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MINERALIDENTIFICATION TABLE 1 – MINERAL LUSTER
Luster – The most useful subdivision of luster is whether a mineral has metallic or
nonmetallic luster.
Metallic Luster Streak color can help in identification, especially for
hematite. Some observers see more subtle variations in streak color.
Mineral Name Specific Luster Streak Color
Chalcopyrite brass or gold dark gray
Native Copper coppery copper
Galena silvery dark gray
Graphite silvery dark gray
Hematite steel, but see below brick red
Magnetite broken cast iron dark gray
Pyrite brass dark gray
Sphalerite steel, but see below pale yellow
Nonmetallic – All other minerals in the lab set including hematite and
sphalerite have nonmetallic lusters. A more detailed analysis of nonmetallic
luster can help. Some minerals may exhibit more than one type of nonmetallic
luster depending on crystal habit. Following are some examples of specific
lusters. Different observers often have different opinions on luster
appearance. Semantics aside, experience is the best way to develop expertise.
Specific Luster Example Minerals
Glassy or Vitreous olivine, quartz
Waxy cryptocrystalline quartz, talc
Pearly muscovite, biotite
Earthy or Dull nonmetallic hematite, limonite
Greasy calcium-rich plagioclase feldspar
Resinous nonmetallic sphalerite
Porcelaneous hornblende, potassium feldspar
MINERAL IDENTIFICATION TABLE 2 – HARDNESS
The following table lists mineral hardness using the Mohs scale. TEE Table 4.2,
page 88 lists the index minerals on this scale. Topaz and Diamond are not
included in the mineral identification exercise, but all other minerals in Table 4.2
are. The scale is not linear. In practice, the scratch tests are performed with
commonly available objects. These also are listed in Table 4.2, but not
accurately. Remember that harder materials with higher Mohs number scratch
softer materials with lower Mohs number. Substances of equal hardness scratch
each other with great difficulty.
Other common objects that can be used for hardness testing include copper
penny (Mohs 3.5), wire nail (Mohs 4.5), concrete nail (Mohs 5.5) steel file (Mohs
6.5), streak plate (Mohs 6.5), and quartz (Mohs 7). Nails and streak plates are
provided in the laboratory. Noting the ease or difficulty of scratching a material
will provide a useful clue about the precise hardness of a mineral. Some
pressure is needed to scratch glass with potassium feldspar, but quartz and
harder materials scratch the glass easily.
Minerals harder than a knife blade or glass plate; Mohs > 5.5
Mineral Mohs
Hardness
Comment
Corundum 9 Index mineral on Mohs scale
Garnet 7
Olivine 7
Quartz 7 Index mineral on Mohs scale
Cryptocrystalline Quartz 7 Be sure to test fresh mineral
Pyrite 6-6.5
Magnetite 6
Plagioclase Feldspar 6
Potassium Feldspar 6 Index mineral on Mohs scale
Hematite, metallic 6.5
Augite 5.5-6
Hornblende 5.5 Very difficult to scratch glass
Minerals softer than a glass plate but harder than a fingernail; Mohs 2.5-5.5
Mineral Mohs
Hardness
Comment
Hornblende 5.5 Very difficult to scratch glass
Hematite, nonmetallic 5
Goethite 5-5.5
Limonite 1.5-5.5 Varies because of structure
Apatite 5 Index mineral on Mohs scale
Fluorite 4 Index mineral on Mohs scale
Chalcopyrite 3.5-4
Azurite 3.5-4
Sphalerite 2.5-4
Dolomite 3.5-4
Barite 3-3.5
Calcite 3 Index mineral on Mohs scale
Biotite 2.5-3
Copper 2.5-3
Galena 2.5
Halite 2.5 Crumbles – false low hardness
Muscovite 2-2.5
Minerals softer than a fingernail; Mohs < 2.5
Muscovite 2-2.5 Difficult to scratch
Chlorite 2-2.5 Difficult to scratch
Gypsum 2 Index mineral on Mohs scale
Limonite 1.5-5.5 Fine grained crumbly form is soft
Graphite 1 Messy to handle
Talc 1 Index mineral on Mohs scale
MINERAL IDENTIFICATION TABLE 3 – CLEAVAGE
Minerals can be classified into two broad groups by whether or not cleavage is
obvious. This works well with large mineral grains with obvious broken
surfaces. Determining if cleavage is present can be difficult in fine-grained
specimens, though use of magnification can help. When uncertain as to the
presence of cleavage, ignore that property and try to identify the mineral with
other diagnostic properties.
If cleavage is observed, then determining the number of directions of cleavages
and the angle between cleavages will help to define the mineral. For example,
fluorite is the only mineral we study with 4 directions of cleavage.
TEE does not use the term direction of cleavage. Cleavage planes in a direction
of cleavage will all be parallel to each other. Any other directions of cleavage in
the mineral grain will have different orientations. When rotating a single mineral
grain under a light, the parallel planes of a direction of cleavage will all flash at
the same orientation of the mineral grain.
Cleavage can be classified further as to its quality and ease of forming. This
ranges from fair through good to perfect. In the latter case, cleavages will be
smooth and easily formed. This is less so with good cleavage, and poor cleavage
is difficult to form.
Two other types of surfaces occur on mineral grains. Fractures are irregular
surfaces from breakage on other than cleavage planes. The nature of the fracture
may be useful in determining the mineral name. When minerals grow
unrestricted, they develop crystals with well defined crystal faces. A crystal form
is all crystal faces related by symmetry. Some information about crystal forms
will be acquired in the basic study of minerals, and can be very useful for
identification. The study of crystal symmetry is beyond the scope of GO110,
though it is a fascinating subject.
Minerals that do not exhibit cleavage
Mineral Name Comments
Apatite Conchoidal fracture
Chalcopyrite Poor cleavage rarely seen
Copper Hackly fracture tends to grab skin when rubbed
Corundum
Garnet Widely spaced parting sometimes confused with cleavage
Goethite/Limonite
Hematite Brick red streak is diagnostic!
Magnetite Magnetic property is diagnostic!
Olivine Conchoidal fracture
Pyrite
Quartz Conchoidal fracture
Quartz, cryptoxln Conchoidal fracture
Talc has one direction of cleavage, but it often is to fine-grained to
show cleavage
Minerals that do exhibit obvious cleavage
Mineral
Name
Cleavage Directions Comments
Augite 2 at nearly 900
Barite 2 good; 1 excellent Cleavage results in platy appearance
Biotite 1 excellent Easy to cleave
Calcite 3 excellent not at 900 Rhombohedral cleavage
Chlorite 1 excellent Difficult to see when fine grained
Dolomite 3 excellent not at 900 Similar to calcite, usually small grains
Feldspars 2 good at 900
Fluorite 4 excellent Octahedral cleavage; diagnostic!
Galena 3 excellent at 900 Cubic cleavage
Graphite 1 excellent Excellent lubricant
Gypsum 2 poor; 1 excellent Difficult to see when fine grained
Halite 3 excellent at 900 Cubic cleavage
Hornblende 2 good at 600 and 1200
Muscovite 1 excellent Easy to cleave
Sphalerite 6 excellent Diagnostic, but finding all 6 is difficult
Geology is science that studies all physical aspects of the planet Earth and other planets. It is the
study of the history of the planet, the composition, the internal structure and the surface features of
that planet. We study our planet by field work, sample collecting and mapping. The samples can be
analyzed via chemical means as well as by optical means. Fossils are identified and correlated to age
and paleo-environments. Other related Earth sciences that are closely related to geology are
meteorology, oceanography and ecology. Subfields of geology are geochemistry, geophysics and
geo-biology. Other planets we study via remote sensing and by studying analysis sent by rovers of
samples that they have collected.
While geology is lumped as earth science and taught in high school to those who can’t handle
science like biology, chemistry or physics. In reality geology is a hybrid science requiring calculus,
chemistry through at least p-chem., and physics. On top of this knowledge add to it an
understanding of earth processes, mineralogy, and optics. Paleontology also requires knowledge of
biology as well.
Geologists are taxed with finding oil and coal for energy to power our modern economy. But this
is not only task that the geologist serves. Every bit of metal you use, every mine that is dug depends
of geologist. Modern batteries that may give us energy independence, materials used to build
photovoltaic cells and wind turbines are courtesy of geologist.
How does a geologist study the earth? Are we out there with rock hammers in hand and a pack
mule behind us? We use the same method as all other scientist- the scientific method. This method
calls for a hypothesis (possible explanation) for an event or series of events that have been
observed. A coherent set of hypothesis back by a large by of data both observational and by
laboratory measurements gives theory to explain a series of events. These theories obey the
physical laws or principles of how the universe works. Please notice that one part of the scientific
method is missing- experimentation in a controlled environment. It is impossible to do this when the
events we are theorizing about are global in nature and take place of millions to billions of years. To
deal with that we turn to the scientific model and create mathematical models that we try to
simulate what the Earth will do during time.
We use knowledge the data that formed the hypothesis and theories into a mathematical model
run through a computer to test the validity of our theories. But remember it is the same technique
that forecast the local weather. To improve the theory and model we continue to gather data. This
data is not only based on observations in the field (our geologist with the mule) but by using remote
sensing using satellite data, chemical and optical analysis of rocks.
From observation can the principle of uniformitarianism that can be paraphrased by the
statement that the present is the key to the past. The forces acting on our planet today are the same
forces that have acted on it in the past. This covers the slow steady process such as river moving
sediment to the ocean and the wind eroding a rock into strange shapes. It also allows for the
cataclysmic event like the earthquake, volcanic eruption and meteor impact. Most geologic process
takes thousands to millions of years, though.
The first part of understanding geology is understanding the makeup of the planet itself. The
size of the planet was determined 250 BC by a Greek performing an experiment. His estimate of
40,000 km with a radius of 6370 km and was surprisingly accurate. A more precise measurement
was achieved in the 1800’s found that the earth is not a perfect sphere. It bulges out slightly at the
equator and is smashed slightly at the poles.
The Earth is not a smooth sphere as it appears in
space. Instead it is broken by mountains and valleys.
This is called topography and is measured with respect to
sea level (sea level is considered 0). Our planet varies
from 0 to 1 km above sea level and 4 to 5 km below sea
level. This topography was created by forces acting in the
Earths depths.
The Earth’s density gave the first clue to the different
layers in the interior of the earth. Our planets volume was
known by knowing its radius (πr2) discovered in 250 BC. Its density was calculated in 1789 by from
the force of gravity that pulls objects to the surface. A German physicist Emil Wechert knew that
the average crustal rock like granite had a density of
2.7g/cm
3
. The planets density was measured at
5.5g/cm
3
. The heaviest rocks found on the planet was
found to be rich in iron and brought to the surface by
volcanoes. It only had a density of 3.5g/cm3.
He looked down at his feet and proposed an idea that would explain this difference. He knew
that some meteorites were a mixture of two heavy atoms, nickel and iron (density of 8g/cm
3
). If the
outer portion of the planet was light then the inner portion must be composed of the heavier
materials. The idea of a light crust on the planet surrounding a heavier core was proven by
examining the seismographs to see if there were changes in the waves as they passed into the earth.
This theory was proven correct in 1906 by Englishman Robert Oldham with the use of seismic
waves. There are two seismic waves that can penetrate the body of the planet. One of these waves
can penetrate both solids and liquids, the other only through liquid. (assignment 1,
http://www.classzone.com/books/earth_science/terc/content/investigations/es0402/es0402page0
1.cfm) By noting the missing seismic wave at different seismograms he could map a liquid core. He
discovered that the fact that deep in the earth was a molten layer. Here the planet was hot enough
to melt nickel and iron but not silicate based rocks. This explained the discrepancies in mass.
Prior to this was discovered another phenomena- the Moho Discontinuety. This was a change in
seismic wave speed as it entered the earth’s depths. Seismic waves travel slower in less dense
material. Our crust was made up of material that is less dense then the material beneath it. This
outer material, the mantel, is made up of minerals containing magnesium and iron, while the crust
minerals were mainly made up of silicate mineral composed of potassium and aluminum. Our crust
was surrounding a much denser material and our continents were found to “float” one this denser
mantle.
Courtesy of the cold war and seismic stations spread throughout the world (to monitor nuclear
testing) all of the major Earth layers had been discovered. The composition of these layers was
estimated by their densities. This density was determined by the change in speeds of the seismic
waves. Look at the figure on page 41 to see what makes up our planet. These are the main
elements that compose the planets with the other elements being present in trace amounts.
Our planet then is constantly changing under the influence of two “engines”. The internal engine
is powered by the internal heat from our planet. The external or surface of our planet is powered by
the Sun. The combination of these two driving forces comprises the Earth system. This system is an
open ended system exchanging mass and energy within itself and with its surrounding
Solar energy powers our weather creating weathering and erosion of the earth’s surface and
powers the growth of plants. Our climate is controlled by a balance of solar energy and the energy
of Earth radiates back into space. Mass is exchanged back and forth between Earth and space. We
get mass from meteors. In fact our planets origin is as an assembly of small bodies such as meteors,
asteroids and comets. The Earth, in turn, loses molecules such as water escape into space.
Earth is divided into spheres. The external spheres are energized by solar radiation. The external
spheres are the following: 1. atmosphere, 2.hydrosphere, 3. cryosphere, 4. biosphere. The internal
spheres are the following: 1. lithosphere (rock layer)
composed of the upper ridged mantel and the crust, 2.
asthenosphere or plastic sphere, 3. deep mantel, 4.
inner and outer core.
The last two spheres the deep mantel and the core
form the Geodynamo System. This system controls the
magnetic field. The first two internal spheres control
the formation of new sea floors, new land masses,
mountains and can increase the sea level. These
processes explain the phenomena known as Plate
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http://www.classzone.com/books/earth_science/terc/content/investigations/es0402/es0402page01.cfm
Tectonics.
The magnetic field is formed by the heat transfer from the inner core to the outer core which
creates convection currents in the outer core. The rapid motions from the currents stir the liquid
inner core up and electric currents form in the liquid outer core. The magnetic fields create both the
north magnetic pole which is off the earth’s axis by 11 degrees and the South Pole. This magnetic
field protects us from solar radiation. This increase radiation would increase cancers in adults, kill
phytoplankton, and injure all life forms. The interesting thing about this is that this field reverses
itself every tens of thousands to millions of years. There is no full extinction event seen at these
reversals.
Plate tectonics is driven by the internal heat, just as the geodynamo. Here the heat passes up
from the inner core, through the lower mantel to the asthenosphere. This plastic layer
(asthenosphere) develops convection currents that splits the lithosphere. It also causes volcanoes
and earthquakes while building land masses and destroying sea floor. The mountains that where
created impacts the uppers spheres, areas of the atmosphere, hydrosphere, cryosphere and
biosphere. The formation of new sea floor and its destruction impacts the biosphere and
hydrosphere.
Instead of going straight to plate tectonics I choose to lecture on rocks and minerals because
these are what we, the geologist, study to get a better understanding of the Earth. Minerals are the
building blocks of rocks. They are naturally occurring solid crystalline substance, usually inorganic
with a specific chemical composition. Minerals cannot be divided mechanically into smaller
components without losing their properties (you go down to the ions that compose them).
These characteristics that define what a mineral is can be easily explained. Naturally occurring
minerals means that these can’t be “man-made” but found in nature. Solid crystalline substance,
minerals are neither liquid or gas but a solid. Crystalline means that atoms that compose the mineral are
arranged in orderly repeating three dimensional arrays. This website will demonstrate this principle.
http://www.classzone.com/books/earth_science/terc/content/investigations/es0506/es0506page
02.cfm . Solid materials without this atomic arrangement are called glassy or amorphous and are not
minerals. Glass is such a thing either naturally occurring or windowpane. The “usually” inorganic means
that there is no organic carbon bonds as if sound in organisms living or dead. Minerals may be secreted
by organisms. Shells such as those in clams and oysters are composed of the mineral calcite. Specific
chemical composition is defined as the chemical formula that allows for only small variation that is
defined by a fixed ratio. The ratio never varies. For example olivine is composed of iron, magnesium,
oxygen and silicon and has a fixed ratio. The number of iron and magnesium may vary but the number
of those two atoms together compared to silicon atoms can’t.
The structure of mater is defined by its smallest unit that retains the physical and chemical
properties of an element. This is the atom. By this time in your education you should be aware of what
an atom is and how bonds form. Review this be reading pages 78-80 in your text book.
The crystalline structures are determined by how the anions are arranged and how the cations fit
between them. A cation’s size allows for substitutions in the mineral matrix. This size similarity creates
the variation as talked about in the chemical composition of minerals. Crystallization can occur as the
different ions order themselves during either the cooling of molten magma or during evaporation as the
ions are brought closer and closer together
Large crystals in magma grow by slow temperature changes. As the temperatures in magma drops
ions slow their movement and now can bond together and hold. If the temperature drops slowly crystals
grow larger and larger, also if the pressure increases ions are forced together crystals will also form. If
the temperature slows to fast then the ions either can’t bond together and an amorphous material is
formed. If the temperature is slow enough to allow small crystals to grow then the material may or may
http://www.classzone.com/books/earth_science/terc/content/investigations/es0506/es0506page02.cfm
http://www.classzone.com/books/earth_science/terc/content/investigations/es0506/es0506page02.cfm
not have easily visible crystals.
Crystals will form “faces”. These are the boundaries of the natural repeat surfaces. So the crystal
faces are the external expression of the minerals internal
atomic structure. Since the shape of the crystals is partially
determined by the chemical formula crystal shape can be
used to identify different minerals. Different pressures
and
temperatures will impact the shape and therefore the
properties of the mineral. This creates polymorphs,
minerals with the same chemical formula but different
shapes and different properties. An example of this is
graphite and diamond. Both are made completely
inorganic carbon. Their properties are very different.
Physical properties of minerals are how we identify
them and are determined by both their chemical make-up
and their crystal shape. Covalent bonds are stronger then
ionic bonds, giving different properties. The shape of the
crystal is determined by the different bonds. The
properties that are determined by this are the following: 1.
hardness, 2. cleavage, 3. fracture, 4. luster, 5. density, 6. chemical habit and to a lesser extent and 7.
color.
Hardness is the measure of the ease with which the surface of a mineral can be scratched. To
determine this property a scale of hardness was developed called the Mohs Scale of Hardness. It used
minerals with the softest being talc and the hardest being diamond. Of course geologist won’t carry
valuable minerals in the field so we carry simple common things such as our fingernails, copper coins,
knife blade, window glass and steel file.
Within groups of minerals there can be variation in
hardness. Hardness can also be related to other factors that also
increase the bond strength therefore the strength of the
minerals. These other factors are size, charge and packing of
atoms or ions. Size of the ions plays a major role, the smaller
the ions, the smaller the distance between them and the
greater the attraction and thus the bond. The charge of the ion
determines the attraction between them and therefore, the
strength or hardness of the mineral. Packing of the atoms or
ions refers to the amount of “space” between the ions of a
crystal the smaller the distance between the ions the stronger
the bond in the crystal.
Cleavage is the tendency of crystals to break along where
there is less bond strength. It varies inversely with the bond strength and varies along the different
planes of the crystal. Diamonds have covalent bonds, the strongest that there are. They can cleave along
specific planes along weaker planer surfaces giving perfect flat (planer) surfaces. This cleavage helps
identify the minerals. There is also different cleavage strengths. While some minerals have perfect
smooth cleavage along certain planes others have imperfect or fair cleavage. So minerals can be
assessed by how it splits apart, making specific angles. So minerals can have perfect, good or fair
cleavage. Some minerals have no cleavage what so ever. Instead they fracture because their bonds are
too strong. All minerals will fracture if struck along bond strengths that are distributed along cut across
cleavage planes. Fractures may be conchoidal, showing smooth curved surfaces (like a bullet in hole in
glass), another common fracture surface has the appearance like split wood. This gives a fibrous or
splintery appearance. Again the shapes and appearances of fractures depend on the crystal structure
and composition. This is an excellent website for different cleavages and mineral examples
http://www.rockhounds.com/rockshop/xtal/part3.shtml
Luster is how the surface of a mineral reflects light. Luster is controlled by the kinds of atoms and
http://www.rockhounds.com/rockshop/xtal/part3.shtml
the type of bonds that the minerals have. The crystal structure impacts how the light passes through the
minerals. Ionically bonded crystals tend to have a glassy or vitreous luster. Covalently bonded materials
vary more. Many have adamantine luster like diamond (think sparkle). Metallic lusters are often shown
in pure metals like gold, and copper. Sulfides often have metallic luster. A pearly luster is imparted
minerals as light passes through a clear surface and reflected back from other crystal planes beneath the
surface. This website will demonstrate crystal
luster http://dph1701.tripod.com/geology/properties/luster.html
Density depends on the atomic mass of a mineral’s ions. It is also dependent on the amount of
space between the ions. The closer the ions are packed together in a crystal the more it will “weigh”.
Increases in density caused by pressure will impact how a mineral will transmit light, heat and seismic
waves. This change with density explains how we can use seismic waves to judge the make up of the
interior of the earth.
Color is an elusive property. It is determined by the presence of certain ions in a mineral. The mineral
called olivine has a formula (Fe,Mg)2SiO4. The Iron and Magnesium
cations are similar in size and often form a mixture in the olivine
crystal. If there is only Iron (Fe) present the green since iron reflects all
light except green. If it only has magnesium (Mg) it is colorless. The
mixture of the iron and magnesium make olivine vary in color. This
would indicate that color is a good way to identify a mineral. The
problem happens when there are impurities incorporated into the
mineral. These are called trace elements. An example of this is the
mineral corundum (aluminum oxide). If there is a small amount of
chromium in it we have a ruby. If there is either titanium or iron we have a sapphire.
Streak is a subset of color as well as an indication of hardness. Unglazed porcelain will remove
molecular level crystals are reveal the true nature of a mineral. A classic example of this is the mineral
Iron Pyrite or fool’s gold (FeS2). This mineral is gold inn color and often confused early miners and
hobbyist into thinking that they had found gold. When rubbed across a streak plate instead of leaving
behind a gold streak like true gold would it leaves a black streak.
The last property is crystal habit. This is the shape that and individual crystal or aggregate of crystals
will form. It is diagnostic for some minerals such as quartz which has a distinctive 6 sided column topped
by a 6 sided pyramid like structure. This is dependent on the of ions in the crystals structure. Many
minerals can have more than one habit. An example of this is gypsum- try to find examples of gypsums
different habits.
Unique properties- some minerals have unique properties that can be used to identify them that are
dependent on their chemical formulas. Calcite with the formula of CaCO3 fizzes the presence of acid.
Dolomite, Ca,Mg(CO3)2 must be powdered to fizz. Your clay minerals are plastic when wet and smells
earthy. One group of clay minerals will swell when wet causing tremendous problems in engineering.
There are approximately 30 rock forming minerals that are the building blocks of our planets. Many
http://dph1701.tripod.com/geology/properties/luster.html
of the other minerals occur in veins or form secondarily and are important for providing raw materials
such as copper, rare earth elements, tin etc. The 30 minerals reflect the 8 most common elements that
make up 99% of the Earth’s crust.
The most common of the mineral groups is the silicate group. This group is characterized by the
silicate tetrahedron. Here the silicon ion (Si
4+
) is surrounded by four oxygen (O
2-
). The fact that the
oxygen share charges creates an amazing strong structure. The Si with 4+ is balanced by the four O
2-
so
is neutral. This allows the oxygen with their shared negative charge with a free charge of 1- to bond with
other cations or it the oxygen can be shared with other silicate tetrahedrons.
There are four forms of silicate minerals that
are common as rock forming minerals. They are
the following: 1. Neosilicate, a single silicate
tetrahedren bonded to a cation, 2. Inosilicate,
here a chain of tetrahedrons forms. This chain
can either be a single chain such as the pyroxene
group or a double chain such as amphibole, 3.
Phyllosilicates or sheet silicates here the crystal
growth form thin sheets along one plane
examples of this are the micas (a secondary
mineral formed from micas are the clays and
shares the same structure), and 4. Tectosilicates
(framework) are the complex forms that have a
three dimensional framework. Rocks can have
two types of tectosilicates, one is quartz with no
cleavage planes and feldspars which have two
planes at right angles. While quartz is only made
up of SiO2 other silicates are bonded to cations
such as sodium, potassium, calcium, magnesium
and iron. There can be cation substitution for the
silicon by aluminum in the silicate tetrahedron.
Carbonates are those minerals that combine with the anion complex CO3
2-
. It is probably the second
most second common mineral family in
the Earth’s crust. Carbonates have one
carbon surrounded by three oxygen
giving the group a negative 2 charge.
This group combines with calcium (Ca
2+
)
to give calcite (CaCO3). This forms the
rock limestone. When magnesium
combines with calcium we have the rock
forming mineral dolomite Ca,Mg(CO3)2.
Other carbonates provide valuable
minerals such as copper from MALACHITE, Cu2(CO3)(OH)2 and AZURITE, Cu3(CO3)2(OH)2. Malachite is
green and Azurite is blue
Oxides are minerals formed by oxygen combined with metallic cations such as iron, both Fe
2+
and
Fe
3+
. This group is an important source of ores. Three iron minerals are commonly found in crustal rocks.
The first is magnetite (Fe3O4). This mineral is important not only for ore but in proving an important
geologic theory of plate tectonics and is the source of magnets.
Hematite
(Fe2O3) is a major source of iron ore and an indication of the formation of free
oxygen in our own atmosphere for the first time. It can be seen as a
semiprecious jewel and as the red stain in many soils and as yellow stain in
others. Goethite (limonite) is a hydrated iron oxide FeO(OH).
Sulfides are minerals in which the sulfide anoin S
2-
combining with metallic
cations. Most of these compounds like metals and are a valuable source for
metals that we have used for centuries. Iron pyrite (fool’s gold) is a FeS2 and Galina, (PbS) a major
source of lead and our state mineral.
Sulfates are a tetrahedron with the sulfur surrounded by four oxygen (SO4
2-
). Gypsum that makes up
wallboard, is an evaporite mineral (CaSO4.2H2O). The pure form is anhydrate and the water is missing
from the formula. Sulfate minerals were found as an evaporite from water. We found sulfates on the
surface of Mars, one more indication that Mars was once a water world like Earth early in its history.
Another group of minerals are the Halides. These minerals are evaporites like gypsum. An example
of a halide is halite also known as salt (NaCl). This and sylvite (KCl) can be found in sedimentary rocks.
These deposits often have oil in them since they are porous.
Rock is a naturally occurring solid aggregate of minerals or in special gases nonmineral solid matter.
Concrete may have rocks in it and may look like a type of rock but it is not rock! The minerals in rocks
are joined together in such a way that their individual properties remain. There are three types of rocks:
1. igneous rook formed from solidified molten material (magma), 2. sedimentary rocks made from loose
particles that have been cemented together or made from ions the combine in water to form new
minerals that are cemented together, 3. metamorphic rock is rocks that have undergone temperature
and pressure change. We will be looking at each class of rock and how they form in full chapters later in
this book. Below is an overview of the rock types.
Igneous rocks are associated with molten material. This material can be from the mantel or
melted crust material. Texture, iron content and silica content determine the type of igneous rock. The
texture determines the cooling temperature. Rapid cooling gives small to no crystal formation. This
means that the rock formed on the surface. The main minerals for igneous rocks are quartz, feldspar,
mica pyroxene amphibole and olivine. Mantel rock will have mainly olivine, and pyroxene. Melted
crustal rock has large amounts of quartz in it. Texture is a clue to the environment, internal (intrusive) or
external (extrusive). This was known for 200 years and later confirmed with the developing of polarizing
microscope and the careful grinding of a rock thin section (ground rock so thin light can pass through it).
Texture is created by cooling temperatures. Ions in hot magma have too much energy to bond together
Hematite
and form a crystal. As cooling proceeds the ions lose energy and can then form crystals. Pressure will
also help in this since it forces ions together regardless of the heat energy. If the cooling proceeds slowly
as in intrusive material you have large crystals, if ejected into the surface you get small crystals or no
crystals.
Intrusive rocks are coarse textured (phanorytic.) These rocks
have slow cooling regimes cooling over thousands and thousands of
years in the crust of our planet. The heat is held in by the overlaying
rock. Elephant rocks in Missouri are a perfect example of such a rock.
This is an igneous intrusive body injected into the crust over a billion of
years ago.
Extrusive rocks have different types of appearances, one composed of
fine grained aphenitic another by glassy (no crystals) rocks. These categories
are dependent on how they erupted from the volcanoes. Lavas have a range
of appearances dependent on their chemical make-up and temperature.
Pyroclastic formation is characterized by violent
eruption with lava thrown into high into the air. If
the material is ejected rapidly and cools rapidly it forms volcanic glass- mineral
free since amorphous. Pumice is a form of igneous rock that forms from
volcanic glass with air pockets or vesicles. These vesicles are formed by the
degassing of the molten material (CO2, H2O, etc.). Volcanic ash is composed of
fragmented rocks, lavas and/or volcanic glass. It is thrown high in the air and
smaller fragments will travel around the globe. Bombs have a range of shapes and made up of solidified
lava. They are tossed into the air and fall along the sides of the volcanic cone. The last two are scoria and
pumice. These are gas filed lava. Pumice (to the left) is so light that it floats.
There is one more texture, a mixed texture indicating two different cooling regimes. This is
called porphyritic texture or porphyry. Here a slow cooling regime starts and
large visible crystals form. These are then there is a volcanic eruption before
more large crystals can grow and this gives us the two different crystal types.
Igneous material is also classified by chemical content. We break the material
into four types according to the proportion of silicate minerals. Specific
minerals form at specific temperatures. Minerals with a high proportion of
iron and magnesium and calcium compared to silica form this group and are
called mafic from magnesium. The mafic and felsic mineral suites are on page 112 in your book. The
feldspar group is divided into potassium rich (orthoclase),
sodium rich plagioclase and calcium rich plagioclase. The last
two the sodium and the calcium are end the members of a solid
solution where the plagioclase formula is NaAlSi3O8;CaAl2Si2O8.
The more calcium in the plagioclase the more mafic the forming
rock and the higher the melting temperature to the right is an
Labradorite- tectosilicate
– Ca(50-70%) Na(50-30%) (Al, Si)AlSi2 O8
example of these mixture. The more sodium, the more felsic the magma melt and the lighter the
igneous rock( look at the chart on page 113).
Mafic Rocks a have large amounts of olivine and pyroxenes giving the rocks their characteristic
dark colors. There may be a small to moderate amount of calcium plagioclase. The lava form of this is
called basalt. There are several areas of sheets of basalt such the Columbian Plateau along the Columbia
River in Washington. India as an even larger area, the Deccan Traps, where kilometers thick layers of
basalt contributed to the Cretaceous extinction event and another in Siberia with an area as large if not
larger also associated with an extinction event (Permian).
There is another mafic rock form. This form is called ultra-mafic. The mineral suite is primarily
made up of olivine with a small amount of pyroxene. This is the material that makes up the upper
mantel. The basalt upwelling at the spreading centers formed the ocean crust.
Felsic Rocks are poor in iron and magnesium. They are also poor in calcium. These rocks tend to
be light in color and one of the most abundant intrusive igneous rocks. They contain approximately 70%
silica and are abundant in quartz and orthoclase feldspar with some
sodium feldspar (albeit minerals). The intrusive form of this igneous
rock is Granite, the extrusive form is Rhyolite. These rocks can appear
as light brown, salt and pepper, pink, or orange or in some cases
almost purple (Missouri rhyolite). The Picture on the left is an image of
this rhyolite. Notice that it is porphyritic.
Between
the end members
of these two rock
types are the rocks
that are called the
intermediate.
These rocks have
less silica then the
felsic and more
than the mafic.
They have some
quartz, micas and
may have some pyroxene. They may also have amphiboles. The intermediate is divided into
granodiorite and diorite. Granodiortite is very difficult to differentiate from granite. This is done by
looking at difference in the percentages of quartz, orthoclase and sodium plagioclase. We will then only
talk about diorite and its extrusive form andesite. This material has a mineral suite with pyroxene like
mafic and calcium plagioclase but it also has amphiboles, micas, a mix of the sodium/calcium plagioclase
minerals and some quartz. The variation in the amount of silica gives its melt a variety of properties that
can swing from one extreme to another. The last is the composition factor impacting the melting is the
chemical formula. The more mafic the melt, the higher the melting temperature for mineral formation
and mafic melts are characterized by less silica in the melt and more iron and magnesium. Conversely
the more felsic the melt represented by more silica, the lower the melting temperature.
Sedimentary Rocks
Sedimentary rocks formed from the breakdown of other rocks by surface processes. There are
two types of weathering, physical and chemical. These processes seldom occur alone but are combined.
One type of weathering can dominate depending on the climate.
Physical weathering takes place without the modification of the minerals of the parent rock, it is
a mechanical process. So how does the rock break down? The first method is abrasion. Here a small
fragment of rock strikes the surface of the rock and knocks chips off of the parent rock. These particles
can be transported by air or water. In desert situations the particles transported by wind are silt size and
fine sand size in origin. In water currents can transport smaller
particles and larger ones depending on flow. Think about
sandblasting, wind is dry sand blasting and water is wet sanding.
Abrasion is just one method. Tree roots expand and
contract as water flows through them and is pumped up to leaves
during photosynthesis and stops during respiration. This continued
expansion and contraction fractures boulders. Another form of
physical weathering is frost wedging. You can see this along I55.
Water percolates into cracks during the day in winter. It freezes at
night and expands, fracturing the rocks. Another method of
fracturing rock is heat exchange. During fires a rocks outer surface
expands while the center remains cool for a time. This creates
stresses that can start cracking the rock when the rock cools the
reverse takes place. Here the surface cools over the hotter expanded inner core of the rock.
Chemical weathering takes both heat and water to take place. Heat is necessary for a chemical
reaction to take place and water allows for the ions in minerals to reconfigure into new minerals.
Feldspars can become clays and so can mica. The chemical reaction releases extra silica ions (H4Si4, silicic
acid) that react with water to precipitate out quartz. The amount of water controls the amount of
weathering at a given time (deserts have chemical weathering from dew). The necessity for heat should
tell you that during winter and in cold climates when water is frozen suspends chemical weathering.
Physical and chemical weathering takes place together for the most part. As rocks are broken
down physically more surface area is exposed. This gives more area for chemical weathering to take
place speeding the breakdown of the initial rock and new minerals forming. Smaller particles have
greater surface area to react with the environment.
Transport
This break down produces sediment that is transported to and deposited in low lying areas.
Physically weathered particles are often angled but as they are transported they become progressively
more rounded. This will give clues as to the amount of transportation. Wind transport has very round
particles and they are often frosted from impact with other grains. Clues of the amount of time spent in
transport comes from the particles are well sorted or not. This means that how many different types of
minerals are present. Mature sediment is predominantly quartz in nature since quartz resist chemical
weathering.
Deposition
Once transport is done there is
deposition. This is the time when the velocity
of the transporting media drops to the point
that a particular sized can it can no longer be
carried. When the particles settle out they
settle in roughly horizontal beds. In water
particles can settle out also new minerals form
as ions recombine and settle out.
Burial takes place as layer after layer
of sediment are deposited. This often takes
place in basins. These sediments will remain in the basins until plate tectonics either causes an uplift or
the crust is recycled down in an area of subduction. The burial leads to compaction.
Diagenisis
Diagenisis is the change of sediment into rock. This is also called lithification. The first step in
this is compaction. Here spaces found between the grains of sediment are narrowed. As compaction
proceeds the amount of water that is present in the pore spaces is reduced. This is followed by
cementation. The cement can very and this variation changes the strength of the rock. Cements are
formed by chemical weathering making new compounds. Calcium carbonate is often present in rocks
formed either in shallow ocean basins or in the desert (calcium carbonate is present as any water
evaporates in a desert environment). This cement will dissolve away in the presence of acid (rain water
is a pH 5 and that is without the addition of acids from coal burning). Another cement is hematite, Fe2O6.
This cement is far stronger and the rocks cemented by it will hold up even in the presence of acids of
many acids. Clay is also cement. Clay minerals can be formed by water interacting feldspars and micas.
SiO2 is the strongest cement of all. It is formed by chemical weathering feldspar rich rocks and micas.
Sedimentary rocks are broken down into two types, clastic or siliciclastic and chemical including
biogenic. The clastic rocks are further divided by grain size. Course grained rocks are made up of gravels.
Gravels can range from 2mm (dry pea sized) to larger than 256 mm (diameter of over 7 feet). The
medium grain or sand size has a range of 2-0.062mm (dry pea to granulated sugar). Fine grained size
ranges from 0.062-0.0039mm (granulated sugar to flour) and makes up mud and silt. The finest grain
size is 0.0039 or clay size. This size feels “soapy” when rubbed between your fingers.
The gravels form the rocks conglomerates or breccias. A conglomerate is made up of rounded
grains while a breccia is made up of angular grains. The angulation tells
ou about the distance from the source that the grains have traveled. A
breccia has undergone lithification close to the source, possibly from
the talus at the bottom of a mountain. A conglomerate has traveled
further from the source
becoming more rounded.
The size of a conglomerate’s
grains tells you the strength
of the current that transported them. The grain size of the
conglomerate is not uniform but made up of many different
sizes with the largest being gravel sized.
The medium sized material makes up sandstone.
This can be divided into fine, medium and coarse. A sandstone that is made up predominantly of
medium to fine particles is well sorted an indication of the current strength or wind strength since the
larger particles drop out with the drop in velocity. The sandstone is also graded according to the degree
of roundness. The more round the particles in the sandstone the further the transport from the source
of the grains. The last classification is maturity. If the sandstone is made up of only one type of grain,
quartz, it is called mature and is called a quartz
sandstone. This type of sandstone indicates chemical
weathering and time. All other grains minerals will
breakdown to quartz given enough time. The white
sandstone bluffs along I55 are an example of this. If
there are pebbles (rock fragments) this is called lithic
sandstone. If there are feldspar minerals present it is
an arkrose sandstone ? Greywacke is sandstone that is
mixed with clay. This can be seen in tidal flats and
floodplains.
Clays and silts make siltstone, mudstone and shale. The sources of these sediments and stones
can be lakes, floodplains and the abyssal plain of the ocean. Siltstone and mudstone are thick and blocky
while shale has fine partings making thin layers. When the shale is black it indicates that it is rich in
organic matter and was formed in an anoxic environment. This is the shale that is being “fracked” or
fractured to retrieve the oil and natural gas that is trapped in it.
Chemical and Biological Rocks
Chemical and biological rocks are by products of chemical
weathering. Ions produced by the chemical weathering of different
minerals are recombined to form new minerals. The biological
rocks have animal or plants combining these ions to produce new
minerals that they use for their shells or support structures
(calcareous algae). The ions that biota chooses for their shells determines the raw material available to
form rocks. If CaCO3 is chosen then limestone is formed. If SiO2 is chosen (diatoms) you get a siliceous
sediment and chert is formed. There is another form of biological rock and that is made from plant
material and these are not forming new minerals. Here plant and animal material deposited in swamps
undergoes diagenisis to form coal.
Chemical precipitates forms in water that are saturated in ions. This can take place due to
evaporation found arid environments. Water as it evaporates will
precipitate out first CaCO3, then gypsum (CaSO4), halite (NCl) and
finally silvite (KCl). This forms a ring system with the least soluble
coming out first followed by next less soluble and so on. There is
some mixing between layers; it’s not perfect rings but mix at the
margins. The gypsum forms rock gypsum and the NCl and KCl forms
rock salt.
Limestone or CaCO3 can directly form out of seawater. Ca
2+
in warm seawater will directly combine with CO3
2-
to form CaCO3 or
lime muds. These muds undergo diagenisis forming limestone.
Limestone formed this way can be used to predict a paleoclimate
and plate tectonic activity.
Another rock formed as a precipitate is phosphorite. This rock forms from the breaking down
and release of phosphates from bone and teeth or from hydrothermal vents and weathered igneous
rocks that have phosphate bearing minerals in them.
A modification of limestone is dolomite. Here percolating ground water seeps down through the
rock and magnesium combines with the limestone. It formula is CaMg(CO3)2. While this stone is more
resistant to acids therefore acid rain it is not prized for cement manufacturing and for greenhouse gas
release. Concrete is made from CaCO3 and one CO2 is released in the process. To get the same amount
of CaO for concrete you release twice
as much CO2.
Metamorphic Rocks
Metamorphism is change in
form in response to changes in
temperature and pressure. This
change takes place in the mineral
content of the rock, but it happens in a
solid form and is due to plate
tectonics. By saying this it is implied
that metamorphism takes place at
plate boundaries but that is not totally
true. One form takes place in basins
were sediment is deposited and undergoes increasing pressure at depth.
As different events come to play different states of metamorphism comes to play. An example
of this progression of one form changing into another is below. Yet despite
these changes there is a preservation of their former conditions and the events led to metamorphism in
the minerals that have formed. The image above shows not only a change in particle size with increase
degree of metamorphism but a change in the mineral composition of these rocks. These changes are still
limited by the source rock. Chemical rocks with only one mineral present such as limestone and chert
are restricted in the metamorphic rocks that they can become.
In the process of metamorphism temperature plays a crucial role. As plate tectonics moves the
sediment from the surface to depth there is an increase in temperature (30oC/Km depth). The rocks
adjust to these higher temperatures by recrystallizing. Each mineral is stable at a specific temperature.
We use this information as a geothermometer to reveal the temperature at which the rock formed.
Remember when we talk about stability we are not talking about an immediate change but a geologic
time frame. You can observe high temperature minerals on the surface. These minerals will change at a
faster rate than low temperature minerals. An example is olivine. This mineral will break down to
limonite if exposed to humid climates in a few decades.
Pressure also affects the rocks during metamorphism. Here the rocks are exposed to two types of
pressure, confining and directed pressure. Confining pressure is a pressure that is all around you. You
are surrounded by ~1atm pressure as you read this. If you dive in the ocean the pressure increases
around your entire body until you’re crushed like a crushed bear can. Confining pressure increases with
depth of the Earth.
Directional pressure is forces directed in a particular
direction. This is called differential stress and is
concentrated along discrete planes. Heat softens the rock
and the directional pressures causes severe folding.
Minerals maybe compressed elongated or rotated.
These directional pressures create different textures. The
minerals oriented perpendicular to the forces form
foliation, a set of wavy parallel cleavage planes. This texture is formed by the platy minerals such as the
micas. Metamorphic rocks are classed according to four main criteria, the metamorphic grade, crystal
size, type of foliation and banding.
Fluids have a major role in metamorphism. The source of the water is hydrothermal and introduced
to the depths by water saturated minerals such as clays in convergent plate boundaries and from cracks
in the crust that let surface waters inter the depths of the lithosphere. The fluids bring CO2 and other
substances such as gold copper and silver into contact with the rocks minerals. This allows for
metasomatism or change in the rocks composition by these fluids.
This metasomatism may be the reason for granoblasitic rocks. These are rocks that are
nonfoilated with crystals that are equidimensional in shapes such as spheres or cubes. Examples of this
are marble and quartzite. Here the existing crystals have be altered, increasing in size and intergrowing.
Quartzite, formed from sandstone, is some of the hardest and most weathering resistant rock known.
The last thing I will mention is that
any rock can form any other rock. All
rocks undergo erosion deposition
and lithification. Any rock can
undergo temperature and pressure
change forming a new metamorphic
rock and any rock can melt and cool
forming a new igneous rock.
This will cover how Geologist Study the Earth, Plate Tectonics and Minerals. It is composed of 5 short answer questions, 5 matching and 10 true/false
You should know what determines a mineral and how minerals are identified (think of your experiences in lab)
There are examples of minerals for each mineral class, you should know them. Again this is lecture and lab.
There are proofs of plate tectonics both from Alfred Wagner and from later. You should know them. The later ones you should be able to explain them and how they are formed and what minerals might be involved.
You should know the parts of the different plate boundaries.
You have been given multiple examples of plate boundaries and steps in divergence, you should know them.
You should know the spheres of our planet, both inner and outer and how they interact and what powers them.
There is a guiding principle in science, you should know this principle for proving theories of science and how it has to be modified for geology.
There is a guiding principle in geology that describes how the process of the earth works over time, you should know this.
Plate Tectonics is the driving force of geologic structures that shape the world that we live in. The
first indication that continents might have been linked physically took place as soon as there was
mapping. It was known by the sixteenth and seventeenth century that the continents fitted together like
pieces of a puzzle. This and other proofs listed below lead to the concept of continental drift, that the
large continents had once been linked together. The scale movements of continents across the across
the surface of the planet was mind boggling.
By the close of the nineteenth century the geologist Edward Suess postulated that the southern
continents were joined together. This was from evidence seen by fossil animal distribution. Ancient life
forms have ranges just as modern animals. Tigers are found in India and Siberia, not in North America.
These ancient animals couldn’t get from South America to Africa, from Antarctica to Australia to India.
There were rock layers both sedimentary and igneous that were continuous on all of the five continents.
He christened this large continent Gondwanaland.
In 1915 Alfred Wegener wrote a book on continental drift. He proposed another super continent
called Pangaea. This continent included all of the modern continents. His mechanism for the movement
of the continents was that the continents moved through ocean crust, drifting until they joined then
breaking apart again. Wegener and others argued for their evidence pointing out rock similarities
type, similarities in rock trends and ages using all of the information gathered from Steno’s laws and
fossil assemblages. They also showed that certain distant continents had similar plants and animals that
entered a different evolutionary path after the continents separated. An example of this is the fresh
water fish the Arowana found in both South America and Australia. Another example of this is the large
amount of marsupial
fossils found in South
America and Australia.
Placental mammals
were the dominant
mammalian life forms
in Eurasia, North
America and Africa.
Only in South America
and Australia were
marsupial fossils
dominated. It wasn’t
until 3 million years ago
North and South
America joined
allowing placental
mammals to gain
dominance in South America. Despite this evidence Wegener and his followers mechanism for
continental movement was badly flawed. They proposed that the tidal forces from the Sun and Moon
like the tides in the ocean. This force is much too weak to move a continent so the theory was rejected.
Fossil plants and animal distribution on southern continents
In 1928 a geologist Arthur Holmes proposed the convection currents split the continents and that
these currents were in the mantel. Remember the knowledge of a mantel was already known at this
time. Ironically this was a mechanism had been proposed by Benjamin Franklin in 1782. Many
geologists argued against this and Holmes bowed to their criticism.
It wasn’t until WWII that there was evidence to
show that Holmes (and Franklin) was correct. A
marine geologist, pioneer of marine seismology,
deep ocean photography by the name of Doc Ewing
found that the sea floor was composed of young
basalt, not old granite (we knew absolute age
dating this time) as originally thought. The other
finding was that there was mapping of the
undersea Mid-Atlantic Ridge. In the center of this
ridge was a rift valley. These were mapped by
Marie Tharp and Bruce Heezen. It was found that
the rift was and active feature with volcanic activity
as well as seismic activity. Harry Hess and Robert
Dietz proposed that the Earth’s crust separates
along the rifts and forms new oceanic crust.
At this point in time many thought the Earth was continuously expanding. It took a massive
earthquake (1964 Alaskan Earthquake) to show the destruction of the ocean floor. Here a large portion
of the seafloor moved beneath the Alaskan crust. In 1965 J. Wilson proposed the theory of plate
tectonics. That seafloor was recycled down deep trenches and at these regions there were earthquakes
and volcanoes. This movement explained the rock folds and faults. It also explained many of the
minerals. It was also shown that the rates and directions of movements of crustal rocks were
mathematically consistent. Wilson the proposed plate boundaries. There are three of them, divergence,
convergence and transform.
Divergence, found in the Mid-Continent Rifts, created the new seafloor, calling it seafloor
spreading. Mantel temperature difference drives this. A plume of heat or possible actual upwelling
mantel moves upward to the plastic asthenosphere. Here convection currents developed. The
asthenosphere drags against the lithosphere splitting it. New material from the asthenosphere moves
upward and forms a new ocean crust material. This happens in the rift valleys of the Mid-Continental
Rifts that bisect the ocean floor.
Rift valleys form in the center of continents forming with volcanoes and earthquakes as the continents
tear apart. This can be seen in the Great Rift Valley of East Africa creating parallel valleys. The ocean
plate if produced by basalt (iron rich and silica poor igneous rocks). The minerals found here are olivine
and pryoxine and some feldspar. When rifts are on the continent the upwelling magma first melts
continental crust so you find quartz, mica, and feldspars. An example of a continetal rift valley is the rift
valley found in Iceland. Here new Atlantic Plate is being made as North America moves away from the
Eurasin Plate. Iceland sites on top of the Mid-Atlantic Rift. Not all rifts splits continents. Instead some fail
leaving zones of weakness. An example of this is our own New Madrid Seismic Zone as well as the
Wabash Seismic Zone. These rifts failed hundreds of millions of years ago.
There are continental rifts that you see today that have been successful. Here the rift valley continues
until new seafloor made up of basalt
forms. These flood forming a linear sea
as seen in the Gulf of California (Sea of
Cortez) and the Red Sea. At the Gulf of
California, Baja California splits off of
the North American Plate forming new
seafloor. The Red Sea is the breaking off
of the Arabian Plate from the African
Plate.
The final phase of divergence is the
expanding ocean. The upwellings have
created underwater mountain chains.
The mountain chains are split by the rift
valley where active volcanoes and
magma create new ocean floors.
Convergent boundaries are scattered
over the globe. Here plates collide and
ocean crust is recycled back into the earth for two of these boundaries. There are three convergent
plate boundaries: 1. Ocean-Ocean, 2. Ocean-Continent, 3. Continent-Continent. These convergent zones
are marked by forms of metamorphic rocks and specific minerals. They are also marked by deformation
of rock layers with folds and faults as well as volcanoes and earthquakes.
Ocean-Ocean Convergent plate boundaries has one plate descending beneath another. This is called
subduction. This event takes place along in narrow deep-sea trench. The trenches are the deepest
portion of the ocean. The some descend as deep as 11 km, an example and the deepest the Marianas
Trench of western Pacific. At this an older portion of the oceanic lithosphere sinks into the
asthenosphere and then recycled
into the mantle as the
asthenosphere convection currents
drags it down. The water from the
subducting plate is squeezed out of
the hydrated minerals and rises
into the asthenosphere. There
the water drops the melting
temperature of the asthenosphere
and this molten material creates a chain of volcanoes, an island arc. There are many examples of this,
Indonesia, Philippians, Aleutians, and Japan. Here you can find volcanoes and major earthquakes. Some
of these earthquakes are small and some huge like the magnitude 9 in 2004 at Sumatra and the recent
2011 magnitude 9 at Japan. The minerals here have undergone both mixing with ocean crust and
fractionation and are intermediate in nature.
Ocean-Continent Convergent plate boundaries have subduction again. Here the heavy iron rich ocean
lithosphere sinks beneath the silicone rich continental crust. The continental margin crumples and
deforms and uplifts rocks into mountain chains that parallels to the deep sea trench. As the oceanic
lithosphere moves beneath the continental crust lighter material is scrapped off onto the continental
crust expanding the continent.
Again the water from the hydrated
minerals melts the asthenosphere.
This material rises up and melts
continental crust. Such a boundary
can be seen at the Cascade Range.
It can also be seen in South
America were the Nazca plate dips
beneath the South American Plate
creating the Andes. This is the
location of the largest earthquake in recorded history, magnitude 9.5 in Chili in 1960 and a magnitude
9.2 in Alaska in 1964.
Continent-Continent Convergence takes place when two continents collide. This is how Pangaea and
Gondwanaland were assembled. It happens when one oceanic crust being subducted under a
continental crust has a continent on it. As the subducting plate sinks the continental crust collides and
the two continental lithospheres join. This collision doubles the thickness of the crust at this area. The
Indian and the Eurasian plates have
done just this raising up the Tibetan
Plateau and the highest mountain
range that exist today, the
Himalayan Mountains. Other
example of this is the Alps, and the
Pyrenees Mountains. In the past the
continuous collisions from 440 to
approximately 300 million years ago
formed the Appalachian Mountains on our own continent.
The last boundary is the Transform-Fault Boundary. These boundaries are normally associated with
divergent plate boundaries. They offset the Mid-Oceanic Ridge and allow for differences in magmatic
upwellings. In certain areas this boundary can be found on continents. One such area is the San Andreas
Fault in California and the Anotoli Fault in Turkey. Both have been sites of major earthquakes and
thousands of deaths. Notice there are no volcanoes associated with this plate boundary.
Plates are normally bounded by multiple boundaries. Our plate has the Mid-Atlantic Oceanic
Ridge (divergent plate boundary) and two (or more) transform-fault plate boundary. One of these
transform-fault plate boundaries is between the North American Plate and the Carrabin Plate. This was
the location of the Haitian earthquake in 2010. The other is the transform-fault boundary between the
Pacific Plate and the North American Plate as represented by the San Andreas Fault. We also have a
transform-fault plate boundary in Canada. Our plate overrides the Pacific Plate at the Aleutian Islands
and continues to a portion of Japan (our plate is more then North America). Our plate also boarders
three different convergent plate boundaries, in the Pacific Northwest our plate override’s the Jaun de
Fuca Plate. In the country of Mexico the North American Plate overrides the Cocos Plate. We have a
junction with Eurasia that has not been identified (probably a transform-fault).
The proof of the divergence and seafloor spreading was recorded on the floor of the ocean
itself. Geologist modified the magnetometers developed to detect submarines from the magnetic fields
from their steel hulls. These modified magnetometers were sensitive the magnetic field on theocean
floor. They discovered a magnetic anomaly. The magnetic field alternated between high andlow values
in narrow parallel bands that were symmetrical with the mid-ocean ridge. The reason for the magnetic
field is the mineral magnetite. It is charged and floats free in magma until that magma cools. Once the
magma cools the magnetite will point to magnetic north. The anomaly was an increased field
alternating with a decrease in the field. The
increase in the magnetic fields is oriented
to where the magnetite points to our
magnetic north. The decrease in the field is
when the magnetite points in the opposite
direction. This reversal shows dramatic
change in the geodynamo so that the field
reversed. This stripping on the seafloor
parallels the mid-oceanic ridge. The
stripping where the field is normal or
reversed are called magnetic chroms.
When combined with absolute age dating
this was proof spreading centers.
Deep-sea drilling combined
with absolute age dating gives us the
seafloor isochroms. The youngest rock
is found at the spreading centers. The
oldest rock is found adjacent to
specific continents. The oldest rock is
180 million years old. The continents
are much much older. In our own state
the highest point, Taum Sauk
Mountain, is 1.28 billion years old. This
means that the seafloor is new and now gives credence to Wagner’s Pangaea. We can reconstruct the
plate movement by using the transform-fault plate boundaries we can deduce the movement of the
plates. The isochroms will place the continents at different points of time. Because of subduction the
original seafloor was destroyed by subduction returning to the older rock
information we can rejoin the jigsaw puzzle. Using old mountain belts such as
the Appalachians of North America and the Caledonian Mountains in
Iceland and Scotland we join North America to Europe. The fossils
magnetism of continental fragment record it ancient orientation and
magnetic latitude. This can be augmented by paleoclimate information
from the fossils and minerals. Look at the on pages 70 to 71 to see the
growth of the world as we know it and our potential future position. To
get a picture of the time it will take to get to future world here is a simple
exercise .Assignment Using the fact that the plates in Iceland are moving
apart (creating new land for Iceland) at 25mm per year how long will it take for
Iceland to grow 1 km.
Further proof of this theory of plate tectonics came with GPS (global positioning system). GPS stations
set up over continents record the changes in positions of the continents. While this gives us an accurate
measurement of the speed of the boundaries for this brief period of time it doesn’t give us changes of
plate movement over time. For that do the exercise. Here absolute time calculated by isotope decay and
position curtsy GPS can be combined to give changes in plate movement over time.
The movements of these plates are powered by our geodynamo. Convection currents upwell and to
work on the plates. There are two thoughts of for the convection currents. one proposes one current
circulating between the upper and lower mantel. The other theory is that of two convection currents.
the convection current of the lower mantel and creates a convection current in the upper mantel. The
felling at the moment is that the spreading centers are controlled in their movement by the descent of
the plate at the subduction zones. This is called slab pull ridge push causing a slow upwelling at the
spreading centers along the fracture represented by the rift valley.
There is another upwelling from the mantel. This is the mantel plume. A narrow slender cylinder of fast
rising mantel material. These areas are often far from the spreading centers. An example of mantel
plumes can be seen in the Hawaiian Islands, the Galapagos Islands and Yellowstone.
