Rey writer

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

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

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

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.