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Rock Hound Lab. Identify Major Characteristics of Different Rock Textures. Igneous rocks can be identified by the determination of the composition and texture of the rock. .
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Rock Hound Lab Identify Major Characteristics of Different Rock Textures
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.
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) • Intermediatecompositions - 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
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
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
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)
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
Equigranular Texture • Rocks with equigranular ("EC-wi-GRAN-ular") texture • Have mineral grains that are generally the same size • This example is a granite
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
Porphyry • Porphyry ("PORE-fer-ee") • Is a name used for any igneous rock with conspicuous larger grains— • phenocrysts—floating in a fine-grained groundmass
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
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)]
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 Basalt Obsidian
Obsidian Flow in 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). Obsidian Blocks Obsidian flows develop a blocky surface as their outer shell quickly solidifies Big Obsidian Flow in Newberry Caldera, Oregon Obsidian Tools Obsidian is the best material for making stone tools. The stone doesn't need to be perfect to make useful implements Weathering of Obsidian Water attacks obsidian readily because none of its material is locked up in crystals, making it prone to alteration into claysand related minerals.
Vesicular Texture 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 • 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 Pumice Used for abrasive grit or soil amendments
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
Test Your Igneous Rock Identification IQ • http://facweb.bhc.edu/academics/science/harwoodr/geol101/labs/igneous/Igsamp01.htm • Click on the link above • You will see your 1st rock to identify. • Click on all the images provided for different views of this unknown rock • Identify the Compositionby identifying the color and determine the Textureby 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 Texturefor 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.
#1 - • Composition • Texture
#2 - • Composition • Texture
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#6 – • Composition • Texture
#7 - • Composition • Texture
#8 - • Composition • Texture
#9 - Rhyolite • Composition • Felsic • Texture • Porphyritic
#10 - • Composition • Texture
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#12 – • Composition • Texture
Metamorphic Rocks These are what happens when sedimentaryandigneousrocks become changed, or metamorphosed, by conditions underground
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.
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
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
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
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
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
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
Texture Texture is divided into two groups
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)
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
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
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)
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
Test Your Metamorphic Rock Identification IQ • http://facweb.bhc.edu/academics/science/harwoodr/geol101/labs/metamorf/Meta02.htm • Click on the link above • You will see your 1st rock to identify. • Click on all the images provided for different views of this unknown rock • Identify the correctTexture(Foliated or Non-Foliated) by using the information provided in class about metamorphic rocks • Now, click the appropriate Foliation, Composition, and Parent Rockfor 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.
#1 - • Texture • Foliation • Composition • Parent Rock
#2 - • Texture • Foliation • Composition • Parent Rock