, making it prone to alteration into clays and related minerals.
17 Scoria (Volcanic Cinders) Vesicular TextureRocks with vesicular ("ve-SIC-ular") textureAre full of bubblesIt always indicates a volcanic rock, like this scoriaPumice is basically lava frothAn extrusive rock frozen as its dissolved gases come out of solutionIt looks solid but often floats on waterScoria (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 waterPumiceUsed for abrasive grit or soil amendments
18 Pyroclastic Texture Rocks with pyroclastic ("PY-ro-CLAS-tic") texture Are made of pieces of volcanic materialCreated in an explosive eruptionLike this welded tuffTuff 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 aboveYou will see your 1st rock to identify.Click on all the images provided for different views of this unknown rockIdentify 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 typeDo all 12 samples.
32 Metamorphic RocksThese are what happens when sedimentary and igneous rocks become changed, or metamorphosed, by conditions underground
33 The 4 Main Agents That Metamorphose Rocks HeatPressureFluidsStrainThese 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 pressureThe clay minerals of sedimentary rocks are a good exampleClays 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 feldsparFeldspar & Mica Clays + Heat & Pressure Feldspar & Mica
35 Fluids Important agent of metamorphism Every rock contains some water Sedimentary rocks hold the mostFirst there is the water that was trapped in the sediment as it became rockSecond is the water that is liberated by clay minerals as they change back to feldspar and micaThis fluid may be acidic or alkaline or full of sulfides or carbonates or metals
36 StrainRefers to any change in the shape of rocks due to the force of stressMovement on a fault zone is one exampleIn 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 thickerThe foliated rock types that form under these conditions are called schist or gneiss, depending on their textureSchist is finely foliatedWhereas gneiss is organized in wide bands of minerals
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 cementedSandstone quartziteMixture of sand and clay mudstones schists or gneissesLimestone marble
40 Metamorphic RocksRocks that have undergone a change from their original form due toTemperature, pressure or chemical alterationTheir classification is based on the minerals that are presentAnd the temperature and pressure at which these minerals form
42 Foliated and Unfoliated Rocks First steps towards identifying metamorphic rocks is to determine if it has foliation or notFoliation means banding or alignment of the minerals resulting from pressureFoliated 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 surfacesPhyllitic 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 eyeThe 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 eyePlaty minerals tend to dominateGneissic banding - is the easiest of the foliations to recognizeIt 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 rockAnthracite coal is similar to bituminous coalBoth are black in color , and is composed of carbonAnthracite coal is generally shiny in appearance and breaks with a conchoidal fracture (broken glass also shows this type of fracture)
46 Non-FoliationMetaconglomerate is composed of pebbles and gravel that have been flattened due to directed pressureQuartzite is composed of quartz sand grainsQuartz has a hardness of 7, which makes it difficult to scratchMarble is composed of calcite and will readily react to a small drop of HCl
47 Metamorphic Rock Identification Chart TEXTUREFOLIATIONCOMPOSITIONTYPEPARENT ROCKROCK NAMEFoliatedslatymicaRegionalMudstoneSlatephylliticquartz, mica, chloritePhylliteschistosemica, quartzSchistamphibole, plagioclaseBasalt or GabbroAmphibolitegneissic bandingfeldspar, mica, quartzGneissNon-FoliatedcarbonContact or RegionalBituminous CoalAnthracite Coalquartz, rock fragmentsConglomerateMetaconglomeratecalciteLimestoneMarblequartzSandstoneQuartzite
48 Test Your Metamorphic Rock Identification IQ Click on the link aboveYou will see your 1st rock to identify.Click on all the images provided for different views of this unknown rockIdentify the correct Texture (Foliated or Non-Foliated) by using the information provided in class about metamorphic rocksNow, 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 typeDo all 8 samples.
58 Identification is primarily based on composition Sedimentary RocksComposed of sediment.Sediment is deposited in a number of environments of deposition, by both moving air and moving waterIdentification is primarily based on composition
59 Texture Clastic – Three types of "texture" are considered – Clastic, Chemical, BiologicClastic –Contain clastsFragments or pieces of rock or mineralsThe composition of clastic sedimentary rocks is divided into three types –Clay/silt, sand, and gravelClay and silt are less than 1/16 mmNot visible to the unaided eyeSand is clasts between 1/16 and 2 mm in sizeGravel is greater than 2 mm
60 Texture Chemical – Mineral from which they are composed. Quartz, halite, gypsum and calciteQuartz has a hardness of 7 and is very difficult to scratchGypsum is relatively soft (Hardness =2) and can be scratched easily with a fingernailHalite 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 TextureBiologic –Form as the result of the accumulation of organic material or biologic activityCoal is usually obvious to most peopleThe dark brown to black color is the most obvious charateristicCoquina and limestone are both composed of calciteCoquina is composed almost entirely of shell or fossil fragmentsLimestone may or may not contain fossils fragmentsBoth will react to HClLimestone containing fossils is referred to as fossiliferous limestone
62 Sedimentary Rock Identification Chart TEXTUREGRAIN SIZECOMPOSITIONROCK NAMEClastic>2 mmrounded quartz, feldspar and rock fragmentsConglomerateangular quartz, feldspar and rock fragmentsBreccia1/ mmquartz, feldsparSandstone>1/16 mmfeldspar, quartzArkose<1/16 mmquartz, clay mineralsSiltstone (Mudstone, Shale)<1/256 mmClaystoneChemicalsilica (quartz)ChertdolomiteDolostonecalciteLimestonehaliteRock SaltgypsumRock GypsumBiologicloosely compacted organic material and plant fragmentsPeatdensely compacted organic material and plant fragmentsBituminous Coalcalcite, micro-skeletal fragmentsChalkcalcite, almost entirely shell and skeletal fragmentsCoquinacalcite with some shell and skeletal fragmentsFossiliferous Limestonedolomite with some shell and skeletal fragmentsFossiliferous Dolostone
63 Test Your Sedimentary Rock Identification IQ Click on the link aboveYou will see your 1st rock to identify.Click on all the images provided for different views of this unknown rockIdentify the Texture, Grain Size, Composition and Rock TypeIdentify the correct Texture (Clastic, Chemical, or Biologic) by using the information provided in class about sedimentary rocksNow, click the appropriate Grain Size and CompositionThen click the right Rock NameOnce 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 typeDo all 12 samples.
76 Turn in your answer sheet at the end of the period. 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 FridayTurn in your answer sheet at the end of the period.