The Earth / Introduction
Of all the planets that were discovered so far, the earth is the only one of
which we know with certainty that it sustains life. The earth’s origin goes
back to the formation of our Solar System 4.6 billion years ago. Our planet
is believed to have started off as a conglomeration of colliding blocks.
are currently two theories concerning its origin. The homogeneous theory
states that the rock particles condensed and formed a layered structure of
matter with the lightest matter at its surface.
The heterogeneous theory professesthat there was initially a core of
heavyelements, which attracted lighter matteraround itself. After the
cooling of the crust, the first continents developed. These continental
plates however collapsed and were broken up and molten at the hot interior
of the earth. Such repetitive processes resulted in the separation of the
lighter and heavier elements and the earth gradually obtained its layered
structure. We can today still observe similar processes in the earth’s crust.
Seventy percent of the surface of the earth is covered in water. Water in
liquid form has not been found on any of the other planets. The oceans and
seas of the earth show many interesting and varying features, including
basins, plains, ridges, trenches and mountains and oceanic currents.
Thirty percent of the surface of the earth is made up of land and shows an
even greater variety of structures that the ocean floors. The landscapes of
the earth are partly dominated by the processes in the earth’s interior such
as continental drift and earthquakes, radioactive processes and convection
On the other hand, the surface of the earth is shaped by a variety of
influences such as heat from the sun, wind, precipitation, etc., resulting
in erosion. With rivers, lakes, mountains and valleys, the surface of the
earth is highly structured. The climatic influence presents us with all
types of vegetation and also desert zones.
Composition of the Earth
The earth is the third planet from the sun and is located between the orbits
of Venus and Mars. The moon is its only satellite.
earth is part of the group of the inner planets of our Solar System. The
inner planets consist of three main layers: crust, mantle and core. They are
generally solid rocky bodies.
There is relatively little known of the interior of the earth. Scientists
assume that there is a solid inner core of nickel and iron with temperatures
of up to 4000 °C.At such temperatures, metals are normally in liquid form
but the huge pressure that exists at the earth’s core keeps them in solid
form.This core is believed to have a diameter of approximately 2400
kilometers.The inner core is surrounded by an outer core of approximately
2000 kilometers in thickness and of slightly lower temperatures. It consists
of nickel and iron in liquid form. The convection currents in the liquid
outer core are believed to produce the magnetic field of the earth.
The boundary between the outer core and the earth’s mantle is called the
Gutenberg discontinuity. The mantle is approximately 2800 kilometers thick
and holds most of the earth’s mass. It consists mainly of hot, solid
There is also a distinct boundary between the earth’s mantle and the crust,
which is called the Mohorovicic discontinuity (abbreviated as Moho). It is
named after Andrija Mohorovicic, who, when analyzing earthquake waves in
1909, discovered that there must be an abrupt change of material at this
The crust that surrounds the earth is very thin when compared to the radius
of the planet. Beneath the oceans, its thickness is between 5 and 11
kilometers while it reaches up to 100 kilometers in thickness beneath the
The continental crust is a most versatile layer, containing rocks that are
up to 3.8 million years old.
Erosion, deformation, elevation and depression have resulted in a most
versatile structure. At the very surface of the crust, we often observe
sediments or rocks of volcanic origin. In lower layers, there are foliated
sediment rocks such as granite and foliated metamorphic rocks formed by
chemical reactions, which continuously produce new minerals.
The oceanic crust is more homogenous in structure: a layer of light
sediments containing organic matter extends over the entire floor of the
oceans. This crust usually has a thickness of a maximum three kilometers and
beneath it we find solid rock, mainly of basalt.
The layer of solid rock has a thickness of only about 1.5 kilometers where
it overlaps into a third layer, which is not yet fully explored as boring at
such depths beneath sea level is very difficult. This layer is believed to
have been penetrated from underneath by huge cones of magma originating in
the earth’s mantle. These are the locations where new crust is formed.
The surface of the earth consists of several tectonic plates. They consist
of the crust and the first 100 kilometers of the mantle and are referred to
as the lithosphere (from the Greek for ”world of rocks“). The lithosphere
drifts on the asthenosphere (from the Greek for ”soft rock“), which is a
much hotter and softer layer within the mantle. Depending on whether two
tectonic plates drift apart from each other or collide, mountains are
created or oceanic ridges open with magma drifting upwards to form new crust.
The earth is therefore subject to continuous changes as new crust is built
in several places and, at the same time, oceanic crust sinks back into the
asthenosphere in areas referred to as subduction zones. The rocks melt and
re-enter the cycle of rock formation. Such cycles obviously extend over
millions of years.
Other changes that may influence the nature of the earth are the result of
human activity. Some of the most pressing issues here are the continuous
rise in temperature (green house effect) and the effect referred to as the
hole in the ozone layer.
The earth revolved around the sun and, in the mean time, rotates on its own
axis. The earth completes one full orbit cycle around the sun in one year
(364.25 days) and its rotation period on its own axis is 23 hours and 56
minutes, i.e. one day.
the side of the earthfacing towards the sun, we have daytime. On the side of
the earthfacing away from the sun, we have night-time.
The phenomenon of the seasons is due to the fact that the rotation axis of
the earth is inclined by 23.4. degrees towards the plane of its orbit.
Therefore, the north pole is exposed for nearly six months to the sun while
the south pole is in the dark. This situation is reversed for the following
In the zones near the equator, the climate is much more moderate as the
sun’s light reaches these areas at a much more uniform angle.
The earth is not of perfect spherical shape but is slightly flatted at its
poles. point at the equator moves faster than a point nearer to the poles.
This difference in velocity results in an increased centrifugal force on
particles near the equator and hence the oblate shape of our globe. The
diameter of the earth at the equatorial plane is approximately 43 kilometers
larger than the distance from pole to pole.
The atmosphere, without which life on earth would not be possible, covers
our globe like a protective shield. It consists of 78% nitrogen and 21%
oxygen with 0.9% argon, 0.04% carbon dioxide and traces of neon, hydrogen,
helium, ozone, methane and nitric oxide.
atmosphere is structured into different layers: troposphere, stratosphere,
mesosphere, thermosphere and exosphere.
The troposphere is the layer adjacent to the surface of the earth, i.e. the
innermost layer of the atmosphere. In zones near the equator, it reaches a
thickness of approximately 20 kilometers.
At the poles, the troposphere is only about 10 kilometers thick. The
troposphere is the site of all weather on the earth. In this lowest layer,
rain,snow and cloud formation take place, including storms.
The atmosphere shows a gradual decrease in temperature from the surface of
the earth to the outer limit of the stratosphere. The average temperature on
the earth’s surface is 15°C while it is only minus 60 °C in these upper
damaging ultraviolet radiation from the sun by converting this type of
radiation into heat. The outer limit of the stratosphere is approximately 50
kilometers above us. Within this layer, we observe a slight temperature
increase from minus 60 °C toapproximately 0 °C.
The adjacent mesosphere extends to a height of approximately 80 kilometers
above the ground with a temperature in its upper layers of around minus 80
°C. Meteors from space occasionally penetrate this layer where they
The thermosphere extends up to 450 kilometers above the surface of the earth
and is considerably warmer. It is actually the hottest of the atmosphere
layers. The few gas molecules that are found here absorb the radiation from
the sun. Temperatures therefore can rise as high as 2000 °C.
The outermost layer of the atmosphere is the exosphere, which reaches
approximately 900 kilometers out into space. There is virtually no oxygen
and the atmosphere gradually fades out into space. This is the region where
the man-made satellites are orbiting our planet.
Earth Magnetic Field
Like other planets, the earth has a magnetic field. A magnetic field is an
area where forces exist, brought about by electric currents or electric
magnetic field of the earth has lost strength over the last2000 years.
Scientists predict that it will eventually fully disappear for a limited
time. This has happened several times in the dim and distant past when the
magnetic field changed its poles.
There is relatively little known of its origin but scientists agree today
that many billions of years ago, when the earth was a gaseous sphere,
chargeable particles were trapped by the magnetic field of the sun and
electrons were activated.This resulted in electric currents, inducing both
magnetism and other electricity and, eventually, the magnetic field of the
earth was established. The earth’s magnetic field extends far out into space
where it is severely deformed by the solar wind. The magnetic field, which
would otherwise have the shape of a phere, is flattened at the side exposed
to the sun and elongated at the opposite side. The magnetic field surrounds
the earth like an invisible shield and protects us from the deadly effects
of the solar wind.
The magnetic field of the earth can be imagined as a bar magnet located near
the earth’s center. The ”bar“ is inclined by approximately 11 degrees
towards the rotation axis of the earth. It has two poles and is therefore
also referred to as a dipole magnetic field.
The magnetic north and south poles do not coincide with the geographical
poles. Compasses therefore only indicate the general direction of north and
not the absolute geographical direction.