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
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:
![[ quartz ]](http://www.mii.org/minpics/asbes1.gif)
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 ]](http://www.mii.org/minpics/quartz2a.jpg)
b. Feldspars tend to be light pinkish or beige or tan.
![[ feldspar ]](http://www.mii.org/minpics/albite.jpg)
c. Ferromagnesian minerals tend to be black, dark green, dark red-
brown, or dark grey (e.g., hæmatite, hornblende, olivine,
pyroxene).
![[ hornblende ]](http://www.sdnhm.org/fieldguide/minerals/images/hornblende.jpg)
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.
![[ phaneritic intrusive igneous rock, Wikipedia ]](http://upload.wikimedia.org/wikipedia/commons/f/f0/Rapakivigranite_ss.jpg)
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 ]](http://www.gc.maricopa.edu/earthsci/imagearchive/PEGMATITE_big.jpg)
![[ pegmatite with giant beryl crystals, Giant
Crystal Project, ]](http://giantcrystals.strahlen.org/europe/ascensao1.jpg)
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 ]](http://comp.uark.edu/~sboss/rockbag28.jpg)
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."
![[ glassy igneous rock, Caveman Chemistry]](http://cavemanchemistry.com/oldcave/projects/stone/obsidian.jpg)
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
]](http://www.union.edu/PUBLIC/GEODEPT/COURSES/geo-10/images/Scoria.jpeg)
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 ]](http://www.union.edu/PUBLIC/GEODEPT/COURSES/geo-10/images/Andesite2.jpeg)
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.
![[ sedimentary strata, Cal State L.A., World Builders ]](http://curriculum.calstatela.edu/courses/builders/lessons/less/les2/gifs2-20/sedgifs/layers38044.jpg)
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 ]](http://www.bgsd.k12.wa.us/hml/jr_cam/science/rocks/%20web_rocks/shale.jpg)
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]](http://www.beg.utexas.edu/mainweb/publications/graphics/sandstone400.jpg)
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, physicalgeography.net ]](http://www.physicalgeography.net/fundamentals/images/conglomerate.jpg)
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 ]](http://www.gc.maricopa.edu/earthsci/imagearchive/breccia.jpg)
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 ]](http://www.chem.ox.ac.uk/icl/heyes/structure_of_solids/Scans/Halite(NaCl).JPG)
ii. Gypsum also forms from evaporation.
![[ gypsum, physicalgeography.net ]](http://www.physicalgeography.net/fundamentals/images/gypsum.jpg)
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 ]](http://www.dep.state.fl.us/geology/geologictopics/rocks/anastasia.jpg)
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.
![[ travertine, J. Dostal and S. Hanley, Lincoln Elementary, Dubuque, IO ]](http://www.lincoln.dubuque.k12.ia.us/images/Fourth%20Grade/2002_2003/crystal_lake/larger_images/flowstone.jpg)
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 ]](http://www.lpi.usra.edu/education/fieldtrips/2005/images/tufa_1.jpg)
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, ]](http://www.calstatela.edu/faculty/acolvil/geotime/seven_sisters.jpg)
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 ]](http://www.abdn.ac.uk/rhynie/images/textures/chertblock5b.jpg)
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 ]](http://www.geo.arizona.edu/~jguynn/geology/G14_PetrifiedForest_Roots.jpg)
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 ]](http://static.flickr.com/43/84402858_aaf6bdd4bd_m.jpg)
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:
![[ geode, minerals.net ]](http://www.minerals.net/mineral/silicate/tecto/quartz/images/amtgeod1.jpg)
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.
![[ peat, Cornell U. news release, Feb. '02 ]](http://www.news.cornell.edu/releases/Feb02/peat.72.jpg)
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 ]](http://www.mchenry.edu/Faculty/phamill/common_graphics/eas170/rocks/lignite.jpg)
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 ]](http://www.appaltree.net/aba/images/coal/coal.jpg)
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 ]](http://www.evsc.virginia.edu/tours/rockmin/images/anthracite.jpg)
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!)
![[ slate, Library Thinkquest ]](http://library.thinkquest.org/05aug/00461/images/slate.jpg)
![[ old
school blackboard, Harvest of History ]](http://www.harvestofhistory.org/assets/object-images/main/slate.jpg)
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, physicalgeography.net ]](http://www.physicalgeography.net/fundamentals/images/quartzite.jpg)
iii. Limestone can become the beautifully crystalline marble.
![[ marble, greekmarble.com ]](http://www.greekmarble.com/pictures/28.jpg)
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 ]](http://www.geo.umn.edu/mgs/virt_egg/gneiss.jpg)
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 ]](http://www.gc.maricopa.edu/earthsci/imagearchive/DK_MICA_SCHIST_big.jpg)
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 About.com ]](http://z.about.com/d/geology/1/0/4/F/graphite.jpg)
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