Physical Geology Lecture - FacultyWeb Support Center

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Physical Geology Lecture
GEOL 1303
Study Guide for Exam I
I. Define Geology
“Geology” means: Geos = Greek for “Earth”, and Logos = Greek for “the study of”
Geology is considered an “Eclectic” science, drawing upon information from:
 Chemistry
 Physics
 Astronomy
 Biology
 Mathematics
 Ecology
 Etc.
II. Two Main Subdivisions of Geology:
1. Historical Geology (usually 2nd semester) - centered around
Earth’s History
Stratigraphy – the study of the strata or layers of the earth
Sedimentology – study of deposition of eroded earth materials
Paleontology – study of ancient life through the interpretation of “Fossils” (remains or
indications of past life that must be at least 10,000 years old)
Geochronology – the science of dating the earth materials and events throughout
earth’s history
Geo-ecology – interpreting the past ecosystems of the earth: climate, sea level, plant
and animal life, etc.
Tectonics & Mountain Building Processes – the study of the movements of the
earth’s crustal plates, and how these changes affect surface processes and Life.
Historical Geology is concerned with explaining the “history” of the earth in
aspects usually concerning the formation of continents, oceans, etc. and how
those processes effected and still effect Life on the earth.
2. Physical Geology – centered around the Chemical and
Physical aspects of the earth
Geochemistry – the chemical makeup of magma, lava, minerals, rocks, etc.
Mineralogy – the study of the chemical makeup and occurrence of minerals
Petrology – the study of the formation of rocks (which are comprised of minerals)
Vulcanology - the study of volcanics
Seismology – the study of seismic (earthquake) waves
Seismic Tomography – the study of the interior of the earth indirectly by studying the
behavior of seismic waves
Tectonics – the study of the formation of the continental plates and the mechanics of
their movements
Oceanography – the study of the chemical and physical aspects of the earth’s oceans
Glaciology – the study of the cause and occurrence of glacial episodes
Weathering & Erosion –the disintegration or physical and chemical breakdown and
subsequent transportation of earth materials
Geomorphology – the study of the creations of landforms
Soil Sciences – the study of the formation of the various soil types of the world
Economic Resources – the study of the formation and usage of natural resources:
petroleum, natural gas, coal, stone materials, etc.
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III. The Role of the Geologist :
To understand and define:
 The structure and composition of the earth
 All facets of magma, lava, and volcanic activity
 Minerals and rock types
 Surface processes: rivers, streams, glaciers, etc.
 The earth’s past: both structure origin and life’s evolution with
Paleontological investigations (studying fossil remains)
 Geologic features of other planets
Also:
 To use information learned to find fossil fuels and ores
 To learn how to preserve the environment: erosion control,
pollution control
 Geologic information ties in greatly with advances in
technology
IV. The Development of “Scientific Thought”:
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Approximately 10,000 years ago, the development of agricultural
practices and beginnings of the domestication of livestock set the
stage for humans to become more “GREGARIOUS” (= living
together in one or two semi-permanent area)
This gave rise to the development of villages-towns-cities in which
people would share the workload, which increased the amount of
personal “LEISURE TIME”.
With this extra time came the development of aspects of what we
refer to as “CIVILIZATION”.
As increased technology developed in certain areas in the world,
populations grew, increasing the sharing of the workload,
increasing the leisure time, increasing the development of new
ideas and technologies, etc.
Ancient Grecian, Roman, and Egyptian cultures began to develop
rudimentary sciences/technologies, but advances especially in
mathematics sped up these changes.
Contributions of Ancient Greeks such as Socrates (470 BC – 399
BC), Plato (427 BC – 347 BC), & Aristotle (384 BC – 322 BC) laid
down the foundations of Philosophy, Mathematics, and
Metaphysics.
“Aristotelian Thought” centers on the metaphysical concept that
reality (the earth) is surrounded by “spheres of the heavenly
bodies” called “Celestial Spheres”. [This idea is thought to have
arisen from the impression that the night sky gives an observer
even today. It is the feeling of being inside an upside down bowl, or
“half of a sphere” that makes up the night sky.]
Since heavenly bodies were thought to be the only “true” reality,
and the earth’s reality experienced by Aristotle was only a shadow
of the “true Celestial Sphere Reality”, Aristotle proposed that there
was no need to investigate the earth per se as to its make-up since
it was only an illusion of sorts.
Circular motion was considered to be restricted to the Celestial
Spheres (due to the apparent “circular” motion of the night stars),
and therefore impossible to exist on the surface of the earth. Any
appearance of circular motion in everyday life was only an illusion
according to Aristotle.
The Celestial Spheres model of reality is one of the reasons that
the “Ancients” (Greeks, Romans, & Egyptians) never formulated
advanced studies of chemistry or earth sciences.
Since the “lay persons” considered Aristotle somewhat infallible,
this concept of Celestial Spheres was not challenged for almost
1200 years.
As people ventured out to various parts of the world seeking trade,
gold etc., certain new ideas arose to challenge Aristotelian thought.
During the so-called “Dark Ages”, Marco Polo (1254 AD – 1324
AD) in his journeys brought back gunpowder from the east, and
artillery (cannons and guns) soon developed in the west. As
trajectory studies of cannon balls and other projectiles advanced, it
was proven that circular motion does indeed exist on the earth.
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This was the first of many proofs that Aristotle’s Celestial Sphere
concept was flawed.
As Aristotelian thought gave way to a sophisticated “trial and error”
learning process, modern scientific thought had its beginnings.
With the rise of great scientific visionaries such as Nicolaus
Copernicus (1473 – 1543), Johannes Kepler (1571 – 1630), and
Galileo Galilei (1564 – 1642), the basics of planetary motion were
developed that began to infringe on some of the statements made
in the Bible. This caused the Catholic Church at the time to try to
sequester scientific investigation.
The “Age of Enlightenment” or “Aufklarung” around the
beginning of the 17th century ushered in a rapid growth in the arts,
sciences, and mathematics. Sir Isaac Newton (1642 – 1726)
developed the Calculus that allowed a greater investigative power
in mathematics.
There are countless others, but by the mid 18th century scientists
and researchers such as James Hutton, Charles Lyell, William
Smith, Gottlob Werner, Cuvier, Alfred Wallace, Gregor Mendel, and
Charles Darwin laid the foundations of modern geology.
The development of the modern “Scientific Method” arose from
the trials of these founding fathers of science.
V. The Scientific Method: “A way of looking at and describing
reality”
1. State the Problem - To solve any problem it must be clear as
to what actually needs to be solved.
2. Construction of a Hypothesis - By studying the
problem, an educated guess may be formed as to creating a model of
investigation.
3. Experimentation, Testing, and Data
Gathering – the experimental model is tested and records of the results
are kept and compared. The results then may be published and sent worldwide
for scrutiny by others.
4. Development of a Theory (Factual Level of
Science) If an experimental model tends to be correct after extensive
testing and review time and again; it may then be considered a Theory. The
Theory of Gravity, the Theory of Light Refraction, Atomic Structure Theory, etc.
are all considered to be scientific facts. A fact in science is only considered to be
true until it is disproved sometimes in the future. It makes no sense to say that
Evolution if not to be believed because that it is “only a theory”. It is “only a
theory” that gravity or light behave the way they do. To the Lay Person, theory
means a lack of knowledge. This is not the case in science. Theory in science is
considered “FACT”. Most religious dogmas, whether they are Judeo-Christian,
Buddhist, Hindu, etc., are based upon some form of religious writings: the Bible,
Torah, Koran, etc. Religious writings provide a “Fact Level” for describing reality
and it is accepted as true because of its Divine nature. Creationist scientists then
try to find data to support “religious facts” in the writings. This is tantamount to
working the scientific method backwards.
End of Introductory Materials
Universe Beginnings
I. The “Big Bang”
 Event occurring 10 – 15 BYA attributed with the “creation” of
the universe, including all matter, energy, 3-diminsional
space, and time.
 There is no “before” the Big Bang because there was no
“time” as we know it until the Big Bang.
 It starts as an explosion of energy from a single point that
expanded outward (and is still expanding outward according
to many physicists), creating 3-diminsional space into which
energy and subatomic particles were released.
 Gravity is a function of mass…the greater the mass, the
greater the gravitational attraction.
 300,000 years later, the universe was still expanding but was
cool enough to form atoms. As a few subatomic particles
collided and fused, a denser, more massive structure was
formed that had more gravity that attracted more particles,
increasing its mass and gravity…and so on…to the point that
the first atoms of the element Hydrogen was created. At
first, the universe was 100% Hydrogen. Today, by weight,
Hydrogen and Helium comprise 98% of the known universe,
the remaining 2% comprising the rest of the elements
 As more and more hydrogen atoms were created, the
hydrogen began to accrete forming larger and larger masses
of hydrogen, having larger and larger gravitational forces.
 As gravitational forces increased, hydrogen atoms began to
be forced (fused). 4 hydrogen atoms would fuse to form 2
helium atoms (atomic fusion) releasing a tremendous
amount of energy in the form of heat, light…the entire
electromagnetic spectrum. This is the birth of the first
stars. This fusion reaction is the core reaction of today’s
hydrogen bombs. In essence, stars are huge balls of
hydrogen that are constantly converting hydrogen into
helium and other elements.
 As stars “use up” or convert most of their hydrogen into
helium, the helium began to fuse forming nitrogen, carbon,
oxygen, etc…the 92 naturally occurring elements.
 If the star is like the size of our sun, a “main sequence star”
(run of the mill, common type) and begins to run out of
hydrogen (or starts to “die”), it collapses inward from its own
gravity and then begins to swell to a very large size. This
event is called a Nova. It then collapses under its own
mass and forms a small “Brown Dwarf” star, and fizzles out.
The entire process from start to finish of the life of a main
sequence star is about 10 billion years.
 Another type of star is the “Red Giant”, many times the size
of a “main sequence” star. It also fuses hydrogen, but at a
much faster rate – twice as fast or faster.
 When a Red Giant star dies, it collapses in on itself and then
violently explodes in an event known as a Super Nova. The
elements formed during the life of a Red Giant are then
thrown outward into space forming clouds of dust called
Nebulae.
 The particles of elements in the clouds of stardust then start
to accrete as in the Big Bang. The hydrogen can accrete
and form a Protostar that warms the clouds of dust. Later,
the other masses of elemental materials are pulled inward
toward the new star forming globs of hot, molten matter
called Planetesimals, the first stage of the birth of a planet.
II. The Birth of our Solar System
 Our sun, “Sol” formed from nebular stardust as mentioned
above around 5 billion years ago, and has enough hydrogen
left for another 5 billion years.
 As our sun was forming, other smaller clumps of matter
accreted forming the planetesimals that began to circle
around the sun. These would eventually become the 9
planets of our solar system: 1-Mercury, 2- Venus, 3- Earth,
4- Mars, 5- Jupiter, 6- Saturn, 7- Uranus, 8- Neptune, and
9- Pluto.
 The first 4 planets are called “Rocky” or “Terrestrial
Planets.”
 The next 4 planets are called “Gas Giants” or “Jovian
Planets”
 The 9th planet Pluto is an anomaly. It is the smallest of all
planets and may have been trapped by our solar system at a
later time.
III.
The Effects of Solar Winds on our Solar System
 Solar Wind is the accumulative force of all of the
electromagnetic energy (light, magnetism, radiation)
released from the sun (or any other star)
 It is thought that all of the planets started out possessing
thick atmospheres of hydrogen, helium, methane,
ammonia, CO2, and water vapor.
 Solar winds are thought to have blown away the volatile
thick atmospheres of the 4 inner planets leaving behind
their rocky cores (Mercury, Venus, Earth, & Mars).
 It is thought that the Jovian Gas Giants are far enough
away not to have been effected as much by solar winds, as
they still possess their volatile atmospheres. It is also
thought that at the center of the gas giants is a rocky core
(Jupiter, Saturn, Uranus, & Neptune).
 The actions of solar winds is the reason that the solar
system of today has 4 Inner Terrestrial (Rocky) Planets,
and 4 Outer Jovian Planets, with the 9th planet being the
anomalous Pluto.
Structure of the Modern Earth
I. The Earth consists of three concentric layers:
1. the core
2. the mantle
3. the crust
They are formed as a result of:
 density differences between the layers
 variations in composition
 differences in temperature and pressure
II. The Characteristics of the Core:
 The Core is thought to be composed of iron with some
nickel.
 It is spherical in shape with its outer surface lying 2900 km
below the outer surface of the earth.
 The total diameter of the core is 3470 km.
 It has an average density of about 10 to 13 grams/cm3 and
comprises 16% of the earth’s volume.
 Seismic Tomography data (studying the earth’s interior
indirectly by studying the behavior of earthquake waves)
indicate that the core has a small Solid Inner Region (1220
km in diameter) surrounded by an apparently Liquid Outer
Region (2250 km thick).
III. The Characteristics of the Mantle:
 The Mantle surrounds the core and comprises about 83%of
the earth’s volume.
 It is less dense than the core with an average density of
approximately 3.3 – 5.7 grams/cm3.
 It is composed largely of Peridotite, a dark, dense, igneous
rock containing high amounts of iron and magnesium.
 The Mantle can be divided into three regions:
1. The Lower Mantle – This is solid and comprises most
of the volume of the earth’s interior.
2. The Upper Mantle – This consists of the
Asthenosphere and the overlying solid mantle rocks
up to the base of the crust. The asthenosphere
surrounds the lower mantle and has the same peridotite
composition. It behaves plastically and slowly flows.
Partial melting within the asthenosphere generates
Magma, molten rock material, some of which rises to
the surface because it is less dense than the material
from which it was derived.
3. The Lithosphere – This is the solid portion of the upper
mantle and the overlying crust. The lithosphere is
broken into numerous pieces called Plates that move
over the asthenosphere as the result of underlying
Convection Cells (or Mantle Plumes generated from
heat).
IV. The Characteristics of the Crust:
 The Crust is the outermost layer of the earth. It consists of
two types of rock materials:
1. Continental Crust – (20 – 90 km thick) this material
comprises most of the continental plates. It has a
density of 2.7 grams/cm3 and is rich in silica and
aluminum. This type of rock material is referred to as
being “Sialic” or “Felsic”.
2. Oceanic Crust – (5 – 10 km thick) has a density of 3.0
grams/cm3 and is largely comprised of the igneous rock
Basalt, which is rich in iron and magnesium. This
type of rock material as referred to as being “Mafic” or
“Basaltic”.
V. The Refinement of the Earth’s Crust
The early outer crust of the earth was a mixture of sialic and
mafic material. The following processes occurred to
separate the sialic materials from the mafic. This allowed for
the formation of the continents that are mostly sialic (less
dense granitic materials) in composition, from the denser,
more mafic materials that today compose the oceanic crust.
A. Separation of Mafic Materials from Sialic
1. Partial Melting – This is the process whereby hot
mantle plumes rising up from the upper mantle heats
(“partially melts”) the overlying mixture of mafics and
sialics. This causes the denser mafic materials to
separate downward, leaving the less dense sialic
materials above…. Think of the early crust as being a
block of wax (representing the less dense sialic
material) with marbles (representing the denser mafic
material) suspended in it. If this mixture of wax and
marble is placed into a pan and heated up, the marbles
(mafics) will sink to the bottom, leaving the wax (sialics)
on top.
2. Fractional Crystallization – Mafic materials, being
high in iron and magnesium, will crystallize at a higher
temperature than sialic material, that is high in silica
and aluminum. If the entire mixture of mafic and sialic
material is heated to the point of melting and then
allowed to cool, the mafic minerals will crystallize first,
and being denser than the sialic material, will separate
downwards in the melt from the still molten sialic
materials on top…. Think of this as being similar to
placing a cup of lead B-B’s (representing the mafic
materials) and a cup of plastic B-B’s (representing the
sialic materials) randomly in a vessel. Lead has a
melting temperature of around 8000 F and plastic’s
melting temperature, let’s say, is about 2000 – 3000F
dependant upon the type of plastic. If the vessel is
heated, as the temperature reaches the 3000F mark,
the plastic B-B’s would become liquid, but the lead BB’s would remain solid. As the temperature surpasses
8000F, all of the B-B’s would be molten. Now allow the
vessel to cool. As the temperature drops below 8000F,
the lead will begin to solidify (or crystallize first) but
the plastic would remain molten. Since lead has a
greater density, it would move downward in the vessel.
As the temperature drops below 2000F, the plastic
would begin to solidify on top of the lead, completing
the separation.
B. Formation of Continental (Sialic) Plates:
Continental Accretion
As more crustal movement occurred, as the less dense sialic
material was pushed against the denser mafic materials, the
denser mafic material would become subducted or pushed
downwards, underneath the sialic materials, thereby melting as it
was subducted, the hot magma rising upwards through the sialic
materials forming island arcs (“clumps” of sialic material). The
formation of the island arcs perpetuated the refining processes of
partial melting and fractional crystallization.
As the less dense sialic “clumps” formed on the earth’s surface,
spreading centers (divergent “cracks” in the earth’s surface)
pushed the sialic materials together forming larger masses of
sialic “chunks” in a process known as Continental Accretion.
This caused a fusion of the early sialic materials into Sialic or
Granitic Continental Plates. This also accounts for the
composition of the continental plates as being High Grade
Metamorphic Terranes (metamorphic rocks are formed under
intense heat and pressure, the conditions during continental
accretion).
Other Aspects of the Earth
VI. The Atmosphere
 Prior to 4.5 BYA – The atmosphere consisted of hydrogen,
methane, ammonia, hydrogen sulfide, nitrogen, argon, and
water vapor.
 4.5 BYA to 3.0 BYA – The atmosphere consisted of
nitrogen, argon, water vapor, CO2, and sulfur dioxide.
 3.0 BYA to Today - The atmosphere consisted
approximately of 78% nitrogen, 20% oxygen, with the
remaining 1-2% argon, water vapor, CO2, and minor
gasses.
How is it known that the early atmospheres were
composed as mentioned above? Where did the other
gasses go? Where did oxygen come from?
Evidences of the early atmospheres:
1. It is thought that all planets had at their formation
atmospheres similar to the Jovian planets of today.
Because of solar winds, the volatile gasses (hydrogen,
methane, ammonia, hydrogen sulfide, sulfur dioxide, etc.)
were blown off of the inner terrestrial planets, leaving the
rocky core. By studying the composition of the
atmospheres of the Jovian planets today, geologists can
derive the conditions of earth’s early atmosphere.
2. Banded Iron Formations – In certain areas there have
been igneous activity resulting in formations of layers of
iron interspersed between layers of other materials (i.e.
silica) that have formed at the earth’s surface. Those
banded iron formations that date before 3.0 BYA are
composed of elemental, un-oxidized iron, indicating that
they formed in an atmosphere devoid of free oxygen.
Those banded iron formations that date younger than 3.0
BYA consist of iron that is oxidized throughout. This
indicates that these younger iron layers formed in an
atmosphere rich enough in oxygen to cause the complete
oxidization of the iron. Hence, prior to 3.0 BYA there was
not much free oxygen in the earth’s atmosphere, yet after
3.0 these was.
3. Oxygen is not given off in substantial amounts by volcanic
activity today…so where did it come from? One source is
photochemical dissociation. This occurs whenever
oxygen-bearing chemical compounds in the upper
atmosphere are subjected to cosmic radiation
(background radiation from the Big Bang, solar radiation,
etc.) and break apart releasing their oxygen atoms. This
accounts for some of the “free” oxygen in the atmosphere,
but not all. The other great source of oxygen is
photosynthesis. This is the process whereby plants or
plant-like organisms take in CO2 and H2O, and in the
presence of sunlight convert these compounds into sugars
thereby releasing free O2 into the atmosphere.
Photosynthesis is a series of chemical reactions that convert
sunlight energy into chemical energy. The processes occur in the
chloroplasts of plants and algae. The components for raw
photosynthesis are water, CO2 , and light energy. The formula for
photosynthesis is:
light energy & chlorophyll
6CO2 + 12H2O ----------------------- C6H12O6 + 6O2 +
6H2O
The oldest known photosynthetic organisms, and the oldest
known fossils, are Stromatolites. These are inter-tidal bluegreen algae with the oldest to date is 3.6 BYA
VII. The Hydrosphere and the Hydrologic Cycle
Where did the free water on earth come from?
As magma is formed within the earth, chemical compounds begin
to bond eventually forming various compounds and minerals.
Many of these compounds contain water – H2O as part of their
makeup. Sometimes a crystal lattice (a tinker-toy like structure
of bonding atoms) contains enough space within its 3-diminsional
structure for mater molecules to “fit”. As magma rises to the
earth’s surface and is released on the surface as lava, the water
escapes as steam in a process known as out gassing. As the
steam cools in the atmosphere, water precipitates into clouds of
water vapor. As these clouds cool, they loose their water as rain
or other forms of water precipitation upon the surface of the earth.
This volcanic out gassing is the source of most of the free water
that comprises the oceans, lakes, rivers, etc. Over the time of
earth’s existence, volcanism has out gassed enough water to fill
the low-lying areas forming the ocean basins.
Rainwater is naturally acidic, having a pH of about 5.5 to 6.5. As
it hits the rocks and minerals on the surface of the earth, it is a
major source of weathering and erosion of earth materials. As it
runs down to the low-lying areas, it accumulates. As the sun
evaporates the water, it rises as water vapor, forms clouds and
this Hydrologic Cycle continues again and again.
VIII. The Biosphere: The Organization of Life on Earth
The biosphere is the term for all of the living aspects of the earth.
All life as we know it is composed of atoms of various elements.
These atoms bond in various ways to form molecules. Certain
molecules make up cells, or the basic unit of life. This is called
so because the cell exhibits all of the aspects that we consider to
be living (atoms and molecules are not considered to be alive).
Certain cells work together to form tissues, and various tissues
together form organs. Organs work in conjunction to form
systems (circulatory, respiratory, muscular, etc.), and all of
the systems together form the organism, the entity. All
organisms of the same species in a geographic area are called a
biologic community. All of the biologic communities in a
geographic area are called a biologic population. All of the
populations in an area interact with the abiotic (non-living aspects
– soil, air, sunlight, etc.) to form an ecosystem. All of the
ecosystems on earth interact to collectively form the ecosphere
or biosphere.
When an organism dies, certain bacteria and fungi (ecological
decomposers) break down the organism back into molecules
and atoms that are put back into the ecosystem for other
organisms to use. The calcium in your bones came from
foodstuffs (i.e. milk) consumed during your life. The milk
containing the calcium came from the cow…the cow got the
calcium from the grass consumed…the grass got the calcium
from absorbing it from the soil…the soil formed from the
weathering and erosion of calcium containing rocks and minerals,
that came from the earth in the form of cooling magma or lava.
Or the calcium in the soil could have come from the
decomposition of the skeleton of some previous living
organism…You may have calcium in your bones that once was
incorporated into the skeleton of a dinosaur!!!
Geochemistry and the
Formation of Minerals
Mineral – a naturally occurring crystalline solid substance with a
definite (specific) chemical composition (or slight range of
compositions), and a crystalline structure that reflects its atomic or
molecular arrangement.
Mineraloid – a substance that almost fits the definition of a
mineral, and many times considered a mineral, but has a
chemical composition that is a little too variable. (i.e.):
 Bauxite – (Hydrous aluminum oxide) is a mineraloid
because of its variability of water content in its chemical
composition.
 Some oxides of iron such as Limonite having a variable
composition
Snowflakes (frozen water) are technically true minerals.
Rocks are solid materials comprised of minerals, so, technically,
ice is a rock.
Window Glass is not a mineral because it is first of all not
naturally occurring, and secondly, it is amorphous, not having a
crystalline structure at all. It is really a very viscous quasi-liquid.
Minerals are comprised of elements bonded together.
Element – a substance composed of all of the same atoms; it
cannot be changed into another element by ordinary chemical
means. There are 92 naturally occurring elements in nature,
each with their own unique physical properties.
Atom – the smallest fundamental unit of an element that still
retains the unique properties of that element.
Phases of Matter:
Solid – a rigid substance that retains its shape unless distorted by
a force. i.e. – minerals, rocks, iron, wood, ice
Liquid – flows easily and conforms to the shape of the containing
vessel, has a well-defined upper surface and a greater density
than a gas. i.e. – water, lava, wine, blood, gasoline
Gas – flows easily and expands to fill all parts of a containing
vessel; lacks a well defined upper surface, and is compressible.
i.e. – helium, nitrogen, air, water vapor
Plasma – matter composed of charged ions. i.e. – the matter
comprising solar flares.
The Neils Bohr Model of the Atom
The physicist Neils Bohr formulated the following model of the
atom in the early 20th century. It consists of a central nucleus
composed of proton(s) (positively charged sub atomic particles),
neutrons (neutrally or non-charged particles that add to the mass
of the atom), surrounded by electrons or negatively charged
particles that encircle the nucleus in various energy fields, or
levels.
The Periodic Table of Elements
Elements are arranged on the Periodic Table of Elements
according to their atomic make up and physical properties. They
are numbered according to their atomic number and grouped into
“families” according to their reactivity.
Atomic Number – (of an atom) – This is the number of protons (positively charged
particles) in the nucleus of an atom. Hydrogen has an atomic number of “1” because it
has only one proton in its nucleus. Helium has an atomic number of “2” because it has
two protons in its nucleus….Uranium has an atomic number of 92 because it has 92
protons in its nucleus. The elements with atomic numbers past 92 are elements that
are only made under special circumstances usually under laboratory conditions. Some
of the elements past atomic number 92 may exist in nature but are extremely rare.
Mass – pertains to the quantity of matter that an object contains.
Weight - a function of gravity in formulating and measuring the
attraction of an object towards another…Weight can vary due
from the gravitational attraction of one mass to another. Mass
does not vary. I.e. – if you weigh 120 lbs on earth, you would
weigh 1/6th of that on the moon (20 lbs) because the moon has
less mass than the earth, therefore less of a gravitational
attraction, but your mass would remain the same.
Atomic Weight – It has been determined that hydrogen, the
lightest of all elements, has a weight of 1.67 X 10-24 grams, or
0.000,000,000,000,000,000,000,001,67 grams. Since this
measurement is inconvenient, the relative weights of atoms is
used, rather than the actual weights. The relative weights of the
atoms of different elements are known as the ATOMIC
WEIGHTS and are proportional to the actual weights of the
atoms, when compared to the atomic weight of the common
element carbon-12 isotope, which is 12.011, on an arbitrary
scale. Hence, carbon atoms “weigh” about 12 times that of
hydrogen atoms. The atomic weight of oxygen atoms is
15.999, or about 12 times that of hydrogen, having an atomic
weight of 1.0079.
Atomic Mass Number – is the sum of the protons and neutrons
in the nucleus (the mass of the electrons is negligible to the mass
of an atom). The atomic mass number is primarily used in higher
chemical reactions and will not be utilized here.
Equilibrium and Entropy
One of the physical aspects of our reality is entropy. This is the
tendency of matter to “want” to be at its lowest energy level, or at
equilibrium (rest). This can also be described as the tendency
of water “wanting” to flow downhill, a stacked, ordered pack of
playing cards thrown into the middle of a room to “scatter
randomly”…things in our reality tend to “seek” their lowest energy
level. Atoms seek the same. They usually seek the lowest
energy level possible…closest to being at rest.
Atomic Structure and Bonding
Atomic Structure – The central core of an atom is called the
nucleus. It is composed of:
 protons (positively charged particles)
 neutrons (electrically neutral particles) Common
hydrogen has no neutrons.
Surrounding the nucleus in various energy levels or shells are:
 electrons (negatively charged particles)
Except for hydrogen, which has only one proton and one electron,
all other atoms have only two electrons in the first or innermost
energy shell. The other shells contain various numbers of
electrons, but the outermost shell never has more than eight
electrons. It is the electrons in this outermost shell that are
usually involved in chemical bonding.
The Atomic Number of an Element – is the number of protons
in its nucleus. Hydrogen has an atomic number of 1, it has 1
proton in its nucleus…Helium has an atomic number of 2, and it
has 2 protons in its nucleus…and so on.
Naturally Occurring elements – There are 92 naturally occurring
elements in nature, and are numbered accordingly as to their
atomic number: hydrogen – 1, helium – 2, …uranium – 92. The
elements on the periodic table past 92 (93 – 109?) were
discovered under laboratory conditions and are not thought to
occur in abundance in nature.
Native Element – Some elements are found in nature in their
pure, elemental form such as gold, silver, sulfur, and others. In
geology, these are called Native Minerals.
Entropy – The randomness or amount of disorder in a system.
Things in our universe tend to want to be at rest, or at their lowest
equilibrium…water runs downhill, a stacked deck of playing cards
thrown into the air scatter…
Atoms, also being matter, “want” to be at their lowest energy
state. Since we live is a dynamic, changing universe, this
complete “rest” cannot always be the case. So think of the
actions of atoms as doing the “best they can do” to reach the
lowest equilibrium for the time being. If an atom has 2 protons
(each a positive charge) in its nucleus (this would be helium), it
would be “at rest” with 2 electrons in its shells, giving it an overall
charge of “0”. If it were to loose an electron (a negative charge)
its overall charge would be changes to +1. Conversely, if it
gained an extra third electron, its overall charge would be –1. An
atom that has lost or gained an electron is called an Ion. If the
ion is (+) it is called a Cation. If the ion is negative, it is called an
Anion.
Ion– This is an atom that has either lost or gained electrons,
changing its overall charge.
The periodic table has the known elements arranged in order as
to their atomic number, and to their types or to their reactivity.
Look at the periodic table in your text.
The elements in the upper right hand side of the periodic table,
centered around the element F (or fluorine) have atomic
configurations that tends to allow them to receive an extra outer
shell electron, causing them to have an overall negative charge
under certain circumstances. This tendency to become negative
is called electronegativity and the resulting ion is an anion.
The elements in the lower left and left side of the table centered
around Fr (or francium) have atomic configurations that allow
them to loose an outer electron causing them to become
positively charged. This tendency to become positive is called
electropositivity forming ions that are cations.
Atomic Bonding – the interaction of electrons around atoms can
result in two or more atoms joining together.
Ionic Bonds
Positively charged cations are attracted to negatively charged
anions because of the charge difference. Ionic Bonds are
formed by this attractiveness between the cation and the anion
(i.e. sodium chloride salt – the sodium is electropositive giving up
an electron, symbolized Na+1, and the chlorine is electronegative,
symbolized Cl-1, receiving the extra electron. Ionic bonds are
commonly between a metal cation and a nonmetal anion such
as NaCl – table salt (Halite), Fe2O3 – iron oxide (Hematite), etc.
Covalent Bonds
If certain nonmetallic atoms bond, such as Si (silicon) and O
(oxygen), they tend to have their outer electron shells overlap,
resulting an a sharing of electrons called a Covalent Bond in
the mineral quartz or SiO2. Covalent bonds are generally much
stronger bonds and contribute to the strength and overall
hardness of the mineral.
Metallic and Van der Waals Bonds
Metallic Bonding – results from an extreme type of electron
sharing. The electrons of the outermost electron shells of metals
such as gold, silver, and copper readily move about from one
atom to another. This electron mobility accounts for the fact that
metals have a “metallic luster”, they provide good thermal and
electrical conductivity, and are malleable or easily reshaped. (i.e.
Copper is a good conductor of electricity and is easily made into
wire.) Only a few minerals have metallic bonding.
Van der Waals Bonds – some electrically neutral atoms and
molecules do not have electrons available for ionic, covalent, or
metallic bonding. Nevertheless, they have a weak attractive force
between them when in proximity. This weak attractive force is the
Van der Waals or residual bond. The atoms of carbon in the
mineral graphite are covalently bonded to form sheets that are
attracted to each other by Van der Waals bonds. This accounts
for the softness of the mineral graphite and its ease of use in
pencil leads or lubricants.
Chemical compound - If two or more different elements bond
together (ionic or covalent), the resulting substance is called a
chemical compound.
The Formation of a Mineral
I.e. Halite – NaCl – sodium chloride
As the sodium and chlorine atoms begin to bond, they are
stacked to provide the smallest space possible. This threedimensional framework results is an overall neutral charge on the
crystal. In halite, the sodium atoms are bonded in all directions
with the chlorine atoms on all sides, with the chlorine atoms
surrounded by the sodium atoms. The smallest 3-dimensional
framework formed is called a unit cell of a crystal. As more and
more unit cells connect to each other, a framework called a
crystal lattice is formed. This continues to grow until there are
no more unit cells in solution or other chemical conditions change.
The resulting smooth planer surface formed at the terminus is
called a crystal face. The shape of the crystal and the crystal
habit (the usual crystal shape of a certain mineral) is set.
In addition to containing atoms of a single element, many times
minerals contain tightly bonded, charged groups of other
elements known as radicals. Even though the radical is
composed of different elements, it behaves as single units in a
mineral. A good example is the carbonate radical formed when
one carbon atom bonds with three oxygen atoms forming CO3-2,
which acts as an ion with a minus 2 charge. Other common
radicals include:
 the sulfate radical – SO4-2 (having a minus 2 charge)
 the hydroxyl radical – OH-1 (having a minus 1 charge)
 the silicate radical – SiO4-4 (having a minus 4 charge)
As the magma or lava is cooling and containing many minerals
starting to form, the crystals do not have a chance to form wellformed crystals because of the proximity of the other minerals
crowded together. This results in the mosaic look of igneous
rocks such as granite.
Interpreting Chemical Formulas
Some minerals have simple compositions such as NaCl, sodium
chloride, where there is a one-to-one ratio of sodium atoms to
chlorine atoms. Others have a more complex composition such
as orthoclase feldspar – KAlSi3O8. This means that orthoclase is
comprised of potassium, aluminum, 3-silica atoms (hence the
“Si3” subscript), and 8 – oxygen atoms. It is read as “potassium
aluminum silicate” with the “Si3O8” being the radical, acting as
an anion in itself.
The definition of a mineral states “minerals have a specific
chemical composition, or a slight range of compositions”.
Sometimes other atoms can be substituted in a formula such as
iron sometimes can inter-substitute for magnesium because of
their similar sizes and charges.
The mineral Olivine is (Mg,Fe)2SiO4 meaning that it may be found
as magnesium silicate (called Fosterite), iron silicate (called
Fayalite), or a combination of both.
Common Elements in the Earth’s Crust
(Listed in order of abundance)
Element
Oxygen
Silicon
Aluminum
Iron
Calcium
Sodium
Potassium
Magnesium
All others
Symbol
O
Si
Al
Fe
Ca
Na
K
Mg
% by weight
46.6
27.7
8.1
5.0
3.6
2.8
2.6
2.1
1.5
% by atoms
62.6
21.2
6.5
1.9
1.9
2.6
1.4
1.8
0.1
Oxygen and silicon together constitute more than 74% by weight
of the atoms of the earth’s crust, and nearly 84% of the atoms
available to form compounds.
Mineral Groups
Each mineral group contains members that share the same
type of negatively charged ion or radical.
Mineral group
Composition
Carbonates
(CO3)-2
Halides
Cl-1, F-1
Native Element
-------
Oxide
O-2
Silicate
(SiO4)-4
Sulfate
(SO4)-2
2H2O
Sulfide
S-2
Anion
Examples
Calcite
Dolomite
Halite
Fluorite
Gold
Silver
Sulfur
Diamond
Graphite
Hematite
Magnetite
Quartz
K-Spar
Olivine
Anhydrite
Selenite
CaCO3
CaMg(CO3)2
NaCl
CaF2
Au
Ag
S
C
C
Fe2O3
Fe3O4
SiO2
KAlSi3O8
(Mg,Fe)2SiO4
CaSO4
CaSO4 +
Galena
Pyrite
PbS
FeS2
The Silicate Minerals
 A combination of silicon and oxygen is known as “silica”.
Quartz is pure silica (SiO2) because it is entirely composed
of oxygen and silicon.
 Silicate minerals include about one-third of all known
minerals
 Approximately 95% of the earth’s crust is composed of
silicates.
 The basic building block of all silicates is the silica
tetrahedron consisting of one silicon atom and four oxygen
atoms. This forms a four-faced pyramidal structure called a
tetrahedron. The silicon atom has a positive charge of +4,
and each of the four oxygen atoms has a negative charge of
–2. Positive 4 plus (negative times 4) = negative 4…So, the
silica tetrahedron is written as:
(SiO4)-4
 Because of its negative charge, it does not exist in nature
as an isolated group. It combines with various positively
charged cations. Other silicate minerals have the addition
of one or more additional elements:
 orthoclase – KAlSi3O8
 olivine – (Mg,Fe)2SiO4
 mica – KAl2(AlSi3)O10(OH)2
Five Arrangements of Silica Tetrahedra
I. Isolated Tetrahedra
Olivine, (Mg,Fe)2SiO4, an ultramafic mineral of the earth’s mantle
with the negative charge of minus 4 of the radical (SiO4)-4 being
offset by the positive 2 charge of iron (Fe) and magnesium (Mg),
also positive 2 charge.
II. Single Chains
Silica tetrahedra may also be arranged in continuous chains
whereby each tetrahedron shares two of its oxygen atoms with
the adjacent tetrahedra. This results in a silicon to oxygen
ratio of 3:1 as seen in the pyroxene mineral Enstatite, MgSiO3.
The resulting net charge of each chain is -2, so to offset this, the
parallel chains are linked together by magnesium atoms, Mg+2.
III. Double Chains
Double chains of silica tetrahedra in which alternate tetrahedra, in
which two parallel rows, are cross-linked characterize the
amphibole group. This results in a silicon to oxygen ratio of
4:11. The resulting net charge of the double chain is a minus six
(-6) electrical charge. Mg+2, Fe+2, and Al+2 are usually the cations
that link the double chains together, creating a net charge of zero.
i.e. Hornblende(Ca,Na)2-3(Mg,Fe+2,Fe+3,Al)5(Al,Si)8O22(OH)2
IV. Sheet Silicates (Continuous Sheets)
In sheet structure silicates, three oxygens of each
tetrahedron are shared by adjacent tetrahedra. This results
in continuous “sheets”, with a silicon to oxygen ratio of 2:5.
The sheets also have a net negative electrical charge that is
offset by positive ions located between the sheets. The
micas, such as biotite and muscovite, and clay minerals are
examples of sheet silicates.
i.e. Muscovite Mica - KAl2(AlSi3)O10(OH)2
V. Three-Dimensional Network Silicates
These form whenever all four oxygens of the silica tetrahedron
are shared by adjacent tetrahedra. This sharing of oxygen atoms
results in a silicon to oxygen ratio of 1:2, which is electrically
neutral.
i.e. Quartz – SiO2
The Two broad groups of Silicate Minerals
1. Ferromagnesium Silicates are those silicate minerals rich in
iron (Fe) and magnesium (Mg). I.e. olivine, amphibole, pyroxene,
and biotite mica. These are commonly dark colored and more
dense (3 grams/cm3) than non-ferromagnesium silicates.
Common in MAFIC ROCKS.
2. Non-ferromagnesium silicates are those silicate minerals
lacking Fe and Mg. I.e. quartz, microcline, and muscovite mica.
These are commonly light colored and less dense (2.7
grams/cm3) than ferromagnesium silicates. Common in
SIALIC ROCKS.
The Two Feldspar Groups of Silicate minerals
1. Potassium-Rich – The potassium feldspars represented by
microcline and orthoclase (KAlSi3O8) are common in
igneous, metamorphic, and some sedimentary rocks. They
are typically light colored (pink, blue, light-green, or creamcolored). If they are in abundance in igneous rocks, this is
an indication that the original magma (or lava) was NOT rich
in abundant Fe and Mg, making the rock SIALIC or FELSIC.
2. Calcium/Sodium Rich – The plagioclase feldspars range
from calcium rich (CaAl2Si2O8), to sodium rich (NaAlSi3O8)
creating several varieties. They are typically white, cream
colored, to medium gray. They can be distinguished from the
potassium feldspars by parallel lines on the crystal faces called
striations. When in abundance in an igneous rock it is an
indication that the parent magma was intermediate to mafic in
origin.
The Carbonate Minerals
These minerals contain the negatively charged radical (CO3)-2.
Calcite (rhombohedral crystals) and aragonite (elongated
prismatic crystals) are both CaCO3, but aragonite usually changes
into calcite. These are the main component of the sedimentary
rock LIMESTONE. There are many carbonate minerals but we
will be concerned with only one other: dolomite (Ca,Mg) (CO3)2.
This forms the rock DOLOSTONE from the conversion of calcite
into dolomite by the addition of magnesium.
Other Mineral Groups
Oxides – an element is combined with oxygen. I.e. iron oxides –
hematite Fe2O3 or magnetite Fe3O4 form major iron deposits in
the world. Aluminum oxides (bauxite) form aluminum ores
Sulfides – a positively charged ion bonds with sulfur (S-2). I.e.
galena (PbS), iron pyrite (FeS2), sphalerite (ZnS), etc. Many of
these sulfides of metals are important ore minerals.
Sulfates – an element is combined with the anion (SO4)-2 forming
minerals such as gypsum CaSO4 +2H2O.
Halides – These minerals contain halogen (meaning “salt
formers”) elements such as chlorine (Cl-1), fluorine (F-1), etc.
Halite (NaCl) and fluorite (CaF2) are common halide minerals.
Properties of Minerals Used for Identification
Of the 3500 known minerals, no two minerals have the exact set
of identifying properties.
Crystal Habit – the typical form of the crystal of
a mineral that is reflective of its internal atomic
arrangement –
For example:
 Halite, Pyrite, Galena form cubic crystals
 Micas form “book” crystals of thin sheets
 Quartz forms hexagonal di-pyramidal prisms
 Fluorite forms octahedra
Cleavage – a property of minerals whereby they
split or break along closely spaced, smooth
planes. The number of these cleavage planes
can vary from one mineral type to another
mineral, but is consistent within an individual
mineral:
Quartz – No cleavage planes at all
Muscovite, biotite – 1 cleavage plane (a sheet silicate)
Feldspars, amphiboles – 2 cleavage planes
Calcite – 3 planes not at right angles
Halite, galena – 3 planes at 900 (“cubic cleavage”)
Fluorite – 4 cleavage planes
Hardness – a test for the relative hardness of one mineral
compared to another. This uses the Moh’s Hardness Scale:
10 – diamond
9 – corundum
8 – topaz
7 – quartz
6.5 steel nail
6 – feldspar
5.5 window glass
5 – apatite
4 – fluorite
3 – calcite
3.0 copper penny
2.5 – 3.0 fingernail
2 – gypsum
1 – talc
Luster – the description of how light is reflected from the surface
of the mineral:
Dull, earthy – i.e. ochre hematite
Glassy – i.e. quartz
Metallic – i.e. pyrite
Pearly – i.e. talc
Color – the visible color of the mineral to the eye. This is the
least reliable property due to the fact that just a small amount of
impurities can change the color of the mineral, but not the
chemical composition.
i.e. Varieties of Quartz – all being SiO2
rock quartz – clear
amethyst – purple
citrine – yellow
rose quartz - pink
Streak – the color of the powdered form of the mineral
accomplished by rubbing the mineral across a streak plate (an
unglazed ceramic tile). This is a more reliable indicator of the
mineral in question.
Specific Gravity – This is a ratio of the weight of a mineral to the
weight of its volume in water; a measure of density. A mineral
with a SG of 3.0 has an SG 3 times that of water. I.e. a cm3 of
galena (lead sulfide) has a higher specific gravity than a cm3 of
quartz.
Special Properties:
Magnetism – Some iron compounds are naturally magnetic such
as magnetite.
Reaction with Acid (effervescence) - Carbonate minerals react
(fizz) in the presence of acid releasing CO2
Phosphorescence under UV light – Under UV light, many
minerals phosphoresce or glow.
Optical Properties – Minerals such as calcite doubly refract light
passing through it.
Igneous Rocks and Intrusive Igneous Activity
Magma –
 a mobile, silicate melt formed in the upper mantle or
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lower crust, as much as 100 to 300 kilometers below
the surface.
It accumulates at depths in reservoirs called magma
chambers.
Magma chambers may be only a few km below the
surface at spreading centers and below the oceanic
crust or only a few tens of km below oceanic
subductions or continental plates.
Magma may slowly cool in place forming intrusive
igneous rocks.
Or, magma may breech the surface in the form of a
volcano forming extrusive igneous rocks.
Lava is the term for magma that has breeched the
earth’s surface.
Magma Types:
Ultramafic
 Comprises the upper mantle (asthenosphere)
 Very low in silica (45% or less) & low viscosity
 Does not breech the earth’s surface usually
 Only forms intrusive igneous rocks
 High in ultramafic minerals such as olivines
 Forms dark greenish-colored rocks
Basaltic or Mafic Magma
 Low viscosity; fluid-like and flows easily
 Low silica content (47 – 50% silica)
 Temperature range of 900 – 12000C
 High in mafic minerals: amphibole, pyroxene, olivine
 Forms dark colored rocks
Intermediate Magma
 A “mixture” of mafic and sialic magmas
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Medium viscosity
Silica Content of 50 – 59%
Temperature range of 800 – 10000C
May contain both mafic and sialic intermediate minerals
Forms medium, grayish-colored rocks
Felsic or Sialic Magma
 Higher viscosity, thicker, gooier
 High silica content (65 – 70% silica)
 Temperature range of less than 8000C
 High in sialic minerals: quartz, microcline, muscovite
 Forms light colored rocks
Bowen’s Reaction Series  A series of reactions based upon fractional
crystallization. Mafic minerals have a higher point
of crystallization and crystallize first, followed by
intermediates, followed by sialics, as the magma
cools.
 Discontinuous Branch – a succession of
ferromagnesian silicates crystallize as the temperature
of the magma drops: Olivine to pyroxene to amphibole
to biotite.
 Continuous Branch – Plagioclase feldspars with
increasing amounts of sodium crystallize.
 This leaves the higher silicate minerals to
crystallize last, at the lowest temperatures:
potassium feldspars, muscovite, and quartz.
Effects of Silica Content on Magma
 Silica tetrahedra in magma link together to form
polymers
 The more silica polymers, the thicker, gooier the
magma
 The thicker, gooier the magma, the more explosive a
volcano may be
 The more silica polymers, the more sialic the magma
creating lighter colored rocks
 The more sialic the magma, the lower the temperature
at which it crystallizes (less than 8000C)
 The reverse is true for more mafic magmas…
 The fewer the silica polymers, the thinner, less viscous
the magma and the more free-flowing the magma
 The less viscous the magma, the less explosive the
volcano
 The more mafic the magma is creates darker
ferromagnesian minerals and darker rocks
 The more mafic the magma, the higher the temperature
at which it crystallizes (800 – 10000C)
Effects of Pressure on Magma
 In the earth, pressure on rocks (or a magma body) from
the surrounding rocks (or overburden – the weight of
the rocks above the structure) keeps it from expanding
and prevents melting.
 A drop in pressure causes hot rocks to melt.
 An increase in pressure causes melting rocks to slow
their melting.
 A decrease in pressure can arise from the erosion or
removal of the overburden, causing the rocks to melt.
 An increase in pressure can arise from tectonic
activity or an increase in pressure from the magma
chamber.
 As magma is rising upwards through the rocks,
pressure is decreasing as it nears the surface,
preventing it from solidifying and thus possibly
forming a volcano.
Effects of Water Content on Magma
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Water lowers the melting point of magma
“Dry Magmas” have a water content of less than 10%
“Wet Magmas” have a water content of 10 – 15% water
Water at high temperatures is very volatile
At high temperatures water tends to escape as a gas
(superheated water vapor)
High pressure keeps water from escaping
Cracks in the overburden may allow water to escape
Sialic (Granitic) magmas usually solidify below the
surface intrusively
Mafic magmas being less viscous and many times
reach the surface as lava
The water content in most mafic magmas is very low: 12%
Pressure keeps water from expanding
Near “mafic” melts usually contain 1 – 2 % H2O,
causing it to remain molten and it easily reaches the
surface.
Igneous Rock Textures
The texture of an igneous rock refers to the size, shape and
arrangement of the constituent mineral grains and reflects the
rate of cooling.
IGNEOUS ROCK TEXTEURES:
I. Glassy Texture –
 “resembling man-made glass”
 having “concoidal fracture”
 possessing a “randomness” of crystal lattices
 Ions have no time to migrate to form crystals
 Indicates a rapid rate of extrusive cooling
 Example = Obsidian, Volcanic Glass
II. Aphanetic Texture –
 “a” meaning “without”; “phaneros” meaning “visible” –
overall meaning is that the rock has crystals, but they can
not be identified with the naked eye.
 Tiny crystals have formed, but require magnification to
identify
 Indicates relatively “quick” extrusive cooling but at a rate
slower than that required for a glassy texture
 Cooled at, or near the earth’s surface.
 Example = Rhyolite, Andesite, or Basalt
III. Phaneritic Texture –
 “Phaneros” meaning “vivible” – whereby there are large
crystals present that are easily identified with the naked eye.
 An individual large crystal is referred to as a Phenocryst.
 The presence of phenocrysts indicates a slow rate of
intrusive cooling occurring deep within the earth’s crust.
 Pegmatites are igneous rocks that cool extremely slow
resulting in “giant” phenocrysts.
 Examples = Granite, Diorite, Gabbro, or Peridotite
IV. Porphyritic Texture –
 This texture indicates that the igneous rock had two cooling
periods – the first one slow and the second one quicker.
 The larger phenocrysts form during the first cooling period
while the magma is at an intrusive depth in the crust. If it
were to continue cooling at this deprt, it would have formed a
phaneritic texture. Something occurred to move the still
molten magma (containing the first formed phenocrysts)
closer to the surface whereby the still molten material cools
at a faster rate.
 This gives the rock two distinct crystal sizes: the larger
phenocrysts that formed first are set in a finer “groundmass”
(matrix) of either porphyritic, aphanetic, or glassy textures.
 Rock texture may be porphyritic/aphanetic, meaning that
phenocrysts formed first and the still molten ground mass
rapidly cooled. Or, the rock may be porphyritic/phaneritic
meaning the the phenocrysts formed and the ground mass
cooled having a phaneritic texture.
 Examples: Basalt Porphyry, Granite Porphry
V. Pyroclastic Texture –
 Means “fire-broken” – formed from volcanic ejecta: the ash,
cinders, and “bombs” expelled from eruptions of volcanoes.
 All pyroclastic textured rocks are extrusive being produced
by explosive volcanic eruptions.
 Aside from any pre-formed crystals, pyroclastics are
generally categorized as to the particle size:
fine ash X < 0.06 mm
coarse ash - 0.06mm – 2.0mm
cinders 2.0mm – 64.0mm
“bombs” X > 64.0mm
 Being thrown into the air and later settling out on the ground,
they may cool before they settle out forming ”unwelded”
pyroclastics, or they may remain glowing hot as they settle
out forming “welded” pyroclastics.
 “Tuffaceous rocks” is the term for the rocks resulting from
the settling of pyroclastic particles, and may result in the
formation of “unwelded tuffs” or “welded tuffs”.
Igneous Intrusive Bodies – “Plutons”
Pluton – an intrusive igneous body that cools and crystallizes
deep within the earth’s crust.
The geometric shape of plutons may be:
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massive or irregular
tabular
cylindrical
mushroom shaped
Plutons are also described as to whether they are concordant or
disconcordant.
 Concordant pluton - has boundaries parallel to the to the
layering of the country rock (the surrounding rock)
 Disconcordant pluton – has boundaries that cut across
the layering of the country rock.
Pluton Types:
I. Dikes – discordant intrusive bodies usually emplaced in preexisting fractures cutting across the country rock as the magma
rises.
Characteristics:
 They are discordant cutting across the layering of the
country rock in zones of weakness, such as cracks.
 Most are 1 to 2 meters wide, but they can range from a
few centimeters to more than 100 meters thick.
 They form whenever magma is forced into pre-existing
fractures of the country rock, or when the fluid pressure
in the dike itself creates its own fractures.
 Many can form “wall-like” structures radiating outward
from some volcanoes like the spokes on a wheel.
II. Sills – concordant intrusive bodies that are sheet-like
emplaced between layers of the country rock.
Characteristics:
 Sills are concordant emplaced whenever fluid
pressure is so great that it lifts the overlying rocks, filling
in with magma in a horizontal manner.
 They are tabular or disk-like in shape with many usually
a meter or less thick. Some are much thicker, up to
300 meters or more (i.e. the Palisades of NY and New
Jersey)
 Most have intruded into sedimentary rock, but many are
also commonly found injected into piles of volcanic
rock.
 Sill inflation prior to a volcano erupting may account for
volcanoes swelling just before exploding.
III. Laccoliths – sill-like in that they are concordant, but with a
“mushroom shape”.
Characteristics:
 Laccoliths are concordant mushroom-shaped intrusive
bodies.
 They tend to have a flat floor with a domed up center.
 Like sills, they lift up the overlying strata of the country
rock, but usually being larger than sills, the overlying
strata bends to conform to the curved shape.
 They also are relatively shallow intrusions.
IV. Volcanic Pipes and Necks – discordant cylindrical conduits
of volcanoes.
Characteristics:
 A volcanic pipe is the term for the actual conduit of
magma upward from the magma chamber deep below.
 Through this structure magma rises to the surface.
 When a volcano ceases to erupt, surface processes
begin to erode the cone while the once molten volcanic
pipe solidifies.
 Whenever the solidified volcanic pipe is exposed by
erosion, it is termed a volcanic neck. (i.e. ”Shiprock”
in northwestern New Mexico)
V. Batholiths and Stocks – are the largest of all plutons.
Characteristics:
 These are very large intrusions created by repeated,
forceful injections and voluminous intrusions of
magma in the same area. Many times these
intrusions continue for millions upon millions of
years (i.e. the coastal batholiths of Peru took about 60
million years; and the Llano Uplift of Central Texas).
 To be called a batholith, the body must be greater
than 100 km2 of total surface area.
 A stock is similar in formation but has a surface area
less than 100 km2.
 Some stocks are simply parts of large plutons that
once exposed by erosion are batholiths (the (tip of the
iceberg”)
 Most are granite in composition, but some may be
diorite. (mostly sialic magmas, with some
intermediate magmas)
 Most are formed near continental margins during
episodes of mountain building or great uplift (during an
orogeny or tectonic activity).
 As the solutions at the tops of the intrusions penetrate
cracks in the overlying strata, concentrations of
minerals dissolved in the solutions may become
concentrated. (i.e. gold, copper, silver)
 Granitization – the process whereby the surrounding
country rock is transformed into granite in a severe
form of metamorphism. This may account for the
great amounts of granite formed in some batholiths
that shows a gradation from granite into some other
rock at the borders.
 Some batholiths show a direct igneous origin of its
granite since its borders are “sharp” at the transition
from granite to country rock.
 The presence of inclusions of country rock
especially at the top of batholiths indicates that it was
igneous in origin.
Volcanism
Volcanism – the process whereby lava and its contained gasses,
and pyroclastic materials are expelled upon the earth’s surface, or
into the earth’s atmosphere.
Volcano – a conical mountain formed around a vent where lave,
pyroclastic materials, and gasses have erupted. One purpose of
volcanoes is to help rid the interior of the earth of excess heat
buildup.
Volcanoes and Religion –
 Native Americans of the northwest tell of a titanic
battle between the volcano gods Skel and Llao
accounting for the huge volcanic eruptions that
occurred ca. 6600 BP.
 In Hawaii, Pele is the goddess residing in the crater of
Kilauea responsible for the eruptions and earthquakes
there.
 Ancient Greeks believed that the god Pluto or Vulcan
was responsible for eruptions there.
Active Volcanoes –
 There are approximately 550 active volcanoes on
earth today. (i.e. Mt. St. Helens, Mauna Loa, Kilauea,
etc.)
 At any one time there are about 12 volcanoes
erupting somewhere on earth.
Dormant Volcanoes –
 There are numerous volcanoes that have erupted in
the recent geologic past and probably will erupt again
in the future.
 Mt. Vesuvius in Italy, Mt Pinatubo in the Philippines
Extinct Volcanoes –
 These are volcanoes that have erupted in historic times
but show no sign of erupting again.
Volcanic Gasses –
 The gasses released from the magma as it moves
upward.
 In sialic magma, expansion if restricted due to the high
viscosity (high silica content) and gas pressure
increases greatly causing explosions releasing ash and
other pyroclastics.
 In mafic magmas, expansion occurs due to the low
viscosity (low silica content) allowing gasses to expand
and escape easily…creating a quieter eruption.
Composition of volcanic gasses –
 50 – 80% of all volcanic gasses are water vapor,
with lesser amounts of CO2, N2, and sulfur gasses –
sulfur dioxides and hydrogen sulfides.
 Very small amounts of CO, H, and Cl are released.
 The Blue Haze Famine – Iceland, 1783 – gasses
(probably sulfur dioxide) escaped from the Laki
Fissure, causing 75% of livestock to die. The gas
caused the overall temperature to drop causing crop
failures causing 25% of the human population to die.
 Cameroon, Africa, 1986 – A cloud of volcanic CO2
was emitted from under Lake Nyos (that sits atop a
volcano) killing by asphyxiation ALL ANIMAL LIFE for
miles around, including 1746 humans.
++++++++++++++++++++++++++++++++++++++++++++++++++
Lava Flows
I. Sialic (high viscosity flows) - tend to be thicker, “lobe-shaped
flows with distinct margins.
II. Mafic (low viscosity flows) – tend to be comparatively
thinner, fluid flows that are widespread.
III. Types of Lava Flows – (Hawaiian Terms)
1. Aa – a flow characterized by a surface consisting of
rough, angular, jagged blocks and fragments. This is due to a
higher silica content than:
2. Pahoehoe – a flow characterized by a smooth, ropy
surface, almost like taffy. This is due to relatively lower silica
content than Aa.
Speed of Lava Flows
 Not very quick…the fastest low viscosity in Hawaii
ever measured had a speed of 9.5 kilometers/hour.
 Faster speeds occur whenever the lava flow is
insulated on all sides forming a lava tube or conduit.
After the eruption and the lava tube drains, sometimes
the roof of the tube collapses forming a skylight into
the tube.
IV. Pressure Ridges and Spatter Cones  Pressure Ridges – As the surface of a flow begins to
solidify, pressure from the flow causes the surface to
buckle, forming ripples of sorts called pressure ridges.
 Spatter Cones – Gasses escaping from the flow hurl
globs of lava into the sir. These globs fall back to the
surface and stick together forming small, steep-sided
spatter cones. These may rise several meters above
the flow.
V. Columnar Jointing  Common in flows that are relatively “thick” – several
tens of meters to 100 meters thick.
 They form because of differential cooling of the flow –
the outer surfaces “freeze” while the inside is still
molten. This results in the vertical “splitting” of the
flow into roughly pentagonal prism-like columns –
hence “columnar jointing”.
 Examples include: Devil’s Postpile National
Monument, California, Devil’s Tower, Wyoming.
VI. Pillow Lava –
 Bulbous masses of basalt that resemble “pillows” result
from mafic magma cooling under water. This is
VERY common at the submarine spreading centers
such as at the MOR (mid-oceanic ridge)
 Anywhere pillow lava is found on the surface of the
earth indicates that there was mafic, basaltic lava that
cooled underwater.
VII. Pyroclastics  Meaning “fire-broken” (pyros = fire; clastic = broken) –
these are all of the volcanic ejecta or materials thrown
from the volcano during the eruption.
 These pyroclastics are categorized as to size:
fine ash –
X < 0.06mm
coarse ash
0.06mm – 2.0mm
cinders
2.0mm – 64.0mm
bombs
X > 64mm
 Sometimes ejecta in the size range of 2.0mm –
64.0mm is termed lapilli.
 Ash in the atmosphere is dangerous to the flight of
commercial jetliners. Since 1980 about 80 jetliners
have been damaged by flying into volcanic ash clouds.
It causes their engines to seize up. In 1989 over the
Redoubt Volcano Alaska, KLM Flight 867 developed
clogged engines from ash and fell 3 km before the
crew restarted the engines!
VIII. Distinguishing Flows from Sills in Cross-section  Chilled Margin – The solidified outer edge of a lava
flow or an intrusive body such as a sill.
 As a lava flow flows across the surface of the earth, the
first to solidify is the “margin that touches the relatively
cold ground. In a sill, the “chilled margin” encircles the
intrusion since it “froze” at all contacts with the country
rock.
 Gasses that are released in a flow tend to form
“bubbles called “vesicles”. In a flow, these vesicles
accumulate at the top of the solidification much like the
foam of a beer poured into a glass. Vesicles usually
are not as abundant in a sill since it cooled
intrusively.
 Altered Country rock – Upon coming into contact with
the surface of the ground, a flow usually alters the
country rock on the bottom surface only, whereas a sill
alters the country rock on all edges.
IX.
Flood Basalts –
 Occur as high volume, low viscosity, mafic flows over a
broad, flat area, many times resulting in “flood basalts’
up to 100 meters thick or greater.
 Columnar jointing is common.
 Lava “Plateaus” may be formed.
X. Anatomy of a Volcano  Supplying the volcano is a magma chamber deep
beneath the surface vent where lave begins to pour
forth..
 Magma reaches the volcano via a volcanic pipe or
conduit.
 The depression at the top of the volcano is the crater or
main vent.
 Along the side, there may be smaller conduits forming
side vents or side cones.
 Long slits may occur along the sides of the cone
forming fissures or fissure eruptions.
XI. Types of Volcanoes 1. Shield Volcano Characteristics:
 Usually mafic resulting in forming basaltic rocks
 Low viscosity of the magma due to low silica content
 Low slope angle of the cone – usually 6 - 120
 Many fissure eruptions, lava tubes, conduits, etc.
 Typical of the Hawaiian Islands or MOR volcanoes.
2. Cinder Cone Characteristics:
 Quickly formed (sometimes overnight)
 Symmetrical cone with relatively steep sides
 Usually less than 300m high
 Composed entirely of pyroclastic materials (i.e.
cinders and ash)
 Generally mafic to intermediate in composition
 Erodes away easily and quickly
3. Stratovolcano or Composite Cone Characteristics:
 Formed by an alternating series of lava flows followed
by pyroclastic flows.
 Usually intermediate to sialic magmas.
 Very explosive due to their high silica content and
viscosity.
 I.e. Mt. St. Helens

 Many times after going extinct, the cone erodes away
leaving the volcanic neck or volcanic pipe exposed.
This may contain the rock Kimberlite associated with
the formation of diamonds and other gems.
XII. Violent Magmas – Ash Flow Tuffs and Calderas
Characteristics of Violent Magmas:
 “Dry”, granitic magmas
 less than 10% H20
 Will not solidify before reaching the surface
 Volatiles separate from magma and form a frothy mixture of
super hot gasses and magma
 Ensuing eruption is extremely violent
 “Nuee ardente” super hot gas and ash flow clouds are
produced
 Many pyroclastics are produced – pumice, tuffaceous rocks,
etc.
 Calderas are formed. This occurs after the eruption,
whereby the top of the volcano falls back into itself creating a
circular depression. These create very good mining districts
for precious metals. I.e. Creede, Colorado.
XIII. Hot Spots Characteristics:
 Bodies of magma that have risen near the surface creating a
localized zone of melting below the lithosphere.
 This results in surface eruptions over long periods of time.
 I.e. Hawaiian Islands, Yellowstone National Park
The Hawaiian Island chain is composed of islands dating from:
 Hawaii (The Big Island) – 0.7 MYA to today
 Maui – 0.8 – 1.3 MYA
 Molokai – 1.3 – 1.8 MYA
 Oahu – 2.3 – 3.3 MYA
 Kauai – 3.8 – 5.6 MYA
These islands are also listed as to size: Hawaii the largest, and
Kauai the smallest. Hawaii is over the hot spot and is active. The
other islands have either dormant or extinct volcanoes. The
Pacific Plate has been slowly moving to the northwest where 5.6
MYA Kauai was over the hotspot and was active while the other
islands had not even formed yet.
There is a new volcanic seamount (an underwater volcanic
mountain) called Loihi is forming and is 940 meters below the
surface. It will someday take the place over the hotspot and
become the largest island, while Hawaii erodes smaller as the
other islands have done.
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