The Geologic, Thermal, and Hydrologic State of the Earth

Abstract

To a first approximation, the earth is a 12,700 km diameter 5.976 × 10²⁷ g sphere that rotates once a day on its own axis, follows an elliptical path around the sun once a year, and is circled by the moon about once every 28 days. It was formed as a more or less homogeneous mass about 4.5 billion years ago, but soon began to undergo gravitational differentiation, the lighter materials coming to the surface and the heavier ones settling down toward the center. It now has a dense liquid iron-nickel 6900 km diameter core that contains an even more dense solid 2700 km diameter inner core. The liquid core is in turn surrounded by a 2900 km thick, much less dense solid mantle that accounts for about two-thirds of the total mass of the earth. A still less dense crust varying in thickness from 5 to 70 km, overlies the mantle, the thinnest parts of which lie under the ocean. Thicker sections form the continents and the thickest parts lie beneath the mountains (see Fig. 2–1).