Geography 140
Introduction to Physical Geography

Lecture: Composition of the Earth's Crust

III. The composition of the earth's crust is the concern of this lecture: the 
     materials we encounter here on the surface.
     A. Elements are the basic building blocks of all matter we normally 
        1. Elements are substances made up of one particular kind of atom, or, 
           more precisely, substances made up of atoms with particular atomic 
           a. Atoms are particular combinations of protons and, usually, 
              neutrons in their nuclei and electrons in various orbital levels 
              around their nuclei.  The weight of the various numbers of 
              protons, neutrons, and electons give each element an atomic 
              weight, and the number of protons is its atomic number.  It's 
              the number of protons, actually, that determines each element's 
              chemical behavior ultimately.
                i. The electrons form a trivial amount of the atom's mass but 
                   each one carries as much negative charge as the much 
                   heavier protons carry a positive charge (neutrons have no 
                   charge).  The total positive charge of the protons is 
                   balanced by the total negative charge of all the electrons 
                   orbiting in the various orbital levels around the nucleus, 
                   and an electrically unbalanced atom is called an "ion." 
               ii. The first level (or shell) can hold up to two electrons 
                   (depending on the atomic number of the nucleus) and any 
                   others farther out can hold more and more electrons.  The 
                   maximum number of electrons for a shell can be found by 
                   squaring the shell's count and then multiplying that answer 
                   by two.  So, the second shell could hold 22 
                   times 2 or 8.  The third shell could hold 32 
                   times 2 or 18.  Atoms are most stable and unreactive if 
                   their outermost or valence shell is full or holding a 
                   multiple of 8 electrons (the "octet rule"), unless it only 
                   has one shell (where 2 is the most stable number).
              iii. Each shell is saturated with electrons before another shell 
                   is organized by the positive charge of the nucleus, and 
                   that outer shell tries to get hold of a full set of electrons.
                   a. Those with just one electron in the outer shell tend to 
                      lose it to atoms that are short of just one electron, 
                      which makes both of them ions in the process.  Since the 
                      electron loser has a positive charge and the electron 
                      stealer now has a negative charge, the two ions hang out 
                      together to balance their charges and thereby form a 
                      compound. Sodium and chloride do this to form common 
                      table salt.
                   b. Those with a lot of electrons in the last shell can't be 
                      "bullied," so they often share electrons with other one 
                      or more atoms in order to complete the outer shell, and 
                      this also becomes the basis for chemical compounds of 
                      different elements (covalent bonding). Water is an 
                      example of a compound with covalent bonds between oxygen 
                      (with two empty slots in its outer shell) and two 
                      hydrogen atoms (each with one empty slot in its only 
           b. Some examples of atoms (and I don't expect you to memorize all 
              these details, just have a sense for how this works):
                i. Hydrogen (H) has 1 measly proton in its nucleus and 1 
                   electron in its single shell (giving it an atomic number of 
                   1), while helium (He) has 2 protons (atomic weight of 2) 
                   and 2 electrons in its single shell (and these two elements 
                   are the most common elements in the universe).
               ii. Oxygen (O) has 8 protons and neutrons, with 2 electrons in 
                   the inner orbital shell and 6 in the outer shell (so, it's 
                   two electrons short of a full shell and, so, it is very 
                   reactive and favors covalent bonds, as with hydrogen to 
                   make water).
              iii. Carbon (C) has 6 protons and neutrons, with 2 electons in 
                   the inner shell and 4 in the outer.  It forms covalent 
                   bonds with all sorts of stuff, including silicon (which 
                   also has only 4 electrons in its outer shell, but 8 in the 
                   middle shell and 2 in the innermost shell) -- to form 
                   silica (sand, many rocks).
           c. Atoms, depending on the number of electrons in their outermost 
              shells, can combine with others to form molecules through ionic 
              or covalent bonding.  Sometimes they bond with one another, such 
              as atmopheric nitrogen (N2) and oxygen 
              (O2).  Other times they combine with different types 
              of atoms (e.g., water, which is H2O).
        2. All matter in the earth's crust is based on elements (in various 
           chemical combinations).
           a. There are 92 elements that occur naturally in the earth's crust 
              (and a few very short-lived others that have been produced in 
           b. These can be arranged by atomic number and by number of electron 
              shells into something called the periodic table, which 
              conveniently organizes all these elements by their behavior into 
              several groups. There is a pretty clear discussion of the periodic
              table in the online encyclopædia, Wikipedia.
           c. A very nice online periodic table can be found at: 
           d. You can hear Professor Tom Lehrer's famous and hysterical 
              singing rendition of the periodic table at 
              (if you have the QuickTime software plug-in).
           e. You will be relieved to know that only about eight of these 92 
              elements are at all common in the rocks of the earth's crust 
              (had you worried, didn't I?):
                i. Oxygen (O), with two shells and an atomic number of 8, is 
                   far and away the most common element in the earth's crust, 
                   making up about 47 percent of it by weight.  It hangs out 
                   the most with silicon, to form silica and the silicate 
                   rocks that dominate the mantle and crust.
               ii. Silicon (Si), with three shells and an atomic number of 14, 
                   is the second most common element, making up about 28 
                   percent of the earth's crust by weight.  It hangs out with 
                   oxygen to form silica (SiO2, which accounts for 
                   just under 75 percent of the earth's crust!
              iii. Aluminum (Al) is a light metal with three shells and an 
                   atomic weight of 13 (pretty light). It makes up about 8 
                   percent of the earth's crust, often as a part of silicate 
                   minerals in the upper continental crust (e.g., feldspars, 
                   remember "felsic" rock?).
               iv. Sodium (Na) is another relatively light metal, with three 
                   shells and an atomic number of eleven. It makes up not 
                   quite three percent of the earth's crust and is often found 
                   in silicate rocks (notably the plagioclase version of 
                   feldspar, a light mineral in the upper continental crust).  
                   It can also create an ionic bond with chlorine to form 
                   table salt (or "halite"), and there is beaucoup of 
                   it in the oceans (and, heck, your blood, your own portable 
                v. Calcium (Ca), with four electron shells and an atomic 
                   number of 20, makes up just under four percent of the 
                   earth's crust.  It often hangs out in silicate minerals, 
                   such as plagioclase feldspar, and it often associates with 
                   carbon and oxygen to form calcium carbonate (major 
                   component of limestone and marble).
               vi. Potassium (K) is another light metal with four shells and 
                   an atomic number of 19.  It makes up less than three 
                   percent of the earth's crust and, like aluminum, it hangs 
                   out in feldspar (but in a different form called 
                   "orthoclase").  So, it is more common in the upper 
                   continental crust.
              vii. Iron (Fe) is a relatively heavier metal, with four electron 
                   orbital shells and an atomic weight of 26.  It makes up 
                   about five percent of the earth's crust (becoming more 
                   common with depth). As we saw earlier, it completely 
                   dominates the earth's core (with nickel).  It is also a 
                   very common component of the mantle rocks and the oceanic 
                   crust and the lower continental crust.  There, it is 
                   incorporated (often with magnesium) in the silicate 
                   "ferromagnesian" minerals (such as olivine, pyroxene, 
                   hornblende, and biotite).
             viii. Magnesium (Mg) is a fairly light metal, with three shells 
                   and an atomic number of 12.  It only makes up about two 
                   percent of the earth's crust, and it hangs out with iron a 
                   lot, though, and is found in the "ferromagnesium" silicate 
               ix. The remaining 84 naturally-occurring elements, then, only 
                   make up 1.4 percent of the crust.    
     B. Minerals are natural occurrences of one or more elements in a solid 
        state at room temperature. 
        1. If they aren't solid at room temperature, oh, about 20-25° C, 
           they're called "mineraloids," and examples include water and 
           mercury. If they are solid, metallic, and artificial, we call them 
        2. Minerals have definite chemical "recipes."
           a. Some of these are very particular: Silica is SiO2, 
              calcite (or calcium carbonate) is CaCO3, 
              hæmatite is Fe2O3. 
           b. Others are more variable:  Hornblende includes calcium, sodium, 
              magnesium, iron, aluminum, titanium, silicon, oxygen, and 
              hydrogen in a range of combinations, and feldspar includes 
              silicon and oxygen, aluminum, and may include calcium in some 
              forms and sodium in others.
           c. Of course, if you're out in the field looking at some rocks, 
              it's not like it would be easy to identify the minerals by their 
              chemical composition! 
        3. One characteristic you can use to help narrow down what you're 
           probably looking at is hardness.  Minerals vary in relative 
           hardness from talc and gypsum (which can be scratched by your 
           fingernails!) to diamonds, depending on the strength of the binding 
           force holding the molecules together. Back in 1824, a guy named F. 
           Mohs arranged minerals into a hardness scale based on which one can 
           scratch which others.  This is called the Mohs Hardness Scale, and 
           it gives us a way of characterizing all kinds of minerals by how 
           they scratch or get scratched by the ten ranked minerals.  You 
           basically compare the lump you have with minerals or objects for 
           which you know the hardness.  The harder the mineral, the higher 
           its Mohs reading.  The ten minerals on which the scale is based 
           MINERAL          HARDNESS (comments)
           Talc                    1
           Gypsum                  2 (fingernail is ~ 2.5)
           Calcite                 3 (copper penny is ~ 3.5)
           Fluorite                4
           Apatite                 5 (window glass/knife blade at under 5.5)
           Orthoclase              6 (good steel file just over 6.5)
           Quartz                  7  
           Topaz                   8
           Corundum                9 (sapphires, rubies are corundum)
           Diamond                10

        4. Another useful trait out in the field is something called "streak." 
           This is the color a mineral leaves behind when you scratch or rub 
           it across a piece of unglazed white porcelain, called a "streak 
           plate."  Porcelain is about 7 on the Mohs scale, so you can't use 
           it on the very hardest materials.
           a. Calcite would leave a white or colorless streak, as would 
              gypsum, quartz, serpentine, and sulphur.
           b. Azurite (a copper ore) leaves a blue streak (gosh, like cussing 
              a blue streak?)
           c. Reddish, orangish, or brownish streaks are left by 
              hæmatite (iron oxide in it) and copper.
           d. A grey streak would be left by straight iron, lead, graphite 
              (your pencil lead), pyrite (fool's gold).
        5. You can narrow down your choices by looking at a mineral's 
           cleavage, or how it tends to break when you hit it hard at one 
           focussed point.  The cleavage pattern is related to the molecular 
           lattice formed by the atoms in the mineral and their patterns of 
           weaker bonds.  
           a. This can be characterized as perfect (a face exposed by cleaving 
              is perfectly smooth with no rough spots) through good (mostly 
              smooth, some rough areas) and poor or indistinct (you can hardly 
              make out the smooth crystal faces) to none.
           b. You can also note whether a mineral cleaves in one, two, or 
              three directions (if at all).
           c. The angle of cleavage planes varies, depending on the mineral.
        6. Fracture is another characteristic used in the field to identify 
           a. Some minerals shatter off in shell-like circular patterns of 
              greater and greater depth, the way a chunk of glass will.  This 
              is called "conchoidal" fracture ("shell-like").
           b. Others just sort of crumble ("crumbly").
           c. Still others show a jagged surface (many metals do this).
           d. Some others are splintery, as in asbestos:

              [ quartz ]

        7. Minerals typically occur in a characteristic range of colors, which 
           can help you diagnose what they are:
           a. Quartz is transparent or milky white, but it has variants that 
              are pink (rose quartz), lavender or purple (amethyst), or 
              grey/brown (smoky quartz).

              [ quartz ]

           b. Feldspars tend to be light pinkish or beige or tan.

              [ feldspar ]

           c. Ferromagnesian minerals tend to be black, dark green, dark red-
              brown, or dark grey (e.g., hæmatite, hornblende, olivine, 

              [ hornblende ]

        8. They vary also in their densities:  Some are heavier (e.g., 
           ferromagnesian minerals) than others (e.g., quartz or feldspar).
        9. I do not expect you to memorize mineral colors, compositions, 
           streaks, hardnesses, cleavages, and fractures.  What I want you to 
           come away with are these points:
           a. It takes a laboratory to identify minerals by actual chemical 
           b. You can make pretty good field identifications by using color, 
              density, cleavage and fracture patterns, and hardness.
           c. Remember the Mohs Hardness Scale, though, and its order.
           d. Basically, remember that, depending on the types of minerals 
              involved, rocks resulting from the gravity-layering of our 
              planet and brought to the surface by tectonic processes sort out 
              into three big categories:
                i. Mafic rocks (ferromagnesian in chemical composition, dark 
                   in color, and dense and heavy)
               ii. Intermediate rocks (some ferromagnesian minerals involved 
                   and some felsic minerals, intermediate colors and 
              iii. Felsic rocks (dominated by quartz and feldspars, light in 
                   color, and light in density)
           e. These basic types are related to the earth's structure and 
              general chemical composition, mafic rocks being associated with 
              formation in the lower crust and felsic rocks in the upper 
              crust, due to gravity stratification.
           f. Enough of minerals:  on to rocks.
     C. Rocks are natural minerals, nearly always natural mixtures of 
        different minerals.
        1. When you think of the 92 naturally occurring elements in the 
           earth's crust and the thousands of minerals they can produce in 
           their combinations and then the jillions of possible rocks that 
           could be formed from the combinations of all those minerals, the 
           mind fairly boggles (even accepting that there are only eight 
           elements that are at all common and that minerals tend to form in 
           restricted combinations).  Well, trying to classify rocks by 
           chemical composition is not likely to organize things very well. 
           So, let's try another approach, a genetic approach to rock 
        2. You'll remember back in lecture 24 that the Linnæan binomial 
           classification and the cladistic approach are genetic 
           classification systems, that is, they classify living things by 
           their common evolutionary histories.  Well, we can do the same 
           thing with rocks:  classify THEM by their evolutionary histories, 
           modifying the basic framework by chemical composition or structure 
           as appopriate.
        3. The genetic classification of rocks divides crustal rocks into 
           three great categories:  igneous, sedimentary, and metamorphic.
        4. Igneous rocks are those formed when magma solidifies.
           a. Magma is molten mineral matter melted by high temperatures under 
              the crust due mainly to pressure and the heat released by 
              radioactive decay.
           b. The rock that forms when magma solidifies is the most abundant 
              type of rock in the earth's crust, making up about 95 percent of 
              the crust.
           c. Igneous rock is ancestral to the other rock types, giving rise 
              to them by weathering, erosion, transport, and redeposition (in 
              the case of sedimentary rocks) or by "cooking," compression, and 
              chemical substitution (in the case of metamorphic rocks).
           d. Igneous rocks can be further classified by the rate at which the 
              parent magmas cooled:  slow or fast.
                i. If a magma cools at a very, very slow rate (years), each 
                   mineral in it slowly approaches its solidification 
                   temperature and begins to change state.  Each solid crystal 
                   then has the time to grow as other molten molecules of the 
                   same mineral "freeze" onto it.  As a result, large crystals 
                   can form, and the resulting rock will have a coarse-grained 
                   (or "phaneritic") texture.  
                   [ phaneritic intrusive igneous rock, Wikipedia ]

                   a. This happens when a magma never makes it to the surface 
                      of the earth but solidifies deep down inside the crust.
                   b. The solid rock masses that form inside the crust in this 
                      way are called "plutons" (after Pluto, the Roman God of 
                      Hell, which is supposed to be "way down there") and the 
                      associated phaneritic rocks are called "plutonic" rocks.
                   c. Because the magma intrusion is way down there, these 
                      rocks are also called "intrusive igneous rocks."
                   d. Some intrusions produce just humongous crystals (more 
                      than 2 cm and sometimes even a couple METERS in size) -- 
                      these are called "pegmatites."  The one on the right is 
                      a cool one, showing an intrusive vein of igneous rock 
                      building a group of huge beryl crystals.
                      [ pegmatite, Maricopa Community College ] [ pegmatite with giant beryl crystals, Giant 
                      Crystal Project,   ]
               ii. If a magma cools at a really rapid rate (e.g., hours or 
                   days), the minerals in it will not have much time as they 
                   reach their solidification temperature to drift around and 
                   grow into sizable crystals.  The resulting rock will have a 
                   very fine or aphanitic texture (you won't be able to see 
                   the individual crystal facets with your nekkid eye).  It's 
                   sort of boring looking, actually.
                   [ aphanitic extrusive igneous rock, Stephen Boss, University of Arkansas ]

                   a. This happens when a magma is extruded onto or near the 
                      surface by vulcanism, so this kind of rock is called 
                   b. It is also known as "extrusive igneous rock."
                   c. In some cases, the magma is shot into the air or flows 
                      into water and solidifies virtually instantly, with no 
                      opportunity for any crystals to form at all.  These 
                      rocks have a glassy texture. 
                      1. One example is obsidian, sometimes called "volcanic 
                         [ glassy igneous rock, Caveman Chemistry]

                      2. Another is pumice, which is a frothy glass formed 
                         when a gassy, acidic (felsic) magma is shot into the 
                         air by a volcano, and scoria is a dark vesicled glass 
                         of a more mafic composition.
                      [ scoria, extrusive igneous rock, K. Hollocher 

                   d. Sometimes you find rocks with large crystals here and 
                      there, with broad areas of aphanitic material in between 
                      them.  This kind of texture is called "porphyritic," and 
                      it suggests that slow cooling in an intruded magma body 
                      was beginning to allow the formation of large crystals.  
                      Then, that mass was moved up to or near the surface by 
                      vulcanism, and the rest of the minerals froze in place 
                      before getting to grow into large crystals. 
                      [ porphyritic extrusive igneous rock, K. 
Hollocher ]

           e. Igneous rocks can also be classified by their general chemical 
              composition, which has to do with the temperature of a magma 
              when minerals start crystallizing out of it.  
                i. The sequence of temperature-relaed events is called the 
                   "Bowen Reaction Series."  

                   [ Bowen Reaction Series, 

                   a. Among the first minerals to crystallize out of a magma 
                      (around 1,400 ° C) is olivine, an ultramafic 
                      mineral.  This makes the remaining molten material 
                      relatively enriched in silica.  Now, this is funny, but 
                      the silicic material and the olivine start reacting with 
                      one another to produce pyroxene, a mafic mineral.  It, 
                      too, has trouble with the even more silicic (and cooler) 
                      molten magma, and reacts with it to form another mineral 
                      called "amphibole."  This one, too, starts reacting with 
                      the even more silicic magma to produce biotite (a weird 
                      dark cellophane-like mica) around 1,100° C.
                   b. Meanwhile, back at 1,400° C, another mafic mineral 
                      has started to solidify: calcium-rich plagioclase (a 
                      kind of feldspar called "anorthite").  This stuff is the 
                      most common type of feldspar in mafic rock.  As the 
                      magma continues cooling, the feldspars with more sodium 
                      in them start to solidify, producing a sodium rich 
                      plagioclase called "albite"). Albite is the most common 
                      feldspar in igneous rock of intermediate composition.  
                   c. Somewhere around 1,000° C or so, these two branches 
                      of the Bowen Series merge and the magma starts to see 
                      orthoclase settling out of what's left.  Orthoclase is 
                      yet another feldspar, with a relatively high proportion 
                      of potassium compared to the plagioclases and with 
                      aluminum in it.
                   d. As the magma continues cooling, muscovite starts to 
                      solidify (this is a white or clear cellophane-like 
                      mineral, kind of like bleached, transparent biotite).
                   e. Finally, the last mineral to solidify out is quartz.
               ii. Now, to relate all this to general chemical composition:
                   a. Mafic minerals settle out first at the hottest 
                      temperatures (e.g., pyroxene and anorthite) and produce 
                      dark, dense rocks.
                   b. Intermediate minerals settle out next (e.g., amphibole, 
                      biotite, and albite) and make rocks of intermediate 
                      color and density.
                   c. Felsic minerals settle out last at the coolest 
                      temperatures for magma (e.g., quartz, muscovite, and 
                      orthoclase) and produce light weight, light-colored 
           f. The two subdivisions of the igneous category can be put together 
              into a matrix or spreadsheet.  The columns are the general 
              chemical composition and the rows are the rate of cooling.  Each 
              cell has a different name, which I want you to memorize.

                                             GENERAL CHEMICAL COMPOSITION
              RATE OF COOLING                Felsic     Intermediate    Mafic
                  glassy                     obsidian
                  frothy glass               pumice       scoria        scoria
              Fast (extrusive)               - - - - - - - - - - - - - - - - - 
                  aphanitic (fine)           rhyolite     andesite      basalt
              Slow (intrusive) 
                  phaneritic (coarse)        granite      diorite       gabbro   

        5. Sedimentary rocks are lithified sediments, or rock bits cemented 
           into new rock.  They generally form layered beds or strata. They 
           can be further classified by the source of their parent sediments.

           [ sedimentary strata, Cal State L.A., World Builders ]

           a. Clastic sedimentary rocks are those formed from weathered down 
              pre-existing rock (whether the pre-existing rock was igneous, 
              metamorphic, or sedimentary), which has been eroded, 
              transported, and deposited somewhere where they can accumulate 
              and lithify into rock strata or layers.  Clastics are further 
              broken down by the size of the clasts or rock bits:

                i. Really fine materials (clay and silt, which are smaller 
                   than about 0.06 mm) form mud, which becomes shale 
                   (some types of which are less gracefully called 
                   "mudstone").  Shale is often more narrowly used to describe 
                   mudstone that breaks off in sheets or layers.

                   [ shale, Battle Ground School District ]

               ii. Materials somewhat coarser are sand (which is smaller than 
                   about 2 mm), which becomes sandstone when its 
                   interstitial spaces are filled with a cementing compound 
                   (e.g., calcium carbonate).

                   [ sandstone, Bureau of Economic Geology, Texas]

              iii. Materials dominated by rounded clasts ranging anywhere from 
                   gravel to boulder size (i.e., larger than about 2 mm) 
                   become conglomerates when they're cemented together.  
                   The rounded nature of the gravels, pebbles, and rocks tells 
                   you the rock was formed in a turbulent and energetic 
                   environment, such as a stream.

                   [ conglomerate, ]

               iv. A rock formed of a mix of gravel to boulder sized pieces 
                   that are mostly angular and broken-looking is called a 
                   breccia. Since they weren't worked over and smoothed 
                   out over a long time, you can infer that they didn't travel 
                   too far from where they originally broke up before they 
                   were lithified into new rock.

                   [ breccia, Glendale Community College, Arizona ]

           b. Chemical precipitates form when dissolved chemical compounds or 
              the completely decomposed mineral skeletons of small organisms 
              "precipitate" (or settle) out of water.  Commonly these include 
              various carbonate, chloride, and sulphate solutions and 
              sometimes even dissolved silica, as well as the tiny silicious 
              or carbonate hard parts of diatoms and other small creatures.
                i. Rock salt or halite forms by the evaporation of sea water.

                   [ halite, S.J. Heyes, Chemistry, Oxford ]

               ii. Gypsum also forms from evaporation.

                   [ gypsum, ]

              iii. Limestone forms from calcium carbonate which may include 
                   mineralized skeletons and shells.
                   a. Reef limestone comes from coral 
                   b. Coquina is a form with distinct shells still visible.

                   [ coquina, Department of State, Florida ]

                   c. Travertine is a calcium carbonate rock formed as 
                      dripstone in caverns (e.g., stalactites on the ceilings 
                      of caves, stalagmites below).  A chunk would look like 
                      the limestone it essentially is, but it shows a ringed 

                      [ travertine, J. Dostal and S. Hanley, Lincoln Elementary, Dubuque, IO ]

                   d. Tufa is limestone deposited by springs (often right over 
                      plants and plant litter and the ground), which builds up 
                      mounds or aprons of limestone there.

                      [ tufa, Lunar and Planetary Institute, USRA ]

                   e. Chalk is a soft limestone made up of the calcium 
                      carbonate from the skeletons and shells of tiny marine 
                      organisms (jillions of little critters gave their lives 
                      for your blackboard edification:  A moment of respectful 
                      silence to honor their sacrifice, please!).  Here are 
                      the famous white chalk cliffs of Dover, England:

                      [ chalk cliffs of Dover, ]

               iv. Chert and flint form from dissolved silica. 
                   a. Chert often forms in deep seawater, from the 
                      precipitation of the siliceous skeletons of microscopic 
                      organisms, such as diatoms.  This can result in massive 
                      beds of chert.  

                      [ chert, Chert Textures, University of 
                      Aberdeen ]

                   b. Both flint and chert can also form from the microscopic 
                      deposition of silica in place of calcium carbonate when 
                      groundwater dissolves the calcium carbonate in limestone 
                      rock, so you often find nodules or small flint or chert 
                      nodules in limestone layers.  
                   c. This is also the material that replaces dead organic 
                      material to form petrified wood. 

                      [ petrified tree, Jerome Guynn, University of 
                      Arizona ]

                   d. Flint is a dark version of this rock (it includes 
                      unoxidized iron) and chert is usually light colored.  
                      Flint often forms as nodules in chalk or limestone beds 
                      and, being more resistant, are exposed by the erosion of 
                      these beds and drop as nodules to the foot of eroding 
                      cliffs, as here.

                      [ flint, Branscombe, East Devon, England, 
                      Focalplane Travelblog ]

                   e. Chalcedony, jasper (reddish due to oxidized iron 
                      impurities), and agate are gem versions of this 
                   f. Gathering and hunting peoples loved this stuff, because 
                      it can be flaked very precisely into stone arrowheads, 
                      knives, spearpoints, sickle teeth, and other tools (it 
                      has the same shell-like conchoidal fracture that glass 
                      and obsidian do).
                   g. If you look closely at geodes (those weird rocks that, 
                      when cut in half, show an internal void with quartz and 
                      amethyst and other crystals growing into it), you'll see 
                      the area right around the crystals is made of agate or 
                      banded chert.  Aw, heck -- here's one:

                      [ geode, ]

                v. Phosphate rock derives from vertebrate bones and teeth.  
               vi. Dolomite is a carbonate like limestone but it contains 
                   magnesium as well as calcium.
           c. Organic sedimentary rocks are those that form from plant and 
              animal tissues that accumulate somewhere where they don't rot 
              down completely after death:  They may get gross and gothic but 
              they still have food (hydrocarbon) value (yum!). A typical 
              situation for this series to form is at the bottom of stagnant 
              bogs and wetlands where anærobic conditions preclude 
              complete decomposition.
                i. Peat is the first form in the series, and it is typically 
                   light brown and includes identifiable leaves and stems.  It 
                   burns readily because it's about 60 percent carbon, but it 
                   is very sooty due to all the other non-combustible 
                   materials (impurities) in it.  This has been the only 
                   viable fuel source in many of the colder and wetter parts 
                   of the world where trees are not common enough to cut down 
                   for fuel (e.g., Ireland, Scotland, and Wales).  What people 
                   do is cut this stuff out of the bottom of wetlands and then 
                   stack it somewhere where it can dry out some before being 

                   [ peat, Cornell U. news release, Feb. '02 ]

               ii. Lignite is the next stage.  This is sometimes called "brown 
                   coal," and it is brown or dark grey.  You will sometimes 
                   make out plant fibers in it.  It also burns and makes a lot 
                   of soot.  It's about 70 percent carbon.

                   [ lignite, P. Hamill, McHenry County College ]

              iii. Bituminous coal is next.  It is dark grey to black and it 
                   leaves a powdery soot in your hands as you hold it.  At 
                   this point, the carbon content is pretty high, and you 
                   can't make out any of the source plant parts. It burns 
                   dirtily (it's about 80-90 percent carbon) and is a very 
                   significant cause of acid rain.  Bituminous coal is very 
                   abundant and pretty cheap, so it is commonly used for power 
                   generation these days and you still see it used for 
                   residential heating in a few places.  It is also used in 
                   industrial production.  

                   [ bituminous coal, Pennsylvania coal, Pitt ]

               iv. Anthracite coal is another matter.  It is jet, jet black 
                   and very shiny, like obsidian, but with a beautiful golden 
                   sheen to it.  It is clean to hold and relatively clean to 
                   burn, having been reduced to nearly pure carbon (about 95 
                   percent).  Unfortunately for the world environment, it is 
                   much rarer than bituminous coal and much more expensive.  
                   My own grandfather was a "semi-anthracite" coal miner, who 
                   died at age 41 from the grim conditions of the coal mines 
                   of Pennsylvania -- he kept getting pneumonia, and the third 
                   time killed him back in 1918.  

                   [ anthracite, Env Sci, U. VA ]

                v. The coal series continues in metamorphic rock, so more 
                   about that later.
        6. Metamorphic rock derives from other rocks, which can be igneous 
           rocks, sedimentary rocks, or even other metamorphic rocks.
           a. These rocks are transformed (that's what "meta-morphic" means: 
              "trans-formed," but, since the Latin-derived word, 
              "transformed," is so easy to understand, my theory is we switch 
              to the Greek -- "metamorphic").
           b. Agents of metamorphosis:
                i. Tremendous heat, very commonly in the crust near an 
                   invading magma body.  The heat is not quite enough to melt 
                   the rock, but it is enough to "cook" it, basically.  This 
                   cooking induces chemical and structural changes in the 
               ii. Tremendous pressure can also metamorphose rock by altering 
                   its crystal lattices.  This can happen near faults.
              iii. Very hot groundwater forms near magma bodies and dissolves 
                   all sorts of minerals and acids into itself.  As the water 
                   moves through the country rock in the region near the magma 
                   intrusion, it can dissolve minerals in the rock and deposit 
                   others in their place, fundamentally altering the 
                   composition and structure of the rock.
           c. All metamorphic rocks are crystalline but, unlike phaneritic and 
              aphanitic igneous rocks, they did not crystallize from a molten 
           d. Another characteristic is that the crystals in metamorphic rock 
              tend to line up to a certain degree, which you don't see in 
              igneous rocks.
           e. There are correspondences between metamorphic rock types and 
              their common source rock types:
                i. Shale can metamorphose into slate (a rock that lines up in 
                   thin sheets, which can be used for flagstones, walkways, 
                   and, in the olden days, blackboards!)

                   [ slate, Library Thinkquest ] [ old 
                   school blackboard, Harvest of History ]

               ii. Sandstone can become quartzite, which looks like sandstone 
                   a lot, but the grains have fused and the rock isn't as 
                   scratchy as sandstone.

                   [ quartzite, ]

              iii. Limestone can become the beautifully crystalline marble.

                   [ marble, ]

               iv. Intrusive igneous rocks and clastic sedimentary rocks can 
                   become gneiss (pronounced "nice"), which looks a lot like 
                   granite on one side but on the ends it looks like granite 
                   with its crystals pulled out like taffy!

                   [ gneiss, Minnesota Geological Survey ]

                v. Slate can be further metamorphosed into schist (which 
                   reminds me of a baaaaaad joke by Dr. John Carthew out at 
                   Pierce College, from whom I took this very class -- he said 
                   "rocks are kind of like people: Under pressure, some 
                   develop gneiss personalities and others develop schisty 
                   personalities.").  Schist has crystals arranged in layers, 
                   often kind of glittery.

                   [ schist, Glendale College, Maricopa ]

               vi. Graphite is a metamorphic rock derived from coal. It is 
                   very soft and has a greasy, slippery texture and leaves a 
                   dark grey streak.  This is what we refer to as "lead" in 
                   "lead" pencils:  graphite mixed with clay to produce 
                   different hardnesses (more clay for a hard pencil, less 
                   clay for a soft pencil).

                   [ graphite, Geology ]

        7. All these different rocks and processes can be linked into 
           something called the "rock cycle" or the "cycle of rock 
           transformation." Here is a simple schematic of the rock cycle (a 
           more elaborate one is presented in your textbook at the beginning 
           of Ch. 12).  Capital letters designate the three major rock types 
           in the genetic classification of rocks, and small letters indicate 
           processes involved in their production.

 .-----------------  IGNEOUS      weathering                       chemical 
 |   extrusion --->   ROCK  ---> and erosion ---> transport ---> precipitation--. 
 |       ^            ^  | |           ^              |            |    ^       |
 |       |            |  | |           |              |            |    |       |
 |       |            |  | '--------.  |              |   .--------'    |       |
 |       |  .---------'  |          |  |              |   |             |       |
 |       |  |            v          v  v              v   v             |       |
 |   intrusion <---  melting      uncovering       deposition         decay     |
 |                    ^  ^        ^       ^           |   ^             ^       |
 |                    |  |        |       |           |   |             |       |
 |                    |  '--------+----.  |           |   '---------.   |       |
 |                    |     .-----'    |  |           |             |   |       |
 v                    |     |          v  |           v             |   |       |
 metamorphosis <--> METAMORPHIC   SEDIMENTARY <-- lithification   life & death  |
      ^                ROCK          ROCK     <---.                     |       |
      |                                |          |                     |       |
      '--------------------------------'          '---------------------'-------'                                  

That's a wrap for rocks, minerals, and elements, the basic building blocks of 
the earth's crustal materials.  With elements, be aware of the difference 
between ionic bonds and covalent bonds.  Be sure to make the connection 
between mineral types and the gravity layering of the planet.  Know the Mohs 
Hardness Scale and how some other characteristics can help you differentiate 
minerals in the field far from any laboratory. Make sure you understand why 
rock classification is basically a genetic system.  Know the main rock types 
and subtypes and, for metamorphic rocks, be able to link each metamorphic 
subtype with its source rocks.  For igneous rocks, come away with a sense of 
how the Bowen Reaction Series works.  


Document and © maintained by Dr. Rodrigue
First placed on web: 11/18/00
Last revised: 07/04/07