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2 The Earth’s Structure 2 • series of layers or spheres which differ in density, chemistry (or composition) and physical properties. Density 2 • is the relative heaviness of a substance • Or how heavy something is for its size • defined as the mass per unit volume • usually expressed in g/cm3 • EARTH HAS DENSITY STRATIFICATION 2 Evidence of the Earth’s Interior Composition • Can only drill about 7.5 miles (earth’s radius = 4000 mi) • Vents, volcanoes, variation in pull of gravity, etc. • study of the shocks from distant earthquakes Seismic Waves reveal that Earth’s Interior Is Layered If Earth were uniform (homogeneous) throughout 2 If the density, or rigidity, of Earth increased evenly with depth If Earth were layered inside 2 Compositional Layers of the Earth • the Crust – thin outermost layer • the Mantle – thick middle layer • the Core – densest inner layer – composed mainly or iron (90%) 2 2 2-3 2 Continents and Ocean Basins Differ • Continental crust – is mainly composed of granite, a light colored, lower density (2.7 gm/cm3) rock – thicker • Oceanic crust – is composed of basalt, a dark colored, higher density (2.9 gm/cm3) volcanic rock – thinner 2 2 Figure 1.17 2 2 Physical Properties of the Layers • Not determined only by chemical composition • The behavior of the rock (brittle or plastic) is determined mainly by temperature and pressure 2-1 2 Effects of Pressure and Temperature on Physical State of Layers • Increasing pressure raises the melting point of a material. • Increasing temperature provides additional energy causing material to melt. • Both pressure and temperature increase toward the center of the Earth, but at different rates. 2-1 2 Divisions of the Earth Based Upon Physical State • the Lithosphere – cool, rigid outer layer – comprised of continental crust, oceanic crust and the uppermost cool, rigid portion of the mantle • the Asthenosphere – hot, slowly flowing layer of the upper mantle • the Mesosphere (Lower Mantle) – rigid layer, similar chemically to the asthenosphere but very different physically • the Outer core – thick liquid layer • the Inner core – solid layer (due to tremendous pressure) 2 2 What Are the Sources of Internal Heat That Keep the Asthenosphere Plastic? • Radioactive decay (atoms of elements give off heat when their nuclei break apart) • Internal heat moves to the surface by conduction – slow migration of heat through a material by collision of atoms • And by convection – movement of heat in a fluid as it expands, becomes less dense and rises. 2 Fig. 3-11, p. 57 2 Isostatic Adjustment http://media.pearsoncmg.com/bc/bc_0media_geo/g eo_animations/isostatic-adjustment/isostaticadjustment.html