Introduction: spheres of the Earth

This chapter is an overview of the nature of the Earth's lithosphere, atmosphere, hydrosphere, and biosphere, and the interactions among them at the Earth's surface today. The long-term evolution of these “spheres” throughout Earth's history is also discussed briefly. The material in this chapter forms the basis of the more detailed information in the remainder of the book.

The Earth's shape closely approximates an oblate spheroid with a polar radius of 6,357 km and an equatorial radius of 6,378 km. A sphere with the same volume as the Earth has a radius of 6,370 km. The center of the Earth comprises a spherical core with a radius of approximately 3,470 km. Outside the core are several more or less concentric spheres. From inside out, these spheres are the mantle, lithosphere (composed of the uppermost mantle and overlying crust), hydrosphere (mostly the liquid ocean), and gaseous atmosphere (Figure 2.1). In addition, the biosphere is concentrated at the boundaries between the crust, ocean, and atmosphere. The biosphere extends down into the underlying crust for as much as 5 km, and to some as yet unknown depth into the crust beneath the oceans. The biosphere also extends up into the atmosphere. Coincidently, the surface of the Earth discussed in this book corresponds more or less to the distribution of the biosphere. The masses of the various spheres of the Earth are given in Table 2.1.