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Earth Structure and Systems

The planet Earth is made up of several different layers. From the centre of the core to the surface, there is a distance of approximately 6,370 kilometres. The very middle of the planet, the core, has two different components. The solid inner core is made of up what is believed to be a composite of nickel and iron, and the liquid outer core which is a mass of molten metal which circulates and creates the magnetic field of the planet. The Mantle, which makes up approximately 80% of the mass of the earth, and is made up of dense magnesium and iron silicates, which is solid but displays plasticity with convection currents running through. The mantle can be divided into different sections, with the area closest to the outer core called the Mesosphere, which is thought to be approximately 350 kilometres in depth. The Astenosphere, which is above the Mesosphere, is hot and plastic. The Lithosphere, or the crust, is brittle, cool and rigid 1). Please see picture below:

Earth Internal Structure

Formation

The most accepted theory on the formation of Earth and the solar system is the nebular hypothesis. This hypothesis was first discussed in the 1700's and is also a theory about the original formation of other galaxies and solar systems. It is thought that clouds of gas and particles, called a nebula, start to slowly swirl around and begins to contract, drawing the solid sections towards the centre. As the nebula continues to rotate, the solid mass starts to heat up and form what we know as a sun. Some of the other solid particles start to collide to create planetary masses at different points within the nebula. At points closer to the sun where the temperature is high enough to stop the condensation of water rockier, hotter planets are formed. The further the planetary mass from the sun, the hight the proportion of gas and ice, creating differently structured masses. The early composition of Earth' structure is thought to be made up of gases and molten rock. Due to its weight, the iron and nickel sunk to the centre, and the lighter substances moved to the outer sections. As a result, the Earth has ended up with several different layers, all of which continue to evolve as time goes on. 2).

nebula.jpg

Nebula

The creation of the Earth's magnetic field is evidence of a liquid core, as solid magnets have been proven to lose their magnetism at temperatures great than 500 degrees Celsius and can show decay over long periods of time. When an electrical current circulates, and does so at a rapid speed, a magnetic field can be created, and this is what is believed to be occurring on Earth. As the solid inner core continues to gain size due to the slow cooling in the centre, this provides the energy which is required to keep the liquid molten core circulating. Generally there are three rules when it comes to creating a magnetic field. Firstly you need a fluid which is conductive, secondly the liquid must move in a rapid motion and thirdly a 'seed' magnetic field is required. The seed is believed to come from the magnetic force from the sun, thus giving the Earth the three components required to create a magnetic field 3).

System Structures

There are three types of structures when it comes to systems. Firstly, there is the isolated system, in which energy and matter remain enclosed inside the system, similar to inside a sealed vacuum, with no influence from outside forces. The second system is what is called an closed system, where energy can flow through the system but the matter remains sealed inside. This could be best described as a sealed glass pool, where energy can heat it but matter or physical can not get in or out. The third system is an open system, where there is unrestricted flow of energy and matter through the system. The earth would be best described as a closed system, where energy (sunlight) can flow through but generally speaking no matter flows out of the system 4).

Within the earth system, there are several sub-systems. The scale of these sub-systems can range from a small pond to continental scale systems. These systems can be affected in a series of different ways, and can react differently each time. The cause of these effects can vary from anthropogenic reasons to natural climate changes. The current state of a system is called the state of equilibrium, and when this equilibrium is altered, it can cause the creation of a new equilibrium, which can either be in a positive or negative manner. A new equilibrium may not necessarily be created, as sometimes when the threshold is broken, there can be a runaway effect due to positive or negative feedbacks. For example, the warming in the ocean melts sea ice, resulting in more water volume, which gets heated and melts even more ice and so on. This is an example of a runaway positive feedback event 5).

Changes in systems need to follow a set of laws, which are called the laws of thermodynamics. There are three basic laws of thermodynamics, plus the 'Zeroth Law'. The first law is that energy cannot be created or destroyed, it can only be changed to a different type of energy. This is called entropy. The second law is that as disorder starts to form inside a system, it tends to increase disorder rather than decrease it, and no transformation of energy is ever at one hundred percent. The third law is that as temperature decreases towards absolute zero, the amount of disorder within the system approaches a minimum which tends to try and remain constant 6).

There are fundamental forces which act on earth systems. These forces can be gravitational force, such as the force of the earth's gravity, electromagnetic force which is the force that comes from the poles of the earth, strong nuclear force which hold subatomic particles together and weak nuclear force which contributes to the decay of particles. These are the four fundamental forces, and it is due to these forces that matter interacts and creates different types of energy. Energy is driver of all environmental systems on earth, has a near infinite supply and flows through systems. Matter is acted upon by energy and can be recycled within environmental systems however has a finite supply 7).

The Gaia Hypothesis

Prominent scientist and environmentalist James Lovelock in conjunction with a microbiologist, Lynn Margulis, came up with a theory in the 1970's called the 'Gaia Hypothesis'. They argued that the planet Earth is a single entity upon itself which acts in a way in which it can self regulate the life processes and influence the chemical and abiotic processes so that it can stay 'alive'. They also went on to say that life forms on earth interfered with these systems and that eventually natural selection will see the threatening life forms removed from the planet. There has been much conjecture whether this is a scientific or philosophical theory, as there is no way to prove that the earth is a living entity as a whole. There has been scientific research into the theory, with scientists focusing on how different life forms contribute to things such as global temperatures, the amount of oxygen in the atmosphere and ocean salinity levels. The planet has shown a tendency to keep itself stable in a state which can contribute to life being formed 8).

Earth Sciences

1) Hubbard, M. 2002, 'Solid earth: Structure and tectonics', GeoTimes, vol.47, no.7, p.15
2) Papaloizou, J. C. B., & Terquem, C., 2006, 'Planet formation and migration', Reports on Progress in Physics, vol.69, no.1, pp.119-180
3) McElhinny, M. W., 2000, 'The earth's magnetic field', American Geophysical Union, vol. 81, no.17, p.187
4) Vaughan, A. P. M, 2011, 'Putting the earth into earth system science. Antarctic Science', vol.23, no.4, p.321
5) Roy, W. R, 1997, 'Environmental Geology', Journal of Environmental Quality, vol.26, no.1, p.320
6) Gubbins, K. E. 1997, 'Thermodynamics', American Institute of Chemical Engineers, AIChE Journal, vol.43, no.1, p.285
7) Lui, A. T. Y., & Kamide, Y., 2007, 'Energy transfer in the earth-sun system', Transactions American Geophysical Union, vol.88, no.8, p.98
8) Schneider, S. H. 2002, 'Special theme: The gaia hypothesis', Climatic Change, vol.52, no.4, pp.3-4

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