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.
One of the distinguishing property of normal household (like iron, gold, aluminium etc) is that they have a specific melting temperature. Like melting point of iron is 15380 c. Beyond this temperature, iron will be completely melted, and below this it is completely solid. Rocks are different in this aspect. They do not have a specific melting point, but melts over a range of temperature. For example, a rock called basalt starts to melt some where near 9800 c and completely melts at 12600 c at ~50 km depth. So, below 9800 c, basalt is completely solid. Beyond this temperature, some part of it melts and still some part remain as solid. This temperature, where a rock just starts to melt is called “solidus temperature” (Figure 1). Above 12600c, basalt is completely molten, not a single solid crystal exists here. This temperature is called “liquidus temperature” (Figure 1). So, to melt a rock, temperature has to rise above solidus temperature.
Figure 1: Schematic diagram of solidus an liquidus temperature of a rock. Dark blue color indicates solid rock, red color indicates molten rock. At a certain depth, with increasing temperature, rock starts to melt. The first melt appears at solidus temperature. Then amount of solid portion reduces gradually with more temperature. The rock completely melts at liquidus. Beyond liquidus, not a single solid crystal exists.
Mantle rocks have a solidus and liquidus temperature, within which mantle will melt. Temperature of mantle at each depth can be calculated from some mathematical formula or experimental data. This depth versus temperature can be drawn as a curve which is called “geotherm” (Figure 2). In normal condition, geotherm is always well below mantle solidus temperature, so mantle does not melt. However, in some particular area below earth, this geotherm is perturbed (Figure 3) due to excess heat production. In those cases, geotherm cross the solidus temperature and mantle starts melting. The melt, being lighter material, rises above because of its buoyancy and erupts as magma.
Figure 3: Normal geotherm is fadded and the prominent white dotted line is presenting the perturbed geotherm. Because of different geological settings, in some places, temperature rises above mantle solidus temperature. Once the geotherm of that region crosses solidus, some amount of melt can be produced.
So, the solid mantle melts, and it melts in specific condition when the geotherm is disturbed by excess temperature. The reason of rising of temperature is different in different places. We will look forward into our next article to explore those places and how temperature is raised to produce melt.
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