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Rocks - UMKC School of Education

By Tim Edwards,2014-09-30 04:26
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Rocks - UMKC School of Education

The Earth Rocks

Exploration Phase

    Topic: Rock Types Igneous, Sedimentary, and Metamorphic

    Activity I:

    Rock Identification Using Rock Kits

    1. Using the rock kits provided determine which rocks samples are igneous, sedimentary

    and metamorphic.

    2. Use the nail and magnifying lens to help you test the properties of each rock.

    3. Place each rock you have identified on the appropriate corresponding sheet of paper

    labeled Igneous, Sedimentary or Metamorphic Rock.

    4. Leave any rock samples you were not able to classify in the rock kit box.

    Activity II:

    Peer Review

1. Share your findings and review another group’s results from the initial rock identification.

    2. Make suggestions as to which rocks may have been identified incorrectly and discuss the

    rock classifications amongst yourselves.

    3. As we discuss the rock samples in class, note which rocks you identified correctly and

    which were incorrect.

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Content Review:

    Igneous Rocks

    Igneous rocks rocks that form when molten rock (rock liquefied by intense heat and pressure) cools to a solid state

     - when molten rock cools, it always forms a mass of intergrown crystals and/or

    glass; therefore, all igneous rocks have a crystalline or glassy texture

Igneous Processes

    Intrusive vs. Extrusive Igneous Rocks

     Intrusive rocks that form by the cooling of intrusions within the earth

     Extrusive igneous rocks that form at the planet’s surface; they are extruded or

    violently ejected onto the surface

Bodies of Igneous Rocks

    2Batholith massive igneous intrusions (covering regions of 100 km)

    Sill sheet-like intrusions that force their way between layers of rock

    Laccolith blister-like sills

    Pipe vertical tubes that feed volcanoes

    Dike sheet-like intrusions that cut across layers of rock (sheet dike, ring dike, radial dike)

Textures - dependent on nucleation; the atoms are mobile in a magma and free to nucleate

    nucleation - initial formation of a microscopic crystals to which other atoms progressively bond

    - generally if magma cools more slowly, the atoms in the magma have time to grow

     - if the magma cooled quickly, the mineral crystals will be smaller because they

    didn’t have time to grown and nucleate

     obsidian - formed when the intense heat of a volcano fuses masses of silica together very

    quickly forming the hard glass

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Examples of Igneous Rock Textures

    Pegmatitic crystals > 1cm, very slow cooling

     Phaneritic crystals 1-10mm, slow cooling

    Porphyritic large and small crystals (phenocrysts = large crystals, matrix = smaller

    more numerous minerals

    Aphanitic crystals < 1 mm, rapid cooling, fluid lava

    Glassy rapid cooling

    Vesicular rapid cooling of gas charged lava, gas bubbles in lava (scoria, pumice)

    Pyroclastic particles or fragments emitted from volcanoes (tuff volcanic ash, volcanic

    breccia)

Mineral Composition

    Mafic vs. Felsic Minerals

    Mafic minerals (dark-colored) ferromagnesian minerals (iron and magnesium)

     Biotite Mica (black)

     Amphibole (dark gray)

     Pyroxene (dark green)

     Olivine (green)

    Felsic minerals (light-colored)

     Quartz (gray)

     Plagioclase feldspar (white)

     Potassium Feldspar (K-spar) (pink)

     Muscovite mica (brown)

    Color Index (CI) is the percentage (by volume) of mafic mineral crystals in the rock

    - a rough measure of the proportions of mafic and felsic minerals

Felsic igneous rocks: 0-15% mafic minerals

    Intermediate igneous rocks: 16 45% mafic minerals

    Mafic igneous rocks: 46-85% mafic minerals

    Ultramafic igneous rocks: 86-100% mafic minerals

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    Sedimentary Rocks

How do sedimentary rocks form?

    They form when sediments accumulate and are lithified (meaning hardened by

    compression or cementation), or when masses of intergrown mineral crystals precipitate

    from aqueous solutions

    - Lithification of sediments

    - Cementation thin films of chemical residues “glue” grains together

    - Precipitation of aqueous solutions (i.e. rock salt)

    Sediments these are loose grains and chemical residues of rock fragments,

    mineral grains, animals or plants, and rust (hydrated iron oxide

    residue)

     - Sediments are the products of weathering processes

    - All sediments have a source; either produced by biochemical (organic) processes

    of plants and animals or by chemical or physical weathering processes of

    organic and inorganic materials

    Physical weathering the cracking, scratching, crushing, abrasion, or other physical

    disintegration

    - Large rocks will be broken down into individual clasts (broken pieces of rock fragments

    and minerals)

    - Plant matter, logs and animal shells will be broken down into peat and shell gravel

    Chemical weathering - the chemical decomposition or dissolution of earth materials

    - Feldspar and mica -- broken down into clay minerals

    - Calcite -- goes to calcium and bicarbonate ions in solution

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Classification of Sedimentary Rocks

    A) Clastic (a.k.a. Detrital) rocks made up of mostly rock fragments, quartz grains, feldspar

    grains, or clay minerals

    B) Biochemical (a.k.a. Bioclastic) rocks made mostly of grains that are fragments or shells

    of organisms (plant/animals)

    C) Chemical made mostly of mineral crystals precipitated from aqueous solutions and/or

    chemical residues (e.g. rust)

Texture of a Sedimentary Rock

    - Texture is a description of its constituent parts and their sizes, shapes and

    arrangements

    - Sediments can be transported great distances by wind, water and ice

    - This causes the sediment grains to be dragged, bounced, rolled, and

    carried and causes grains to be scratched, broken, rounded and worn

    (glacial action)

Grain Shape

    Angular grains

    Subangular

    Subrounded

    Rounded

Grain Sizes

    Gravel - includes grains larger than 2 mm in diameter (granules, pebbles, cobbles, boulders)

    Sand - all grains are visible and feel gritty between your fingers (1/16mm-2mm in diameter)

    Silt - grains usually too small to see (grains from 1/256mm to 1/16mm in diameter)

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     - You can feel them as tiny grits between your fingers or teeth

     Clay - grains too small to see w/out microscope (grains less than 1/256mm in diameter)

     - Feels smooth when rubbed between your fingers

     Sorting of Sediments (Grain Arrangements) wind and water currents transport and naturally

    separate sediments into different grains sizes and densities

    Well sorted composed of sediments that are of similar sizes; usually well-rounded

    Poorly sorted many different sizes and/or densities of sediment grains mixed together

    Metamorphic Rocks

    Metamorphic Rocks rocks that are changed from one form to another by intense heat and

    pressure or by the action of watery hot fluids inside of the earth

     - Examples: marble, quartzite, slate, phyllite, schist, gneiss,

    metaconglomerate, anthracitic coal, etc.

Parent Rock the original rock that was metamorphosed into a new rock; also known as protolith;

    can be any of the three types of rocks (igneous, sedimentary or metamorphic)

     - Examples: limestone, sandstone, shale, conglomerate, mafic or ultramafic igneous

    rx

Common Metamorphic Rock forming Minerals:

     Quartz, Calcite, Dolomite, Feldspars, Muscovite, Biotite, Chlorite, Garnet,

     Tourmaline, Serpentine, Talc, Kyanite, Sillimanite, Amphibole (Hornblende)

    Contact metamorphism occurs locally, adjacent to igneous intrusions and along fractures that

    are in contact with watery hot fluids (hydrothermal metamorphism)

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     - Hydrothermal Metamorphism involves the condensation of gases

    to form liquids which can precipitate mineral crystals along

    fractures

    - Zones of contact metamorphism are narrow from millimeters to tens

    of meters thick

    - Caused by conditions of moderate pressure and heating for days to

    thousands of years

    Regional metamorphism occurs over very large areas (regions), such as deep within the cores

    of rising mountain ranges, and is generally accompanied by folding

    and shearing of rock layers

    - Caused by 1) large igneous intrusions that form and cool over long

    periods, 2) extreme pressure and heat, and/or 3) widespread

    migration of hot fluids

     * Note: most major intrusions are preceded by contact metamorphism

    and then followed by regional metamorphism

    Recrystallization a process where small crystals of one mineral will slowly convert to fewer,

    larger crystals of the same mineral, without melting the rock

Metamorphic Rock Textures

    1) Foliated: rocks with a layered appearance

     Slaty rock cleavage very flat foliation developed along closely spaced shear planes (slate)

     Phyllite texture wavy to wrinkled foliation of cryptocrystalline platy minerals (phyllite)

    Schistosity a scaly, glittery layering of visible platy minerals (medium-to-

    coarse-grained), and/or linear alignment of long prismatic crystals (schist,

    garnet schist)

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     Gneissic banding alternating layers of light and dark medium-to-coarse-grained minerals

     (gneiss)

2) Non-Foliated: no obvious layering present

     Crystalline texture aggregate of intergrown crystals, medium-to-coarse grained (marble)

     Microcrystalline texture very fined-grained aggregate of intergrown crystals (hornfels)

     Sandy texture medium-to-coarse-grained aggregate of fused sand-sized grains (quartzite)

     Glassy texture no visible grains (anthracite coal)

3) Other Metamorphic Features

    Stretched or sheared grains deformed pebbles, fossils, or mineral crystals that have

    been stretched out (i.e. metaconglomerate)

     Porphyroblastic texture large crystals set in a finer-grained groundmass

     Hydrothermal veins fractures healed by minerals that precipitated from hydrothermal fluids

     Folds bends and buckles in rock layers that were originally flat

     Lineations lines on rock at the edges of foliation, shear planes, etc.

    Application Phase

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Activity:

    1. Pass around and discuss the rock hand samples (larger classroom samples) and determine

    what minerals comprise each rock.

    2. Classify the hand samples by type (igneous, sedimentary or metamorphic).

    Terms: Parent rock, igneous, metamorphic, sedimentary, batholiths, intrusion, mafic, felsic, pyroclastics, lithifaction, foliation, color index, sedimentation, weathering, metamorphosis, extrusion, layering, pressure, heat, texture.

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