Till now, we have understood a few truth regarding mantle. One of the is that Mantle is a solid body. However, in a large time scale it convects, thus behaves like a fluid. But, in any case it is not a liquid like molten iron or water (last article) . Now, interesting thing is that we see volcanism, where, molten materials are coming from the deep earth. Practically it is has been proved that those materials are coming from different depth levels of mantle. Certain question arrives if mantle is solid, from where the melt is coming from. Straightforward answer to this question is that under certain situation solid mantle also melts. Let us discuss how and when mantle produces melt.
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Mantle convection is probably a fairly common word for most of the people now-a-days. Even from school days, we read a glimpse about the mantle convection, and generally come up with a picture in our mind. The picture is something like the following. This type of figure is widely spread in various webs or even in a few geology textbooks. This often tells that mantle is molten (or semi solid or any other imaginary words) which is being heated up from below, as core is supplying heat there. As it is heated up, material becomes light. So, it up-wells (simple convection principle). Above mantle there is crust, which is cold, hence they are very heavy. So, they start to sink down. All this is mantle convection. However, in reality, mantle convection is far more complex process. Moreover, the simple scenario imprinted in our mind, is wrong to a large extent. Here, we can discuss about the real mantle convection scenario and how it differs from basic convection model. Myth #1: Heat Source of Mantle Convection is driven by heat. This is an obvious fact. As the mantle is convecting, it must have a heat source. Where does this heat come from? Most of us believe that mantle is being heated up from the base. There, core is supplying the heat. This heat source is driving the mantle convection. However, this fact is not probably true. From several calculations it has been shown that the main source of heat is radioactive decay. Radioactive elements (mainly uranium, thorium and potassium) are such that they decay into smaller elements after certain time interval. During this decay, they release heat energy. These radioactive elements are present throughout the mantle. Their decay supplies dominant heat (around 80%) for convection. Basal heating from core is too negligible compared to radioactive heating. Question arises that, crust (Chemically distinct layer from mantle, also the topmost layer of the earth) contains radioactive elements more in concentration, but why is crust unable to produce convection like mantle? Answer to that, the total volume of the crust is way lesser than the mantle. So, total amount of radioactive elements is much more in mantle. That is why, radioactive elements cannot produce sufficient heat in crust, whereas voluminous radioactive decay provides the essential source for mantle convection. For Details: Turcotte, D.L. and G. Schubert, Geodynamics, John Wiley & Sons, New York, 1982. |
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