Chapter 4 Rocks and Minerals: Documents that Record Earth's History
What can Minerals Tell Us? 1. Minerals may contain radioactive elements that can be used for radiometric age dating. 2. Minerals that crystallize from magmas and lavas can provide information about temperatures, as well as viscosity of the magma, type of volcano, and tectonic setting. 3. Minerals that form under metamorphic conditions can provide information about temperatures and pressures, from which we can determine the depth at which metamorphism occurred, and information about the history of the formation of mountain ranges. 4. Minerals that form by evaporation in arid climates can tell us about paleoclimatic conditions. Since some climates are controlled by latitude, we can make general inferences about latitude. 5. Minerals that form in sea water tell us about the nature of ancient seas.
What can Minerals Tell Us? 6. Minerals which contain iron can record the orientation of the Earth's magnetic field, which yields information on latitude, and provides evidence for drifting continents, sea floor spreading, and movement and reversal of the Earth's magnetic poles. 7. Minerals in sedimentary rocks can provide information on the tectonic setting, amount of relief, paleoclimate, and types of rocks that are eroding in the source area. 8. Minerals can also tell us about the changing chemistry of the atmosphere, for example, the presence or absence of oxygen.
Minerals By definition, minerals are: 1. Naturally occurring 2. Inorganic 3. Solid 4. Definite chemical composition 5. Orderly internal crystal structure
Some Physical Properties of Minerals color streak luster hardness density crystal form cleavage fracture magnetism reaction to acid taste flexibility feel
Physical Properties of Minerals Color - the color or range of colors of a mineral as it appears to the eye in reflected light. Examples: Quartz may be colorless, white, pink, purple, dark brown, green or blue. Pyrite is always gold colored.
Physical Properties of Minerals Streak - the color of a mineral when it is ground to a powder. Streak color may be quite different from the whole mineral color. Examples: Hematite may be silver or gray, but it has a reddish brown streak. Pyrite is gold, but is has a black streak.
Physical Properties of Minerals Luster - the character of the light reflected from the mineral. A mineral may have a metallic luster or a non-metallic luster.
Physical Properties of Minerals Hardness - the resistance of a mineral to scratching. Hardness is measured on a scale of 1-10 called Mohs Hardness Scale. Hardness of minerals can also be compared to common objects (fingernail, copper penny, nail, glass).
Mohs Hardness Scale 1. Talc (softest) 2. Gypsum fingernail 3. Calcite penny (copper) 4. Fluorite nail 4. Apatite glass 6. Orthoclase feldspar (potassium feldspar) 7. Quartz 8. Topaz 9. Corundum 10. Diamond (hardest)
Physical Properties of Minerals Density - how heavy a mineral is for its size. The mass of a mineral divided by its volume is a measure of its density. Usually given as specific gravity (mass compared to water). Examples: Quartz has a density of 2.65 g/cm 3. Gold has a density of 19.3 g/cm 3.
Physical Properties of Minerals Crystal form - some minerals are in the form of crystals. Crystal shape is related to the structural arrangement of atoms within the mineral. Perfect crystals are rare because minerals typically grow close together in confined spaces, producing a mass of interlocking crystals.
Physical Properties of Minerals Cleavage - the tendency of a mineral to break along flat surfaces related to planes of weakness in its crystal structure. Some minerals tend to cleave or break into flat sheets (the micas: muscovite and biotite). Others break into cubes (halite), or into rhombs (calcite and dolomite).
Physical Properties of Minerals Fracture - irregular breakage, not related to planes of weakness in the mineral. Some minerals, such as quartz and olivine, do not have cleavage. They have a type of fracture called conchoidal fracture. Conchoidal fracture produces curved breakage surfaces, as seen on arrowheads or chipped glass.
Physical Properties of Minerals Magnetism - A few minerals are magnetic. They are attracted to a magnet, or they act as a natural magnet, attracting small steel objects such as paperclips. Example: Magnetite.
Physical Properties of Minerals Reaction to acid - The carbonate minerals react with diluted hydrochloric acid (HCl) by effervescing or fizzing, producing bubbles of carbon dioxide gas. Examples: Calcite fizzes readily in hydrochloric acid. Dolomite will fizz if it is first scratched and powdered.
Physical Properties of Minerals Taste - Some minerals have a distinctive taste. Example: Halite has a salty taste. It is used as table salt.
Physical Properties of Minerals Flexibility - Some minerals can be bent. Examples: Muscovite and biotite mica are elastic. When bent they return to their original shape. Gypsum is flexible. It bends and stays bent.
Physical Properties of Minerals Feel - Some minerals have a distinctive feel to the fingers. Example: Talc has a soapy feel.
Rock-Forming Minerals There are more than 3000 minerals on the Earth, but only a few are common and make up most of the rocks. The common rock-forming minerals can be divided into two groups: Silicates Non-silicates
Silicate Minerals Earth's crust is dominated by 2 chemical elements: Oxygen (46.6% by weight) Silicon (27.7% by weight) These elements help make up the dominant group of rock-forming minerals, the silicate minerals. Examples: quartz, feldspar, mica
Silicate Minerals - Feldspar Dominant mineral in Earth's crust. Two directions of cleavage at 90 o Flat, glassy rectangular surfaces. Color may be white, pink, gray, green. Common in igneous rocks such as granite and basalt. red orthoclase in granite
Two major types: Silicate Minerals - Feldspar Orthoclase (potassium feldspar) - KAlSi 3 O 8 Plagioclase - A range of compositions with sodium and calcium. Calcium-rich = anorthite (CaAl 2 Si 2 O 8 ) Sodium-rich = albite (NaAlSi 3 O 8 )
Silicate Minerals - Quartz Second-most abundant mineral in Earth's crust. Color varies - colorless, white (milky quartz), gray to brown (smoky quartz), pink (rose quartz), purple (amethyst), blue, or green. Hard (scratches glass) Glassy luster Conchoidal fracture. Six-sided, elongated crystals.
Silicate Minerals - Quartz Common in granite Resists weathering; common in some sands in humid areas Major constituent of quartz sandstone and quartzite. Chert is composed of microcrystalline quartz.
Perfect cleavage in one direction causing it to split into thin sheets. Two types: Silicate Minerals - Mica Muscovite - Colorless or silver-colored mica. Biotite - Black or dark brown mica (contains Mg and Fe).
Olive green color Glassy texture. No cleavage. Conchoidal fracture. Contains Mg and Fe. Silicate Minerals - Olivine Main constituent of the ultramafic rock, peridotite (birthstone = peridot).
Non-silicate Minerals Non-silicate minerals comprise about 8% of the Earth's crust. Carbonate minerals are the most widespread. Types: native elements oxides sulfides sulfates carbonates halides phosphates, etc.
Calcium carbonate. Calcite (CaCO 3 ) Aragonite (CaCO 3 ) different crystal form Calcium magnesium carbonate. Dolomite (CaMg(CO 3 ) 2 ) Carbonate minerals
Calcite Main constituent of limestone and marble. Shells of some marine organisms. Fizzes in hydrochloric acid. Has rhombohedral cleavage (three directions not at 90 o ). Cleavage fragments are rhombs.
Dolomite Has rhombohedral cleavage like calcite. Will fizz in acid only when scratched or powdered. Main constituent of sedimentary rock dolostone or dolomite. Forms from alteration of limestone through the addition of Mg.
Evaporite minerals Halite (NaCl) Gypsum (CaSO. 4 2H 2 O) Anhydrite (CaSO 4 )
Halite Major constituent of rock salt (and table salt). Cubic cleavage Salty taste. Typically colorless to white or pink.
Major constituent of rock gypsum. Used in Plaster of Paris and drywall. Soft - can be scratched by fingernail. Typically white or colorless to pink. Varieties: Selenite - clear crystals with rhombohedral cleavage Alabaster - fine-grained and massive Satin spar - fibrous Gypsum
Rocks A rock is an aggregate of one or more minerals. Rocks are the building blocks of the Earth's crust.
Rocks 3 Types 1. Igneous - Crystallized from hot, molten rock. Examples: granite, basalt 2. Sedimentary - Fragments of sediment laid down by water or wind become compressed or cemented over time Examples: sandstone, shale, limestone 3. Metamorphic - Rocks changed by heat and/or pressure or chemical activity Examples: gneiss, schist, slate, marble
Through the rock cycle, one type of rock can be converted into another. The Rock Cycle
Igneous Rocks The word igneous means "fire-formed." Igneous rocks crystallized from hot, molten magma or lava, as it cooled. Magma is hot, molten rock beneath the surface of the Earth. Lava is hot, molten rock which has flowed out on the surface of the Earth. Igneous rocks make up more than 90% of Earth's crust, by volume.
Extrusive Igneous Rocks Extrusive or volcanic rocks form from lava, which cooled on the Earth's surface. Examples: Basalt, rhyolite, andesite, obsidian
Intrusive Igneous Rocks Intrusive or plutonic igneous rocks form from magma which cooled beneath the surface. Examples: Granite, gabbro, diorite
Cooling History and Grain Size The texture of a rock is a description of its grain size. Cooling rates influence the texture of the igneous rock. Lava cools much more quickly than magma because lava is on the surface of the Earth, where temperatures are much lower than they are at depth. Extrusive rocks = quick cooling = fine grained Intrusive rocks = slow cooling = coarse grained
Extrusive vs. Intrusive Rhyolite - fine-grained, extrusive igneous rock. Granite - coarse-grained, intrusive igneous rock.
Igneous Rock Classification Igneous rocks are classified on the basis of: 1. Texture (or grain size) 2. Composition
Igneous Rock Composition Groups 1. Silica-rich 2. Intermediate 3. Silica-poor
1. High percent silica (quartz). 2. Light-colored. Silica-rich Rocks 3. Has light-colored minerals such as quartz and potassium feldspar. Examples: granite, rhyolite.
Intermediate Rocks 1. Intermediate in composition between silica-rich and silicapoor. 2. Mixture of light and dark minerals. Examples: diorite, andesite.
Silica-poor Rocks 1. Iron and magnesium rich. 2. Dark-colored. 3. Has dark minerals such as olivine, pyroxene, and amphibole. Examples: gabbro, basalt.
Very silica-poor Rocks 1. Very iron and magnesium rich. 2. Typically green in color due to abundant olivine. Example: Peridotite.
Igneous Rock Classification Silica-rich (silicic) Intermediate Silica-poor (mafic) Very silica poor (ultramafic) F i n e Rhyolite Andesite Basalt C o a r s e Granite Diorite Gabbro Peridotite
Basalt The most common igneous rock. Ocean crust is dominated by basalt. Covers about 70% of Earth's surface. Islands like Hawaii and Iceland are made of basalt. Fine-grained texture Dark color because it contains ferromagnesian (Fe and Mg) minerals, along with feldspar.
Granite Earth's continental crust is dominated by granite. Coarse-grained texture. Light color because it is dominated by light-colored minerals like quartz and feldspar.
Bowen's Reaction Series Minerals in igneous rocks crystallize in a particular order, at particular temperatures.
Sedimentary Rocks Cover about 75% of the world's land area. Form when loose sediment (gravel, sand, silt or clay) becomes compacted and/or cemented to form rock. The process of converting sediment to sedimentary rock is called lithification.
Sedimentary Rocks Sediment is deposited in horizontal layers. A major characteristic of sedimentary rock is layering, also called bedding or strata.
Sedimentary Rocks Sedimentary rocks contain the fossil record, which preserves the evolving story of life on Earth.
What can sedimentary rocks tell us? Locations of ancient sedimentary environments (seas, reefs, deltas, beaches, rivers, lakes deserts, glaciers, and mountains). Ancient climates humid tropical coal swamps, dry windswept deserts, glacial ice sheets, high temperatures and high sea levels. Sedimentary rocks contain the fossil record, which preserves the evolving story of life on Earth.
Sedimentary Rocks Sedimentary rocks contain the fossil record, which preserves the evolving story of life on Earth. Sedimentary rocks also hold the fossil fuels and energy resources on which our culture depends - coal, oil, natural gas. Careful reading of the rock record allows exploration geologists to find these critical resources.
How is sediment formed? Sediment forms from the weathering and erosion of rocks, as part of the rock cycle.
Types of Sedimentary Rocks Clastic Sedimentary Rocks (also called terrigenous or detrital) Chemical / biochemical Sedimentary Rocks Organic Sedimentary Rocks (Coal)
Types of Sedimentary Rocks 1. Clastic sedimentary rocks (also called terrigenous or detrital) Conglomerate or Breccia Sandstone Siltstone Shale or Claystone 2. Chemical/biochemical sedimentary rocks Evaporites Carbonate sedimentary rocks (limestone and dolostone or dolomite) Siliceous sedimentary rocks (chert, diatomite) 3. Organic sedimentary rocks (coal) Peat Lignite Bituminous coal Anthracite coal
Clastic Sedimentary Rocks Clastic sedimentary rocks are derived from the weathering of pre-existing rocks, which have been transported to the depositional basin.
Clastic Texture Clasts (larger pieces, such as sand or gravel) Matrix (mud or fine-grained sediment surrounding the clasts) Cement (the chemical "glue" that holds it all together) Types of cement: Calcite Iron oxide Silica
Clastic Sedimentary Rocks Classified by Grain Size Gravel - Grain size greater than 2 mm Sand - Grain size 1/16 to 2 mm Silt - Grain size 1/256 to 1/16 mm Clay - Grain size less than 1/256 mm
Clastic Sedimentary Rocks are classified by grain size Grain size Rock name Gravel Conglomerate = rounded clasts Breccia = angular clasts Sand Sandstone Silt Siltstone Clay Shale = fissile Claystone = massive
Chemical/Biochemical Sedimentary Rocks Form within the depositional basin from chemical components dissolved in the seawater. or Chemicals are removed from seawater and made into rocks by chemical processes, or biological processes (such as shell growth).
Chemical/Biochemical Sedimentary Rocks 1. Evaporites - form from the evaporation of water 2. Carbonate rocks - form by chemical processes and biochemical processes 3. Siliceous rocks - form from chemical processes (silica replacing limestone) or biochemical processes (silicasecreting organisms)
Evaporites 1. Rock salt - composed of halite (NaCl). 2. Rock gypsum - composed of gypsum (CaSO 4. 2H 2 O) 3. Travertine - composed of calcium carbonate (CaCO 3 ) a carbonate rock; forms in caves and around hot springs.
Carbonate Rocks 1. Limestones Micrite (microcrystalline limestone) Oolitic limestone Fossiliferous limestone Coquina Chalk Crystalline limestone Others 2. Dolostones or dolomites
Siliceous rocks Diatomite - made of microscopic planktonic organisms called diatoms. Resembles chalk, but does not fizz in acid. Chert - massive and hard, microcrystalline quartz. May be dark or light in color. Often replaces limestone. Does not fizz in acid.
Organic Sedimentary Rocks - Coal Composed of organic matter (plant fragments). Forms in swamps. With increasing depth of burial (temperature and pressure): Peat Lignite Bituminous coal Anthracite coal
Organic Sedimentary Rocks - Coal Coal is a fossil fuel. Electric utility companies use more than 90% of the coal mined in the U.S. Chemicals derived from coal are used in making plastics, tar, synthetic fibers, fertilizers, and medicines. Releases more greenhouse gas than any other fossil fuel. Contains many other pollutants like uranium and mercury.
Metamorphic Rocks Metamorphic means "changed form." Metamorphism causes changes in the texture and mineralogy of rocks. Rocks are changed or metamorphosed by: 1. High temperatures 2. High pressures 3. Chemical reactions caused by solutions and hot gases
1. Contact metamorphism Types of Metamorphism Alteration of rock by heat adjacent to hot molten lava or magma. 2. Regional metamorphism Alteration of rock over a large area by heat and pressure due to deep burial or tectonic processes.
Types of Metamorphic Rocks Metamorphic rocks are separated into two groups on the basis of texture. Foliated Non-foliated (or granular) Foliation = Laminated structure in a metamorphic rock resulting from the parallel alignment of sheet-like minerals (usually micas).
Foliated Metamorphic Rocks In order of increasing grade of metamorphism: Slate Phyllite Schist Gneiss
Foliated Metamorphic Rocks Slate - Mica flakes are microscopic in size. Derived from the regional metamorphism of shale. Note the relict sedimentary bedding (vertical). Phyllite - Mica flakes are very finegrained; other minerals such as garnet or staurolite may also be present. Derived from the regional metamorphism of shale.
Foliated Metamorphic Rocks Schist - Mica flakes are visible to the unaided eye. Derived from the regional metamorphism of shales or fine-grained volcanic rocks.
Foliated Metamorphic Rocks Gneiss - Coarse-grained rock with minerals segregated into light and dark layers or bands. Derived from the regional metamorphism of high-silica igneous rocks, and muddy sandstones.
Non-foliated Metamorphic Rocks Marble - Composed of finely- to coarsely-crystalline calcite or dolomite. Derived from the metamorphism of limestone or dolostone. Commonly white or gray. May be pink.
Non-foliated Metamorphic Rocks Quartzite - Composed of finely- to coarsely-crystalline quartz. Derived from the metamorphism of quartz sandstone.
Non-foliated Metamorphic Rocks Greenstone - Contains iron and magnesium rich green minerals such as chlorite and epidote. Fine grained texture. Derived from the low-grade metamorphism of basalt.
Metamorphic Index Minerals Certain minerals form during metamorphism, under specific pressure and temperature conditions. These minerals can be used as a guide to metamorphic pressures and temperatures. They are called metamorphic index minerals. Chlorite and muscovite form at relatively low temperatures. Biotite and garnet form at somewhat higher temperatures and pressures. Staurolite and kyanite form at intermediate to high temperatures and pressures. Sillimanite forms at the highest temperatures and pressures.
Metamorphic Index Minerals