Introduction to Physical Geography
Lecture: Composition of the Earth's CrustIII. 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 encounter. 1. Elements are substances made up of one particular kind of atom, or, more precisely, substances made up of atoms with particular atomic numbers. 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 shell). 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 laboratories). 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: http://www.chemicalelements.com/. d. You can hear Professor Tom Lehrer's famous and hysterical singing rendition of the periodic table at http://dcbwww.unibe.ch/groups/ward/pictures/ELEMENTS.AIF (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 ocean). 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 minerals. 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 "alloys." 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 are: 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 minerals. 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: 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). b. Feldspars tend to be light pinkish or beige or tan. c. Ferromagnesian minerals tend to be black, dark green, dark red- brown, or dark grey (e.g., hæmatite, hornblende, olivine, pyroxene). 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 composition. 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 densities) 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 classification. 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. 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. 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. a. This happens when a magma is extruded onto or near the surface by vulcanism, so this kind of rock is called "volcanic." 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 glass." 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. 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. 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." 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 rocks. 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. 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. 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). 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. 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. 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. ii. Gypsum also forms from evaporation. 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. 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 structure. 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. 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: 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. 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. 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. e. Chalcedony, jasper (reddish due to oxidized iron impurities), and agate are gem versions of this material. 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: 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 burnt. 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. 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. 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. 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 rock. 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 condition. 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!) 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. iii. Limestone can become the beautifully crystalline 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! 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. 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). 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