DEFINITION OF EARTH

The third planet from the center of our solar system,

located between Venus and Mars at a distance of 93 million

miles (150 × 106 km) from the Sun. It has a mean radius of

3,960 miles (6,371 km), a surface area of 5.101 × 108 km2,

and an average density of 5.5 grams per cubic centimeter. It is

one of the terrestrial planets (Mercury, Venus, Earth, and

Mars), composed of solid rock, with silicate minerals being

the most abundant in the outer layers and a dense iron-nickel

alloy forming the core material.

The Earth and eight other planets condensed from a

solar nebula about 5 billion years ago. In this process a

swirling cloud of hot dust, gas, and protoplanets collided

with each other, eventually forming the main planets. The

accretion of the Earth was a high-temperature process that

allowed melting of the early Earth and segregation of the

heavier metallic elements such as iron (Fe) and nickel (Ni) to

sink to the core, and for the lighter rocky elements to float

upward. This process led to the differentiation of the Earth

into several different concentric shells of contrasting density

and composition and was the main control on the large-scale

structure of the Earth today.

The main shells of the Earth include the crust, a light

outer shell 3–43 miles (5–70 km) thick. This is followed

inward by the mantle, a solid rocky layer extending to 1,802

miles (2,900 km). The outer core is a molten metallic layer

extending to 3,170 miles (5,100 km) depth, and the inner

core is a solid metallic layer extending to 3,958 miles (6,370

km). With the recognition of plate tectonics in the 1960s,

geologists recognized that the outer parts of the Earth were

also divided into several zones that had very different

mechanical properties. It was recognized that the outer shell

of the Earth was divided into many different rigid plates all

moving with respect to each other, and some of them carrying

continents in continental drift. This outer rigid layer became

known as the lithosphere and is 45–95 miles (75–150 km)

thick. The lithosphere is essentially floating on a denser, but

partially molten layer of rock in the upper mantle known as

the asthenosphere (or weak sphere). It is the weakness of this

layer that allows the plates on the surface of the Earth to

move about.

The most basic division of the Earth’s surface shows that

it is divided into continents and ocean basins, with oceans

occupying about 60 percent of the surface and continents 40

percent. Mountains are elevated portions of the continents.

Shorelines are where the land meets the sea. Continental

shelves are broad to narrow areas underlain by continental

crust, covered by shallow water. Continental slopes are steep

drop-offs from the edge of a shelf to the deep ocean basin,

and the continental rise is where the slope flattens to merge

with the deep ocean abyssal plains. Ocean ridge systems are

subaquatic mountain ranges where new ocean crust is being

created by seafloor spreading. Mountain belts on the Earth

are of two basic types. Orogenic belts are linear chains of

mountains, largely on the continents, that contain highly

deformed, contorted rocks that represent places where lithospheric

plates have collided or slid past one another. The midocean

ridge system is a 40,000-mile (65,000-km) long

mountain ridge that represents vast outpourings of young

lava on the ocean floor and places where new oceanic crust is

being generated by plate tectonics. After it is formed, it

moves away from the ridge crests, and new magmatic plates

fill the space created by the plates drifting apart. The oceanic

basins also contain long, linear, deep ocean trenches that are

up to several kilometers deeper than the surrounding ocean

floor and locally reach depths of 7 miles (14 km) below the

sea surface. These represent places where the oceanic crust is

sinking back into the mantle of the earth, completing the

plate tectonic cycle for oceanic crust.

External layers of the Earth include the hydrosphere,

consisting of the oceans, lakes, streams, and the atmosphere.

The air/water interface is very active, for here erosion breaks

rocks down into loose debris called the regolith.

The hydrosphere is a dynamic mass of liquid, continuously

on the move. It includes all the water in oceans, lakes,

streams, glaciers, and groundwater, although most water is in

the oceans. The hydrologic cycle describes changes, both long

and short term, in the Earth’s hydrosphere. It is powered by

heat from the Sun, which causes evaporation and transpiration.

This water then moves in the atmosphere and precipitates

as rain or snow, which then drains off in streams,

evaporates, or moves as groundwater, eventually to begin the

cycle over and over again.

The atmosphere is the sphere around the Earth consisting

of the mixture of gases we call air. It is hundreds of kilometers

thick and is always moving, because more of the Sun’s

heat is received per unit area at the equator than at the poles.

The heated air expands and rises to where it spreads out,

cools and sinks, and gradually returns to the equator. The

effects of the Earth’s rotation modify this simple picture of

the atmosphere’s circulation. The Coriolis effect causes any

freely moving body in the Northern Hemisphere to veer to

the right, and toward the left in the Southern Hemisphere.

The biosphere is the totality of Earth’s living matter and

partially decomposed dead plants and animals. The biosphere

is made up largely of the elements carbon, hydrogen, and

oxygen. When these organic elements decay they may become

part of the regolith and be returned through geological processes

back to the lithosphere, atmosphere, or hydrosphere.

See also ATMOSPHERE; BIOSPHERE; LITHOSPHERE; MANTLE.

earthflow See MASS WASTING.

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