Chapter 20: Rocks and Minerals
Objectives:
Working through this chapter of the study guide will enable you to:
1. Name and describe some of the most abundant minerals found in Earth's
crust.
2. State the basic physical properties that can be used to identify mineral
samples.
3. Distinguish the physical properties that are useful in identifying minerals.
4. Distinguish the chemical characteristics that are useful in identifying
minerals.
5. Distinguish between rocks and minerals.
6. List and distinguish among the common silicate minerals.
7. Describe the chemical classification of minerals.
8. List and describe the characteristics of intrusive/ extrusive igneous rocks.
including textures.
9. Explain how sedimentary rocks are classified.
10. List and describe the characteristics of foliated/ non-foliated
metamorphic rocks, including textures and composition.
11. Explain how new sedimentary rocks can be formed from the remains of
previous rocks.
12. Distinguish among igneous, sedimentary, and metamorphic rocks.
13. Compare the origins of the fine-grained and coarse-grained igneous
rocks and give several examples of each type.
14. Describe several fragmental sedimentary rocks.
15. Describe several metamorphic rocks and give their origins.
16. Describe the processes by which sediments become rock.
17. Draw a diagram that shows the rock cycle.
Lecture Outline:
The composition and structure of the planet that we live on has long been of
great interest to scientists and laymen alike. Much of the raw materials that we
use to enhance our existence comes from the crust of Earth itself. Mineral
formations are not only pretty to look at, as in gem quality stones for personal
adornment, but also are the basis for many of the chemical and industrial
products that we use in our daily lives. The finding and identification of these
materials can be an interesting and rewarding experience, and many people
today make a living doing just that.
Although many of the ideas that are covered in this chapter are somewhat
complex, the overall discussion of Earth's crust and its physical composition is
quite straightforward and can be easily grasped. Understanding this material is
an important stepping stone for the next chapter in the textbook, which deals
with the overall structure of Earth and the way that movements in its crust and
mantle can produce significant, and sometimes catastrophic, events.
Composition of the Crust
A. The outer part of the earth is called the crust.
B. The two most abundant elements of the earth's crust are oxygen and
silicon.
C. Most crustal rocks are composed of silicon compounds.
1. Silica is silicon dioxide (SiO2). Silicates are compounds of silicon
with oxygen and one or more metals.
2. Silicates are crystalline solids whose basic structural unit is the
SiO44– tetrahedron.
The wide variety of silicate minerals is due to many different ways the SiO44–
tetrahedron can combine with metal ions.
Minerals
Minerals are naturally occurring, inorganic, crystalline materials that have a well-
defined chemical composition. They can be either elements or compounds, and
each has a distinctive set of physical properties. Only about two dozen common
minerals make up the majority of Earth's crust; however, in excess of 2000 have
been found in small quantities. Over 98% of the crust is made up of only eight
elements. In order of decreasing abundance by mass, they are oxygen, silicon,
aluminum, iron, calcium, magnesium, sodium, and potassium. See Figure 11.9 in
Chapter 11 of the textbook for a graphical representation of this distribution.
The term mineral is also used when referring to the elements in foods that are
necessary, in small quantities, for the proper functioning of the human body.
These are often discussed at the same time as other vital organic substances
known collectively as vitamins, so it is quite common to be concerned, for
example, about the vitamin and mineral content of your morning breakfast cereal.
Some minerals fitting this classification are iron, sodium, iodine, manganese,
magnesium, and copper.
Consolidated mixtures of crystalline minerals are called rocks. Most rocks are
composed primarily of oxygen and silicon, much of which is in the form of silicon
dioxide, commonly called silica. Combinations of these elements in which the
oxygen-silicon ratios are greater than 2:1 are known as silicates. For example,
quartz is composed of pure silica (SiO2), whereas olivine is made up of the
silicate tetrahedron (SiO4)4- in combination with several trace metals.
The most abundant mineral group in Earth's crust is the feldspar family. The
members of this family are composed of oxygen, silicon, and aluminum plus at
least one additional metallic element. The two main sub-classes of the feldspar
family are plagioclase feldspar, which contains calcium or sodium, and orthoclase
feldspar, which contains potassium.
Many non-silicate minerals are found in Earth's crust, among which are
carbonates such as calcite, oxides such as hematite, and sulfides such as galena.
There are also rare but important pure elements such as gold, silver, sulfur,
copper, and diamond (carbon). Most of these minerals, however, appear as ores
in which the important minerals make up only a small percentage. The discovery,
mining, and refining of these ore deposits is the basis for a large portion of the
valuable and useful minerals needed in our modern industrial society.
When discussing the physical properties of minerals, we use terms like crystal
form, hardness (as measured on the Mohs' hardness scale), cleavage, fracture,
color, and streak. Well-defined tests and observations can be made of these
properties to help identify mineral samples. Applying these procedures to
samples obtained from Earth's upper crust is quite easy and can even be fun.
Many people have small personal collections of rocks and minerals that they
have acquired through inexpensive purchase or simply by picking them up from
the ground and bringing them home.
Minerals
A. Minerals are naturally occurring, crystalline, inorganic solids having fairly
specific chemical compositions.
B. More than 2000 minerals are known, but most are rare.
C. Some physical properties of minerals are color, hardness, crystal form,
and cleavage (tendency to split along certain planes).
D. Examples of six common minerals are:
1. Quartz (SiO2) crystals are often found in narrow deposits called
veins.
2. Feldspars are silicate minerals having very similar properties and
are the most abundant single constituent of rocks. Varieties include
orthoclase (silicate of K and Al) and plagioclase (silicate of Na, Ca,
and Al).
3. Mica varieties include white mica (silicate of H, K, and Al) and
black mica (silicate of H, K, Al, Mg, and Fe).
4. Ferromagnesian minerals are silicates of Fe, Mg, and usually
other metals such as K, Al, and Ca.
5. Clay minerals are silicates of Al, some with a little Mg, Fe, and K.
6. Calcite (CaCO3) is the chief mineral in limestone and marble.
Mineral
1. Naturally occurring
2. Inorganic
3. Solid
4. Definite chemical composition
5. Orderly internal crystal structure
What are some examples of some mineral names?
1. Feldspar is the most abundant mineral in the Earth's crust. There are
several types of feldspar including:
a. Orthoclase feldspar
b. Microcline feldspar
c. Plagioclase feldspar (has striations - tiny parallel hairline grooves)
2. Quartz is the second most abundant mineral in the Earth's crust
3. Other common rock-forming minerals include:
a. Muscovite mica
b. Biotite mica
c. Hornblende (part of the amphibole group of minerals)
d. Augite (part of the pyroxene group of minerals)
e. Olivine
f. Calcite is common in some sedimentary rocks
Rock
A rock is an aggregate of one or more minerals
Polymorphs
Two minerals with the same chemical composition, but a different crystal
structure.
Physical Properties of Minerals
1. Color
not always diagnostic (feldspar, quartz, fluorite come in many colors)
Feldspar can be green, pink white, gray, etc.
2. Luster
metallic
non-metallic
glassy or vitreous, dull, pearly,
resinous, waxy, adamantine, silky
3. Streak
unglazed porcelain plate
note color, odor if any
Both of these samples are hematite; both have a reddish-brown streak
4. Hardness
scale of 1 to 10 (Mohs Scale)
1. Talc
2. Gypsum
________ fingernail
3. Calcite
________ penny (copper)
4. Fluorite
_________ nail
5. Apatite
_________ glass
6. Orthoclase feldspar (K feldspar)
7. Quartz
8. Topaz
9. Corundum
10. Diamond
5. Cleavage
Breakage along planes
Related to crystal structure
1 direction
(muscovite, biotite)
Muscovite (left) Biotite (right)
2 directions at 90°
(feldspar, pyroxene)
Pyroxene
2 directions not at 90°
(amphibole at 60° and 120°)
3 directions at 90° (cubic)
(halite, galena)
Halite
3 directions not at 90° (rhombohedral)
(calcite, dolomite)
Cleavage fragments of calcite
4 directions (octahedral)
(fluorite)
Cleavage fragments of fluorite
6. Fracture
irregular breakage (no cleavage)
breakage not along smooth planes
Conchoidal fracture
smooth curved fracture surfaces
occurs in quartz, chert, obsidian, glass
Rose quartz lacks cleavage; it has conchoidal fracture
Conchoidal fracture in the igneous rock, obsidian
7. Crystal form
Some minerals that may or may not have cleavage GROW (not break) into
crystals with flat sides.
Examples
quartz
pyrite
Quartz crystals
8. Specific gravity
(similar to density)
Weight of a mineral divided by weight of an equal volume of water.
Chemical classification of minerals
1. Native elements (metal)
native gold (Au), native copper (Cu), native sulfur (S), native silver
(Ag), graphite (C), diamond (C)
2. Sulfides (metal + S)
pyrite (FeS2), galena (PbS), sphalerite (ZnS)
3. Sulfates (metal + SO4)
gypsum (CaSO4 · 2H2O), anhydrite (CaSO4), barite (BaSO4)
4. Oxides (metal + O)
water ice (H2O), hematite (Fe2O3), magnetite (Fe3O4), corundum
(Al2O3)
5. Halides (metal + Cl or F halogens)
halite (NaCl), fluorite (CaF2)
6. Carbonates (metal + CO3)
calcite (CaCO3), dolomite (CaMg(CO3)2)
7. Other - borates, urananates
8. Silicates (metal + Si and O)
quartz (SiO2),
potassium feldspar (KAlSi3O8),
Ca Plagioclase feldspar (CaAl2Si2O8),
Na Plagioclase feldspar (NaAlSi3O8)
Silicate Structures
Based on silicate tetrahedron
4 oxygen atoms and 1 silicon atom
9. Single tetrahedra
olivine
10. Single chains
pyroxene
11. Double chains
amphibole
12. Sheets
muscovite
biotite
13. Frameworks
quartz
feldspar
potassium feldspars (orthoclase and microcline)
plagioclase feldspars
The four most common elements in the Earth's crust:
Element
Approx. Weight %
Oxygen (O)
46.6
Silicon (Si)
27.7
Aluminum (Al)
8.1
Iron (Fe)
5.0
Rocks
Most minerals are not found in a pure state either on or beneath the surface of
Earth. Large quantities of these materials are combined into aggregates called
rocks. Rocks are classified by the three basic ways in which they are formed.
Aggregates that have cooled from molten magma, whether on the surface or
deep in Earth's interior, are known as igneous rocks. Those formed from the
fragments of old rocks that have been deposited after erosion by air, water, or
other chemical or mechanical means are called sedimentary rocks. Finally, any
rocks that have changed their structures due to heat and/or pressure after the
time they were initially formed are referred to as metamorphic rocks. These
three types of rock are closely interrelated, as can be seen by studying the rock
cycle, which is depicted in the spotlight feature in Chapter 21 of the textbook.
Volcanoes
A. The processes of vulcanism (the movements of molten rock) and
diastrophism or tectonism (the movements of the solid materials of
the earth's crust) act in opposition to the processes that would level the
earth's surface.
B. A volcano is an opening in the earth's crust through which molten rock
(called magma while underground, lava above ground) pours forth.
C. A volcano usually has a depression, or a crater, at its summit.
D. The two main factors that determine whether an eruption will be quiet or
explosive are:
1. Viscosity (resistance to flow) of the magma
2. Amount of dissolved gases, such as water vapor (the most
prominent), carbon dioxide, nitrogen, hydrogen, and various sulfur
compounds in the magma
E. Magmas rich in silica (the most viscous) and dissolved gases result in
explosive eruptions, while magmas with modest gas and silica contents
result in quiet eruptions.
F. Lava hardens into one or another type of volcanic rock including:
1. Basalt (the most common)
2. Rhyolite (the most silica rich)
3. Pumice (light and porous)
Most active volcanoes occur around the borders of the Pacific Ocean, on some of
the Pacific Islands, in Iceland, and in East Africa.
Origin of Igneous Rocks
The word igneous means "fire-formed".
Igneous rocks form by cooling and crystallizing from hot molten magma or lava.

Magma - hot molten rock below the Earth's surface

Lava - hot molten rock that erupts onto the Earth's surface through a
volcano or crack (fissure)
Igneous rocks that cool and crystallize beneath the Earth's surface are called
intrusive igneous rocks.
Another name for intrusive igneous rocks is plutonic igneous rocks (named for
Pluto, Roman god of the underworld).
Igneous rocks that cool and crystallize on the Earth's surface are called
extrusive igneous rocks.
Another name for extrusive igneous rocks is volcanic igneous rocks (named for
Vulcan, Roman god of the fire and forge).
Cooling Rates
Cooling rates influence the texture of the igneous rock:

Quick cooling = fine grains.
Extrusive igneous rocks tend to be fine grained or glassy.
Sometimes they are vesicular with tiny holes formed by the release of
trapped gases.
Other extrusive igneous rocks have a fragmental or pyroclastic texture
composed of pieces of volcanic rock and ash welded together by heat.
Lava cools more quickly because it is on the surface.

Slow cooling = coarse grains.
Intrusive rocks tend to be coarse grained with intergrown crystals
ranging from several millimeters to several inches in diameter.
Magma cools more slowly because it is deep within the Earth where
temperatures are high.
Igneous Rocks
It is believed that Earth began as a molten sphere of material, and thus the first
rocks were formed from the cooling of hot magma into solid rock. This means
that igneous rocks can be considered the basis of all other types of rocks.
Today, igneous rock forms either deep beneath Earth's surface or at Earth's
surface as the result of volcanic activity. Molten rock called magma is often
forced to the surface by volcanic action. Once it reaches the surface, molten rock
is referred to as lava. Cooled lava retains this name even after it has solidified
into igneous rock.
Igneous rock makes up about 80% of Earth's crust. Magma that cools and
solidifies as lava at or near the surface of Earth is called extrusive rock, and
magma that solidifies deep within Earth's interior is referred to as intrusive rock.
The outermost layer of Earth is a thin, rigid outer shell called the lithosphere,
which is split up into giant surface plates. The lithosphere rides on top of a
semimolten region known as the asthenosphere. The rigid surface plates are
driven across the asthenosphere by the slow convective circulation of this easily
deformable plastic material as heat works its way outward from Earth's central
core region. The motion of these large lithospheric plates of solid rock across
Earth's surface is explained by a theory called plate tectonics.
As plate tectonics occurs, the plates may bump into each other at convergent
boundaries, pull away from each other at divergent boundaries, or slip past each
other at transform boundaries. It is at these boundaries that we find most
volcanic activity, and it is also where the majority of earthquakes occur.
The actual interaction of plates in quite complex, as will be seen in even more
detail in Chapter 22, but the basic ideas needed in this chapter are that new rock
can form in regions where plates are diverging, because of the upwelling of
magma from the asthenosphere, and that one plate often is driven under
another plate when they converge, producing a subduction zone in which the
descending plate melts and its lighter, volatile material works its way back to the
surface as volcanic gases and lava.
The texture of igneous rock is mainly determined by the type and size of mineral
grains that form as it cools. Different minerals crystallize out of magma at
different temperatures as the molten material cools. The order in which the most
common minerals crystallize is related to the magma's temperature, and the size
of the crystals that form from the various minerals present is dependent on the
rate of cooling. When magma reaches the surface, a volcano is formed and lava
can flow across Earth's surface. The texture, or grain size, in igneous rock
created from this lava also depends on the rate of cooling, with the smallest
grain size occurring in the fastest-cooling materials. On the other hand, if magma
never reaches the surface, it tends to cool at a much slower rate, and the
crystals formed in these subterranean regions will be much larger in size.
The color of igneous rock is determined primarily by the silica content of the rock.
Rocks that are rich in silica often contain sodium and potassium. Granite and
similar silica-rich rock is generally light in color and low in density. Rock that has
lower silica content usually contains iron, magnesium, and calcium, which make
it darker in color and also denser than silica-rich rock. Rocks, such as basalt,
make up the material found in low-lying crustal regions such as ocean basins.
Because of its lower density and high-silica content, granite is the most common
rock found at higher elevations in volcanic islands or in continental mountain
ranges.
Igneous Rocks Characteristics
A. Igneous rocks, such as granite, basalt, and obsidian (natural glass),
are rocks that have cooled from a molten state.
Fine-grained igneous rocks (examples: rhyolite, andesite, basalt) have cooled
rapidly at or near the earth's surface. Coarse-grained ones (examples: granite,
diorite, gabbro) have cooled slowly well below the earth's surface.
Igneous Activity
Igneous rock that has cooled slowly below Earth's surface is found in formations
called plutons. Such igneous bodies can be either concordant, if they form
parallel to the grain in the surrounding rock, or discordant if they cut across this
grain. Examples of concordant formations are sills and laccoliths, whereas
examples of discordant bodies are batholiths and dikes.
Volcanoes appear where magma is squeezed all the way to Earth's surface. In
these cases, steam and other gases are often expelled together, with molten
rock in the form of lava. Solid material ranging in size from dust particles to
boulders is also a common product of volcanic eruptions. Violent eruptions occur
in areas where great pressure is built up by gases that are trapped because the
emerging lava is thick and does not flow easily. If the lava is more fluid (has a
lower viscosity), volatile gases can more easily escape and the eruptions are
usually less violent. This results in lava rivers or fountains that can often be
safely observed close at hand, such as the brilliant pyrotechnic displays that
occur frequently in the Hawaiian Islands. The viscosity of lava depends on its
temperature and on its silica content, with the lower temperature and silica-rich
lava being the thickest and most viscous.
Volcanic eruptions can occur from long fractures in Earth's surface in the form of
fissure eruptions, or in a more localized manner from the top or sides of large
volcanic mountains. Fissure eruptions happen in continental regions where
extensive surrounding areas can be covered by flood basalts, a very low-viscosity
form of lava that can form flows several hundred meters thick, or they can occur
in ocean basins at diverging plate boundaries where even more extensive flows
can produce layers of lava several thousand meters thick.
Volcanic mountains can take on several forms. Shield volcanoes are low-profile
mountains with gently sloping sides and extremely wide bases formed from the
flow of non-viscous basaltic lava. Another common type is the stratovolcano,
which forms from high-viscosity lava eruptions in which the violent activity
produces composite layers of lava and tephra in a characteristic high, steepsided formation. Yet another type of volcano can be formed almost completely
out of tephra in a low, steeply sloped structure usually only a few hundred
meters high, called a cinder cone.
The eruption of any specific volcano is, for the most part, quite unpredictable.
New volcanoes can form suddenly, and old dormant ones may spring to life with
little warning. However, in some areas of Earth's surface such activity is quite
commonplace and can be expected to occur quite regularly. The most famous of
these areas is the outer rim of the Pacific Ocean basin, which forms nearly a
complete circle of currently active or recently active volcanoes. This volcanic
region is commonly referred to as the Ring of Fire. Other more localized regions
of continual volcanic activity are the famous hot spots, over one of which the
Hawaiian Islands are still forming. Another very active region is the mid-oceanic
trench on the floor of the Atlantic Ocean about halfway between the Americas
and the mainlands of Europe and Africa, where seafloor spreading was first
discovered and where it is still taking place today.
Intrusive Rocks
A. Plutons are intrusive bodies formed by the solidification of magma under
the earth's surface.
B. Because plutons cool slowly, the resulting intrusive igneous rocks tend to
be coarse-grained. An example is granite.
C. A dike is a wall of intrusive igneous rock that cuts across existing rock
layers.
D. A sill is a pluton formation that lies between and parallel to existing rock
strata.
E. A laccolith is a pluton formation that forms a mushroom-shaped intrusion
that pushes up overlying rock strata.
F. A batholith is a very large pluton that can cover hundreds of thousands
of square km. Batholiths are always associated with mountain ranges, past or
present.
Composition of Igneous Rocks
Igneous rocks can be placed into four groups based on their chemical
compositions:
1. Granitic
1. Dominated by silicon and aluminum (SiAl)
2. Usually light in color
3. Characteristic of continental crust
4. Forms a stiff (viscous) lava or magma
5. Rock types include:
1. Granite
Granite
2. Rhyolite
Rhyolite
2. Andesitic
1. Intermediate in composition between sialic and mafic
Diorite
3. Basaltic
1. Contains abundant ferromagnesian minerals (magnesium and iron
silicates)
2. Usually dark in color (dark gray to black)
3. Characteristic of Earth's oceanic crust, Hawaiian volcanoes
4. Forms a runny (low viscosity) lava
5. Also found on the Moon, Mars, and Venus
6. Rock types include:
1. Basalt
Basalt
Classification of Igneous Rocks:
Igneous rocks are classified or named on the basis of their texture and their
composition.
Igneous rocks cannot be classified by their process of formation, because the
processes are interpreted from the rocks.
Classification of rocks is always based on objective, observable, measurable
data (such as grain size and the percentages of various minerals), and not on
interpretations.
The igneous rocks classification diagram below shows varying percentages of
minerals in each of the four major categories of igneous rocks (sialic,
intermediate, mafic and ultramafic). The table below shows the rock names for
the various textures, combined with the mineral information.
Texture
Aphanitic
Rhyolite
Andesite
Basalt
--
Phaneritic
Granite
Diorite
Diabase
Gabbro
Periodotite
Pegmatitic
Granite Pegmatite
Diorite Pegmatite
Gabbro Pegmatite
--
Porphyritic
Aphanitic Phaneritic
Porphyritic
Rhyolite
Porphyritic Granite
Porphyritic Andesite
Porphyritic Diorite
Porphyritic Basalt
Porphyritic Gabbro
Porphyritic Periodotite
Pumice
Pumice
Vesicular
--
Vesicular
Texture
Basalt
& Scoria
Glassy
Obsidian
--
--
--
Fragmental
Tuff (ash)
Volcanic
Breccia
Tuff (ash)
Volcanic
Breccia
--
--
0 - 15
20 - 40
50 - 60
95 - 100
COLOR INDEX
(% Dark Minerals)
Sedimentary Rocks
Loose sediment is created by various forms of decomposition and erosion that
will be discussed in detail in Chapter 23. The consolidation of this sediment forms
sedimentary rock. This type of rock makes up only about 5% of Earth's entire
crust, but it covers the surfaces of more than 75% of the continents and ocean
basins. This means that sedimentary rock forms in relatively thin layers.
Sedimentary deposits are also important because they contain abundant supplies
of oil and coal, as well as metal deposits and other materials used primarily in
the construction industry.
Sedimentary rock is made up of detrital sediments (rock and mineral fragments),
organic sediments, and chemical sediments. These materials are transported by
streams and rivers and eventually deposited in layers at the bottom of lakes and
oceans. The consolidation of these layers leads to the formation of sedimentary
rock. The conditions under which sedimentary rock was formed can be
determined by studying characteristics such as color, sorting, rounding, bedding,
fossil content, ripples, and mud cracks. Tables 21.5 and 21.6 in the textbook
show some of the most common classifications of sedimentary rock and describe
their properties.
Sedimentation
A. The eroded material transported by the agents of erosion is eventually
deposited to form sediments.
B. The most widespread sediments collect near continental margins.
C. There are four common sites of deposition:
1. Flood-deposited debris in stream gravel banks and sandbars
2. The flood plains of meandering rivers
3. Alluvial fans, which are deposits of sediments where streams
emerge from steep mountain valleys and flow onto plains
4. Deltas, which are deposits of sediments where a stream enters a
lake or sea
D. Moraines are piles of debris that accumulate around the ends and along
the sides of glaciers. This material is called till.
E. The most important agents of deposition are ocean currents because of
the large volume of sediment they carry and deposit.
F. Examples of groundwater deposition include:
1. Deposition of minerals in veins within rock
2. Cave deposits including stalactites (hanging from cave ceilings)
and stalagmites (rising from cave floors)
3. Deposits around hot springs and geysers
G. Lithification is the process by which sediments become rock.
H. Lithification includes:
1. Compaction, in which the sediment grains are squeezed together
under the pressure of overlying deposits
Concentration, in which the sediment grains are bound together by chemical
changes brought about by circulating groundwater
Sedimentary Rocks
A. Sedimentary rocks have consolidated from materials derived from the
disintegration or solution of other rocks and deposited by water, wind, or
glaciers.
B. Sedimentary rocks are divided into two categories:
1. Fragmental rocks
2. Chemical and biochemical precipitates
C. Three types of fragmental sedimentary rocks are:
1. Conglomerate (cemented gravel)
2. Sandstone (cemented sand grains)
3. Shale (consolidated mud or silt)
D. Examples of precipitated sedimentary rocks are:
1. Limestone (mostly the mineral calcite)
Sediment and Sedimentary Rocks
Sediment = loose particulate material (clay, sand, gravel, etc.).
Most sediment is derived from the weathering (breakdown) of pre-existing rocks.
Some sediment is formed through chemical and biochemical processes acting in
the depositional basin, For example, broken shell fragments or lime mud formed
from calcareous algae would be sediment formed through biochemical processes.
Sediment becomes sedimentary rock through lithification, which involves:
1. Compaction due to pressure or weight of overlying sediments
2. Cementation by deposition of minerals in pore spaces from waters
carrying ions in solution
Types of sedimentary rocks


Clastic (terrigenous or detrital) sedimentary rocks
o
Conglomerate or Breccia
o
Sandstone
o
Siltstone
o
Shale
Non-clastic (chemical/biochemical) sedimentary rocks
o
Carbonate sedimentary rocks (limestones and dolostone)
A. Clastic sedimentary rocks (also called terrigenous or detrital)
Clastic sedimentary rocks are derived from the weathering of pre-existing rocks,
whch have been transported to the depositional basin.
They have a clastic (broken or fragmental) texture consisting of:
1. Clasts (larger pieces, such as sand or gravel)
Clasts and matrix (labelled),
and iron oxide cement
(reddish brown color)
Clastic sedimentary rocks are classified according to their texture (grain
size, as well as shape for gravel-sized clasts):

Gravel: Grain size greater than 2 mm
o
If rounded clasts = conglomerate
o
If angular clasts = breccia
Conglomerate
Breccia


Sand: Grain size 1/16 to 2 mm
o
Sandstone
o
If dominated by quartz grains = quartz sandstone (also called
quartz arenite)
Quartz Sandstone
o

Silt: Grain size 1/256 to 1/16 mm (gritty)
o
Siltstone
Siltstone


Clay: Grain size less than 1/256 mm (smooth)
o
Shale (if fissile)
o
Claystone (if massive)
Note: Mud is technically a mixture of silt and clay. It forms a rock
called mudstone (or mudshale if fissile).
Shale - fissile
B. Non-clastic sedimentary rocks (also called chemical and biochemical
sedimentary rocks)
This group includes the carbonates (limestones and dolostone)
1. Carbonates - The carbonate sedimentary rocks are formed through both
chemical and biochemical processes. They include the limestones (many
types) and dolostones.
1. Two minerals are dominant in carbonate rocks:
1. Calcite (CaCO3)
2. Dolomite (CaMg(CO3)2)
3. Remember which of these fizzes readily, and which of these
must be scratched or powdered!
Metamorphic Rocks
Metamorphism means "changed form". Metamorphic rocks are important
because they remind us that rocks are changeable. The rock cycle describes how
one type of rock can be converted into another. Metamorphic rocks are formed
primarily by the action of heat and/or pressure on other types of rocks.
In many parts of the U.S., metamorphic rocks are deeply buried beneath layers
of sedimentary rocks. In Georgia, however, metamorphic rocks are exposed
throughout much of the central and northeastern part of the state in the
Piedmont and Blue Ridge physiographic provinces.
Metamorphic processes are difficult to observe because they generally occur
deep within the Earth, unlike the deposition of sediment or the eruption of
volcanic igneous rocks. Much of what we know about metamorphic processes is
the result of laboratory experiments and theoretical models that simulate
conditions deep within the Earth.
Metamorphic rocks, and the minerals found within them, are economically value.
They include marble and soapstone for carving, slate for billiard tables and
roofing, various types of building stone, crushed stone for road construction,
asbestos for fire-retardant materials, talc for talcum powder, graphite for
lubricants and pencil "leads", garnet for abrasives, and a number of types of
semi-precious stones including garnet and tourmaline. Economically valuable
mineral deposits can be found in metamorphic rocks, including gold, copper, iron,
lead, and zinc. In addition,anthracite, a type of coal (used for heating or
electricity generation), is a metamorphic rock.
Metamorphic rocks have been changed under the influence of high temperature
and/or extreme pressure deep beneath the Earth's surface. Sedimentary, igneous,
and metamorphic rock can all undergo metamorphic changes, but sedimentary
rock is especially susceptible to such processes. Metamorphic changes can be
either mechanical or chemical, and they may involve only the materials present
in the original rock, or, in some cases, may have additional minerals introduced
into the formation by the metamorphic process itself.
There are three basic types of metamorphism: (1) contact metamorphism,
caused by thermal processes, (2) shear metamorphism, induced by highpressure conditions, and (3) regional metamorphism, involving a combination of
both thermal and high-pressure influences over large areas deep underground.
Once the changes have occurred, metamorphic rock is classified according to its
texture, mineral composition, and ability to split along smooth planes. In
advanced metamorphism this splitting is referred to as foliation. Marked foliation
is most often associated with extensive regional metamorphic activity. Table 21.7
in the textbook shows the common types of metamorphic rock, together with
some of the identifying characteristics associated with each of these types.
Metamorphic Rocks
A. Metamorphic rocks are formed from preexisting rocks that have been
altered by heat and/or pressure.
B. Many metamorphic rocks display foliation, which is an arrangement of
flat or elongated mineral grains in parallel layers that give the rocks a
banded or layered appearance.
C. Examples of foliated metamorphic rocks are:
1. Slate (weakly metamorphosed shale)
2. Schist (severely metamorphosed shale or fine-grained igneous
rock)
3. Gneiss (severely metamorphosed rocks, except pure limestone
and pure quartz sandstone)
D. Examples of nonfoliated or weakly foliated metamorphic rocks are:
1. Marble (metamorphosed limestone)
2. Quartzite (metamorphosed sandstone)
Agents of Metamorphism
Changes occur to rocks because of the action of:

Heat

Pressure

Chemical fluids
Rocks adjust to become more stable under new, higher temperatures and
pressures.
1. Heat
There are several sources of heat for metamorphism.
1. Geothermal gradient
Temperature increases with depth at a rate of 20 - 30 degrees C
per km in the crust.
Ultimate source of the heat? Radioactive decay.
Increase of temperature and pressure with depth causes Regional
Metamorphism
Heat may come from large bodies of molten rock rising under a
wide geographic area.
2. Intrusions of hot magma can bake rocks as it intrudes them. Lava
flows can also bake rocks on the ground surface.
Lava or magma in contact with other rock causes Contact
Metamorphism.
2. Pressure
3. Chemical Fluids
In some metamorphic settings, new materials are introduced by the action
of hydrothermal solutions (hot water with dissolved ions). Many
metallic ore deposits form in this way.
Hydrothermal (hot water) solutions associated with magma bodies can
introduce new ions and remove existing ions (ion exchange)
How do rocks change?
Metamorphism causes changes in:
1. Texture
2. Mineralogy
The original rock (usually sedimentary or igneous) which is changed by
metamorphism is referred to as the parent rock.
Texture
The processes of compaction and recrystallization change the texture of
rocks during metamorphism.
1. Compaction
o
The grains move closer together.
o
The rock becomes more dense.
o
Porosity is reduced.
o
Example: clay to shale to slate
2. Recrystallization
Growth of new crystals. No changes in overall chemistry. New crystals
grow from the minerals already present.
A preferred orientation of minerals commonly develops under applied
pressure. Platy or sheet-like minerals such as muscovite and biotite
become oriented perpendicular to the direction of force. This preferred
orientation is called foliation.
Metamorphic Textures

Foliation is a broad term referring to the alignment of sheet-like minerals
(such as the micas, muscovite and biotite).
Examples of foliated metamorphic rocks
(see pictures and more detailed information below):
1. Slate
2. Phyllite
3. Schist
4. Gneiss

Non-foliated or granular metamorphic rocks are those which are
composed of equidimensional grains (such as quartz or calcite). The
grains form a mosaic.
Examples of non-foliated metamorphic rocks
(see pictures and more detailed information below):
1. Quartzite derived from the parent rock, quartz sandstone
2. Marble derived from the parent rock, limestone.
Note: Not all quartzites and marbles are pure. Some contain
impurities that were originally mud interlayered with (or mixed
with) the original quartz sand or lime mud. These clay impurities
metamorphose to form layers of micas or other minerals, which
may give marble a banded, gneissic appearance, or which may give
a slight foliation to some quartzites.
The foliated metamorphic rocks
As shale is subjected to increasing grade of metamorphism (increasing
temperatures and pressures), it undergoes successive changes in texture
associated with an increase in the size of the mica grains.
1. Slate - very fine grained rock. Resembles shale. Has slaty cleavage which
may be at an angle to the original bedding. Relict bedding may be seen
on cleavage planes. Often dark gray in color. "Rings" when you strike it.
(Unlike shale, which makes a dull sound. Temperature about 200 degrees
C; Depth of burial about 10 km. Shale is the parent rock of slate.
Slate
2. Schist - metamorphic rock containing abundant obvious micas, several
millimeters across. Shale is the parent rock of schist.
Muscovite Schist
3. Gneiss - (pronounced "nice") - a banded or striped rock with alternating
layers of dark and light minerals. The dark layers commonly contain
biotite, and the light layers commonly contain quartz and feldspar. Shale
is the usual parent rock of gneiss. Granite is Shale is the parent rock of
the parent rock of some gneisses.
Gneiss
The non-foliated metamorphic rocks
1. Marble - fizzes in acid because its dominant minerals is calcite (or
dolomite). The parent rock is limestone.
Marble
2. Quartzite - interlocking grains of quartz. Scratches glass. The rock
fractures through the grains (rather than between the grains as it does in
sandstone). The parent rock is quartz sandstone.
Quartzite
Mineral changes in metamorphic rocks
1. Recrystallization - rearrangement of crystal structure of existing
minerals. Commonly many small crystals merge to form larger crystals,
such as the clay in shale becoming micas in slate, phyllite, and schist.
Also, fine-grained calcite in limestone recrystallizes to the coarse-grained
calcite mosaic in marble.
2. Formation of new minerals - there are a number of metamorphic
minerals which form during metamorphism and are found exclusively (or
almost exclusively) in metamorphic rocks:
o
Garnet - dark red dodecahedrons (12 sides)
o
Talc - white or pale green and soft.
o
Graphite - metamorphosed carbon
o
Micas - muscovite (silvery), biotite (dark brown), phlogopite (light
brown)
Principle of Uniform Change
A. Seventeenth-century theologian Bishop Ussher, using stories in the Bible,
determined that earth was created at 9 o'clock in the morning of October
12, 4004 B.C.
B. The German geologist Abraham Gottlob Werner believed that all rocks
were sedimentary rocks and that the geologic history of the earth
consisted of three sudden precipitations from an ancient ocean that were
followed by the disappearance of the water.
C. The French biologist Georges Cuvier regarded the earth's history as a
succession of catastrophes. Cuvier based his ideas of earth's history on
the study of fossils, the remains or traces of organisms preserved in
rocks.
D. The Scottish geologist James Hutton proposed that earth's history could
be understood in terms of processes under way in the present-day world.
E. The English geologist Charles Lyell (1797-1875) modified and expanded
Hutton's ideas and proposed the principle of uniform change, which
stated that geologic processes in the past were the same as those in the
present.
Rock Formations
A. It is often possible to reconstruct past geologic events in terms of
processes still at work reshaping the earth's surface.
B. The science of geology is faced with two fundamental problems:
1. To arrange in order the events recorded in the rocks of a single
outcrop or small region
2. To correlate events in various regions of the world to give a
connected history of the earth
C. Basic principles of historic geology include:
1. In a sequence of sedimentary rocks, the lowest bed is the oldest
and the highest bed is the youngest.
2. Sedimentary beds were originally deposited in more or less
horizontal layers.
3. Tectonic movement took place after the deposition of the youngest
bed affected.
4. An igneous rock is younger than the youngest bed it intrudes.
D. An unconformity is a buried surface of erosion and involves at least four
geologic events:
1. The deposition of the oldest strata
2. Tectonic movement that raises and perhaps tilts the existing strata
3. Erosion of the elevated strata to produce an irregular surface
4. A new period of deposition that buries the eroded surface
The Rock Cycle
A. Rocks can change from one kind to another in a variety of ways.
The rock cycle is a never-ending process.
Here is another diagram of the rock cycle. How is it like the one above? How is it
different?
1. Magma cools and crystallizes to form igneous rock.
2. Igneous rock undergoes weathering (or breakdown) to form sediment.
The sediment is transported and deposited somewhere (such as at
the beach or in a delta, or in the deep sea).
3. The deposited sediment undergoes lithification (the processes that turn
it into a rock). These include cementation and compaction.
4. As the sedimentary rock is buried under more and more sediment, the
heat and pressure of burial cause metamorphism to occur. This
transforms the sedimentary rock into a metamorphic rock.
5. As the metamorphic rock is buried more deeply (or as it is squeezed by
plate tectonic pressures), temperatures and pressures continue to
rise. If the temperature becomes hot enough, the metamorphic rock
undergoes melting. The molten rock is called magma. This completes
the cycle.
Now if you look at the processes going on in the middle of the diagram, note
that:
1. Any rock type can undergo weathering (breakdown) to form
sediment, followed by transportation and deposition of the sediment. Both
metamorphic and sedimentary rocks can undergo weathering.
2. Igneous rocks can undergo metamorphism (as a result of heat and
pressure) to form metamorphic rocks.