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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Author : Sreemoyee Chakraborty email
Image Source : USGS Plate Tectonic Mapping The Convergent Boundary - There can be 3 different types of interaction in this case -> Oceanic plate-oceanic plate convergence, continental plate-oceanic plate convergence and continental plate-continental plate convergence. In the first two cases volcanic activity is well observed. For these two cases when the plates collide, the denser plate subducts. The oceanic plate consists of many hydrated minerals along with sediments which carry hydrated minerals and as the plate subducts phase transition occurs resulting in dehydration of the minerals. The dehydration leads to release of water which lowers the melting temperature of the minerals present in the mantle wedge thus leading to melting of the same. The molten material then rises through the lithosphere leading to volcanic eruptions. The Divergent Boundary - Along the divergent boundaries two cases may occur – Either two oceanic plates diverge or two continental plates diverge. In case of the divergence of two continental plates initially rifts and then rift valleys are generated with no volcanic eruptions. However when two oceanic plates diverge a long, linear mid oceanic ridge is generated and this is where volcanic eruptions occur. The main cause for volcanism in this region is decompression melting – as the plates diverge there is a release of pressure which lowers the melting temperature of the mantle material leading to melting and upwelling of magma. This is also known as fissure eruption. The Transform Boundary - Volcanism is not much genetically associated along this boundary.
So, if you have read this article carefully then you probably know a little something about Plate Tectonics as well as about the related volcanic eruptions.. |
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