In the last article we got an idea that for convection heat is required. In idealistic case, heat is supplied from below to sustain convection. However, for earth hear source is uniformly distributed throughout the mantle by means of radioactive decay. But, what is the exact role of the heat in convection? How does it help in this physical process? Let us discuss about density. Density is an inherent property of a material. It can tell us how heavy a material is. If the material is heavy, it tends to sink in to a lighter material (like an iron ball can sink in a bucket of water). This happens because, iron (Fe) is much more dense (in general term, heavier) than water. Similarly, oil floats on water because oil is less dense (or lighter) than water. Mathematically, density is expressed as mass per unit volume. To visualize this, we can take an example of iron and cotton. 1 kg of iron will take very less volume, but 1 kg of cotton will take a huge volume, in spite of the fact that both have same weight. So, density of material is dependent on mass and volume. If the mass of the material is kept constant, but somehow the volume is changed, density of the material will change. Same material of same weight, can become heavy or light depending on its volume. Can someone change the volume of a material without touching it? YES! The materials that we see out daily do not change their volume, like iron plate does not change its volume. This is because, we live in a very short range of temperature. Hottest day of the hottest area on the earth and coldest night in the coldest area of the earth may have a temperature difference of maximum 100-130 degree Centigrade. In daily life we see much less temperature variation. But in scientific experiments it has been observed that, for a large change of temperature (implemented by applying more heat energy) can change the volume of material. However, this effect of volume change on solid due to variation of temperature is almost negligible. Although, in case of fluid temperature has significant effect on volume change. If the temperature is very high, fluid volume increases significantly. The fluid having larger volume, with same mass inside it, would have less density, hence lighter. Now using this basic knowledge of relationship between material density and temperature, the overall process of the so-called mantle convection can explained. In typical convection, the idea is a fluid is being heated from below, and it becomes lighter at the base of the fluid layer. With respect to this, material above will be heavier, hence they will feel more gravitational pull. Eventually, lighter material will go up and heavier material will sink down (see the first paragraph). Upon reaching the top, hot and lighter material will liberate the heat they had and will cool down eventually, whereas, the heavier material which has come down, will be heated. As soon as the heavier material is sufficiently heated, they will become less dense and will tend to move upward, and the earlier lighter material by now must have liberated heat and become dense enough to sink. This cycle continues, until the fluid attains a uniform temperature. Simply, if there is no change of temperature, there will be no density difference. With uniform density, there will be not lighter or heavier part. So, there will be no motion within the fluid. Figure : Steps of Convection process(a) A fluid is heated from the base. Hot and cold fluid is qualitatively described by the color code. (b) Hot fluid starts to up well from bottom, and cold fluid starts to sank towards bottom. (c) Complete overturn of temperature, upper part is hot, lower part of the fluid is cold, (d) Upper part liberated heat and became cold, lower part of the fluid is heated again. The cycle continues This process, known as convection, is only possible in fluids. As we discussed that density of solid is not much effected by temperature change, significant density anomaly is not possible to produce within a solid to sustain convection. Similarly, one cannot even visualize that in a chunk of iron, some part is moving upward and some part coming down. It is simply impossible to have convection within solid medium. From the above discussion, it might be clear that to sustain a convection, heat is the most important thing which can change the density of medium. Most important, this density change is significant to sustain convection only if the medium is fluid. For solid it is not at all possible. So, what is the state of mantle, the largest part within the earth, covering 84% of the total volume of our planet? Is it simply a fluid? Probably not. Will find the answer in our next articles.
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