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  • 1. Igneous Rocks and Melt Chemistry
  • 2. Earth Materials: ROCKS Most rocks consist of one or more minerals (although volcanic glass and organic matter can also form rocks, 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 older rocks, 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.
  • 3.  
  • 4. One type of rock can be transformed over geologic time into another type. This process is known as the rock cycle .
  • 5. IGNEOUS ROCKS Any 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) and MINERAL COMPOSITION (the types and abundance of minerals present). Examining the rock CHEMICAL (MINERAL) COMPOSITION and the rock TEXTURE can tell you where and how the rock formed.
  • 6. Igneous Rock Chemistry
    • MINERAL 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-colored mafic rocks .
  • 7. Igneous Rock Chemistry - Felsic
    • FELSIC igneous rocks contain felsic minerals with a high Si and O concentration with little or no Fe or Mg.
    • What are some silicate minerals we would expect to find in Felsic igneous rocks?
  • 8. Igneous Rock Chemistry - Felsic
    • 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 lighter color (remember Fe and Mg cause darker colors).
    • Where do you think we would find our lowest density rocks on earth?
  • 9. Igneous Rock Chemistry - Felsic
    • The minerals in felsic igneous rocks are high in silica which means the tetrahedra share many or all of the oxygens.
    • What does large amounts of covalent bonding tell us about the stability of felsic igneous rocks?
  • 10. Igneous Rock Chemistry - Felsic
    • Felsic melts (molten rock) tend to be ‘thick’ or viscous due to the high sharing of oxygens among tetrahedra.
    • 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.
  • 11. Common Felsic Igneous Rocks
    • Pumice (left), Rhyolite (bottom left), Granite (bottom right).
    • Granite can come in many colors, depending on the chemistry of the melt.
    • You can see the light minerals, pink (orthoclase), white (plagioclase) and a few dark minerals (augite and hornblende). Overall, the rocks are made of felsic minerals.
  • 12. Felsic Igneous Rocks: Are lighter in color, have high amounts of silica and oxygen (lower amounts of iron and magnesium). Because of their chemistry, they tend to be of lower density than their mafic counterparts. Felsic igneous rocks tend to form at the continental crust! PERIDOTITE GABBRO DIORITE GRANITE Phaneritic Large XLS X BASALT ANDESITE RHYOLITE Aphanitic XLS too small to see with naked eye X SCORIA OBSIDIAN PUMICE OBSIDIAN Glassy No XLS Ultramafic LOW SiO 2 HIGH Fe, Mg Mafic low SiO 2 high Fe, Mg Intermediate high SiO 2 low Fe, Mg Felsic HIGH SiO 2 LOW Fe, Mg Chemistry  Texture 
  • 13. 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 the tetrahedra, ions such as Fe and Mg are needed.
    • The ions form IONIC bonds with the tetrahedra. How would we expect the stability of mafic minerals to be different from felsic?
    • Which silicate minerals make up mafic igneous rocks?
  • 14. Igneous Rock Chemistry - Mafic
    • The darker ones! Augite, Hornblende and Olivine.
    • Lower percentages of silica tetrahedra sharing oxygen results in a less viscous melt. Mafic rocks are usually associated with non-violent volcanic eruptions.
    • Mafic minerals are denser than felsic minerals. So where do you think we would find them on earth?
  • 15. Common Mafic Igneous Rocks
    • Scoria (left), Basalt (bottom left), Gabbro (bottom).
  • 16. Mafic rocks are composed of mafic minerals, have higher percentages of iron and magnesium and as such, are denser than felsic rocks. MAFIC igneous rocks form oceanic crust (recall basalt). PERIDOTITE GABBRO DIORITE GRANITE Phaneritic Large XLS X BASALT ANDESITE RHYOLITE Aphanitic XLS too small to see with naked eye X SCORIA OBSIDIAN PUMICE OBSIDIAN Glassy No XLS Ultramafic LOW SiO 2 HIGH Fe, Mg Mafic low SiO 2 high Fe, Mg Intermediate high SiO 2 low Fe, Mg Felsic HIGH SiO 2 LOW Fe, Mg Chemistry  Texture 
  • 17. Igneous 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 intermediate composition.
    • See diorite, it has many dark minerals (augite, hornblende) and also light ones too (feldspars). Gives the salt and pepper appearance.
  • 18. Intermediate rocks have a composition somewhere between felsic and mafic, and therefore contain some felsic and some mafic minerals. They have more Si and O than mafic but more Fe and Mg than felsic minerals. INTERMEDIATE igneous rocks tend to form where you have felsic and mafic melts MIX (such as a continental oceanic collision boundary). What is ANDESITE named after? PERIDOTITE GABBRO DIORITE GRANITE Phaneritic Large XLS X BASALT ANDESITE RHYOLITE Aphanitic XLS too small to see with naked eye X SCORIA OBSIDIAN PUMICE OBSIDIAN Glassy No XLS Ultramafic LOW SiO 2 HIGH Fe, Mg Mafic low SiO 2 high Fe, Mg Intermediate high SiO 2 low Fe, Mg Felsic HIGH SiO 2 LOW Fe, Mg Chemistry  Texture 
  • 19. 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 shared between tetrahedra? What was the net overall charge on the tetrahedra?
    • In comparison to other silicate minerals, what would the stability of olivine be?
  • 20. Igneous Rock Chemistry - Ultramafic
    • Ultramafic igneous rocks are found exclusively to form in a layer of the earth called the mantle, below the 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 isolated tetrahedra are held together by ionic bonds (from Fe and Mg). Therefore these minerals are easy to break down.
  • 21. Peridotite (made mostly of olivine) is the only ultramafic rock that we will be concerned with. It has the highest amounts of iron and magnesium. ULTRAMAFIC rocks are the densest and predominate in mantle rocks.
  • 22. Ultramafic igneous rocks have the highest amounts of iron and magnesium, rendering them the densest. PERIDOTITE GABBRO DIORITE GRANITE Phaneritic Large XLS X BASALT ANDESITE RHYOLITE Aphanitic XLS too small to see with naked eye X SCORIA OBSIDIAN PUMICE OBSIDIAN Glassy No XLS Ultramafic LOW SiO 2 HIGH Fe, Mg Mafic low SiO 2 high Fe, Mg Intermediate high SiO 2 low Fe, Mg Felsic HIGH SiO 2 LOW Fe, Mg Chemistry  Texture 
  • 23. Igneous Rock Texture
    • Igneous Rocks can also be classified by texture. Texture involves the size of the minerals in the igneous rock.
    • As a general rule the SLOWER the melt cools, the LARGER the crystals, and vice versa.
  • 24. Phaneritic Igneous Rocks PHANERITIC rocks have large crystals, way large enough to see with the naked eye. These rocks cool slowly 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 (left)
    • Gabbro (bottom left)
    • Peridotite (bottom)
    felsic mafic ultramafic
  • 25. Phaneritic Texture - Large Mineral Crystals. PERIDOTITE GABBRO DIORITE GRANITE Phaneritic Large XLS X BASALT ANDESITE RHYOLITE Aphanitic XLS too small to see with naked eye X SCORIA OBSIDIAN PUMICE OBSIDIAN Glassy No XLS Ultramafic LOW SiO 2 HIGH Fe, Mg Mafic low SiO 2 high Fe, Mg Intermediate high SiO 2 low Fe, Mg Felsic HIGH SiO 2 LOW Fe, Mg Chemistry  Texture 
  • 26. Aphanitic Igneous Rocks
    • Rhyolite (left)
    • Andesite (bottom left)
    • Basalt (bottom right)
    APHANITIC rocks have small crystals. Mineral crysals are present but too small to be visible with the naked eye. These rocks cool at an intermediate rate - slow enough to form crystals but not slow enough for the crystals to be large enough to see. Aphanitic texure most commonly forms in rocks that cool from lava at the earth’s surface called extrusive igneous rocks. felsic intermediate mafic
  • 27. APHANITIC TEXTURE - Rocks have crystals, however, they may be too small to see with the naked eye. PERIDOTITE GABBRO DIORITE GRANITE Phaneritic Large XLS X BASALT ANDESITE RHYOLITE Aphanitic XLS too small to see with naked eye X SCORIA OBSIDIAN PUMICE OBSIDIAN Glassy No XLS Ultramafic LOW SiO 2 HIGH Fe, Mg Mafic low SiO 2 high Fe, Mg Intermediate high SiO 2 low Fe, Mg Felsic HIGH SiO 2 LOW Fe, Mg Chemistry  Texture 
  • 28. Glassy Igneous Rocks
    • Pumice (left)
    • Scoria (bottom left)
    • Obsidian (bottom)
    • Note gasses in the lava can cause fine holes called vesicles as seen in the pumice and scoria.
    Glassy Igneous Rocks cool so rapidly, that atoms don’t have enough time to get together, bond and form crystals. To cool this quickly the rocks MUST be extrusive (cool at the earth’s surface).
  • 29. Glassy Texture - NO Mineral Crystals Present PERIDOTITE GABBRO DIORITE GRANITE Phaneritic Large XLS X BASALT ANDESITE RHYOLITE Aphanitic XLS too small to see with naked eye X SCORIA OBSIDIAN PUMICE OBSIDIAN Glassy No XLS Ultramafic LOW SiO 2 HIGH Fe, Mg Mafic low SiO 2 high Fe, Mg Intermediate high SiO 2 low Fe, Mg Felsic HIGH SiO 2 LOW Fe, Mg Chemistry  Texture 
  • 30. This diagram shows how texture and mineral composition are used together to determine the type of igneous rock. For example, a rock rich in the minerals olivine and pyroxene, with a phaneritic texture (large crystals), is called peridotite . A rock rich in the minerals quartz and potassium feldspar, with an aphanitic texture (small crystals) is called rhyolite .
  • 31. 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.
  • 32. 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 may have formed?
    • Which of the rocks would you expect to be intrusive? Extrusive?
  • 33. Igneous Rock Chemistry
    • If 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.
  • 34. 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.
  • 35. How can we melt rock?
    • 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.
  • 36. How can we melt rock?
    • 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.
  • 37. Magma Crystallization
    • Crystallization 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.
  • 38. Magma Crystallization
    • 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.
    • Pay close attention here.
  • 39. BOWEN’S REACTION SERIES FOR MINERAL CRYSTALLIZATION
  • 40.
    • As 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.
  • 41. BOWEN’S REACTION SERIES FOR MINERAL CRYSTALLIZATION
  • 42. Discontinuous Branch
    • As 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.
  • 43. Discontinuous Branch
    • 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.
  • 44. Discontinuous Branch
    • 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.
  • 45. Discontinuous Branch
    • 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.
  • 46. Discontinuous Branch
    • Okay– if the initial melt is low in Fe and Mg to begin with, 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.
  • 47. Continuous Branch
    • AT 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.
  • 48.
    • 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.
    • Felsic rich melts will result in quartz form. Mafic rich melts will not.
    • 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 in the 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 forming increasingly complex silicate minerals.
  • 49. Changing Magma Composition
    • If 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?
  • 50. Changing Magma Composition
    • Crystal Settling
    • The 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.
  • 51.  
  • 52. Changing Magma Composition
    • Assimilation
    • A 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.
  • 53.  
  • 54. Changing Magma Composition
    • Magma Mixing
    • Two magmas of different composition can mix together to form a new melt within the crust.
  • 55. Changing Magma Composition
    • Partial Melting
    • Minerals 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.
  • 56.  
  • 57. Igneous Rocks: Volcanism and Plutonism
    • The 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!