LEcture4 - So .. working backwards: Tectonic Plates are...

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Unformatted text preview: So .. working backwards: Tectonic Plates are part of the lithosphere .. which is rock made of minerals .. enriched in elements: Si, Al, (K, Na, Ca) -rocks termed silicic or felsic (sialic) Asthenosphere & mesosphere are rock enriched in elements: Mg & Fe -rocks termed (ultra-) mafic or basic Minerals are building blocks of rocks: -naturally occurring -solid -definite chemical composition (elements; expressed by a chemical formula) -characteristic crystal structure (quartz) Elements is a form of matter that cannot be broken down into simpler form by heating, cooling, or chemical reaction (Si, O) Compound – combination of 1 or more elements in specific proportions -atoms bond to form minerals in regular geometric shapes (crystals => SiO2 : quartz) Atoms – smallest particles of an element that retains all of the element's chemical properties (distinguishing characteristic) 1 Composition of minerals Combination of elements to form minerals depends on a few characteristics of atoms of that element: 1) relative abundance of available elements (w/in crust): O : 45% Si : 27% Al : 8% Fe : 5% BIG 8 Ca : <5% Mg : <5% Na : <5% K : <5% Atomic structure • central region called the nucleus – Consists of protons (positive charges) and neutrons (neutral charges) Composition of minerals Atomic structure • Atom is neutral if protons = electrons • So, why do atoms bond and interact? • electrons – Negatively charged particles that surround the nucleus – Located in discrete energy levels called shells • Chemical stability requires 8 electrons in outermost shell (octet rule) 2) T & P at time of formation - controls structural growth of crystal - e.g. C = diamond & graphite ! (polymorphs .. same chemistry, different structure) 3) size & charge of atoms and ions 2 Composition of minerals Electrons Oxygen Valence electrons Second most abundant element in Earth by weight (most abundant in crust) Element % Fe 34.6 O 29.5 Si 15.2 Mg 12.7 Chemical bonding • Formation of a compound by combining two or more elements 1) Ionic bonding • Atoms gain or lose valence electrons to form ions (charged particles) • Ionic compounds consist of an orderly arrangement of oppositely charged ions; neutral / stable 3 Composition of minerals Electrons 2) Covalent bonding • Atoms share electrons to achieve electrical neutrality • Covalent compounds are generally stronger than ionic bonds (C = diamond) Valence electrons Sodium +1 electron Chlorine -1 • Both ionic and covalent bonds typically occur in the same compound (bonds are seldom 100% ionic or covalent in character) Halite (NaCl) – ionic bonding 4 Composition of minerals 3) Other types of bonding a) Metallic bonding – Cloud of valence electrons are free to migrate among atoms – Metals = conductors – Weaker and less common than ionic or covalent bonds b) Van der Wall forces (secondary bonding) – Weak electrostatic forces – Very weak (e.g. graphite .. also C) Composition of minerals Not only is charge a factor in creating compounds, but also ionic radius (distance from nuclei center to outermost electron) atoms competing for space in crystallizing liquid Composition of minerals Certain atoms have same charge & nearly same radius: Fe2+ Mg2+ 0.83 nm 0.78 nm = easily substituted = substitution of 1 atom for another is called SOLID SOLUTION Olivine: Mg2SiO4 What's the difference Fe2SiO4 Change chemistry, not structure between solid sol'n & polymorph? 5 Classification of minerals So now we can start building minerals - silicates: 4000+, 20 are common, BIG 7 (from Big 8 which make 98% of crust) crust: quartz mica potassium feldspar plagioclase feldspar amphibole mantle: pyroxene olivine SiO2 K(Al, Mg, Fe)2-3Si3O10(OH)2 KAlSi3O 8 NaAlSi3O8 - CaAl2 Si2 O 8 (Na, Ca)2 (Mg, Al, Fe)5 (Si, Al)8 O22(OH)2 Classification of minerals minerals everyday… 2) Other minerals: nonsilicates Several major groups exist including – Oxides Fe2O3 (iron ore) – Sulfides PbS (lead ore) – Sulfates CaSO4 (gypsum) – Native Elements Au (gold) – Carbonates CaCO3 (calcite) (Fe, Mg, Ca)Si2 O 6 FeMgSiO4 6 Biominerals P5+ in seawater tooth enamel (apatite: Ca5(PO4 )3 (F,Cl,OH)) bones of vertebrates (apatite) stones in urinary track (calcium & phosphate deposits) shells of molluscs and other invertebrates (calcite) The apatite cycle 7 Igneous Rocks Magma is molten rock and contains crystals & gases within the Earth. Melt is generally only the liquid portion of the process. Lava is molten rock and contains crystals & gases at the surface of the Earth. How does magma form? Subduction & crustal thickening Temperature: • radioactive decay within Earth • early Earth produced during Big Bang • ± frictional heat of tectonic plates ..crustal thickening • (convergent margins) + water 8 How does magma form? Decompression melting Pressure: • Hi-P results in higher T to melt solid (bonds are closer together = stronger) • If quickly release Hi-P, solid will melt • (divergent margins) ..crustal decompression 9 Where does rock come from? Before minerals form: • liquid magma has random atoms suspended in non-crystalline structure During magma cooling: • atomic vibrations slow & bonds begin to form compounds & eventually a crystalline structure After cooling = igneous rock • rock which cooled & crystallized directly from molten material at/near surface Where does rock come from: volcanic... Erupt & flow, or explode at the Earth’s surface Cooling times range from sec to yrs May have no crystals (all glass) or small + large crystals (phenocrysts) Basalt is the archetype - composed of very fine grained minerals (chiefly pyroxene & plagioclase feldspar), forms mostly on the ocean floor Vulcan (god of fire) & Pluto (god of underworld) extrusive / volcanic & intrusive / plutonic surface subsurface 10 Where does rock come from: plutonic.. How do we know? thin sections polarizing microscope Intrude at depths of 0.1's-10's of km within the crust mm-cm scale crystals are generally evident Can form huge masses (plutons/batholiths) that likely cool over a million years Granite is the archetype - composed of potassium + sodium feldspar and quartz and commonly found on the continents 11 Characteristics of magma Characteristics of magma Bowenʼs series crystallization: Nature of magma consists of 3 components: – Liquids (melt) composed of mobile ions – Solids (crystals) • Cooling of magma results in systematic crystallization pattern – Generally silicates, SiO2 : 40-70 wt% + variable amounts of other oxides, e.g. Al2O3, K2O, Na2O, CaO, MgO, FeO etc. – Gases (volatiles) dissolved in the melt from 0.55.0 of wt% – water vapor (H2O) – carbon dioxide (CO2 ) – sulfur dioxide (SO2) • An explanation for the diversity of igneous rock types that appear to have evolved from a single magma source (basalt) • Silicate minerals crystallize in a predictable order!!! Bowenʼs Reaction Series ! ! ! ! ! ! • As they slowly cool in deep reservoirs (magma chambers) different minerals achieve saturation at different temperatures • Crystallization of the minerals changes the chemical composition of the magma Bowenʼs series crystallization occurs in 2 ways: 12 Discontinuous - Fe, Mg minerals crystallize one after another in a specific sequence; composition & structure change (simple -> complex) Bowen’s Reaction Series Continuous - Ca-plagioclase preferentially crystallizes early (hi-T); gradually as magma cools, Na ions continuously replace Ca in crystal; little structural change Bowen’s Reaction Series Bowenʼs reaction series… assumes: 1) Closed system 2) Early minerals would remain in contact with magma, reacting to form new minerals 3) Demonstrates that a full range of igneous rocks could be produced from the same mafic magma But.. reality is: 1) crystals settle to bottom of magma chamber or get stuck on chamber walls = fractional crystallization / magmatic differentiation …analogous to distillation & precipitation of salts from seawater 13 Bowenʼs reaction series… assumes: But.. 2) Assimilation of wallrock material 3) Magma mixing -Involves two bodies of magma intruding one another -Two chemically distinct magmas may produce a composition quite different from either original magma Assimilation & magmatic differentiation compositions 14 Magma compositions: 1) mafic Igneous rocks… silicates dominate!!! • Dark (ferromagnesian) silicates – – – – Olivine Pyroxene Amphibole Biotite mica • 45-55 wt% SiO2 • + MgO, FeO, CaO • 'fluid', low volatile concentration • basalt, gabbro Magma compositions: 2) felsic Igneous rocks… silicates dominate!!! • Light (nonferromagnesian) silicates – Quartz – Muscovite mica – Feldspars • • • • >65 wt% SiO2 + K2O, Al2O3, Na2O viscous granite, rhyolite Magma compositions: others 3) Intermediate (andesitic) composition – Contains 55-65 wt% SiO2; relative to granitic: more FeO & MgO, less Na2O & K2O, less volatile, less viscous – Associated with explosive volcanic activity 4) Ultramafic (komatiite) composition – <45 wt% SiO2; high in MgO & FeO – Rare composition that is composed of olivine and pyroxene; common >2.6 Ga 15 Igneous textures textures Igneous textures Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals Factors affecting crystal size…. Factors affecting crystal size 1) Rate of cooling – Slow rate promotes the growth of fewer but larger crystals – Fast rate forms many small crystals – Very fast rate forms glass 2) Amount of silica (SiO2) present 3) Amount of dissolved gases 16 Igneous textures Igneous rock textures… • Phaneritic (coarse-grained) – Crystals can be seen without a microscope – Why??? • Aphanitic (fine-grained) – Microscopic crystals – Why??? – May contain vesicles (holes from gas bubbles) Igneous textures Igneous rock textures… • Porphyritic – Large + small crystals – Large crystals, called phenocrysts, are embedded in a matrix of smaller crystals, called the groundmass – Why??? Igneous textures Igneous rock textures… • Glassy – No crystals visible – # silica: obsidian; foamy: pumice – Why (glass, foamy)??? 17 Igneous textures Igneous textures Igneous rock textures… • Pyroclastic – Fragments ejected during a violent volcanic eruption – Often appear to be layered or composed of shattered rocks Igneous rock textures… • Pegmatitic – Exceptionally coarse grained – Form in late stages of crystallization of granitic magmas with high volatile content 18 Mafic, intermediate, or felsic? Mafic, intermediate, or felsic? 19 ...
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