GY 111 Lecture Notes D Haywick 2008 09 1 GY 111 Lecture Note Series Crystallization of Magma and Lava Lecture Goals A The chemical composition of the crust B Crystallization of molten rock C Bowen s Reaction Series Reference Press et al 2004 Chapters 5 and 6 Grotzinger et al 2007 Chapter 4 A The chemical composition of the Earth In an earlier lecture we discussed the nature of the interior of the Earth Although it is unlikely that we will be able to visit the Earth s deep interior any time soon we can still resolve chemical and mineralogical changes on the basis of seismic wave behavior Volcanoes also give us some idea of the materials that compose the Earth s interior after all the lava that they erupt has to come from somewhere So what exactly is the composition of the Earth s interior It depends upon the depth that you are interested in The mantle is different from the core and the core is different from the crust Since we live on the crust it is perhaps best to focus on this layer rather than others At this point in our course it is preferable not to use minerals as the basis of composition They are too highly variable Besides there are two general types of crust anyway 1 continental crust and 2 oceanic crust Instead we ll just look at the elemental composition of the Earth s crust The following table lists the 8 most common elements in weight percent and in of atoms of average crust Element Wt of atoms Oxygen 46 6 60 5 Silicon 27 7 20 5 Aluminum 8 1 6 2 Iron 5 0 1 9 Calcium 3 6 1 9 Sodium 2 8 2 5 Potassium 2 6 1 8 Magnesium 2 1 1 4 All other elements 1 5 3 3 If you ever wondered why quartz SiO2 is so common in crustal rocks or why there are so many different silicate minerals these data should answer your questions The more of a particular element that you have to work with the more that element will form minerals The equivalent GY 111 Lecture Notes D Haywick 2008 09 2 number of atoms is particularly useful for estimating mineral composition as it allows you to predict mineral formulas and hence mineral percentages Anyway the gist of the matter is that the crust contains a lot of Si and O and there is a lot of SiO44 available for minerals Now what we need to do is to discuss how these minerals actually grow in molten rock B Crystallization of molten rock Picture a container full of magma that you collected from a pluton deep in the Earth s interior As the magma starts to cools it will start to grow crystals This process is called crystallization and it is analogous to precipitation of crystals from solutions Remember when you had to grow salt crystals in high school science class You took salt dissolved it in a glass of warm water and placed a string in the glass to act as a site of initial crystal growth or nucleation The longer that you left the string in the solution the bigger the crystals became If you were impatient like me you pulled the string out after a few minutes which was of course too soon for any crystals to have formed If you left the string alone for a day or so crystals too small to be seen with the naked eye would have formed seed crystals Over time layer after layer of salt is added to the seed crystals making bigger and bigger crystals After a few weeks you could see the cubic habit of halite Crystallization of magma works the same way Seed crystals form first and the crystals just get bigger and bigger and bigger However there are several major differences between crystallization of magma and precipitation of salt crystals Not the least of which is that magma contains a combination of elements and subsequently will form many different minerals as it crystallizes Salt water is pure and only forms halite as a precipitate Back to our jar of magma Above 1800 degrees C there are essentially no solid components to the melt Everything is liquid As the magma begins to cool seed crystals of olivine begin to form The chemical composition of olivine at least according to your book is Mg Fe 2SiO4 That olivine contains SiO4 should come as no surprise to you The cations are Mg and Fe Any mineral that contains these two elements is called ferromagnesium Ferromagnesium minerals tend to be the first to form from cooling magmas As crystallization continues the olivine crystals get larger and large as layer upon layer is added to the seed crystals The end result is a mineralogical version of the Gob stopper candies that GY 111 Lecture Notes D Haywick 2008 09 3 you used to pop in your mouths You know the ones they were multi layered The longer you sucked them the stranger the colors became The olivine crystals in cross section look exactly like that You often see slight geochemical changes in the olivine crystals as they grow They frequently start off Mg rich but become more Fe rich over time This is called zonation It is necessary at this time to remind you that olivine is actually not a single mineral We just treat it as one but olivine is actually a mineral group There are two end members 1 Fayalite Mg2SiO4 and 2 Forsterite Fe2SiO4 In GY 111 we don t differentiate them but magma crystallization often does As time goes on and the temperature begins to drop more and more other minerals begin to crystallize out of the magma At around 1100 degrees C another ferromagnesium mineral forms Pyroxene chemical composition Fe Mg SiO3 2 crystallizes along with olivine This is an important concept Crystallization of separate minerals is not sequential they overlap one another From time to time 3 or 4 or more minerals are all crystallizing at the same time Once minerals begin to form in a melt they start to settle downward due to the influence of gravity Minerals as solid substances tend to have higher specific gravities than the magma that they are crystallizing from Hence they sink downward The process is called gravitational settling See adjacent sketch The scenario above will produce a lower layer enriched in olivine followed by a layer containing pyroxene and olivine In the lab GY 111 Lecture Notes D Haywick 2008 09 4 component of the course you will see examples of these rocks The olivine rich rock is called Dunite The pyroxene olivine rich rock is called peridotite As the magma continues to cool different minerals start to form The sequence that they crystallize out in is called the Bowen s Reaction Series We ll get to that in a second But first a comment about what happens to the magma as crystals form The first two minerals to form from a cooling magma are olivine and