Igneous Rocks and Melt ChemistryEarth Materials: ROCKSMost rocks consist of one or more minerals (although volcanic glass and organic matter can also formrocks, even though technically neither are minerals). There are three main groups of rocks:1. Igneous rocks form from solidification of magma (molten rock).2. Sedimentary rocks form from solidification of sediment (particles formed from the erosion of olderrocks, or by chemical or biological processes at the earth’s surface).3. Metamorphic rocks form from other rocks changed in a solid state by heat, fluids, and pressure.One type of rock can be transformed over geologic time into another type. This process is known as therock cycle.For example, an igneous rock may form from cooling of some magma. Later it may become eroded intopieces that are washed down rivers, eventually settling and becoming buried and compacted into asedimentary rock. This rock may then eventually be buried so deeply under other layers that it becomeschanged by heat and pressure into a metamorphic rock. This rock might then get exposed at the earth’ssurface and eroded to eventually become a sedimentary rock. Or it might get buried deeply enough tomelt, forming magma that will eventually form a new igneous rock. And on and on it goes…..IGNEOUS ROCKSAny rocks produced from molten magma or lava that cools from a liquid to a solid.Two types of activity could produce igneous rocks: volcanism (lava reaching the surface) and plutonism(magma crystallizing beneath the surface). So what is the difference between magma and lava?Igneous rocks are classified using two criteria: TEXTURE (the size and shape of the minerals) andMINERAL COMPOSITION (the types and abundance of minerals present). Chemistry Felsic Intermediate Mafic Ultramafic HIGH SiO2 high SiO2 low SiO2 LOW SiO2 Texture LOW Fe, Mg low Fe, Mg high Fe, Mg HIGH Fe, Mg Glassy PUMICE SCORIA X No XLS OBSIDIAN OBSIDIAN Aphanitic XLS too small RHYOLITE ANDESITE BASALT X to see with naked eye Phaneritic GRANITE DIORITE GABBRO PERIDOTITE Large XLSExamining the rock CHEMICAL (MINERAL) COMPOSITION and the rock TEXTURE can tell you whereand how the rock formed.
Igneous Rock ChemistryMINERAL COMPOSITION:Recall the silicate minerals we learned about. Silicate minerals have varying percentages of Si, O, Fe and Mg.The chemical composition of the magma determines which types of silicate minerals will form.•Magma rich in silicon (called felsic magma) will form felsic minerals producing light-colored felsic rocks,•Magma rich in iron and magnesium (called mafic magma) will form mafic minerals producing dark-coloredmafic rocks.Igneous Rock Chemistry - Felsic•FELSIC igneous rocks contain felsic minerals with a high Si and O concentration with little or no Fe orMg.•What are some silicate minerals we would expect to find in Felsic igneous rocks? Mostly quartz,plagioclase (white feldspar), orthoclase (pink feldspar) and muscovite mica.•Since these minerals contain abundant Si and O, they tend to have a lower density and have a lightercolor (remember Fe and Mg cause darker colors). Where do you think we would find our lowest densityrocks on earth? Continental Crust.•The minerals in felsic igneous rocks are high in silica which means the tetrahedra share many or all ofthe oxygens.•What does large amounts of covalent bonding tell us about the stability of felsic igneous rocks? Highstability•Felsic melts (molten rock) tend to be ‘thick’ or viscous due to the high sharing of oxygens amongtetrahedra. Tetrahedra form long, thin chains that become tangled in the melt. This results in thick,slow ‘flowing’ (viscous) melt and these rocks tend to be found associated with violent volcanic eruptions.•Common Felsic Igneous Rocks: Pumice. Rhyolite, Granite. Granite can come in many colors, depending onthe chemistry of the melt. You can see the light minerals, pink (orthoclase), white (plagioclase) and a fewdark minerals (augite and hornblende). Overall, the rocks are made of felsic minerals.Igneous Rock Chemistry - Mafic•MAFIC silicate minerals contain Si and O but they have a much higher percentage of Fe and Mg.•Silica tetrahedra do not share as many oxygens in these minerals so to neutralize the charge on thetetrahedra, ions such as Fe and Mg are needed.•The ions form IONIC bonds with the tetrahedra. How would we expect the stability of mafic mineralsto be different from felsic? Low stability•Which silicate minerals make up mafic igneous rocks? The darker ones! Augite, Hornblende and Olivine.•Lower percentages of silica tetrahedra sharing oxygen results in a less viscous melt. Mafic rocks areusually associated with non-violent volcanic eruptions.•Mafic minerals are denser than felsic minerals. So where do you think we would find them on earth?Oceanic Crust.•Common Mafic Igneous Rocks: Scoria, Basalt,GabbroIgneous Rock Chemistry - Intermediate
•INTERMEDIATE igneous rocks have a chemistry somewhere in between felsic and mafic.•They have more Si and O than mafic but more Fe and Mg than felsic minerals.•An intermediate melt may result when both felsic and mafic melts MIX to form an intermediatecomposition. .Igneous Rock Chemistry - Ultramafic•ULTRAMAFIC igneous rocks contain the least amount of Si and O and the most amount of Fe and Mg.•These have the highest densities of all igneous rocks.•The dominant mineral is olivine (what was the structure of olivine again?). How many oxygens are sharedbetween tetrahedra? What was the net overall charge on the tetrahedra? -4•In comparison to other silicate minerals, what would the stability of olivine be? Low•Ultramafic igneous rocks are found exclusively to form in a layer of the earth called the mantle, belowthe earth’s crust (because of the high density due to all of that Fe and Mg).•The most stable portion of the isolated silicate structure is the actual tetrahedron. The isolatedtetrahedra are held together by ionic bonds (from Fe and Mg). Therefore these minerals are easy tobreak down.•Peridotite (made mostly of olivine) is the only ultramafic rock that we will be concerned with. It has thehighest amounts of iron and magnesium.Igneous Rock Texture•Igneous Rocks can also be classified by texture. Texture involves the size of the minerals in the igneousrock.•As a general rule the SLOWER the melt cools, the LARGER the crystals, and vice versa.Phaneritic Igneous RocksPHANERITIC rocks have large crystals, way large enough to see with the naked eye. These rocks coolslowly within the earth’s crust or below the earth’s crust in the mantle.Rocks that cool from magma below the earth’s surface are called intrusive igneous rocks. • Granite • Gabbro • Peridotite.Aphanitic Igneous RocksAPHANITIC rocks have small crystals. Mineral crysals are present but too small to be visible with thenaked eye. These rocks cool at an intermediate rate - slow enough to form crystals but not slow enoughfor the crystals to be large enough to see. Aphanitic texure most commonly forms in rocks that cool fromlava at the earth’s surface called extrusive igneous rocks. • Rhyolite • Andesite • BasaltGlassy Igneous RocksGlassy Igneous Rocks cool so rapidly, that atoms don’t have enough time to get together, bond and formcrystals. To cool this quickly the rocks MUST be extrusive (cool at the earth’s surface).
• Pumice • Scoria • Obsidian • Note gasses in the lava can cause fine holes called vesicles as seen in the pumice and scoria.Make a note:Some of the silicate minerals can go by a few names.Augite=pyroxene (the single chained silicate)Hornblende=Amphibole (the double chained silicate).Also, try to go back to the mineral chapter. Review the silicate structures, percentages of Si, O, Fe, Mg and relate them to which ones are abundant in which igneous rocks. This will also help with the next section on melt chemistry.MELT is a term for liquid rock. I will specify LAVA for melts that flow and crystallize AT the earth’s surface. MAGMA is liquid rock that cools WITHIN the earth’s crust.Can you answer these questions?If you can, you are in good shape.•Why are there no glassy, ultramafic rocks?•Where on earth do you expect gabbro and basalt to occur? Peridotite? Granite?•Where do the holes (vesicles) come from?•Where on earth do you expect to find felsic rocks form?•At what type of boundary would you find intermediate rocks form?•What does crystal size tell us about the cooling rate of the melt? Can it tell us where the rocks mayhave formed?•Which of the rocks would you expect to be intrusive? Extrusive?Igneous Rock ChemistryIf the earth was one big homogenous (same chemistry throughout) blob of liquid rock early in its formation, then how do we derive different igneous rock chemistries? How do we get an ultramafic mantle, mafic oceanic crust, felsic continental crust—and intermediate rocks (somewhere between felsic and mafic) at continental-oceanic collision boundaries?It all has to do with melting rocks, the crystallization (soldifying) of melts under certain circumstances and the chemistry of the melt.Let’s look at how we can melt rock.How can we melt rock?Heat it up. Because earth’s temperature increases with depth, rocks will melt if they are shoved back into the mantle. The earth’s increase in temperature with depth is called the geothermal gradient.Oceanic crust is continuously being recycled along subduction zones where denser, colder, older oceanic crust is pushed beneath either oceanic crust or continental crust.Release Pressure.Remember there is also an increase in pressure within the earth—SOLID rocks under pressure. Confining pressure of atoms at great depths pushes them closer together (than if the rocks were at the earth’s surface). To change from a solid to a liquid requires the atoms to spread apart. Well we can spread apart atoms by heating them (of course) OR by releasing the pressure.Rocks deeper in the earth are solid at a higher temperature due to the high pressure (atoms closer together under the high pressure). Therefore, the melting point of rocks increases with increasing
pressure. (under pressure rocks will remain solid at high temperatures). Cracking the lithosphere at MOR releases pressure ALLOWING the atoms to spread apart which allows for melting without changing the temperature. This is why we have constant volcanism along mid-ocean ridges— continuously spewing out basalt creating oceanic crust.Magma CrystallizationCrystallization occurs when molten rock cools. As the melt begins to cool, movement of atoms slow down to arrange themselves into certain minerals until the crystals are closely packed with no spaces between them.The first chemical bonds to form are Si and O forming the tetrahedra. Then, whatever remaining ions are available, will determine the silicate structure (isolated, single chains, double chains, sheets or framework).High amounts of iron and magnesium in the melt should produce darker, mafic minerals such as augite, hornblende and olivine. Melts with little iron and magnesium (but perhaps K, Na, Ca, Al) will produce felsic minerals.The chemistry of the melt/rock (i.e. how much Si, O, Fe, Mg) will determine:•The temperature it will melt or crystallize.•The order that minerals will melt or crystallize.•Which minerals will crystallize.BOWEN’S REACTION SERIESFOR MINERAL CRYSTALLIZATIONAs a melt cools, minerals will crystallize in a specific ORDER based on their melting points which is related to their SILICATE STRUCTURE.REGARDLESS OF THE COMPOSITION OF THE MELT, minerals will crystallize in this order (first forming simple silicates and getting more and more complex). Olivine (isolated) Augite (single chain) Hornblende (double chain) Biotite (sheet) and finally Quartz (framework).The first elements to BOND as a melt cools is the silica tetrahedron (remember, it is the basic building block). After silica tetrahedra form, Fe and Mg are sucked out of the melt to fill up the tetrahedra. But the goal is to have Fe and Mg equally distributed amongst as many silica tetrahedra as possible.Discontinuous BranchAs the melt cools the first mineral to crystallize is olivine. Isolated tetrahedra surrounded by maximum Fe and Mg that ionically ‘glue’ the tetrahedra together. Remember olivine is ‘isolated’ and Ultramafic with the highest amount of Fe and Mg.If excess silica remains in the melt as the temperature begins to drop, olivine will REACT with the melt. The isolated tetrahedra will link up to form single chains. Recall single chains require less Fe and Mg to neutralize. Olivine will become Augite.If STILL excess silica remains in the melt as the temperature begins to drop, augite will REACT with the melt. The single chains become double chains requiring less Fe and Mg to be neutral. Augite becomes Hornblende.If STILL excess silica remains in the melt as the temperature begins to drop, hornblende will react with
the melt. The double chains will become sheets requiring less Fe and Mg to be neutral. Hornblende becomes Biotite.Okay– if the initial melt is low in Fe and Mg to begin with (felsic), the crystallized igneous rocks should not have mafic minerals in it. They would have reacted with the melt changing the arrangement of the tetrahedra so that less Fe and Mg is required to neutralize the silicate. Felsic rocks therefore do not contain mafic (olivine, augite, hornblende) minerals in them. A felsic melt will crystallize the mafic minerals, however the melt is so rich in Si and O that it must evenly distribute the Fe and Mg so the mafic minerals react to form less mafic minerals.Continuous BranchAT THE SAME TIME the discontinuous branch is running, the continuous branch is focused on forming non ferromagnesian minerals. Here, Si and O are used to form feldspars. At higher temperatures, Feldspars that have large amounts of Ca are formed first until all the Ca is used up.As temperature drops, Na is incorporated into the feldspars until it is used up.Feldspars DO NOT REACT with the melt to change their silicate structure. Whatever feldspars formed will be found in the resultant rock.IF after both series run their course and STILL excess silica remains in the melt, then microcline K feldspar and muscovite will form.IF all you have left is Si and O, then quartz will form.•If the melt is mafic, the series will run only until augite and hornblende forms (and Ca rich feldspar).With so little silicon and oxygen to begin with, the melt runs out of tetrahedra and all that Fe and Mg inthe melt is incorporated into mafic minerals.•THEREFORE a felsic igneous rock may not have olivine or augite in it, but it DID as the melt was cooling.It just so happens the mafic minerals reacted to use up all that left over silicon and oxygen by formingincreasingly complex silicate minerals.Changing Magma CompositionIf all magmas originally derive from the mantle (ultramafic) then how do we get felsic melts?If lava at MOR is mafic, then how do we get felsic continental crust?How do we get FELSIC or MAFIC rocks originating from the mantle if the mantle is ULTRAMAFIC?Crystal SettlingThe downward movement of minerals that are denser than the magma from which it crystallized.As a melt begins to cool, early developed minerals will crystallize leaving the remaining melt more silica rich. If the melt cools slowly, these minerals are denser and will settle out of the ‘reaction’ so they can’t react with the melt.The remaining melt will crystallize in the absence of Fe and Mg that are tied up in the early forming minerals that dropped out of the reaction.You therefore can obtain an intermediate melt from an originally mafic melt; or a felsic melt originally from an intermediate melt.AssimilationA magma body working its way up through the lithosphere may crack and break rocks apart that it is intruding into (like melting ice cubes in iced tea—diluting the tea). We can ‘dilute’ the Fe and Mg if we
add felsic (high Si and O) continental crust rock into a mafic magma.The magma will assimilate other rocks into it, melting them and thus changing the chemical composition. I.e. mafic magma assimilating granite to form a new intermediate melt.Magma MixingTwo magmas of different composition can mix together to form a new melt within the crust.Changing Magma CompositionPartial MeltingMinerals crystallize in an order and they also melt in the reverse order (Bowen’s reaction series backwards).As you slowly heat up rock, the silica rich minerals melt first (having the lowest melting temperature) like quartz, muscovite, etc.Slowly heating up a rock will result in melting felsic minerals first, which become liquid, less dense and rises up through the crust separating itself from the remaining rock.The solid rock residue left behind will have a higher composition of Fe and Mg.Igneous Rocks: Volcanism and PlutonismThe atoms and minerals chapter familiarized you with basic chemistry and bonding and the chemical compositions of felsic and mafic minerals.In the igneous rocks chapter we learned how an amalgamation of silicate minerals produces different igneous rocks (felsic, mafic, intermediate, ultramafic). Rocks are different in overall color which has to do with the percentage of Fe and Mg in the minerals making up these rocks.The different mineral chemistries of these igneous rocks, therefore dictate WHERE they are found on the earth. Densest stuff in the mantle (with the highest Fe and Mg and darker minerals) and least dense at the continental crust (highest Si and O, lowest Fe and Mg—lighter colored rocks).Crystal size tells us whether the rocks cooled quickly (at the earth’s surface—volcanic) or slowly (large crystals indicate slow cooling—plutonic).Taking chemical and textural clues, we can look at an igneous rock and tell where it formed (which layer of the earth and if it was a lava flow at the earth’s surface or a magma body that cooled in the crust).All this information will allow you to understand the next chapter on volcanism and plutonism. Not all volcanoes are alike!