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ECU GEOL 1500 - Earth%27s Internal Structure - Notes - Fall 2014

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AND COMPOSITIONINTERNAL STRUCTURE OF EARTHCOMPOSITION OF THE EARTHGeologists study the Earth’s physical structure and chemical composition for 3 reasons:(1) The Earth’s internal structure has a lot to do with shaping the Earth’s landscape and affects processes that may pose hazardous situations and events for humans.(2) Humans are dependent on the materials of the solid Earth as resources.(2) These Earth materials have distinct physical and chemical properties, which we need to understand because they can affect human society in many ways.THE INSIDE AND OUTSIDE OF THE EARTHThe Earth is a differentiated planetary body. This means that the materials from which the Earth is composed are NOT evenly or homogeneously distributed. Most of this differentiation occurred early in Earth history and is sometimes referred to as the Iron Catastrophe. As a result of the Iron Catastrophe, most of the Earth’s denser materials sankto its center and the lighter, less dense materials rose toward its surface. Escaping gases led to the Earth’s atmosphere and oceans. Ultimately, by about 4.0 billion years ago, the Earth was differentiated into 3 major compositional zones: (1) the Core, which is composed mostly of iron with lesser amounts of nickel and other elements; (2) the Mantle, which is less dense than the core and composed mostly of iron and magnesium silicate and oxide minerals; and (3) the Crust, which is the thinnest and least dense compositional layer and is composed of Aluminum (Al), Potassium (K), Sodium (Na), Calcium (Ca), Iron, (Fe), and Magnesium (Mg) silicate minerals. For an illustration of the Earth's compositional zones, see the diagram immediately below.Cross sectional view of Earth illustrating its heterogeneous layers. (image from Beatty, J. K. and A. Chaikin, eds. The New Solar System. Massachusetts: Sky Publishing, 3rd Edition, 1990.)COMPOSITION AND SUBDIVISIONS OF THE EARTH’S 3 MAJOR COMPOSITIONAL ZONESCORE (~2900 - ~6375 km) Density averages about 10.7 times that of water. Inner Core - Predominantly composed of Iron (Fe) - Solid StateOuter Core - Predominantly composed of Iron (Fe) - Liquid StateMANTLE (5/35 - 2900 km) Density ranges from about 3.3 to 5.5 times that of water. Asthenosphere (~100 - 350 km) {Lies entirely within the Upper Mantle} Hot, weak, and plasticLithosphere (0 - ~100 km) {Contains the uppermost part of the Mantle and the entire Crust} Cold, strong, and rigid/brittleCRUST (0 - ~100 km) Oceanic Crust - Averages 5 km thickness with average density of 2.9-3.0 times that of water (Basaltic)Continental Crust - Average 35 km thickness with average density of 2.7-2.8 times that of water (Granitic)Be Able to Label the Earth’s Compositional Zones in this DiagramAnd Know the Radius and Diameter of the Earth in KilometersSUBDIVISIONS OF THE EARTHEARTH STRUCTURE BASED ON PHYSICAL BEHAVIOR AND COMPOSITIONIn cross section the Earth is divided into a number of compositional zones. At the center of the Earth is the core. It is composed mostly of iron and nickel and exists both in the solid and molten state. Hence, the Inner Core and the Outer Core of the Earth are distinguished based on physical behavior, solid versus liquid state respectively.The rest of the Earth above the core is divided in two ways, one based on composition-density and theother based on physical behavior. These divisions are sometimes confusing because the two sets of divisions have different terms applied to them and the divisions overlap.Based on physical behavior, the upper 350 to 400 kilometers of the Earth is divided into the lithosphere and asthenosphere. The lithosphere is the upper layer, which is cold, brittle, and strong and breaks when stressed. It extends from the Earth’s surface to depths of 70 to 100 kilometers and includes all of the Earth’s crust and the uppermost part of the mantle. The asthenosphere is part of the Upper Mantle that lies directly under the lithosphere and is hot, plastic, and weak and flows very slowly (cm/year) in convection cells. The asthenosphere extends from the base of the lithosphere to depths of 350 to 400 kilometers so it lies completely within the upper mantle.Based on composition and density, the upper 2900 kilometers of the Earth is divided into the crust and mantle. The crust, which is also differentiated based on composition and density into oceanic and continental crust, averages 5 kilometers and 35 kilometers in thickness respectively. However, both the oceanic and continental crust are compositionally different from the mantle and have lower densities than the mantle. Study the illustrations shown below. The mantle extends to a depth of about 2900 kilometers from the base of the Earth’s crust so most of the mantle lies below the asthenosphere. (image from Beatty, J. K. and A. Chaikin, eds. The New Solar System. Massachusetts: Sky Publishing, 3rd Edition, 1990.) INCLUDEPICTURE "http://pubs.usgs.gov/gip/dynamic/graphics/FigS1-1.gif" \* MERGEFORMATINETImage from the


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