Jyotirmoy Paul, Indian Institute of Science
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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.
Author: Sreemoyee Chakraborty, email
Let’s begin the discussion with a short introduction to a Mantle Plume. A Mantle Plume is a column of hot buoyant material rising through the mantle trying to reach the surface. In popular belief, it has a distinct head and a tail. Most mantle plumes are likely to be originated from the core-mantle boundary (See the figure below). According to alternative hypothesis, a shallower origin of mantle plumes also exists. However the concept of Mantle Plume itself is controversial, then the real question of this article is – if mantle plumes exist and from where do they originate. Author : Researcher of Geodynamics, email In the last article of myths of mantle convection, we were put into a confusion that what the state of mantle is. Convection in a medium implied the medium should be fluid, on the other hand propagation of S-wave indicates that the material must be solid. How can a same material possess two states simultaneously? Let us discuss over this matter.
If we observe racks of books shelves for a longer time, will see that the straight racks, made up of wood or metal, get little bit bent. This means that their initial length has increased due to some flow inside the material (this is not elasticity, as the bent woods never get back to original shape). However, their end points remained same. Important to mention, that such bending occurs in a time scale of several years (5-10 years, depending on wight of books and material of shelf). Author : Researcher of Geodynamics, email From the earlier discussion, we came to know that convection is only possible in a fluid medium. The idea and theory of plate-tectonics support that mantle motion contribute a major part to to sustain the tectonic movements. This mantle motion occurs by means of convection within mantle. So correlating this two facts, one may come to a conclusion that mantle is in fluid state. However, mantle is much more complicated than that of this simple idea. Let us explore why!
We are at least familiar with the term earthquake, fortunately, if not with the phenomenon itself. Earthquake has various reasons. For the time being, we will not go into those and think about the horror experience of earthquakes, that destroys everything. From the visual impact of the earthquake terror, we may get an idea that earthquake must have a huge energy that can cause such a large scale destruction. Now, earthquakes begin at depth (may range from few km to maximum 700 km), but we feel the energy pound on the surface. Who carries the energy? If we see any sort of energy, like sound, light etc, they propagate as wave. Earthquake also produces waves which carry the energy from interior of the earth to surface. There are two types of wave that propagate through the interior of the earth. One is called longitudinal wave or in terms of earthquake science (known as Seismology) it is P-Wave. This is same as sound wave that can propagate through any medium, regardless it is solid or fluid. There is another kind of wave called transverse wave, or in terms of seismology it is S-wave. This is same as the circular waves produced when a stone is thrown into a pond (Figure 1). If we observe, these waves do not propagate inside the water-body of the pond, but only remain on the air-water interface. Such is happened because, transverse wave cannot propagate through a fluid medium. (Although light is also a transverse wave and can propagate through fluid medium, but light has electromagnetic properties that must not be confused with non EM waves). Hence, if we get a transverse wave propagating through a medium without any interface with other medium, the simple conclusion is the medium is solid. 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. 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. 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.. Most of us already know that large scale topographic feature, like mountains, are formed by plate-tectonic activates.
If two plates of almost same density collide with each other, the intermediate parts (most likely filled by earlier sediments) compress and bulge up. This kind of phenomenon is occurred in case of continent-continent collision. The Himalayan region is an example of such continent-continent collision. If two plate diverges, then negative space is created. Results in negative topography. This is a popular phenomenon near mid-oceanic ridges, where due to extension, oceans (negative tectonic topography) are formed. . Above said topographies are compensated at a certain depth of mantle. So, they are in isostatic equilibrium. Such topography does not change quickly in geological time scale. However, these are not only way to effect the topography on surface. Mantle flow below the lithospheric plates contributes significant surface topography. This topography may change with time, depending on the mantle flow and its convective stress. That is why, this is named as Dynamic Topography. Dynamic topography is low in amplitude, but wavelength is large. Suppose a rigid plate is floating on viscous mantle. If the underlying mantle is hot and buoyant, it will up well and a divergent flow will create vertical upward force. This will create a positive topography on the surface of the earth. If a mantle plume comes up, positive dynamic topography is expected. Opposite to it, mantle underlying the crust, could be dense. This can create a convergent flow. At the boundary of the crust-mantle, it creates a vertical downward flow followed by depression. Overlying surface follows the depression and gives rise a negative dynamic topography. Dynamic topography is calculated as observed topography minus isostatic topography. That is why, it is also called as residual topography. For details: Braun 2010, Nature Geoscience |
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