The hydrosphere is the layer of water that, in the form of the oceans, covers approximately 70.8 percent of the surface of the earth. The mantle and core are the heavy interior of the earth, making up most of the earth's mass. The hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters. The average depth of the oceans is 3794 m (12,447 ft), more than five times the average height of the continents. The mass of the oceans is approximately 1.35 quintillion (1.35 × 1018) metric tons, or about 1/4400 of the total mass of the earth.
The rocks of the lithosphere have an average density of 2.7 and are almost entirely made up of 11 elements, which together account for about 99.5 percent of its mass. The most abundant is oxygen (about 46.60 percent of the total), followed by silicon (about 27.72 percent), aluminum (8.13 percent), iron (5.0 percent), calcium (3.63 percent), sodium (2.83 percent), potassium (2.59 percent), magnesium (2.09 percent) and titanium, hydrogen, and phosphorus (totaling less than 1 percent). In addition, 11 other elements are present in trace amounts of 0.1 to 0.02 percent. These elements, in order of abundance, are carbon, manganese, sulfur, barium, chlorine, chromium, fluorine, zirconium, nickel, strontium, and vanadium. The elements are present in the lithosphere almost entirely in the form of compounds rather than in their free state. These compounds exist almost entirely in the crystalline state, so they are, by definition, minerals.
The lithosphere comprises two shells-the crust and upper mantle-that are divided into a dozen or so rigid tectonic plates (see Plate Tectonics). The crust itself is divided in two. The sialic or upper crust, of which the continents consist, is made up of igneous and sedimentary rocks whose average chemical composition is similar to that of granite and whose density is about 2.7. The simatic or lower crust, which forms the floors of the ocean basins, is made of darker, heavier igneous rocks such as gabbro and basalt, with an average density of about 3.
The lithosphere also includes the upper mantle. Rocks at these depths have a density of about 3.
