Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University
of Wisconsin - Green Bay
Earth: Vital Statistics
- Mean radius 6371 km (3981 miles)
- Average distance from Sun 149.6 million km (93 million miles)
- Closest to Sun (perihelion) 147.1 million km (91.5 million miles) about Jan. 3
- Farthest from Sun (aphelion) 152.1 million km (94.5 million) miles about July 4
- Rotates on axis in 24 hrs., revolves around Sun in 365.2564 days.
- Diameter: through equator 12756.26 km (7928 miles); through poles 12713.5 km (7899 miles)
- Difference is due to centrifugal force of earth's rotation (at equator, centrifugal
force is about 3/1000 as much as Earth's gravity)
- Surface area: 510 million square km (198 million) square miles, of which about a quarter is land.
- Mass: 5.97x1024 kg or 6,000,000,000,000,000,000,000 (6x1021)
- Of the Earth's mass:
- about 1/3 is in the core
- 2/3 in the mantle
- 4/1000 in the crust
- 2/10,000 in the oceans
- 1/1,000,000 in the atmosphere
- 1/100,000 in ice caps
What Makes the Earth an Unusual Planet
- Life (based on carbon--over 3 million species)
- Atmosphere (79% nitrogen, 20% oxygen, 1% argon, variable amounts of water vapor, carbon
- Presence of Large Amounts of Liquid Water
- Very Active Geologically
- Active erosion by water
- Plate tectonics--metamorphism, deformation, granitic rocks
- Few impact craters visible
- Strong Magnetic Field for Such a Small Planet
- Has an Unusually Large Satellite in Proportion to Its Size
Overview of Earth Systems
The three main components of the Earth are the atmosphere, its gaseous
envelope, the hydrosphere, the surface coating of water, and of course,
the solid earth. All three are subdivided into subsystems. The atmosphere and
hydrosphere get their energy mostly from the Sun, and the solid earth gets its
energy from internal heat, some of which is produced by radioactive decay and
some is left over from the formation of the earth. A tiny amount of energy also
comes from gravitational interactions between the Earth, the Moon, and the Sun.
The atmosphere is driven by unequal solar heating: unequal heating of land and
water, unequal heating between day and night, and unequal seasonal heating. The
hydrosphere is partly driven by winds, which drive ocean currents, but the
hydrosphere is also the earth's principal heat storage system. Evaporation of
water from the hydrosphere, its transport by the atmosphere, its eventual
condensation as rain or snow, and its eventual return to the oceans make up the
The earth's internal heat causes hot material to rise and cool material to sink
in the earth's interior, and this movement causes large slabs of the outer rigid
crust of the earth to move around.
The hydrologic cycle modifies the surface of the earth. Water breaks rocks down
chemically and mechanically, a process called weathering. Water flowing
on the surface carries loose material with it, a process called erosion.
Plate tectonics drives geologic processes like mountain building, earthquakes,
and volcanism. It also contributes to heating and melting of rocks.
Crustal movements uplift or lower the crust. Meanwhile weathering and erosion
wear down mountains and transport the debris to lower areas. Between the two
sets of processes, the earth's landscapes are created - and destroyed. Today's
landscape is the modified remains of yesterday's landscapes, and landscapes of
millions of years ago are lost beyond recovery.
The destruction of rocks by surface processes and their modification by
subterranean heat and pressure results in a constant recycling of rocks called
the rock cycle. Igneous rocks, the result of melting, are
eventually exposed on the surface by uplift and erosion. They are broken down by
weathering, and the debris transported and deposited to make sedimentary
rocks. These in turn can be buried, changed by heat and pressure, and become
metamorphic rocks. Some can even be heated to the point of melting and
creating a new generation of igneous rocks.
Finally, extraterrestrial disturbances can disrupt earth systems. Large
impacts create dramatic geologic effects near the impact, but also can cause
global climatic and environmental effects. If a star within a few light years of
the earth were to go supernova, the radiation effects on earth's life could be
dramatic. Close approaches of other stars to the Sun can cause distant comets to
drop into the inner solar system, possibly increasing the risk of impacts on
The diagram above shows the main components of the earth:
The solid earth consists of concentric shells that differ chemically and
mechanically. The middle shell, the mantle, can be defined differently depending
on whether we use chemical or physical definitions.
The chemically defined shells are:
- The crust, ranging from 5 km thick under the oceans to 40 km
thick under the continents and up to 80 km thick under mountain ranges. The
crust is rich in elements like potassium that don't fit easily in the
minerals that make up the deep earth. The rocks of the crust are the result
of accumulating these elements over time. The crust makes up about 0.3% of the
- The mantle, made of iron and magnesium silicates. The mantle
makes up 2/3 of the mass and 5/6 of the volume of the earth. When we're
talking about the chemistry of the earth, we generally use this definition
of the mantle.
- The core, mostly iron and nickel. The core is very nearly the
size of the planet Mars, a bit over half the diameter of the Earth. It makes
up 1/6 of the volume of the earth and 1/3 of its mass.
The mechanically defined shells are:
- A thin outer layer, the lithosphere, is about 100 km thick. In
comparison to the whole earth, it is about as thick as the skin on an apple.
The lithosphere consists of the crust and some of the upper mantle. This
part of the lithosphere is often called lithospheric mantle. The
lithosphere is what makes up the tectonic plates.
- Mechanically, the mantle begins with the asthenosphere, a thin, weak,
partially molten layer. The asthenosphere is the lubricating layer over
which the plates slide. The remainder of the mantle is solid but flows. It
consists of several layers that probably mark where minerals are compressed
into denser forms by the great pressure.
- The core is the same as the chemically defined core.
Temperatures in the core probably rival the surface of the Sun, but because
of the enormous pressure, the outer core is liquid and the inner core (a bit
smaller than the Moon or about a quarter of the earth's diameter) is solid.
Currents in the outer core probably generate earth's magnetic field.
The domain of life, from several kilometers deep in the lithosphere to 10 km
or so above the surface.
The zone of liquid water on the earth, dominated by the oceans but also
including lakes and rivers and liquid underground water.
The zone of frozen water on the earth, including the Antarctic and Greenland
ice caps, glaciers, and permanently frozen ground (permafrost). The cryosphere
is often considered part of the hydrosphere.
- The bottom 10 km or so is the troposphere, from a Greek word for
"turning." The sun warms the surface, but warm air rises, and as
it rises, it expands and cools. Then the cool air sinks. This constant
churning creates the weather.
- Above 10 km or so, the air is too thin for warm air from the surface to
continue rising. The next layer, the stratosphere, is stable and
stratified, with cold air at the base and warm air above. Because it is so
stable, commercial aircraft fly at the lower boundary of the stratosphere.
Military aircraft and the supersonic airliner the Concorde fly within the
stratosphere. Near the top of the stratosphere, solar ultraviolet causes
oxygen to form ozone (O3). Ozone absorbs solar ultraviolet and
helps protect the surface.
- From 50-80 km, the temperature of the atmosphere again begins to fall.
This layer is the mesosphere. At about 80 km we find the coldest
temperatures in the atmosphere, about -95 C. In the mesosphere, solar
radiation and particles create electrically charged atoms and this screen of
charged atoms (the ionosphere) absorbs radio signals. However, at night most of the charged
atoms recombine with stray electrons and the remaining charged atoms settle
into fairly smooth layers that reflect radio signals. This is why at night
you can often pick up AM stations a thousand miles or more away.
- Above 80 km temperatures again increase. The air is so thin that atoms are
accelerated by sunlight and solar particles. They are moving very fast, so
their temperature is very high, but there are so few atoms that a spacecraft
passing through this layer, the thermosphere, feels no appreciable
heat. The thermosphere tapers off into space, but traces of atmosphere extend
1000 km or more above the earth.
Convection in the Earth
Convection is the transportation of heat by moving hot or cold
material from place to place. Convection works because warm material is light
and rises, while cool material is denser and sinks. As long as there is a
temperature difference with depth, there will be a cycle of rising and sinking
material. A lava lamp is a perfect illustration of convection - if
geophysicists, astronomers, and other scientists who teach about convection had
their way, lava lamps would never go out of style.
In reality, there is a continuous cycle. Material at the base of the mantle
becomes hot and rises. As it rises, it expands and cools, and near the surface,
heat leaks through the crust and escapes. Cooled mantle material begins sinking
and descends to the bottom of the mantle, to be heated again.
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Created 30 July 2008, Last Update
13 September 2018