Introduction to Earth
Do you know where you live? With the hustle and bustle of everyday life, it is easy to forget that the human family lives on a small blue planet named Earth. All around us we see trees, animals, cars, buildings, farms, factories, stores, and other natural and man-made structures.
With all of these every day familiar objects around us and with the vast sky above us, and the deep oceans beneath us, our home planet often feels quite large. Compared to us, it is very large. There is enough space for each of us, our families and friends, our pets, as well as trillions of other life forms to live and enjoy the various experiences of life.
While to us, the Earth appears to be a vast wilderness, compared to other objects in the Universe it is actually quite small, in fact, it is so small, that you could say it is tiny.
Earth, also known as theEarth or Terra. It is the third planet outward from the Sun. It is the largest of the solar system’s terrestrial planets, and the only planetary body that modern science confirms as harbouring life. The planet formed around 4.57 billion (4.57×109) years ago and shortly thereafter acquired its single natural satellite, the Moon. Its dominant species is the human (Homo sapiens).
Physical characteristics of the Earth
Physical characteristics
Shape
The Earth is approximately a slightly oblate spheroid (ellipsoid having a shorter axis and two equal longer axes), with an average diameter of approximately 12,742 km. The maximum deviations from this are the highest point on Earth (Mount Everest, which is only 8,850 m) and the lowest (the bottom of the Mariana Trench, at 10,911 m below sea level). The mass of the Earth is approximately 6 x 1024 kg.
Structure
Geophysical studies have revealed that the Earth has several distinct layers. Each of these layers has its own properties. The outermost layer of the Earth is the crust. This comprises the continents and ocean basins. The crust has a variable thickness, being 35-70 km thick in the continents and 5-10 km thick in the ocean basins. The crust is composed mainly of alumino-silicates.
The next layer is the mantle, which is composed mainly of ferro-magnesium silicates. It is about 2900 km thick, and is separated into the upper and lower mantle. This is where most of the internal heat of the Earth is located. Large convective cells in the mantle circulate heat and may drive plate tectonic processes.
The last layer is the core, which is separated into the liquid outer core and the solid inner core. The outer core is 2300 km thick and the inner core is 1200 km thick. The outer core is composed mainly of a nickel-iron alloy, while the inner core is almost entirely composed of iron. Earth’s magnetic field is believed to be controlled by the liquid outer core.
The Earth is separated into layers based on mechanical properties in addition to composition. The topmost layer is the lithosphere, which is comprised of the crust and solid portion of the upper mantle. The lithosphere is divided into many plates that move in relation to each other due to tectonic forces. The lithosphere essentially floats atop a semi-liquid layer known as the asthenosphere. This layer allows the solid lithosphere to move around since the asthenosphere is much weaker than the lithosphere.
Interior
Interior heat
The interior of Earth reaches temperatures of 5270 kelvins. The planet’s internal heat was originally generated during its accretion, and since then additional heat has continued to be generated by the decay of radioactive elements such as uranium, thorium, and potassium. The heat flow from the interior to the surface is only 1/20,000 as great as the energy received from the Sun.
Structure
Earth’s composition (by depth below surface):
0 to 60 km – Lithosphere (locally varies 5-200 km)
0 to 35 km – Crust (locally varies 5-70 km)
35 to 2890 km – Mantle
100 to 700 km – Asthenosphere
2890 to 5100 km – Outer Core
5100 to 6378 km – Inner Core
The core of the earth
The core
The average density of Earth is 5515 kg/m3, making it the densest planet in the Solar system. Since the average density of surface material is only around 3000 kg/m3, we must conclude that denser materials exist within the core of the Earth.
The core is divided into two parts, a solid inner core with a radius of ~1250 km and a liquid outer core extending beyond it to a radius of ~3500 km. The inner core is generally believed to be solid and composed primarily of iron and some nickel. The outer core surrounds the inner core and is believed to be composed of liquid iron mixed with liquid nickel and trace amounts of lighter elements. It is generally believed that convection in the outer core, combined with stirring caused by the Earth’s rotation gives rise to the Earth’s magnetic field through a process described by the dynamo theory. The solid inner core is too hot to hold a permanent magnetic field but acts to stabilise the magnetic field generated by the liquid outer core.
The mantle
Mantle
Earth’s mantle extends to a depth of 2890 km. It is largely composed of substances rich in iron and magnesium. The melting point of a substance depends on the pressure it is under. As there is intense and increasing pressure as one travels deeper into the mantle, the lower part of this region is thought solid while the upper mantle is semi-molten. Thus, the upper mantle can only flow very slowly.
Crust
The crust ranges from 5 to 70 km in depth. The thin parts are oceanic crust composed of dense iron magnesium silicate rocks and underlie the ocean basins. The thicker crust is continental crust which is less dense and composed of sodium potassium aluminium silicate rocks.
Biosphere
Biosphere
Earth is the only place where life is known to exist. The planet’s life forms are sometimes said to form a “biosphere”. The biosphere is divided into a number of biomes, inhabited by broadly similar flora and fauna. On land, biomes are separated primarily by latitude. Terrestrial biomes lying within the Arctic and Antarctic Circles are relatively barren of plant and animal life, while most of the more populous biomes lie near the Equator.
Atmosphere
Earth has a relatively thick atmosphere composed of 78% nitrogen, 21% oxygen, and 1% argon, plus traces of other gases including carbon dioxide and water vapour. The atmosphere acts as a buffer between Earth and the Sun. The Earth’s atmospheric composition is unstable and is maintained by the biosphere. Namely, the large amount of free diatomic oxygen is maintained through solar energy by the Earth’s plants, and without the plants supplying it, the oxygen in the atmosphere will over geological timescales combine with material from the surface of the Earth.
The layers, troposphere, stratosphere, mesosphere, thermosphere, and the exosphere, vary around the globe and in response to seasonal changes.
TROPOSPHERE
This is the layer of the atmosphere closest to the Earth’s surface, extending up to about 10-15 km above the Earth’s surface. It contains 75% of the atmosphere’s mass. The troposphere is wider at the equator than at the poles. Temperature and pressure drops as you go higher up the troposphere.
STRATOSPHERE
This layer lies directly above the troposphere and is about 35 km deep. It extends from about 15 to 50 km above the Earth’s surface. The lower portion of the stratosphere has a nearly constant temperature with height but in the upper portion the temperature increases with altitude because of absorption of sunlight by ozone. This temperature increase with altitude is the opposite of the situation in the troposphere.
The Ozone Layer: The stratosphere contains a thin layer of ozone which absorbs most of the harmful ultraviolet radiation from the Sun. The ozone layer is being depleted, and is getting thinner over Europe, Asia, North American and Antarctica, “holes” are appearing in the ozone layer.
MESOSPHERE
Directly above the stratosphere, extending from 50 to 80 km above the Earth’s surface, the mesosphere is a cold layer where the temperature generally decreases with increasing altitude. Here in the mesosphere, the atmosphere is very rarefied nevertheless thick enough to slow down meteors hurtling into the atmosphere, where they burn up, leaving fiery trails in the night sky.
THERMOSPHERE
The thermosphere extends from 80 km above the Earth’s surface to outer space. The temperature is hot and may be as high as thousands of degrees as the few molecules that are present in the thermosphere receive extraordinary large amounts of energy from the Sun. However, the thermosphere would actually feel very cold to us because of the probability that these few molecules will hit our skin and transfer enough energy to cause appreciable heat is extremely low.
Hydrosphere
Hydrosphere
Earth is the only planet in our solar system whose surface has liquid water. Water covers 71% of Earth’s surface (97% of it being sea water and 3% fresh water (http://earthobservatory.nasa.gov/Library/Water/) and divides it into five oceans and seven continents. Earth’s solar orbit, gravity, greenhouse effect, magnetic field and oxygen-rich atmosphere seem to combine to make Earth a water planet.
Earth is actually beyond the outer edge of the orbits which would be warm enough to form liquid water. Without some form of a greenhouse effect, Earth’s water would freeze.
On other planets, such as Venus, gaseous water is destroyed by solar ultraviolet radiation, and the hydrogen is ionized and blown away by the solar wind. This effect is slow, but inexorable. This is one hypothesis explains why Venus has no water. Without hydrogen, the oxygen interacts with the surface and is bound up in solid minerals.
In the Earth’s atmosphere, a tenuous layer of ozone within the stratosphere absorbs most of this energetic ultraviolet radiation high in the atmosphere, reducing the cracking effect. The ozone, too, can only be produced in an atmosphere with a large amount of free diatomic oxygen, and so also is dependent on the biosphere. The magnetosphere also shields the ionosphere from direct scouring by the solar wind.
The total mass of the hydrosphere is about 1.4 × 1021 kg, ca. 0.023 % of the Earth’s total mass
Earth in the Solar System
Earth in the Solar System
It takes Earth 23 hours, 56 minutes and 4.091 seconds to rotate around the axis connecting the north pole and the south pole. Earth orbits the Sun every 365.2564 mean solar days. Earth has one natural satellite, the Moon, which orbits around Earth every 27 1/3 days.
The orbital and axial planes are not precisely aligned: Earth’s axis is tilted some 23.5 degrees against the Earth-Sun plane (which causes the seasons), and the Earth-Moon plane is tilted about 5 degrees against the Earth-Sun plane (otherwise there would be an eclipse every month).
The Moon
The Moon
Luna, or simply ‘the Moon’, is a relatively large terrestrial planet-like satellite, about one quarter of Earth’s diameter(3,474 kms). The natural satellites orbiting other planets are called “moons”, after Earth’s Moon.
While there are only two basic types of regions on the Moon’s surface, there are many interesting surface features such as craters, mountain ranges, riles, and lava plains. The structure of the Moon’s interior is more difficult to study. The Moon’s top layer is a rocky solid, perhaps 800 km thick. Beneath this layer is a partially molten zone. Although it is not known for certain, many lunar geologists believe the Moon may have a small iron core, even though the Moon has no magnetic field. By studying the Moon’s surface and interior, geologists can learn about the Moon’s geological history and its formation.
The footprints left by Apollo astronauts will last for centuries because there is no wind on the Moon. The Moon does not possess any atmosphere, so there is no weather as we are used to on Earth. Because there is no atmosphere to trap heat, the temperatures on the Moon are extreme, ranging from 100° C at noon to -173° C at night.
The Moon doesn’t produce its own light, but looks bright because it reflects light from the Sun. Think of the Sun as a light bulb, and the Moon as a mirror, reflecting light from the light bulb. The lunar phase changes as the Moon orbits the Earth and different portions of its surface are illuminated by the Sun.
The gravitational attraction between the Earth and Moon cause the tides on Earth. The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit the Earth. As a result it always presents the same face to the planet.
The Moon is just far enough away to have, when seen from Earth, very nearly the same apparent angular size as the Sun (the Sun is 400 times larger, but the Moon is 400 times closer). This allows total eclipses as well as annular eclipses to occur on Earth. Here is a diagram showing the relative sizes of the Earth and the Moon and the distance between the two.
Biggest geographic subdivision
Area:
Total: 510.073 million km2
Land: 148.94 million km2
Water: 361.132 million km2
Note: 70.8 % of the world’s surface is covered by water, 29.2 % is exposed land
Land boundaries: the land boundaries in the world total 251,480 km (not counting shared boundaries twice)
Coastline: 356,000 km
Climate
Two large areas of polar climates separated by two rather narrow temperate zones from a wide equatorial band of tropical to subtropical climates. Precipitation patterns vary widely, ranging from several metres of water per year to less than a millimetre.
The climate where you live is called regionalclimate. It is the average weather in a place over more than thirty years. To describe the regional climate of a place, people often tell what the temperatures are like over the seasons, how windy it is, and how much rain or snow falls. The climate of a regional depends on many factors including the amount of sunlight it receives, its height above sea level, the shape of the land, and how close it is to oceans. Since the equator receives more sunlight than the poles, climate varies depending on distance from the equator.
However, we can also think about the climate of an entire planet. Globalclimate is a description of the climate of a planet as a whole, with all the regional differences averaged. Overall, global climate depends on the amount of energy received by the Sun and the amount of energy that is trapped in the system. These amounts are different for different planets. Scientists who study Earth’s climate and climate change study the factors that affect the climate of our whole planet.
While the weather can change in just a few hours, climatechanges over longer timeframes. Our Earth is warming more quickly than it has in the past according to the research of scientists. Hot summer days may be quite typical of climates in many regions of the world, but global warming is causing Earth’s average global temperature to increase. The amount of solar radiation, the chemistry of the atmosphere, clouds, and the biosphere all affect Earth’s climate.
Natural and environmental hazards
Large areas are subject to extreme weather such as tropical cyclones, hurricanes, or typhoons that dominate life in those areas. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, and other calamities and disasters.
Many localized areas are subject to human-made pollution of the air and water, acid rain and toxic substances, loss of vegetation, loss of wildlife, species extinction, soil degradation, soil depletion, erosion, and introduction of invasive species.
A scientific consensus exists linking human activities to global warming due to industrial carbon dioxide emissions. This is predicted to produce changes such as the melting of glaciers and ice sheets, more extreme temperature ranges, significant changes in weather conditions and a global rise in average sea levels.
In General
Modern geologists and geophysicists accept that the age of the Earth is around 4.54 billion years (4.54 × 109 years ± 1%). This age has been determined by radiometric age dating of meteorite material and is consistent with the ages of the oldest known terrestrial and lunar samples.
Following the scientific revolution and the development of radiometric age dating, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old. The oldest such minerals analyzed to date, small crystals of zircon from the JackHills of Western Australia, are at least 4.404 billion years old. Comparing the mass and luminosity of the Sun to the multitudes of other stars, it appears that the solar system cannot be much older than those rocks. Ca-Al-rich inclusions (inclusions rich in calcium and aluminium), the oldest known solid constituents within meteorites that are formed within the solar system , are 4.567 billion years old, giving an age for the solar system and an upper limit for the age of Earth. It is hypothesized that the accretion of Earth began soon after the formation of the Ca-Al-rich inclusions and the meteorites. Because the exact accretion time of Earth is not yet known, and the predictions from different accretion models range from a few millions up to about 100 million years, the exact age of Earth is difficult to determine. It is also difficult to determine the exact age of the oldest rocks on Earth, exposed at the surface, as they are aggregates of minerals of possibly different ages. The Acasta Gneiss of Northern Canada may be the oldest known exposed crustal rock.
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