Transcript Seventh lecture - 23 September, 2015

Geology in the news:
Representatives of the
Maldives, Kiribati, the
Seychelles, Palau, and other
low-lying island nations are
going to mount an aggressive
campaign at the U.N.
General Assembly when it
reconvenes on Sept. 28th.
Their goal will be to get
participating nations at
the Paris global-warming
conference in December
to adopt tough antiwarming standards, in
hopes of preserving their
nations' very existence.
See http://tinyurl.com/ohabjwo
This is why the HVO has placed Mauna Loa on a "yellow alert."
On a more local front: There is now a Geology Help Center – in
Mudd 411. Advanced Geology majors will be there to provide help
on Sundays and Tuesdays, from 7-9 p.m. On weeks with GE141
exams, they will have extra Monday shifts, also 7-9 p.m. June Li,
Oriana Battifarano, and Lauren McCarthy will be there to help!
Plate Tectonics Recap
 The Earth's surface is underlain by relatively
RIGID tectonic PLATES of lithosphere, resting atop
plastic, deformable aesthenosphere.
 Tectonic plates DIVERGE at extensional zones
 The process is driven by CONVECTION in the
mantle, resulting in SLAB PULL at subduction
zones, and SLAB PUSH at areas of mantle
upwelling.
 The Mid-Ocean Rise and Ridge System (MORRS)
marks the areas where plate divergence is taking
place.
 Flow rates are measured in cm/year (typically 2-15
cm/yr)
Plate Tectonics Recap II
 There are three kinds of Zones of Convergence
where tectonic plates come together.
 Oceanic subduction beneath a continental margin
– e.g., western South America, or the Pacific
Northwest of the U.S.A., or Kamchatka
 Oceanic subduction beneath other oceanic crust,
creating an island arc system – e.g., the Aleutian,
Kuril, Japanese or Philippine Islands, as well as the
Antilles, Sumatra & Java
 Continent-continent collisions, as in India-Asia.
Plate convergence is what creates most
mountains – and is why major mountain
ranges are linear features.
Plate Tectonics Recap III
 Hawaii is NOT on a plate margin, but owes its
origin to the Pacific Plate moving NW over a
mantle plume, or "hot spot" that produces
volcanism.
 The San Andreas Fault of California is a transform
fault, in which the North American and Pacific
Plates are sliding sideways past each other. This is
along an offset in the East Pacific Rise, part of the
MORRS.
 Accreted terranes are blocks of land that had their
own geologic history, but got "stuck onto" the
continent by convergent plate motions.
Moving right along ...
Chemistry of the Earth
and an
Introduction to Minerals
Only EIGHT elements make up >98%
of the entire Earth:
Iron (Fe)
Oxygen (O)
Silicon (Si)
Magnesium (Mg)
Sulfur (S)
Nickel (Ni)
Calcium (Ca)
Aluminum (Al)
34.8%
29.3%
14.7%
11.3%
3.3%
2.4%
1.4%
1.2%
NOTE that Fe is the most abundant element in the
Earth as a whole!
CONTINENTAL CRUST is enriched in lighter elements
Oxygen
Silicon
Aluminum
Iron
Calcium
Magnesium
Sodium
Potassium
O 45.2% of crust
Si 27.2%
Al 8.0%
Fe 5.8%
Ca 5.1%
Mg 2.8%
Na 2.3%
K 1.7%
O + Si = ~72% of the crust; SILICATE minerals are far and
away the dominant group on Earth.
Quartz (SiO2) is one of the most common minerals (see fig. 3.3,
p. 76; fig. 3.4d, p. 79; fig. 3.7a, p. 80)
ELEMENTS COMBINE via chemical bonding
a) IONIC BONDING occurs when an element gains or loses
an electron to have a complete outer shell; this occurs when
elements have many or very few electrons in outer shell.
(e.g., K, Na, Cl)
Size of elements is also a factor
in mineral formation.
Positive ions (cations) are
usually smaller than negative
ions (anions).
The smaller cations tend to fit in
spaces between larger anions.
Structure of halite - NaCl showing ionic bonding at work
b) COVALENT BONDING occurs when elements have
moderate numbers of electrons (e.g., Si, C, Fe+++), so it's easier
for them to share electrons than to strip or fill an outer shell
per each atom.
Structure of ethane, a gas,
showing covalent C-C and C-H
bonding
Water is also held together
by covalent bonds
Covalent bonding forms strong bonds holding minerals together.
In metallic bonding, important in many native elements (like
gold or copper), the metallic ions form a crystal lattice that is
stabilized by a "sea" of electrons that are free to move from
one ion to another.
These free electrons are why metals are able to convey an
electrical current!
Critically important in many minerals is
IONIC SUBSTITUTION (the substitution
of one ion for another of similar size and
the same or a close electrical charge)
TWO are extremely common and VERY important :
1.
This can be illustrated in the structure of the rock-forming
mineral OLIVINE:
2.
THIS pair of ions is critical - because both silicon and
aluminum are so abundant. This characterizes THE
major mineral group that makes up over half the
entire crust - the feldspars!
HOWEVER, this also sets up a charge imbalance!
This is accommodated by addition of K+, Na+,
Ca++ and other ions to the minerals.
Soooo, what IS a mineral, anyway?
1. Naturally occurring
2. Inorganic solid
3. Has a constant chemical composition, or
one varying within defined, set limits
4. Has a crystalline (ordered) internal
structure. ( This is reflected in the crystal form! )
QUESTION: Is the January ice on
Johnson Pond a mineral ???
Is wood a mineral ?
Is glass a mineral ?
Critical to remember is that ALL minerals
are stable ONLY under specific
temperature-pressure conditions!
Minerals that have both silicon and
oxygen in them are the silicate minerals.
These comprise 95% of the crust - we'll
cover them last, but in some detail.
The NON-silicate minerals (i.e., all the
others) are, save the carbonates, most
important for their economic significance.
1. The first group is the simplest group: Native elements . These
are comprised of a single element, not in combination with
will denote those you'll see in lab!
anything else.
Gold
*
Diamonds
Graphite
Native
copper
Native silver
Native sulfur
2. Second group is the Sulfides. Many are major ores. These are
comprised of a metal plus sulfur.
*
*
Pyrite - NOT an ore
Chalcopyrite  Cu
Bornite  Cu
+ Cinnabar  Hg
Sphalerite  Zn ….
*
Galena  Pb
Molybdenite  Mo
*
Sulfide minerals, and Native Sulfur (from coal seams)
give rise to
,
an environmental disaster if left
untreated. Though some of this
acidic drainage occurred
naturally before mining began,
access to the minerals responsible
was greatly enhanced by mining
activities.
Some of you may remember the acid mine drainage spill into the Animas River of
Colorado this past summer.
3. Third group is the Oxides and Hydroxides. Many of these
are also major ores. These are comprised of a metal plus
either oxygen or an (OH) group, or both.
*
*
*
Magnetite
Hematite
Limonite
The three major ores of iron , mined worldwide
Bauxite is a blend of AlOOH, Al (OH)3, and other
aluminum oxides and hydroxides
A. Is Bauxite a mineral?
B. Why is this specimen red?
C. For what would IT be useful?

*
Other important oxides include:
Corundum - Al2O3
*
Used to be
mined
extensively for
abrasives - now
most important
for the colored
varieties …..
Rubies ….
 sapphires
And the
Oriental
emeralds 
All coloration is due to trace impurities ...
4. The fourth group is the Halides - made up of a metal plus
a halogen - usually Fluorine or Chlorine
*
Halite - NaCl
Halite and Sylvite
show well the
influence of
internal structure
on the ultimate
shape of the
mineral grains.
Sylvite - KCl
Fluorite - CaF2 - is also a very important halide mineral
*
As you can
guess from
these pictures,
fluorite can
be practically
ANY color!
WHY???