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Igneous Rocks and the Rock Cycle What are Rocks? Most rocks are an aggregate of one or more minerals and a few rocks are composed of non- mineral matter. A mineral is solid inorganic material of the Earth that has both a known chemical composition and a crystalline structure that is unique to that mineral There are three major rock types: 1. Igneous 2. Metamorphic 3. Sedimentary These quartz crystals are hexagonal prisms. Gypsum Quartz Major Rock Types Igneous rocks are formed by the cooling of molten magma or lava near, at, or below the Earth’s surface. Sedimentary rocks are formed by the lithification of inorganic and organic sediments deposited near or at the Earth’s surface. Metamorphic rocks are formed when preexisting rocks are transformed into new rocks by heat and pressure, usually below the Earth’s surface. The Rock Cycle The continuous and reversible processes that illustrates how one rock changes to another. Igneous Rocks Igneous rocks are formed by the cooling of molten rock. There are two major states of molten rock: Magma is a form of molten rock that exists below the Earth’s surface. Lava is the term given to magma once it reaches the Earth’s surface, usually in the form of a volcanic eruption. There are two major classifications of igneous rocks: Intrusive igneous rocks are formed by magma that cools below the Earth’s surface. Extrusive igneous rocks are formed by lava that cools at the Earth’s surface. Intrusive igneous rocks generally cool very slowly deep below the earth’s surface or as the magma is rising to the earth’s surface. Extrusive igneous rocks generally cool quickly when they reach the earth’s surface usually through volcanoes or fissure. Crystal Size and Cooling Rates Slower cooling rates produce larger individual crystals in the rock Intrusive igneous rocks generally cool very slowly and tend to have large crystals that produce a course-grained rock. Phaneritic rocks are coarse-grained rocks which contain individual crystals that are relatively even in size and large enough for scientists to identify the different mineral grains that compose the rock. Faster cooling rates produce smaller individual crystals in the rock Extrusive igneous rocks tend to cool quickly and are characterized by smaller grains that produce a fine-grained rock. Aphanitic is the term used to describe very fine grained rocks. Porphyritic textured rocks contain both a coarse and fine-grained texture. The coarse grains in a porphyritic rock begin to develop as the magma is cooling below the surface of the earth. Following eruption or exposure to lower temperatures, the remaining magma or lava cools very quickly and forms minerals with fine-grained textures. As a result, porphyritic textures contain both coarse- and fine–grained minerals. Texture Texture is a term used to describe the size, shape, and arrangement of interlocking crystallized mineral grains in an igneous rock. Two major factors affect the size of crystal grains in an igneous rock: 1) Rate at which molten rock cools; slow or fast 2) Amount of dissolved gases or fluids in the magma. Texture Igneous minerals vary greatly in grain size. Grain-size classes are similar to the sedimentary scale, but there are fewer divisions with a greater range of size. Phenocrysts are grains in an igneous rock that are larger than the other grains that make up the rest of the rock. Grain Size Categories Grain Size Divisions fine grained = < 1 mm medium grained = 1 mm < 5 mm coarse grained = 5 mm < 3 cm very coarse-grained = > 3 cm Phenocrysts Texture microphenocrysts = 0.03 mm – 0.3 mm phenocrysts = 0.3 mm – 5 mm megaphenocrysts = > 5 mm Igneous Rock Textures Phaneritic (Intrusive) Aphanitic (Extrusive) Porphyritic (Intrusive and Extrusive) Glassy (Extrusive) Pegmatitic (Intrusive) Pyroclastic Materials (Extrusive) Phaneritic Texture Phaneritic (Intrusive) Phaneritic rocks are coarse-grained rocks which form below the Earth’s surface. The individual crystals are relatively even-sized and large enough for scientists to identify the different mineral grains that compose the rock. Granite rock with a phaneritic texture Quartz Crystals: (White) Feldspar Crystals: (Pink) Biotite Crystals: (Black) Copyright © Dr. Richard Busch Aphanitic Texture Aphanitic (Extrusive) Aphanitic rocks are very fine-grained and contain crystals that are too small to distinguish Basalt rock with an aphanitic without the aid of a magnifying lens. and vesicular texture Aphanitic rocks are often described by how light or dark the rock appears. Lighter colored aphanitic rocks contain mostly nonferromagnesian silicate minerals. Darker colored aphanitic rocks contain mostly ferromagnesian silicate minerals. Aphanitic rocks may also contain vesicles of remnant gas that give the rock a vesicular texture. Vesicles form when the rock cools very quickly and preserves the openings formed by Copyright © Dr. Richard Busch the expansion of trapped gas bubbles. Porphyritic Texture Porphyritic (Intrusive and Extrusive) Porphyritic rocks contain both coarse- and fine-grained textures indicating different environmental conditions which formed the rock. The coarse grains in a porphyritic rock develop as the magma is cooling below the surface of the earth. The fine-grained component of a porphyriic rocks forms when the magma or lava cools faster. The large coarse-grained crystals are referred to as phenocrysts. The small fine-grained crystals are referred to as groundmass. Rhyolite rock with porphyritic texture containing phenocrysts of olivine and pyroxene and a gabbro groundmass. Glassy Texture Glassy (Extrusive) Glassy textured rocks are formed by very rapid cooling of magma. Glassy rocks often form from magmas with high silica content that arranges into long chainlike structures before crystallization occurs. These silica chains increase the viscosity of the magma and it once it eventually cools it forms a glassy textured rock. Obsidian rock with a Glassy rocks can be considered amorphous because they glassy texture and have no crystalline structure. conchoidal fractures Glassy rocks are classified by the amount of glass contained by the rock: Glass-bearing: 0-20% glass Glass-rich: 20-50% glass Glassy: 50 – 100% glass Obsidian is a common glassy rock. Copyright © Dr. Richard Bush Pegmatitic Texture Pegmatitic (Intrusive) Pegmatitic rocks contain large interlocking crystalline grains > 1-2 centimeter in diameter. Pegmatites are commonly composed of quartz, feldspar, and mica minerals. Pegmatities form from a combination of hydrothermal and igneous processes; and is dependant on the presence of fluids and volatiles such as water, chlorine, bromine, sulfur, and fluorine. Pegmatites form late in the crystallization process when there are a lot of fluids present in the molten rock. The fluids enable individual ions to move around more freely, ultimately bonding to form very large and sometimes exotic crystals. Pegmatitic dikes form around the margins of intrusive plutons, or occasionally as veins of rock which extend into the pluton. Pyroclastic Materials Pyroclastic (Extrusive) Pyroclastic materials form when individual rock fragments are ejected during a violent volcanic eruption and consolidate into larger rock composites when they deposit on the surface. Pyroclastic rocks contain at least 75% pyroclastic fragments with the remainder consisting of other inorganic sediments or organic materials. Pyroclastic rocks contain a mixture of different types of particles that are not cohesively joined by interlocking crystals, but instead are consolidated masses of multiple rock fragments. Pyroclastic Fragments Pyroclastic materials do not conform well to igneous grain size classifications. Instead, geologists use the following terms to describe pyroclastic fragments: Juvenile fragments: volcanic rocks formed from cooled magma before it is deposited. Cognate fragments: volcanic rocks which formed during previous volcanic activity and are ejected by a later eruption. Accidental fragments: rocks which are disrupted, dislodged, and possibly even ejected by the volcano, but are not of volcanic origin (although they may be igneous). Aa Lava Aa Lava (Extrusive) Aa is a basaltic lava flow that has a rough surface, Aa lava in Hawaii’s Volcanoes National Park. characterized by sharp, jagged blocks and protruding spines of volcanic rock. Aa flows move slowly (5-50 meters per hour) and are often several meters thick. As aa lava flows, the outer surface and advancing edge cools first. The molten material pushes through the cooled rocks and breaks the fragments even more. As a result the lava flow appears more Copyright © Bruce Molina, Terra Photographics like a mass of advancing rubble as apposed to a viscous flow. Photo Courtesy USGS Hawaiian Volcano Observator Bowen’s Reaction Series Discontinuous Series Continuous Series 1400 ºC Calcium rich Olivine Mafic Crystallization Temperature Pyroxene 800 ºC Amphibole Biotite Sodium rich Orthoclase Feldspar Muscovite mica Quartz Intermediate Felsic Earth elements Granite Rhyolite Basalt vesicular texture Pegmatite (very coarse-grained) Sedimentary Rocks Rocks formed by consolidation of sediments Classified by the type of sediments Sedimentary rocks Two Major Types of Sediments Clastic sediments are physically deposited particles derived from weathered rocks Chemical and biochemical sediments include minerals carried in solution such as calcite and halite From Sediments to Solid Rocks Lithification is the process of converting sediments into solid rocks (will cover in detail in another lecture) What is a sedimentary rock? Sedimentary rocks result from mechanical and chemical weathering Comprise ~ 5% of Earth’s upper crust About 75% of rocks at surface Contain evidence of past environments Record how sediment is transported Often contain fossils Metamorphic rocks Metamorphic Rocks Definitions • Metamorphic - -- "Meta"= Change "Morph"= form Rock (Grk) (Grk) a rock that has been changed from its original form ( parent ) by heat , pressure , and fluid activity into a new rock ( daughter ). Heat Sources Include..... Magma ••Magma -- temperature temperature of of magma magma -- composition composition of of magma magma Geothermal gradient gradient ••Geothermal --- temperature increases increases with with depth depth of of burial burial temperature core of of Earth Earth is is warmer warmer than than outer outer crust crust core Uniform Pressure vs Directed Pressure Uniform Pressure Lithostatic •• Lithostatic ---- "Lithos"= rock, rock, static= static= unchanged unchanged (pressure) (pressure) "Lithos"= uniform (aka (aka non-directed) non-directed) uniform equal intensity intensity from from all all directions directions by by rocks rocks equal Directed Pressure one direction of squeezing is much stronger than the others Minerals align themselves to reduce stress. Types of Metamorphism Contact •• Contact -- caused by by igneous igneous activity activity caused Dynamic •• Dynamic -- associated with with faults faults & & earthquake earthquake zones zones associated Regional •• Regional -- caused by by tremendous tremendous pressures pressures caused associated with with tectonic tectonic plate plate activity activity associated Contact Metamorphism Igneous Intrusions Intrusions •• Igneous --- size and and type type of of magma magma important important size >> mafic magma magma hotter hotter than than felsic felsic mafic heat decreases decreases away away from from magma magma heat >> forms aa zone zone of of altered altered country country rocks rocks called called Aureoles Aureoles forms Sometimes creates a metamorphic rock called a hornfels -in essence a “cooked” rock Dynamic Metamorphism • Cataclastic Metamorphism associated with Fault Zones associated with • -- Places Places where where the the Earth's Earth's crust crust ruptured ruptured -- Rock Rock pulverized pulverized >> heat and and pressure pressure come come from from movement movement along along the the heat Fault Fault resultant rock is is known known as a Mylonite resultant rock • Regional Metamorphism Most common common form form of of metamorphism metamorphism •• Most caused by by large large scale scale forces forces •• caused -- lithospheric plate plate collision collision lithospheric covers very very large large areas areas •• covers --- metamorphic belts belts or or zones zones metamorphic Zones are are characterized characterized by by Index Index Minerals Minerals Zones >> >> form under under specific specific temperatures temperatures and and pressures pressures form metamorphic facies facies metamorphic commonly associated associated with with •• commonly -- shields: stable areas of crystalline rocks Metamorphic Textures Foliated • Foliated --- -- Folios == page page or or leaf-like leaf-like Folios rock has has distinct distinct banding banding or layering rock >> often not not smooth smooth like like in in sedimentary sedimentary rocks rocks often formed under under directed directed pressure pressure formed Non-foliated • Non-foliated ---- no distinct distinct layering layering character character no often aa massive massive crystalline crystalline texture texture often formed under under uniform uniform pressures pressures formed Non-Foliated Texture Contact Metamorphism (due to heating) Foliated Texture Regional Metamorphism (due to pressure) Foliated Texture Foliated Textures •• Slatey ----- looks like like blackboard blackboard looks dull surface surface >> dull smooth, thin thin layering layering smooth, breaks into into flat flat slabs slabs breaks >> -distinct bands of minerals -- visible grains -- referred to to as as slatey slatey cleavage cleavage referred no mineral grains visible Phyllitic •• Schistose •• -- >> garnets, staurolites may have shiny appearance >> due to mica minerals •• Gneissic -- larger larger grains grains has aa "sheen" "sheen" to to itit >> has -may look look like like igneous igneous rock rock may -may have have little little "waves" "waves" on on may may have have crude crude banding banding may surface surface >> intensely distorted -- looks like like waxed waxed surface surface looks >> referred to as crenulations some small small grains grains visible visible some -- different minerals minerals than than different schistose schistose Foliated MM Rocks slate phyllite schist gneiss MM MM Rocks Rocks that that could could form form as as aa shale shale (sedimentary) (sedimentary) parent parent rock rock is is exposed to increasing directed pressure and temperature Non-foliated Rocks • Marble: - metamorphosed limestone •• Quartzite: -- metamorphosed quartz sandstone Metamorphic Zones • Metamorphism is common along most plate boundaries like this. Plate tectonic theory نظرية حركية األلواح ■ تعتبر النظرية أن القشرة األرضية عبارة عن طبقة رقيقة نسبياً تسمى . Lithosphere ■ Lithosphereمقسم إلى عدد من األلواح Platesصغيرة و كبيرة. ■ يسمى اللوح على حسب نوع القشرة -قشرة محيطية أو قشرة قارية. ■ تحت Lithosphereتوجد طبقة سائلة نسبيا ً نتيجة إلرتفاع درجة الحرارة تسمى . Asthenosphere ■ Asthenosphereيسلك مسلك الوسط اللزج حيث بسمح بتحرك االلواح الموجودة فوقها. ■ حركة هذه األلواح وتفاعلها مع بعض هى المسئولة عن كل ما يالحظ على سطح األرض مثل البراكين ,الزالزل ,الفوالق ,الجبال.....إلخ. Tectonic boundaries أنواع الحدود الحركية . )Converging boundary ( ■ نطاق التقارب .)Diverging boundary( ■ نطاق التباعد )Conservative boundary( ■ الحدود المحافظة )Converging boundary ( نطاق التقارب Plate Tectonics- Andesite Line Diverging boundary نطاق التباعد Diverging boundary نطاق التباعد Diverging boundary نطاق التباعد Decompression Melting Magma under lithosphere heats and cracks it. Mantle rock is exposed to low pressures – it Conservative boundary( الحدود المحافظة