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Chp10: Earth Interior Structure of the Modern Earth I. The Earth consists of three concentric layers: a. the core b. the mantle c. the crust They are formed as a result of: -density differences between the layers -variations in composition -differences in temperature and pressure II. The Characteristics of the Core: a. The Core is thought to be composed of iron with some nickel. b. It is spherical in shape with its outer surface lying 2900 km below th outer surface of the earth. c. The total diameter of the core is 3470 km. d. It has an average density of about 10 to 13 grams/cm3 and comprises 16% of the earth’s volume. Chp 10: Earth’s Interior Chp10: Earth Interior Characteristics of the Core (continued….) d. Seismic Tomography data (studying the earth’s interior indirectly by studying the behavior of earthquake waves) indicate that the core has a small Solid Inner Region (1220 km in diameter) surrounded by an apparently Liquid Outer Region (2250 km thick). III. The Characteristics of the Mantle: a. The Mantle surrounds the core and comprises about 83%of the earth’s volume. b. It is less dense than the core with an average density of approximately 3.3 – 5.7 grams/cm3. c. It is composed largely of Peridotite, a dark, dense, igneous rock containing high amounts of iron and magnesium. d. The Mantle can be divided into three regions: 1.The Lower Mantle – This is solid and comprises most of the volume of the earth’s interior. 2. The Upper Mantle – This consists of the Asthenosphere and the overlying solid mantle rocks up to the base of the crust. -The asthenosphere surrounds the lower mantle, has the same peridotite composition - It behaves plastically and slowly flows. Partial melting within the asthenosphere generates Magma, molten rock material, some of which rises to the surface because it is less dense than the material from which it was derived. Chp 10: Earth’s Interior Our knowledge of Earth’s interior largely comes from studying wave propagation from earthquakes…..the Relative density of a rock can be estimated based upon the travel time of wave from the focus to the seismograph station. Chp 10: Earth’s Interior Recall that S waves cannot pass through liquids (like outer core) Chp10: Earth Interior hree Subdivisions of the Mantle, continued…. . The Lithosphere – This is the solid portion of the upper mantle and the overlying crust. The lithosphere is broken into numerous pieces called Plates that move over the asthenosphere as the result of underlying Convection Cells (or Mantle Plumes generated from heat). IV. The Characteristics of the Crust: The Crust is the outermost layer of the earth. It consists of two types of rock materials: a. Continental Crust – (20 – 90 km thick) this material comprises most of the continental plates. It has a density of 2.7 grams/cm3, Granite, is rich in silica, aluminu This type of rock material is referred to as being “Sialic” or “Felsic”. b. Oceanic Crust – (5 – 10 km thick) has a density of 3.0 grams/cm3 and is largely comprised of the igneous rock Basalt, which is rich in iron and magnesium. This type of rock material as referred to as being “Mafic” or “Basaltic”. V. The Refinement of the Earth’s Crust a. The early outer crust of the earth was a mixture of sialic and mafic material. b. The following processes occurred to separate the sialic materials from the mafic. This allowed continents to differentiate from oceanic material. Sial=granite Chp 10: Earth’s Interior Mafic=basalt Chp10: Earth Interior A. Separation of Mafic Materials from Sialic 1. Partial Melting – This is the process whereby hot mantle plumes rising up from the upper mantle heats (“partially melts”) the overlying mixture of mafics and sialics. This causes the denser mafic materials to separate downward, leaving the less dense sialic materials above…. 2. Fractional Crystallization – Mafic materials, being high in iron and magnesium, will crystallize at a higher temperature than sialic material, that is high in silica and aluminum. If the entire mixture of mafic and sialic material is heated to the point of melting and then allowed to cool, the mafic minerals will crystallize first, and being denser than the sialic material, will separate downwards in the melt from the still molten sialic materials on top…. B. Formation of Continental (Sialic) Plates: Continental Accretion As more crustal movement occurred, as the less dense sialic material was pushed against the denser mafic materials, the denser mafic material would become subducted or pushed downwards, underneath the sialic materials, thereby melting as it was subducted, the hot magma rising upwards through the sialic materials forming island arcs (“clumps” of sialic material). The formation of the island arcs perpetuated the refining processes of partial melting and fractional crystallization. Continental accretion-material literally scraped off oceanic plate and ‘accreted’ to continental plate…accretion=growth! Related to subduction of oceanic plate beneath continental Chp10: Earth Interior 1.As the less dense sialic “clumps” formed on the earth’s surface, spreading centers (divergent “cracks” in the earth’s surface) pushed the sialic materials together forming larger masses of sialic “chunks” in a process known as Continental Accretion. 2. This caused a fusion of the early sialic materials into Sialic or Granitic Continental Plates. 3. This also accounts for the composition of the continental plates as being High Grade Metamorphic Terranes (metamorphic rocks are formed under intense heat and pressure, the conditions during continental accretion). Three basic questions that geology can help to answer (specifically using what we know about the Earth’s Interior): 1. 2. 3. What is the origin of the Earth’s atmosphere? What is the origin of the water in the oceans? What is the origin of life on Earth? Chp10: Earth Interior Other Aspects of the Earth VI. The Atmosphere a. Prior to 4.5 BYA – The atmosphere consisted of hydrogen, methane, ammonia, hydrogen sulfide, nitrogen, argon, and water vapor. b.4.5 BYA to 3.0 BYA – The atmosphere consisted of nitrogen, argon, water vapor, CO2, and sulfur dioxide. c. 3.0 BYA to Today - The atmosphere consists approximately of 78% nitrogen, 20% oxygen, with the remaining 1-2% argon, water vapor, CO2, and minor gasses. How is it known that the early atmospheres were composed as mentioned above? Where did the other gasses go? Where did oxygen come from? Evidences of the early atmospheres: It is thought that all planets had at their formation atmospheres similar to the Jovian planets of today. Because of solar winds, the volatile gasses (hydrogen, methane, ammonia, hydrogen sulfide, sulfur dioxide, etc.) were blown off of the inner terrestrial planets, leaving the rocky core. Chp10: Earth Interior By studying the composition of the atmospheres of the Jovian planets today, geologists can derive the conditions of earth’s early atmosphere. 1. Banded Iron Formations – In certain areas there have been igneous activity resulting in formations of layers of iron interspersed between layers of other materials (i.e. silica) that have formed at the earth’s surface. Those banded iron formations that date before 3.0 BYA are composed of elemental, un-oxidized iron, indicating that they formed in an atmosphere devoid of free oxygen. Those banded iron formations that date younger than 3.0 BYA consist of iron that is oxidized throughout. This indicates that these younger iron layers formed in an atmosphere rich enough in oxygen to cause the complete oxidization of the iron. Hence, prior to 3.0 BYA there was not much free oxygen in the earth’s atmosphere, after 3.0 BYA there was. (BYA = Billion years ago…) 2. Oxygen is not given off in substantial amounts by volcanic activity today…so where did it come from? One source is photochemical dissociation. This occurs whenever oxygen-bearing chemical compounds in the upper atmosphere are subjected to cosmic radiation (background radiation from the Big Bang, solar radiation, etc.) and break apart releasing their oxygen atoms. This accounts for some of the “free” oxygen in the atmosphere, but not all. The other great source of oxygen is photosynthesis. Banded Iron Formations Chp10: Earth Interior This is the process whereby plants or plant-like organisms take in CO2 and H2O, and in the presence of sunlight convert these compounds into sugars thereby releasing free O2 into the atmosphere. Photosynthesis is a series of chemical reactions that convert sunlight energy into chemical energy. The processes occur in the chloroplasts of plants and algae. The components for raw photosynthesis are water, CO2 , and light energy. The formula for photosynthesis is: light energy & chlorophyll 6CO2 + 12H2O ----------------------- C6H12O6 + 6O2 + 6H2O The oldest known photosynthetic organisms, and the oldest known fossils, are Stromatolites. These are inter-tidal blue-green algae with the oldest to date is 3.6 BYA VII. The Hydrosphere and the Hydrologic Cycle Where did the free water on earth come from? As magma is formed within the earth, chemical compounds begin to bond eventually forming various compounds and minerals. Many of these compounds contain water – H2O as part of their makeup. Stromatolites-ancient and recent. note the bulbous, concentric growth patterns 1.80BYA-Side view Shark Bay, Australia ..growing today Saratoga Springs, NY 500 MY old-top view Chp10: Earth Interior Sometimes a crystal lattice (a tinker-toy like structure of bonding atoms) contains enough space within its 3-diminsional structure for water molecules to “fit”. Origin of the oceans….. 1. out gassing- As magma rises to the earth’s surface and is released on the surface as lava, the water escapes as steam. 2. As the steam cools in the atmosphere, water precipitates into clouds of water vapor. As these clouds cool, they loose their water as rain or other forms of water precipitation upon the surface of the earth. This volcanic out gassing is the source of most of the free water that comprises the oceans, lakes, rivers, etc. Over the time of earth’s existence, volcanism has out gassed enough water to fill the low-lying areas forming ocean basins. 3. Rainwater is naturally acidic, having a pH of about 5.5 to 6.5. As it hits the rocks and minerals on the surface of the earth, it is a major source of weathering and erosion of earth materials. As it runs down to the low-lying areas, it accumulates. As the sun evaporates the water, it rises as water vapor, forms clouds and this Hydrologic Cycle continues again and again. …the whole process is constantly occurring, repeating itself through geologic time… Chp10: Earth Interior-Summary I. The Earth consists of three concentric layers: a. the core: iron and nickel, 2 parts-solid inner, liquid outer; very dense b. the mantle: lower, upper=Asthenosphere, lower Lithosphere; Lithosphere c. the crust: Continental and Oceanic: 1. Continental Crust – (20 – 90 km thick) this material comprises most of the continental plates. It has a density of 2.7 grams/cm3 and is rich in silica, aluminum. This type of rock material is referred to as being “Sialic” or “Felsic”= granite. 2. Oceanic Crust – (5 – 10 km thick) has a density of 3.0 grams/cm3 and is largely comprised of the igneous rock Basalt, which is rich in iron and magnesium. This type of rock material as referred to as being “Mafic” or “Basaltic”. II. Refinement of Earth’s Crust- 2 processes a. Partial melting: hot mantle plumes rise within Mantle heating (i.e. partial melting) the material. Mafic minerals denser so they settle out, leaving less dense sial minerals behind. b. Fractional crystallization: Mafic minerals crystallize at higher temperatures than Sialic minerals, so Mafic minerals will crystallize first and separate from Mafic minerals III. ‘Growth’ of Continental Plates by accretion Process related to subduction of oceanic plate beneath continental plate. Sediments and basal literally scraped off subducting oceanic plate, plastered to edge of continental plate. Essential this material is ‘glued’ to the edge of the continental plate, resulting in that plate ‘growing’… Chp10: Earth Interior-Summary The Interior of the Earth gives us some clues as to the origin of: I. Atmosphere a. Evolution of gases through time- evidence in Banded Iron Formations. Generally some of oldest rocks on surface of Earth (1.0 BY or more….) b. Photosynethesis- stromatolites are evidence of bacterial action very early in preCambrian 600+ MYA II. Hydrosphere -Water originally came from volcanic activity- steam associated with outgassing as lava is extruded onto surface of Earth. -Steam is converted into water vapor in atmosphere. -Water vapor condenses in atmosphere and falls back to surface of Earth as rainfall, which erodes rocks -Water accumulates in rivers and streams, is evaporated by sunlight as water vapor into atmosphere and the entire process begins again…..The Hydrologic Cycle…. Chp10: Earth Interior-Summary III. The Biosphere The smallest particles are atoms which combine together to form molecules, which are the basic building blocks for all elements 1.The basic elements that serve as nutrients for life are derived from the Earth’s Interior and sunligh 2.These elements combine to form molecules, 3.Molecules combine to form cells, which are the basis for all Life as we know it… 4.These cells form tissues, which in turn combine to form organs. 5.Organs work in conjunction to form systems (circulatory, respiratory, muscular, etc.), and all of the systems together form the organism, the entity. 6.All organisms of the same species in a geographic area are called a biologic community 7.All of the biologic communities in a geographic area are called a biologic population. 8.All of the populations in an area interact with the abiotic (non-living aspects – soil, air, sunlight, etc.) to form an ecosystem. 9.All of the ecosystems on earth interact to collectively form the ecosphere or biosphere. Chp 6: Sedimentary Rocks- Summary A. Sedimentary Rocks are derived from weathering of igneous rocks B. Two types of Sedimentary rocks: -Chemical precipitates: Calcium carbonate -Clastic rocks: sandstones, shales, etc. C. 4 Steps to Formation of Sedimentary Rocks: -Erosion: various weathering processes -Transportation: air, glaciers, running water, etc -Deposition -Compaction: overburden, burial D. Characteristics of Clastic Sedimentary Rocks -Roundness: well rounded vs angular -Sorting: well sorted vs poorly sorted -Color: red=exposed to air; glauconite=green=marine -Particle Size (see following diagram) Table 6-1, p.152 Chp 6: Sedimentary Rocks- Summary II. Chemical Precipitates -Limestone: calcium carbonate - Dolostone: calcium and magnesium carbonate -Bioclastic deposits: form around remains of marine organisms. e.g. chalk, coquina (shells), -Evaporites: halite, gypsum, etc. Evaporitic conditions -Coal: peat, lignite, bituminous coal, anthracite Bedding/Layering of Sedimentary Rocks -Aeolian: wind, cross bedding -Fluvial: cross bedding, laminations, etc. -Turbidites: graded bedding. -Marine: currents-ripples -Marginal marine to terrestrial: mud cracks Laws of: Superposition and horizontality: oldest on bottom, flat Chp 6-Sedimentary Rocks: Summary Vertical Succession of Facies -Transgressive: sea level rise -Regressive: sea level fall Economic Uses of Sedimentary Rocks: -iron ore from fluvial deposits -placer (fluvial) deposits: precious minerals-gold and silver -Drilling for hydrocarbons (oil and gas) stratigraphic traps, structural traps, salt domes -gravel pits for road use Chp 7- Metamorphic Rocks-Summary Metamorphic Rocks form as a result of ‘metamorphism’…an alteration of rock characteristics and chemical composition due to application of heat and/or pressure, or chemically active fluids. “Parent rock” is term applied to the rock being metamorphosed-it may be igneous, sedimentary or even another metacmorphic rock. Metamorphic rocks commonly occur in-core of mountain ranges -continental shields (sedimentary rocks commonly deposited on top of them…) -original continental accretion in PreCambrian Factors applied during metamorphism: -Heat -Pressure -Chemically active fluids Table 7-1, p.192 Chp 7- Metamorphic Rocks-Summary Types of Metamorphism: A. Contact metamorphism: results from heat and fluids -metamorphic ‘halo’ known as aureole is generated (shale) -baked zones common -hydrothermal effects occur.. B. Regional burial: occurs over large area -gradation of minerals common as a result of high pressure -specific minerals indicate different levels of pressure/temperature C. Dynamic metamorphism: usually associated with fault zones - mylonites Economic uses- mining slate, hydrothermal minerals suggest proximity to gold or silver?? Chp 7- Metamorphic Rocks-Summary Metamorphic textures: A. Foliated-results from contact metamorphism -varies from coarse to fine slate, phyllite, schist, gneiss, amphibolite, migmatite B. Non-Foliated- no preferred orientation to minerals -2 types: single mineral, grains too small to be seen with naked eye marble, quartzite, greenstone, hornfels, anthracite Metamorphic Zones/Facies: metamorphic rocks characterized by specific mineral assemblages that reflect pressure-temperature regime rock experienced: 1. Greenschist: contain chlorite, low temperature, lo pressure 2.Granulite/Amphibolite: similar but higher pressure 3. Blueschist: fairly low temperature, high pressure. Indicative of subduction zones. Glaucophane mineral…. Metamorphic facies produced along oceanic-continental boundary Fig. 7-19, p.201 Summary • Early Christian theologians viewed time – as linear and decided that Earth – was very young (about 6000 years old) • A variety of ages for Earth were estimated – during the 18th and 19th centuries – using scientific evidence, – ages now known to be too young • Neptunism and catastrophism were popular – during the 17th, 18th and early 19th centuries – because of their consistency with scripture, – but were not supported by evidence Summary • James Hutton viewed Earth history – as cyclical and very long – His observations were instrumental – in establishing the principle of uniformitarianism • Charles Lyell articulated uniformitarianism – in a way that soon made it – the guiding doctrine of geology • Uniformitarianism holds that – – – – the laws of nature have been constant through time and that the same processes operating today have operated in the past, although not necessarily at the same rates Summary • The principles of superposition, – – – – – original horizontality, lateral continuity and cross-cutting relationships are basic for determining relative geologic ages and for interpreting Earth history • Radioactivity was discovered – – – – during the late 19th century and lead to radiometric dating, which allowed geologists to determine absolute ages for geologic events Summary • Types of unconformities– A disconformity is a surface • separating younger from older rocks, • both of which are parallel to one another – A nonconformity is an erosional surface • cut into metamorphic or intrusive rocks • and covered by sedimentary rocks – An angular unconformity is an erosional surface • on tilted or folded strata • over which younger rocks were deposited – Hiatus= interval of time NOT represented by rock record Summary • Geologists determine how many half-lives – of a radioactive parent isotope – have elapsed since the sample crystallized • Half-life is the length of time – – – – it takes for one-half of the radioactive parent isotope to decay to a stable daughter isotope of a different element Summary • The most accurate radiometric dates – – – – are obtained from long-lived radioactive isotope/daughter pairs in igneous rocks Common pairs include: • • • • • uranium 238 – lead 206 uranium 235 – lead 207 thorium 232 – lead 208 rubidium87 – strontium 87 potassium 40 – argon 40 Summary • The most reliable radiometric ages – are obtained using two different pairs – in the same rock • Carbon 14 dating can be used – – – – only for organic matter such as wood, bones, and shells and is effective back to about 70,000 years Chp 9: Earthquakes STUDING THE INTERIOR OF THE EARTH A. Seismic Tomography 1. Geologists use seismic waves to study the interior of the Earth 2. Properties of waves a. In a uniform homogeneous medium, a wave radiates velocity b. The velocity of a seismic wave depends on the nature of the material that travels through; i.e. its rigidity and density c. When waves pass form one material to another, they refract (bend) d. P-waves are compressional waves and travel through all media—gases, liquids, and solids—whereas 3-waves are only transmitted through solids. 3. In a uniform and homogeneous Earth waves will radiate form the focus of an earthquake in concentric spheres and travel uniformly through the planet. Uniform propagation of waves is not observed B. Discovery of the core 1. Shadow Zone a. No direct waves are defected beyond 105 degrees from the focus b. Caused by a discontinuity deep in the interior of the Earth c. The shadow zone exists because of refraction of the p-waves at the mantle-core boundary and the failure of s-waves to pass through Chp 9: Earthquakes 2. The failure of s-waves to pass through the outer core, and the refraction of p-waves shows that the core is composed of an inner solid sphere surrounded by an outer liquid sphere C. Discovery of the Crust-Mantle Boundary Mohorovicic Discontinuity (Moho): the boundary between the crust and the mantle was first identified as the boundary between different types of rock that transmit waves at different velocities. THE EARTH’S INTERIOR Seismic Tomography: The study of the Earth’s interior indirectly by studying the action of seismic waves. This information was discovered by seismic interpretation A. The Crust 1. The outer shell of the Earth 2. Oceanic crust 7-10 km thick -p-waves travel through oceanic crust (basaltic composition) at 7 km/sec 3. Continental crust -20-70 km thick -p-waves travel through continental crust (granitic composition) at 6km/sec Chp 10: Earth’s Interior Chp 9: Earthquakes B. The Mantle 1. 2900 km thick and comprises about 80% of the volume of the Earth 2. Large quantities of basalt magma originate in the mantle 3. The layers within the mantle a.Upper mantle --Extends from the base of the curst downward to about 670 km beneath the surfa --composed primarily of peridotite -- subdivided into three layers 1. lithosphere—crust and uppermost part of the mantle—where most earthquakes occur 2. asthenosphere – extends from the base of the lithosphere to a depth of 350 km 3. low-velocity layer—separates lithosphere form asthenosphere C. Outer Core – known to be liquid because of the behavior of seismic waves. It is speculated that the material is liquid iron and nickel. D. Inner Core – known to be solid because of the behavior of seismic waves and is thought to be solid iron and nickel. Chp 9: Earthquakes: Summary Earthquakes – causes of ….and locations of… a. release of accumulated stress along faults b. commonly occur along plate boundaries: circum-Pacific and Mediterranean region Earthquakes: location determined bya. minimum of 3 operating seismograph stations b. measure travel time to each station, plot distance as radius of circle c. where 3 circles intersect is the Epicenter-the location on the surface of the Earth directly above earthquake center Earthquake energy transmitted as WAVES…various types of waves: a. P waves: primary, distort grains parallel to motion of wave b. S waves; secondary, distort grains perpendicular to motion of wave Tonga volcanic arc, Pacific O: focal depth increases in well-defined Zone that defines subducting oceanic plate…called Benioff Zone…. Subducting oceanic plate… Fig. 9-5, p.248 Chp 9: Earthquakes: Summary Types of S waves: (surface) a. Rayleigh waves: material distorted in elliptical path b. Love waves: side to side motion Richter Scale: a. an increase of 1 unit on scale represents 10 fold increase in amplitude b. an increase of 1 unit on scale represents 30 fold increase in energy Seismic gap: useful for prediction today (not accurate) a. An immobile region of a fault bounded by moving segments b. Rock within the seismic gap is accumulating elastic deformation and will eventually fracture producing a major earthquake Other predictive tools: a. foreshock swarm intensity increases dramatically b. radon gas increase c. animal behavior Chp 9: Earthquakes: Summary Propogation of seismic waves from earthquakes has greatly aided our understanding of composition of interior of Earth: A. The Crust 1. The outer shell of the Earth 2. Oceanic crust -7-10 km thick -p-waves travel through oceanic crust (basaltic composition) at 7 km/sec 3. Continental crust -20-70 km thick -p-waves travel through continental crust (granitic composition) at 6km/sec B. The Mantle 1. 2900 km thick and comprises about 80% of the volume of the Earth 2. Large quantities of basalt magma originate in the mantle 3. The layers within the mantle Chp 9: Earthquakes: Summary a.Upper mantle --Extends from the base of the curst downward to about 670 km beneath the surface --composed primarily of peridotite -- subdivided into three layers 1. lithosphere—crust and uppermost part of the mantle—where most earthquakes occur 2. asthenosphere – extends from the base of the lithosphere to a depth o km 3. low-velocity layer—separates lithosphere form asthenosphere C. Outer Core – known to be liquid because of the behavior of seismic waves. It is speculated that the material is liquid iron and nickel. Inner Core – known to be solid because of the behavior of seismic waves and is thought to be solid iron and nickel. Chp 10: Earth’s Interior Chp 10: Earth’s Interior Chp 10: Earth’s Interior Chp 10: Earth’s Interior Chp 10: Earth’s Interior Chp 10: Earth’s Interior Interactions in Earth’s Subsystems Gases from respiration Transport of seeds and spores Chp10: Earth Interior VIII. The Biosphere: The Organization of Life on Earth The biosphere is the term for all of the living aspects of the earth. 1.All life as we know it is composed of atoms of various elements. 2. These atoms bond in various ways to form molecules. 3.Certain molecules make up cells, or the basic unit of life. This is called so because the cell exhibits all of the aspects that we consider to be living (atoms and molecules are not considered to be alive). 4. Certain cells work together to form tissues, and various tissues together form organs. 5.Organs work in conjunction to form systems (circulatory, respiratory, muscular, etc.), and all of the systems together form the organism, the entity. 6. All organisms of the same species in a geographic area are called a biologic community. 7. All of the biologic communities in a geographic area are called a biologic population. 8. All of the populations in an area interact with the abiotic (non-living aspects – soil, air, sunlight, etc.) to form an ecosystem. 9. All of the ecosystems on earth interact to collectively form the ecosphere or biosphere. Chp10: Earth Interior When an organism dies, certain bacteria and fungi (ecological decomposers) break down the organism back into molecules and atoms that are put back into the ecosystem for other organisms to use. The calcium in your bones came from foodstuffs (i.e. milk) consumed during your life. The milk containing the calcium came from the cow…the cow got the calcium from the grass consumed…the grass got the calcium from absorbing it from the soil…the soil formed from the weathering and erosion of calcium containing rocks and minerals, that came from the earth in the form of cooling magma or lava. Or the calcium in the soil could have come from the decomposition of the skeleton of some previous living organism…You may have calcium in your bones that once was incorporated into the skeleton of a dinosaur!!!