lollyone said:
Can somone explain to me about Magmas which have different chemical compositions have different temperatures i.e basaltic magmas erupt at about 1200 °C whereas rhyolitic magmas erupt at about 700 °C. what is the difference in chemical composition between these two? also why can the composition of magma change from mafic to felsic with decreasing temperature? thank you,
L. harris,
Hey lollyone, there are far more compositions than one can imagine; however, I'll see what I can do to help you understand the differences between basaltic magmas and rhyolitic magmas.
(just a quick note: Your question relates to introductory igneous petrology. Some topics that may further your understanding are Bowen's Reactions Series, and images of different volcanic systems)
Basaltic magmas (melts of varying composition, such as E-MORB, T-MORB, N-MORB, OIBs, Alkali and Tholeiitic basalts) are believed to be formed chiefly from the fractionation of the upper mantle, where a dense rock known as peridotite makes up the majority of the upper mantle. As the peridotite (composed of minerals such as clino and ortho pyroxene, and olivine) begins to melt, specific minerals will form prior to others. The melt that forms from the harzburgite (a type of peridotite) is what is known as basalt. There are many types of basalt as mentioned previously; however, for the sake of simplicity we'll say that basalt is low in volatiles and silica, containing minerals rich in Mg and Fe (which give rise to its dark, or mafic, colour). The chemical properties of the composition determine what temperature it melts at (relating to Bowens Reaction Series). Because basalts form from the differentiation/fractionation of peridotite rocks, it contains little in the way of volatiles.
Rhyolitic melts are equivalent to granitic melts, in other words, gabbro is to basalt as granite is to rhyolite (intrusive vs extrusive). Rhyolotic or granitic melts, contain higher concentrations of silicate minerals and volatiles due, in part, to their protolith source (subduction zone/continental rock). As the basaltic, oceanic crust, approaches a subduction zone, weathered material (continental derived sediments) accumulate on the sea floor. All of these sediments, along with oceanic water are subducted below the continent; however, keep in mind that water lowers the melting temperature of rocks. Therefore, a "wet" melt has a lower melting temperature than does a "dry" melt. The basalt was formed from the differentiation/fractionation of peridotite and as it gets subducted (pressure and temperature increase) it under goes slab dehydration; where all of the volatiles basically begin to boil off, for lack of better terminology. The sediments, along with a small portion of the basalt are then differentiated/fractionated to form another type of melt. This body of magma then rises (due to density changes), through the continental crust. However, as it rises, it must consume the rock that it is passing through in order to continue to rise. This increases the amount of silica and volatiles present in the new, granitic melt. The lower eruption temperature is in fact due to the presence of higher volatiles and silica content vs. that of basalt, which contains less silica and volatiles.
The composition of magma can change because, as previously mentioned, different minerals form at different temperatures. If I have a bag of red and blue smarties, (the blue melts at 700 degrees and the red at 900) and I have an initial temperature of 1200 degrees, the magma will have a chemical composition of both of those, red and blue. However, if I decrease the temperature to 850 degrees, the red will start to form crystals while the blue has yet to form. The red will then "steal" the necessary elements so it can form crystals of "x" composition and therefore when and if blue crystallizes, will not have the same elements available to it to form as they have been used up by the red crystals during their formation . If erupted at this temperature, I will only have a magma of blue crystals as the red crystals have formed solids and may be left behind.
I hope that helped, and if not, let me know what I can clear up as it certainly may not be entirely intuitive.
