Week 4 Geology Notes And In Class Review
Week 4 Geology Notes And In Class Review Geos 1113
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This 12 page Class Notes was uploaded by Brandon Notetaker on Tuesday September 20, 2016. The Class Notes belongs to Geos 1113 at University of Arkansas taught by Mohamed Aly in Fall 2016. Since its upload, it has received 10 views. For similar materials see General Geology in Geology at University of Arkansas.
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Date Created: 09/20/16
Week 4 Class Notes: Magma, Igneous Rocks & Intrusive Activity Magma: Parent Material of Igneous Rock 1. Igneous rocks form as molten rock (magma) cools and solidifies 2. General characteristics of magma: Parent material of igneous rocks Forms from partial melting of rocks Magma at surface is called lava 3. Magma consists of three components: Liquid portion = melt Solids, if any, are crystals of silicate minerals Volatiles dissolved gases in the melt that vaporize at surface pressure 4. Most common volatiles in magma: Water vapor (H2O), Carbon dioxide (CO2), and Sulfur dioxide (SO2) 5. From Magma to Crystalline Rock Crystallization cooling of magma which results in the systematic arrangement of ions into orderly patterns Silicon and oxygen atoms link together first to form a silicon−oxygen tetrahedron (the basic building block of silicate minerals) 6. Igneous Processes Crystallization of magma at depth forms plutonic or intrusive igneous rocks § These rocks are observed at the surface following periods of uplifting and erosion of overlying rocks Solidification of lava or volcanic debris at surface forms volcanic or extrusive igneous rocks Igneous Compositions 1. Igneous rocks are composed primarily of silicate minerals Dark (or ferromagnesian) silicates – Rich in iron and/or magnesium (e.g., olivine, pyroxene, amphibole, and biotite mica) Light (or nonferromagnesian) silicates – Contain more potassium, sodium, or calcium than iron and magnesium (e.g., quartz, muscovite mica, and feldspars) 2. Four Compositional Groups: Granitic (or felsic) composition – Lightcolored silicates (Composed almost entirely of quartz and potassium feldspar), Termed felsic (feldspar and silica) in composition, High silica (SiO2) content, Major constituent of continental crust Basaltic (or mafic) composition – Dark silicates and calciumrich feldspar, Termed mafic (magnesium and ferrum, for iron) in composition, Higher density than granitic rocks, Comprise the ocean floor and many volcanic islands 3. Andesitic (or intermediate) composition Contain 25% or more dark silicate minerals (amphibole, pyroxene, and biotite mica) Associated with volcanic island arcs 4. Ultramafic composition Rare composition of mostly olivine and pyroxene Composed almost entirely of ferromagnesium minerals (peridotite is an example) The main constituent of the upper mantle Silica content as an indicator of composition 1. The chemical makeup of an igneous rock can be inferred from the silica content 2. Crustal rocks exhibit a considerable range (~40% 70%) of silica 3. Silica content influences the behavior of magma Granitic magmas have high silica content, are viscous (thick), and erupt at a lower temperature Basaltic magmas have much lower silica content, more fluidlike behavior, and erupt at a higher temperature Igneous Texture 1. Texture is the overall appearance of a rock based on the size, shape, and arrangement of mineral grains 2. Factors affecting crystal size: Rate of cooling – Slow rate = fewer but larger crystals – Fast rate = many small crystals Amount of silica Amount of dissolved gases 3. Types of Igneous Textures Aphanitic (finegrained) texture – Rapid rate of cooling with Microscopic crystals Phaneritic (coarsegrained) texture – Slow cooling with Large, visible crystals Porphyritic texture – Some minerals can grow large before others form from the magma. The magma can move to a different environment which causes the remaining minerals to form quickly. Large crystals (phenocrysts) are embedded in a matrix of smaller crystals (groundmass) Vesicular texture – Rocks contain voids left by gas bubbles in the lava. Common feature of an extrusive igneous rock Glassy texture – Very rapid cooling. Ions are frozen in place before they can unite in an orderly crystalline structure Pyroclastic (fragmental) texture – Forms from the consolidation of individual rock fragments ejected during explosive eruptions Pegmatitictexture – Exceptionally coarsegrained. Form in late stages of crystallization of magmas. Rocks with this texture are called pegmatites Naming Igneous Rocks 1. Igneous Rocks Classification Based on texture & mineralogical composition: Texture influenced by cooling history Mineralogical composition influenced by the chemical composition of the parent magma 2. Granitic (Felsic) Igneous Rocks Granite Coursegrained (phaneritic). One of the best known igneous rocks. Very abundant. Natural beauty, especially when polished. 10−20% quartz, roughly 50% potassium feldspar. Small amounts of dark silicates. Some granites have a porphyritic texture (contain elongated feldspar crystals) Rhyolite Extrusive equivalent of granite. Composed essentially of lightcolored silicates. Typically buff to pink or light gray in color. Less common and less voluminous than granite Obsidian Darkcolored, glassy rock § Forms when silicarich lava cools quickly at Earth’s surface § Usually black to reddishbrown in color § Similar chemical composition of granite § Dark color is the result of small amounts of metallic ions in an otherwise clear, glassy substance Pumice Glassy textured rock that forms when large amounts of gas escape from the lava. Voids are quite noticeable. Resembles fine shards of intertwined glass. Typically found in deposits with obsidian. Will float when placed in water 3. Andesitic (Intermediate) Igneous Rocks Andesite Mediumgray, finegrained rock. Volcanic origin. Commonly exhibits a porphyritic texture Diorite. Intrusive equivalent of andesite. Coarsegrained rock. Looks like gray granite, but lacks visible quartz crystals. Can have a saltandpepper appearance 4. Basaltic (Mafic) Igneous Rocks Basalt Very dark green to black, finedgrained rock. Composed mostly of pyroxene and calciumrich plagioclase feldspar. When porphyritic, contains small, lightcolored feldspar phenocrysts. Most common extrusive igneous rock. Upper layers of oceanic crust are composed of basalt Gabbro Intrusive equivalent of basalt. Very dark green to black, finegrained rock. Composed mostly of pyroxene and calciumrich plagioclase feldspar. Uncommon on the continental crust, but makes up a significant portion of the oceanic crust 5. Pyroclastic Rocks Composed of fragments ejected during a volcanic eruption Tuff, Common pyroclastic rock, Composed of ashsized fragments cemented together Welded tuff, Ash particles are hot enough to fuse together, Can contain walnut sized pieces of pumice and other rock fragments, Covers vast portion of previous volcanically active areas of the western United States Volcanic breccia, Composed of particles larger than ash Names do not imply mineral composition and are identified with a modifier (Example: rhyolitic tuff) Origin of Magma 1. Earth’s crust and mantle are primarily composed of solid rocks 2. Magma is generated in the uppermost mantle Greatest amounts are produced at divergent plate boundaries Lessamounts are produced at subduction zones Can also be generated when crustal rocksare heated sufficiently to melt 3. Geothermal Gradient: temperatures in the upper crust increase with depth about 25oC per kilometer 4. Rocks in the lower crust and upper mantle are near their melting points 5. Tectonic processes trigger magma production by reducing the melting point Decrease in pressure Addition of water Increase in temperature of crustal rocks 6. Generating Magma from Solid Rock Decrease in pressure (decompression melting), Melting occurs at higher temperatures with increasing depth, Reducing pressure lowers the melting temperature, Solid, hot mantle rocks will ascend to regions of lower pressure, inducing melting at: Divergent plate boundaries, and Mantle plumes at hot spots Addition of water. Occurs mainly at subduction zones. As an oceanic plate sinks, heat and pressure drive water from the crust and overlying sediments. Fluids migrate into the overlying wedge of mantle. The addition of water lowers the melting temperature of the mantle rocks to trigger partial melting. Water and other volatiles act as salt does to melt ice – Causes rock to melt at lower temperatures Temperature increase: melting crustal rocks. Mantlederived basaltic magma buoyantly rises toward the surface. Heat from these magma sources can melt the surrounding crustal rocks. Crustal rocks can also melt from heat generated during continental collisions that result in the formation of large mountain belts How Magma Evolves 1. A single volcano may extrude lavas that vary in composition 2. Bowen’s reaction series Describes how an entire suite of silicate minerals can form from a single basaltic magma as it cools and crystallizes Minerals crystallize in a systematic fashion based on their melting points As minerals crystallize, the composition of the liquid portion of the magma continually changes 3. Magmatic Differentiation and Crystal Settling Magmatic differentiation The formation of one or more secondary magmas from a single parent magma Crystal settling Earlierformed minerals are denser than the liquid portion of the magma and sink to the base of the magma chamber. When the remaining magma solidifies, the mineralogy will be different from the parent magma 4. Assimilation and Magma Mixing Assimilation As magma migrates through the crust, it may incorporate some of the surrounding rock into the chamber, melting and changing the chemical composition. Magma mixing During the ascent of two chemically different magma bodies, the more buoyant mass may overtake the slowerrising body, merging them, and their melts mixing by convective flow. 5. Incomplete melting of rocks is known as partial melting This process produces most magmas During partial melting, the melt is enriched in ions from minerals with the lowest melting temperature. Partial melting of ultramafic rocks yields mafic magmas. Partial melting of mafic rocks yields intermediate magmas. Partial melting of intermediate rocks yields felsic magmas Partial Melting & Magma Composition 1. Formation of Basaltic Magmas Most magma that erupts is basaltic (mafic) magma Most originate from partial melting of mantle rocks at oceanic ridges. These melts are called primary or primitive magmas because they have not yet evolved 2. Formation of Andesitic and Granitic Magmas Andesitic magma Magmatic differentiation of mantlederived basaltic magma. Can also form when basaltic magmas assimilate crustal rocks Granitic magmas Most form when basaltic magma ponds beneath the continental crust § Melted crustal rocks alter the magma composition. Can form from magmatic differentiation of andesitic magma Intrusive Igneous Activity 1. Most magma is emplaced at depth in Earth 2. Nature of Intrusive Bodies A plutonis cooled, emplaced magma into preexisting rocks Classification of plutons Plutons are classified by their orientation to the surrounding rock 3. Classification of plutons Tabular (tablelike), Discordant(cut across existing structures), Concordant(are parallel to features like sedimentary) strata Massive (Irregularly shaped) 4. Tabular Intrusive Bodies Dike (a tabular, discordant pluton). Serves as tabular conduits to transport magma. Parallel groups are called dike swarms Sill (a tabular, concordant pluton) – Tend to accumulate magma and increase in thickness. Closely resembles buried lava flows. May exhibit columnar jointing (occurs when igneous rocks cool and develop shrinkage fractures that produce elongated, pillarlike columns with 6 sides) 5. Massive Intrusive Bodies Batholith Largest intrusive body. Surface exposure of >100 sqkm (smaller bodies are termed stocks). While expansive, most are <10 km thick Emplacement of batholiths Magma at depth is much less dense than the surrounding rock. In the mantle, the more buoyant magma pushes aside the host rock and rises in Earth through a process called shouldering. Near to Earth’s surface, the rocks are cooler and brittle. Upward movement is accomplished by a process called stoping, where the overlying blocks of country rock sink through the magma Xenolithsare suspended blocks of country rocks found in plutons Stocks Smaller bodies (<100 sq km) with similar characteristics are termed stocks Laccoliths Forcibly injected between sedimentary strata. Causes the overlying strata to arch upward Week 4 Class Notes: Volcanoes & Volcanic Hazards Nature of volcanic eruptions 1. Volcanic activities involve magma and/or lava Magma is molten rock that usually contains some crystals and varying amounts of dissolved gases Lava is the erupted magma 2. The behavior of magma is determined by its: Temperature, Composition, and Dissolved gases 3. The above three factors control the viscosityof magma, which in turn controls the nature of eruption Nature of Volcanic Eruptions 4. Viscosity is a measure of a material’s resistance to flow The more viscous the material, the greater its resistance to flow (syrup is more viscous than water) 5. Factors affecting viscosity Temperature–hotter magmas are less viscous Composition–silica (SiO2) content – Highsilica content magmas are more viscous (e.g., rhyolitic and andesitic satellitess(tetrahedra) start to link together in long chains early in the crystallization process Dissolved gases –Dissolved water vapor in magma reduces its viscosity by inhibiting formation of silica tetrahedrachains. Gases expand within a magma as it nears the Earth’s surface due to decreasing pressure. The violence of an eruption is somehow related to how easily gases escape from magma Nature of Volcanic Eruptions 6. Two Types of Volcanic Eruptions Quiescent Hawaiian Type Eruptions – Involves fluid basaltic lavas. Eruptions are characterized by outpouring of lava that can last weeks, months, or even years Explosive Eruptions – Associated with highly viscous magmas. Eruptions expel particles of fragmented lava and gases at supersonic speeds that evolve into eruption columns Materials Extruded During an Eruption 1. Lava Lava Flows – 90% of lava is basaltic lava, <10% of lava is andesitic lava, 1% of lava is rhyolitic lava 2. Types of Lava Flows Aa and Pahoehoe Flows – Composed of basaltic lava. Aa flows have surfaces of rough jagged blocks. Pahoehoe flows have smooth surfaces and resemble twisted braids of rope B. Block Lavas – Composed of andesitic and rhyolitic lava. Upper surface consists of massive, detached blocks Pillow Lavas – Composed of basaltic lavas extruded underwater. Flow is composed of tubelike structures stacked one atop the other 3. Gases Gases make up 1‒ 6%of the total weight of a magma As the magma reaches the surface and the pressure is reduced, the gases expand and escape 4. Pyroclastic Materials (Tephra) Volcanoes eject pulverized rock and lava fragments called pyroclastic materials Particles range in size from fine dust, to sandsized ash, to very large rocks 5. Types of Pyroclastic Materials Volcanic ash –fine glassy fragments. Welded tuff fused ash Lapilli –walnutsized material Cinders–peasized material Blocks–hardened or cooled lava Bombs–ejected as hot lava Materials Extruded During an Eruption Pumice–light gray or pink porous rock from frothy andesitic and rhyolitic lava Scoria–reddishbrown porous rock from frothy basaltic and andesitic lava Anatomy of a Volcano 1. General Features Conduit– a fissure that magma moves through to reach the surface Vent– the surface opening of a conduit Volcanic cone – a cone of material created by successive eruptions of lava and pyroclastic material Crater– a funnelshaped depression at the summit of most volcanic cones, generally <1 km in diameter Caldera – a volcanic crater that has a diameter of >1 km and is produced by a collapse following a massive eruption Parasitic cones – a flank vent that emits lava and pyroclastic material Fumaroles – a flank vent that emits gases Types of Volcanoes 1. Shield Volcanoes Broad, slightly domeshaped Examples Mauna Loa is the largest shield volcano on Earth Covers large areas Produced by mild eruptions of large volumes of basaltic lava Most begin on the seafloor as seamounts; only a few grow large enough to form a volcanic island Examples the Hawaiian Islands, the Canary Islands, the Galapagos, and the Easter Island 2. Cinder Cones (Scoria cones) Built from ejected lava fragments Flanks have steep slope angles Rather small size (usually <300 m) Frequently occur in groups Sometimes associated with extensive lava fields Example: Paricutin (located 320 km west of Mexico City) Types of Volcanoes 3. Composite Volcanoes – Also called stratovolcanoes Large, classicshaped volcano (symmetrical cone, thousands of feet high and several miles wide at the base) Composed of interbedded lavaflows and layers of pyroclasticdebris Many are located adjacent to the Pacific Ocean in the Ring of Fire Examples: Mount St. Helens and Mount Etna Types of Volcanoes Volcanic Hazards 1. Pyroclastic Flows Pyroclastic flow is a mixture of hot gases infused with incandescent ash and lava fragments that flows down a volcanic slope 2. Lahar is mudflow on an active or inactive volcano 3. Other hazards Volcanorelated tsunamis Volcanic ash – a hazard to airplanes Volcanic gases – a respiratory health hazard Effects of volcanoes on climate 4. Pyroclastic Flows – Also called a nuée ardente Propelled by gravity and move similarly to snow avalanches Material is propelled from the vent at high speeds (can exceed 100 km per hour). Pyroclastic flows are typically generated by the collapse of tall eruption columns Surgeis a small amount of ash that separates from the main body of the pyroclastic flow. Occasionally, these surges have enough force to knock over buildings and move automobiles In 1902, the town of St. Pierre was destroyed by a pyroclastic flow from Mount Pelée 5. Lahars – lahar is mudflow on an active or inactive volcano Volcanic debris becomes saturated with water and rapidly moves down a volcanic slope Some lahars are triggered when magma nears the surface of a volcano covered in ice and snow and causes it to melt In 1985, lahars formed during the eruption of Nevado del Ruiz, killing 25,000 people 6. Other hazards Volcanorelated tsunamis Destructive sea waves can form after the sudden collapse of a flank of a volcano Volcanic ash Jet engines can be damaged when flying through a cloud of volcanic ash. In 2010, the eruption of Iceland’s Eyjafjallaöku created a thick plume of ash over Europe, stranding hundreds of thousands of travelers Volcanic gases § Volcanoes can emit poisonous gases, endangering humans and livestock 7. Effects of volcanoes on climate Ash particle released from volcanoes can reflect solar energy back into space. The ash from the eruption of Mount Tambora in 1815 led to the “year without summer”(1816) Other Volcanic Landforms 1. Calderas relatively circular, steepsided depressions with a diameter >1 km 2. Three different types: Crater Laketype calderas: Form from the collapse of the summit of a large composite volcano following an eruption; these calderas eventually fill with rainwater Hawaiiantype calderas: Form gradually from the collapse of the summit of a shield volcano following the subterranean drainage of the central magma chamber Yellowstonetype calderas: Form from the collapse of a large area after the discharge of large volumes of silicarich pumice and ash; these calderas tend to exhibit a complex history 3. Formation of Crater Lake–type calderas: About 7000 years ago, a violent eruption partly emptied the magma chamber of former Mount Mazama, causing its summit to collapse. Rainfall and groundwater contributed to form Crater Lake, the deepest lake in the United States (594 m deep) and the ninth deepest in the world. 4. Large Igneous Provinces Large igneous provinces cover a large area with basaltic lava Basaltic lava extruded from fissures blanket a large area, called a large igneous provinces or basalt plateaus Examples: the Colombia Plateau and the Deccan Traps 5. Lava Domes A lava dome is a small domeshaped mass composed of rhyoliticlava 6. Volcanic Necks and Pipes A volcanic neck is the remains of magma that solidified in a volcanic conduit Example: Shiprock in New Mexico Plate Tectonics and Volcanic Activity 1. Volcanism at convergent plate boundaries Occurs at subduction zones, where two plates converge and the oceanic lithosphere descends into the mantle Volcanic arcs develop parallel to the associated subduction zone trench Examples of volcanic island arcs: the Aleutians, the Tongas, and the Marianas Example of a continental volcanic arc: the Cascade Range Most active volcanoes are found along the circumPacific Ring of Fire Eruptions tend to be explosive and associated with volatilerich, andesitic magma 60%of Earth’s yearly output of magma is from spreading centers Characterized by a vast outpouring of fluid, basaltic lavas 2. Intraplate volcanism Volcanoes that occur thousands of kilometers from plate boundaries Occurs when a mantle plume ascends towards the surface Examples: the Hawaiian Islands, the Columbia River Basalts, and the Galapagos Islands Monitoring Volcanic Activity 1. Efforts aimed at detecting movement of magma from a subterranean reservoir Changes in patterns of earthquakes Inflation of the volcano related to rising magma Changes in the amount and/or composition of gases released from the volcano Increase in ground temperature 2. Remote sensing devices aid in monitoring limitedaccessibility volcanoes 3. A volcano must be monitored for a long time to recognize a difference between “resting state” and “active state” In Class Review: Magma, Igneous Rocks, and Intrusive Activity 1. Lava flows are typically finer grained than intrusive igneous rocks because the lava flows cool quickly on the earth’s surface so the mineral grains do not have time to grow. 2. Most magma comes from the upper mantle 3. Decompression melting occurs because the melting temperature of hot material is lowered as the cooling pressure decreases 4. Bowens reaction series is based on lab experiments involving melting rocks 5. Granite has a lot of quartz I it because it is an intrusive rock that formed from the cooling of relatively highsilicate magma. 6. The most important factor to weather magma cools quickly or slowly is the environment. The higher the temp. the slower the magma cooling. 7. The difference between primitive and second degree magma is a parent from which the secondary magma form through differentiation. 8. Glassy igneous rocks form when the magma cools so fast that mineral grains cannot crystalize and grow.