2 The Earth’s Structure series of layers or spheres which differ in density, chemistry (or composition) and physical properties.

2 Density is the relative heaviness of a substance defined as the mass per unit volume usually expressed in g/cm 3 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

2 Earth’s Interior Is Layered What evidence supports the idea that Earth has layers? The behavior of seismic waves generated by earthquakes give scientists some of the best evidence about the structure of Earth. (a) If Earth were uniform (homogeneous) throughout, seismic waves would radiate from the site of an earthquake in straight lines. (b) If the density, or rigidity, of Earth increased evenly with depth, seismic wave velocity would increase with depth, and the waves would bend smoothly upward toward the surface. (c) If Earth were layered inside, some seismic waves would be reflected at the boundaries between layers while others were bent. Seismic evidence shows that Earth is layered.

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 Figure 1.14

2 Figure 1.16

2 Continents and Ocean Basins Differ Continental crust –is mainly composed of granite, a light colored, lower density (2.7 gm/cm 3 ) rock –thicker Oceanic crust –is composed of basalt, a dark colored, higher density (2.9 gm/cm 3 ) volcanic rock –thinner 2-3

2 Table 1.1

2

2 Figure 1.17

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 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) 2-1

2 Figure 1.14

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