EARTH

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.

There 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 flows.

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.

Orbit and 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.

The 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 silicate rocks.

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 depth.

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 continents.

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.

Earth Rotation

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.

On 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 six months.
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.

Earth Atmosphere

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.

The 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 dissipate.

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 fields.

The 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.

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