Earth

I

Introduction

Earth, third planet from the Sun and the fifth-largest of the eight planets. The mean distance of the Earth from the Sun is 149,503,000 km (92,897,000 mi). It is the only planet known to support life, although some of the other planets have atmospheres and contain water.

The Earth is not a perfect sphere but is slightly pear-shaped. Calculations based on perturbations in the orbits of artificial satellites reveal that the Earth is an imperfect sphere, because the equator bulges, or is distended, by 21 km (13 mi); the North Pole bulges by 10 m (33 ft); and the South Pole is depressed by about 31 m (100 ft).

II

Motion

In common with the entire solar system, the Earth is moving through space at the rate of approximately 20.1 km/s or 72,360 km/h (approximately 12.5 mi/s or 45,000 mph) towards the constellation of Hercules. The Milky Way galaxy as a whole, however, is moving towards the constellation Leo at about 600 km/s (375 mi/s). The Earth and its satellite, the Moon, also move together in an elliptical orbit about the Sun. The eccentricity of the orbit is slight, so that the orbit is virtually circular. The approximate circumference of the Earth’s orbit is 938,900,000 km (583,400,000 mi), and the Earth travels along it at a velocity of about 106,000 km/h (66,000 mph). The Earth rotates on its axis once every 23 hr 56 min 4.1 sec. A point on the equator therefore rotates at a rate of a little more than 1,600 km/h (about 1,000 mph), and a point on the Earth at a latitude of 45° North, rotates at about 1,073 km/h (667 mph).

In addition to these primary motions, there are other components of the total motion of the Earth: these include the precession of the equinoxes (see Ecliptic) and nutation, a periodic variation in the inclination of the Earth’s axis caused by the gravitational pulls of the Sun and Moon.

III

Composition

The Earth may be regarded as consisting of five parts: the first, the atmosphere, is gaseous; the second, the hydrosphere, is liquid; the third, fourth, and fifth, the lithosphere, mantle, and core, are largely solid. The atmosphere is the gaseous envelope that surrounds the solid body of the planet. Although it has a thickness of more than 1,100 km (700 mi), about half its mass is concentrated in the lower 5.6 km (3.5 mi). The lithosphere, consisting mainly of the cold, rigid, rocky crust of the Earth, extends to depths of 100 km (60 mi). The hydrosphere is the layer of water that, in the form of the oceans, covers approximately 70.8 per cent 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 3,794 m (12,447 ft), more than five times the average height of the continents. The mass of the oceans is approximately 1,350,000,000,000,000,000 (1.35 × 10¹⁸) tonnes, or about 1/4400 of the total mass of the Earth.

The rocks of the lithosphere have an average density of 2.7 times that of water and are almost entirely made up of 11 elements, which together account for about 99.5 per cent of its mass. The most abundant is oxygen (about 46.60 per cent of the total), followed by silicon (about 27.72 per cent), aluminium (8.13 per cent), iron (5.0 per cent), calcium (3.63 per cent), sodium (2.83 per cent), potassium (2.59 per cent), magnesium (2.09 per cent) and titanium, hydrogen (mostly combined with oxygen in the form of water), and phosphorus total less than 1 per cent between them. In addition, 11 other elements are present in trace amounts of from 0.1 to 0.02 per cent. These elements, in order of abundance, are carbon, manganese, sulphur, 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.

The lithosphere comprises two shells—the crust and upper mantle—that are divided into a dozen or so rigid tectonic plates. These are constantly in movement, driven by the flow of heat from the interior. The plates move like conveyor belts, being drawn downward into the crust at some margins and being replaced with rising molten rock at others. Continents are carried on the plates, at rates of movement measured in centimetres per year.

The crust itself has two parts. The sialic, or upper, crust, of which the continents consist, is made up of igneous, metamorphic, 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 3.0.

The lithosphere also includes the upper mantle. Rocks at these depths have a density of about 3.3. The upper mantle is separated from the crust above by a seismic discontinuity, called the Moho, and from the lower mantle by a zone of weakness known as the asthenosphere. Shearing of the plastic, partially molten rocks of the asthenosphere, 100 km (60 mi) thick, enables the continents to drift across the Earth’s surface and oceans to open and close.

The dense, heavy interior of the Earth is divided into a thick shell, the mantle, surrounding an innermost spherical core. The mantle extends from the base of the crust to a depth of about 2,900 km (1,800 mi). Except for the zone known as the asthenosphere, it is solid, and its density, increasing with depth, ranges from 3.3 to 6. The upper mantle is composed of iron and magnesium silicates, as typified by the mineral olivine. The lower part may consist of a mixture of oxides of magnesium, silicon, and iron.

Seismological research has shown that the core has an outer shell about 2,225 km (1,380 mi) thick with an average density of 10. This shell behaves like a fluid: studies show that its outer surface has depressions and peaks, the latter forming where warm material rises. In contrast, the inner core, which has a radius of about 1,275 km (795 mi), is solid. Both core layers are thought to consist largely of iron, with a small percentage of nickel and other elements. New research, published in 2002, appears to suggest that within the inner core there might exist an innermost centre, measuring approximately 600 km (370 mi) in diameter. Temperatures in the inner core have been estimated at 5,500° C (9,932° F), while the average density is estimated to be 13.

IV

Internal Heat Flow

Intense heat from the inner core is continually radiated outward, through the several concentric shells that form the solid portion of the planet. The source of this heat is thought to be energy released by the radioactive decay of uranium and other radioactive elements. Convection currents within the mantle transfer most of this heat energy from deep within the Earth to the surface and are the driving force behind continental drift. Convective flow supplies hot, molten rock to the worldwide system of mid-ocean ridges (see Ocean and Oceanography) and feeds the lava that erupts from volcanoes on land.