Historical Geology
GEOL 1304
Exam I
I. Introductory/Foundational Material
“Geology” – from Greek “Geos” or “Gaia”
= “mother earth”, & “Logos” = “the
study of”
A. Define GEOLOGY…the study of the planet Earth including:
 The materials of which it is made
 The processes that act on these materials
 The products formed
 The history of internal & surface features
 The history of the planet and how it formed
 The origin of life forms & how they change as the environment changes
Geology is considered an “Eclectic” science, drawing upon information from:
 Chemistry
 Physics
 Astronomy
 Biology
 Mathematics
 Ecology
 Etc.
B. Major Branches of Geology: Physical & Historical
1. Physical Geology – study of the physical and chemical aspects of the earth:
 Magma & lava chemistry & make-up
 Mineralogy - How minerals form
 Petrology - How rocks form
 Geomorphology - Formation of surface features: mountains, valleys, etc.
 Weathering and erosion of exposed rocks
 Vulcanology - Volcanoes
 Seismology- Earthquakes
 Glaciers
 Sedimentology - Deposition of sediments
 Oceanography- the chemical, physical, & biotic factors of the seas
 Etc.
2. Historical Geology – the study of the historical development of the earth and the
evolution of life forms:
 Stratigraphy – science of rock strata (layers) aspects, interpretation, and
mode of origin
 Paleontology – “Palaios” means “ancient”: the study of ancient life
through the interpretation of fossils
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Paleoecology/Paleoclimatology – the study of how environments and
climates have changed over time
Geochronology – the science of dating rocks and events in the earth’s
past
Tectonics & Mountain Building Processes – the study of the
movements of the earth’s crustal plates, and how these changes affect
surface processes and Life.
C. 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
D. The Development of “Scientific Thought”:
 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 semipermanent 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
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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. 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.
D. 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.
Founders of Historical Geology
Leonardo Da Vinci – (1452 – 1519) - He is attributed with:
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the first recorded reference to “fossils” actually being the remains of
once living organisms.
He also claimed that when the remains of sea creatures are found on
mountain tops that it shows the impermanence of oceans as well as
mountains.
Prior to and during his claims, there were wide ranging explanations for
seashells on mountains; from being left by the Deluge, to fossils having
placed on mountains to by Satan to sway the non-faithful into believing
heresy.
Nicolaus Steno – (1638 – 1687) – Niels Stensen was a Danish physician to the Grand
Duke of Tuscany who after settling down in Tuscany, Italy, “latinized” his name to Nicolaus
Steno. He did not have much work to do, as attending the medical needs of the Duke took
very little time. During his frequent walks and hiking excursions, he began to notice certain
patterns and repetitions in some of the rock outcrops he passed. After considerable
pondering, he developed what have come to be known as “Steno’s Three Principles of
Geology”:
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Principle of Superposition – In any sequence of undisturbed
strata, the oldest layer is at the bottom and the successively
higher layers are successfully younger. (“Undisturbed” refers
to them not having been compressed to the point that older
material is folded on top of the younger material.
Principle of Original Horizontality – Steno claimed that “Most
sediment must have been deposited in layers that were nearly
horizontal and parallel to the earth’ surface on which they were
accumulating. (Exceptions Steno did not know about: 1. The
terminal ends of river deltas show a near vertical deposition
as deposits are dropped into deeper water. 2. “Crossbedding” of strata in riverbeds (“Fluvial”=”water carried” cross-bedding showing embrication of grains due to the
more consistent flow of water) and sand dunes (“Aeolian” =
“wind carried” cross-bedding showing non-embricated,
scattering of the grains).
Principle of Lateral Continuity – As originally deposited, strata
either extend in all directions until they: 1. “thin” to a feather
edge at the margin of a depositional basin; 2. end abruptly
against some barrier to deposition (i.e. mountain range); 3. or
grade laterally into a different kind of sediment (i.e. the marine
lithofacies)
Steno’s Principles form the foundational principles of the branch of geology called
Stratigraphy – the study of layered sedimentary rocks, including texture, composition,
arrangement, and correlation from place to place.
Steno’s Principles were lost for a while after his death, but were “rediscovered” and put to use
by late 17th and early 18th century European scientists. These contributors to geology are
referred to as the “Interpreters of Geologic Successions”.
Interpreters of Geologic Successions
John Strachey – (1671 – 1743) First recorded use of Steno’s Principles to successfully
and repeatedly find coal bearing strata in England.
Giovanni Arduino – (1714 – 1795) [and others] First to classify mountains as to rock
type found there. He defined “Primary” mountains as those of a crystalline nature (later
known as Igneous & Metamorphic) and “Secondary” Mountains as those of layered, wellconsolidated, fossiliferous rocks (later known as Sedimentary).
Abraham Gottlob Werner – (1750 – 1817) A keen mineralogist, but better
remembered for his views on geology: He insisted that all rocks of the earth were deposited or
precipitated from a great ocean that once enveloped the entire planet in a concept known as
“Neptunism” after the Roman god of the Sea. The major criticism is where did this large body
of water go? When it was proven by the French geologist, J. F. D’Aubisson de Voisins (1769 –
1832) that basaltic layers in Werner’s sequences were created by volcanic activity, this formed
an opposing view of “Neptunism” (“rocks formed in water”) called “Plutonism” (rocks formed
in fire” after the Roman god of the Underworld – Pluto)
James Hutton – (1726 – 1797) One of the earliest “Plutonist” whose staunch opposition
of Neptunism, and astute observations of the earth led to the concept of “Uniformitarianism”.
This concept states that:
 the earth is an ever-changing, dynamic system, whereby mountains rise
up and are eroded down
 volcanoes erupt and the igneous rocks formed weather to other
sedimentary rocks
 rivers change their courses depending upon the ever-changing terrain
 seas inundate the lowlands, only to recede or to evaporate, leaving behind
series of sediments
 rivers can eventually cut through mountains and erode them into particles
carried off to the sea
 etc.
 “These physical and chemical processes that alter and change the
earth’s surface today also went on in the geologic past”
 Hutton published his concepts and findings in a famous 1875 book entitled
Theory of the Earth. This book became the standard for geologic study
even into the early 20th century.
 James Hutton is considered to be the “Father of Modern Geology”
…“The past history of our globe must be explained by what can be seen to be
happening now.”
Charles Lyell – (1797 – 1875) – As a student and proponent of Hutton, he was an early
advocate of Uniformitarianism, and was against Neptunism. In the early 1800’s, he published
a book entitled Principles of Geology in which he added his observations to those of Hutton.
His additions were:
 The Principle of Cross-cutting Relationships – “An igneous
intrusion, a break, or a fault cutting across preexisting strata, must
be younger than the strata it cuts across”. In other words,
something must first be in existence prior to anything happening to
it, making the “something” necessarily older than the “happening”.
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The Principle of Inclusions – “Fragments of rocks within larger
rock masses are older than the rocks they are contained within”.
For example, an arkose sandstone (chiefly composed of quartz
particles) layer may be 200 million years old. Since the quartz
grains were originally formed in igneous rocks that were eventually
eroded into sediments, which were carried off by the elements, to
eventually settle in a basin of deposition (such as a seafloor), the
age of the individual grains that comprise the arkose sandstone
must necessarily be older than the sandstone in its entirety.
From the Uniformitarianism views of both Hutton and Lyell, very thick strata of sediments
would take a very long time to accumulate.
…Therefore, the age of the earth is many times greater than the few thousands (10,000
years or so) of years as proposed by the major religious writings: namely the Bible.
Uniformitarianism was considered to be an assault upon religious
doctrine and was thought of as heresy by many at that time.
William “Strata” Smith – (1769 – 1839) He was an English land surveyor whose
contributions include:
 Making the first known Geologic Map of England and Wales in 1815,
showing outcrops of various strata (most of which is correct even today).
 Being able to use geologic principles to predict subsurface strata,
especially coal.
 Made extensive studies of fossil assemblages and used geologic
principles to formulate Correlation Techniques to find continuances of
eroded layers.
 Using his findings, he developed the “Principle of Biologic
Succession”.
The Principle of Biologic Succession states:
“…Life of each age in the earth’s history is unique for that particular period.”
This can be understood if you think of all of the 40 million or so life forms alive today:
elephants, red fish, banana trees, humans, mushrooms, amoebas, etc. If you go back in
geologic time, say, 2 million years ago, most of these life forms were probably in existence,
along with a few that have since become extinct. So, if you examine strata of that time, it
would probably have an assortment of fossils that reflect the life forms of that time. If you go
back 400 million years, probably very few of the life forms in existence today would have been
in existence, but a great many organisms [that have subsequently become extinct] would be
represented in the strata of that time preserved in the Fossil Record. Therefore, Smith is
saying that each time frame has its own array of life forms that “represent” that time frame in
the geologic record.
Baron Georges Leopold Cuvier – (1769 – 1832)
AND
Alexander Brongniart – (1770 – 1847) – Both Cuvier and Brongniart were French
scientists: Cuvier, an anatomist, and Brongniart, a naturalist and mineralogist. They were
close associates that also worked with and contributed to aspects of “Biologic Succession”,
and studied fossil sequences in various strata. They are best known for the formulation of the
“anti-uniforminitarianism” view called “Catastrophism”. Their explanation of why there were
fossil seashells on mountain tops, or why there would an abrupt end to a certain fossil life
forms existence is because of a religious concept called “Deus Irae”, which is Latin for the
“Wrath of God”.
Cuvier and Brongniart felt that the [sometimes “violent”] upheavals of mountain ranges, or the
twisting of solid rock layers, or thick layers of salt deposits within rock sequences were
evidence of God’s Wrath unleashed upon the earth as punishment of sinners or the cleansing
of evil by such acts as the Noachian Deluge.
At this time of the mid to late 1800’s, geologists could subscribe to one of two main camps of
thought:
1. Hutton’s Uniformitarianism, or,
2. Cuvier’s Catastrophism
During meetings of the Royal Academy of Science in London, it is written that many violent
fistfights erupted during the debates between these two concepts by the proponents of each
side.
As geologic techniques developed further toward the end of the 19 th century that showed the
the earth was indeed millions of years older that the Biblical account, Catastrophism faded out
of popularity, being replaced by Uniformitarianism based geology. “Creationist Science” of
today can be said to be an offshoot of Catastrophism in many ways.
Gregor Mendel – (1822 – 1884) – He was an Austrian Monk schooled as a
mathematician, particularly in the field of statistics. When he had the time, he enjoyed
gardening, especially the raising of sweet pea plants. Over time, he began to notice and
record differences that arose within the population of plants, especially from certain crosses he
would make. He realized that if Tall Plants were crossed with Dwarf Plants that the offspring
generation would all be Tall. If he crossed Dwarf plants with Dwarf plants they would all be
Dwarf. When he crossed Tall Hybrid plants, the results showed that both Tall and Dwarf plants
would be present in the next generation. Mendel discovered means by which he could
statistically predict the next generation. Because of his findings, Mendel is considered the
“Father of Genetics”.
Alfred Russel Wallace – (1823 – 1913) He worked in the Philippines and parts of
Indonesia describing the inter-relationships of organisms and their environments. He noted
that physical characteristics of organisms changed in response to environmental changes. His
work is very similar to Darwin’s concept of Natural Selection.
Charles Darwin – (1809 – 1882) He began his education in the field of religion, and later
in medicine, never completing either. With his strong interest in all life forms, his father
procured a position on the H.M.S. Beagle that was a research vessel. The 5-year journey was
primarily a mapping mission, but sample collections of life forms were also taken at all ports.
Darwin signed on really as the unpaid assistant to the Captain and 1st Mate. Darwin kept
journals of all things encountered, as well as the environmental conditions in which they were
found.. Whenever they would go into port, Darwin would start collecting specimens of insects,
plants birds, etc. Upon analyzing his journals, he concluded it was the environment that
determined which characteristic trait of an organism’s population would be expressed: hence,
he developed the Concept of Natural Selection and Survival of the Fittest.
Thomas Morgan Hunt – Around 1910, he was first to describe the “chromosome” as
the means in which genetic information is passed from one generation to the next. He also
coined the term “gene” being the discrete unit of a chromosome that controls traits. He is
considered the person that discovered the mechanical transfer of genetic inheritance in
dividing cells.
James Watson and Francis Crick – They discovered the complex shape and
function of deoxyribonucleic acid (DNA) and their research provided the basis of genetics on
the molecular level.
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Planetary Beginnings – Big Bang, etc….
The History of the Universe, Solar System,
& Planets
Be familiar with the Big Bang and the development/formation of the earth
Universe beginnings – the “Big Bang”. Description of the Big Bang:
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Metaphysical event creating “reality” occurring around 10-15 Billion Years Ago.
In a space smaller than an atom, both space and time were set at zero…There is NO
BEFORE BIG BANG! This is because that both space and time are unalterably linked
to form a space-time continuum as per Einstein’s Theory of Relativity…Without 3diminsional space there can be no time
Evidences of the “BIG BANG” – First – The “Doppler Shift” indicates that the universe
is expanding… Galaxies are moving away from each other
Secondly – the universe is permeated by a “background radiation” of 2.7 degrees
above absolute zero (00 Kelvin)…almost no molecular movement at all!
Subatomic particles collide, increasing their mass, that increases their gravitational
attraction , that causes larger particles to form from the fusion of smaller ones. The first
element, hydrogen, formed.
As more hydrogen formed, it began to coalesce forming the first stars.
As these stars “ran out” of hydrogen, other elements formed.
These early stars eventually exploded (supernova) spreading the 92 naturally occurring
elements in nebulae (clouds of stardust).
These clouds coalesced into other stars (i.e. the sun) with the outer clumps of matter
forming the planetesimals that would eventually form the planets.
Age of the universe = 8 – 10 BYA
Age of the Sun = 5BYA
Age of the Earth = 4.6 BYA
Structure of the Modern Earth and Tectonics
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 B-B’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).
Tectonic Plate boundaries:
I. Constructive – “Divergent” creating new earth surface
a. M.O.R – “mid-oceanic ridge”, the spreading centers on
the ocean floors especially the “mid – Atlantic ridge”
b. Continental Rifts – i.e. the Great Rift Valley of eastern
Africa
II. Destructive – “Convergent” – destroying earth’s surface
a. Continental/Continental – two or more continental
plates converging as in India – Asia forming the
Himalayan Mountains
b. Continental/Oceanic – oceanic mafic plate is
thrust under the sialic continental plate in a
process of “subduction” as in the Pacific Rim or
“ring of fire”
c. Oceanic/Oceanic – two mafic oceanic plates
thrust together whereby one dives deeply below
and melts. The rising magma can form volcanic
islands.
III. Shear or Transform – Two plates “slide” past one another as in the
San Andreas Fault. There is no up or down movement, only lateral
movement.
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Geochronology – Dating the Earth
There are two main methods used in dating rocks:
I. Absolute Dating - Utilizing the rate of decay of radioactive isotopes to
determine exact age of when the rock formed.
II. Relative Dating - Rock layers are placed in their proper order or sequence of
events. The ages of the layers are found ONLY relative to each other.
In order to become proficient in relative dating techniques, you must be familiar with the following
principles and terms.
Relative Dating Terminology
Law of Superposition - Geologic Principle stating that in an undisturbed sequence of strata, the
youngest is on top and the oldest is on the bottom.
Principle of Original Horizontality - The layers of sediments are generally deposited in a nearly
horizontal position.
Principle of Crosscutting Relationships - Principle stating that an intrusion or fault that cuts
across a sequence of strata is always younger than the strata it cuts.
Principle of Inclusions - Principle stating that any fragments or particles of country rock in an
igneous intrusive body is always older than the intrusion itself. Also, any sedimentary rock is
made of substances (i.e. particles) that are older than that sedimentary rock.
Unconformities - Periods of no deposition and represent a gap in time in the geologic record.
These appear as erosional surfaces in a sequence of strata.
Types of Unconformities:
i. Angular Unconformity - Strata below the unconformity are folded or tilted before being
eroded.
ii. Disconformity - A type of unconformity where there is an erosional surface between
two groups of sedimentary strata.
iii. Nonconformity - An erosional surface between igneous/metamorphic layers and
sedimentary layers. There is a lithologic change across the unconformity.
Radiometric or Absolute Dating
There are 92 naturally occurring elements in nature. All matter is made up of chemical elements,
with each being composed of extremely small particles called atoms. The nucleus of an atom is
comprised of positively charged particles called protons, neutrally charged particles called
neutrons, with negatively charged particles, called electrons encircling the nucleus in energy
levels or electron shells.
The number of protons in the nucleus of any atom of an element is the atomic number of that
particular element. That is the basis of the numbering of the elements on the Periodic Table of
Elements. For instance, the element Hydrogen has one proton in its nucleus. Therefore it has
an atomic number of 1; Helium has two protons in its nucleus, and therefore has an atomic
number of 2; Uranium has 92 protons in its nucleus and therefore has an atomic number of 92.
The atomic number of an element defines that element. If an element looses a proton by some
means, it is no longer that element. Conversely, if an element gains a proton by some means, it
is no longer that element.
Neutrons in the nucleus of an atom do not affect its charge (since neutrons are neutrally
charged), but neutrons do affect the atomic mass of the element. The atomic mass of an
element is the combined number of protons and neutrons in the nucleus. Not all atoms of the
same element have the same number of neutrons in their nuclei. These variable forms of the
same element are called isotopes. For instance, hydrogen has an atomic number of one: one
proton and one electron. If a neutron is added to the nucleus of hydrogen, it still has the same
atomic number, but you have increased its atomic mass, forming the isotope of hydrogen called
deuterium. If another neutron is added to the nucleus it becomes the isotope of Hydrogen called
tritium. All three, Hydrogen, Deuterium, and Tritium all have an atomic number of one (one
proton in the nucleus), but they are all different isotopes of the same element.
If you could continue to add neutrons to the nucleus of an atom, a point would be reached that
the nucleus would become very unstable. Because of our reality being “ruled” by the processes
of entropy (whereby everything “wants” to be at its lowest point of equilibrium, or its lowest “rest”
state), atoms with unstable nuclei (those with high neutron to proton ratios) begin to emit
particles, which we refer to as radioactivity. Radioactive decay is the process whereby an
unstable atomic nucleus is spontaneously transformed into an atomic nucleus of a different
element.
There are three basic types of radioactive decay:



Alpha Decay
Beta Decay
Electron Capture Decay
Alpha decay occurs when 2 protons and two neutrons are emitted from the nucleus, resulting in
a loss of 2 atomic numbers and 4 atomic mass numbers.
Beta decay occurs when a neutron in the nucleus emits a fast-moving electron, changing that
neutron to a proton and consequently increasing the atomic number by 1, with no resultant
atomic mass number change.
Electron capture decay comes about by a proton in a nucleus capturing an electron from an
electron shell and thereby converting into a neutron, resulting in the loss of one atomic number,
but not changing the atomic mass number.
Some elements undergo only one step to convert from an unstable nucleus to a stable one.
Others require several conversions until a stable state is achieved. For example, the element
rubidium 87 decays to strontium 87 by a single beta emission, and potassium 40 decays to argon
40 by a single electron capture. Uranium 235 decays to lead 207 by seven alpha steps and six
beta steps. Uranium 238 decays to lead 206 by eight alpha and six beta steps.
The half-life of a radioactive element is the time it takes for one-half of the atoms of the original
unstable parent element to decay into atoms of a new, stable daughter element. The daughter
element is the stable element that an unstable element decays or changes into. The half-life of
radioactive elements is constant and can be measured. Each different unstable radioactive
element has a different half-life that can range from less than a billionth of a second to 49 billion
years.
All igneous rocks contain radioactive isotopes. Whenever they solidify (or cool) the
radioactive parent isotope begins to decay into the stable daughter element. So, whenever an
igneous rock of unknown age is found, a field sample of it is taken, and the sample is analyzed as
to which radioactive isotope is present in abundance. When that is determined, a survey of the
daughter element that particular radioactive element decays into is made from the sample in
question. This creates a percentage of radioactive parent isotope to the stable daughter isotope
present in what is called the parent/daughter ratio for that particular rock.
The half-life (a measurement of time) for that particular radioactive element found in abundance
in the field specimen is easily found in physics and chemistry reference books. So, knowing the
percentage of the radioactive parent to the stable daughter element present in the sample of
igneous rock of unknown age, and knowing the half-life for the radioactive isotope in question, the
actual age of the igneous rock can be deduced.
Usually, only igneous rocks can be dated using the following procedures. For metamorphic
rocks, only the age of the actual metamorphism can be determined. In rare instances, some
sedimentary rocks containing Glauconite (a green-colored, radioactive potassium mineral found
in some sedimentary deposits) can give information on the age of deposition of the sedimentary
beds. All igneous rocks can be dated using radiometric techniques.
Absolute dating techniques involve the measurement of the breaking down of certain
radioactive elemental compounds in the rock that have occurred over time. The rate of decay is
known for these radioactive elements from laboratory experimentation.
If a geologist finds an igneous rock layer in the field and needs to know the exact age of the rock,
a sample is taken from the outcrop. This sample is then sent to a laboratory that specializes in
radiometric dating techniques. There the rock is ground into a very fine powder. This powder is
then analyzed as to which radioactive isotopes are present in the rock. This lab must be
equipped with an apparatus called a mass spectrometer. This analytical device allows the
geologist to project purified samples of the rock in question into a strong, fluctuating magnetic
field that has sensors that can detect the presence of different elements that have different atomic
masses. It works similarly to the following scenario. If you turned on a strong fan and stood in
front of the fan with a feather in one hand and a lead ball in the other, and simultaneously let go of
both, what would happen? The feather would go shooting off because of its low weight (low
mass) and the lead ball would fall to the ground because of its high weight (high mass). It’s the
same principle whenever the atoms of different masses are projected through the magnetic field
of the mass spectrometer: the “lighter” elements “fall” through the magnetic field differently than
the “heavier” elements, there fore hitting the sensors at different areas and different rates. This is
how the parent daughter ratio is determined in an unknown sample.
To fully understand this technique, one must be familiar the following terms:

Isotope - Varieties of the same element that have different mass numbers. Their nuclei
contain the same number of protons but different numbers of neutrons.

Parent Isotope - the full amount of isotope in the newly formed igneous rock.

Daughter Isotope - (what the parent isotope will eventually turn into) the amount of altered
parent isotope over time.

Half-life - The time it takes for one-half of the atoms of a radioactive substance (Parent
Material) to decay into another element (Daughter Material). For example, Uranium 238
(Parent) decays to Lead 206 (Daughter). The rate of decay is known for many of the naturally
occurring radioactive elements. So, if the rate of decay is known, and the ratio of parent
material to daughter material is measured in a rock, then the age of the formation of the rock
can be found.
 Mass Spectrometer – the laboratory device used in determining the relative amounts of
Parent and Daughter isotopes.
Sample problems of Radiometric Dating Techniques
1.) In the geologic past, a rock formed from cooling magma, containing 1 gram of radioactive
Uranium 238 and no Lead 206. Many years later a geologist who wants to find the exact age of
this rock collects a sample. If the half-life for U238 is 4.5 billion years and after analysis the
Uranium 238 to Lead 206 ratio (parent/daughter ratio) was 1:1 (50% U & 50% Pb), how old is the
rock?
2.) A rock specimen was found that had a ratio of Potassium-40 to Argon-40, which was 1:7 (1
part Potassium-40 to 7 parts of Argon-40). Potassium-40 has a half-life of 1.3 billion years. How
old is the rock?
3.) A geologist collects a piece of a meteorite rock in the field and wants to know the exact age of
the rock. After close examination of the specimen, it was discovered that the specimen contains
sufficient amounts of the potassium 40 to warrant using the K40 – Ar40 test. Knowing that the halflife of K40 is 1.3 billion years and that there was a ratio of 1 part K40 to 3 parts Ar40, how old is the
rock?
4.) If a rock contained a parent/daughter ratio of “parent element X” to “daughter element Y” of
3:1, and the known age of the rock is 500 million years, what is the half-life of “element X”
Sediments and Sedimentary Rocks
Most fossils are found in sedimentary rocks. This is because the
organic remains of organisms are usually destroyed by the high
temperatures associated with igneous activity or the processes of
metamorphism. The type of sedimentary rock formed in an area reflects
the environment in which it was deposited. The term used by
geologist to describe this aspect of sedimentary beds is “facies”. Much
can be learned about the ancient environments of the earth by studying
various characteristics of sedimentary rocks.
All rocks form initially with the solidification of molten magma or lava.
These newly formed igneous rocks are subsequently subjected to the
surface processes of weathering and erosion (the destructive actions of
running water, wind, glaciers, etc.) These rock fragments eventually
settle out somewhere to form “sediments”. These sediments can
become compacted to form sedimentary rocks. If these “new”
sedimentary rocks are subjected to enough heat and pressure, they may
become changed into “metamorphic” rocks. If the sedimentary rocks
are completely melted by geologic processes, they revert back into a
type of igneous rock upon cooling.
I. The Rock Cycle:
The rock cycle is the conversion of one rock type into another by
melting, pressure deformation, and weathering and erosion. All rocks
are initially igneous (The word “Igneous” means “born of fire”).
II. Rock Types:
 Igneous rocks make up 90% by volume of the earth's
crust. Igneous rocks are formed directly from molten
material having its origin in the interior of the earth. As this
molten material cools in some areas, it solidifies and
hardens to become rock. Intrusive igneous rock forms
below the surface of the earth. Extrusive igneous rocks
form from molten material that has been forced out onto
the surface of the earth (i.e. volcanoes).
 Sedimentary rocks form from the accumulation of eroded
debris of other rocks or chemically from elements in
seawater. Sedimentary rocks make up 75% of all of the
rocks exposed at the earth's surface and are where most
all fossilized remains are found. This makes sedimentary
rocks useful in interpreting the earth's geologic history.
 Metamorphic rocks are formed from pre-existing rocks
that have been altered as the result of intense heat and
pressure. Metamorphism increases the “crystallinity“ and
hardness of the rock; sandstone changes to quartzite;
shale changes to slate, and limestone changes to marble.
III. Types of Sedimentary Rocks:
Since the facies of sedimentary beds tells the geologists so much
information about the geologic past (paleoenvironments, paleoclimates,
and past life forms), sedimentary rocks are emphasized in Historical
Geology. There are 2 basic groups of sedimentary rocks:
1. Chemical Precipitates from the evaporation of seawater, or
from the concentration of ions in water. These include rocks such as
limestone and various salts such as Halite (NaCl), Sylvite (KCl), Gypsum
(CaSO4), etc. The salts usually indicate periods of massive evaporation
of aqueous environments.
2. Clastic Sedimentary Rocks are formed from the accumulation
of debris from the weathering and erosion of other rocks. The 4 stages
of the formation of clastic sedimentary rocks (“clastic” means "broken")
are described on the following pages.
IV. The Four Steps for Formation of Sedimentary
Rocks:
1. Physical and Chemical Weathering of the “Parent Rock” (the
source rock from which the clastic material is being derived). Physical
weathering includes the breaking apart of the parent rock by freezing
and thawing, wind erosion, etc. Chemical weathering includes
dissolution of the parent rock by chemicals in the water (i.e. acid rain).
2. Transportation is the stage where the clastics are
"moved"(“transported”) from the source area by water, wind, gravity,
or ice. The terrain determines the area of transportation. The distance
the particles are moved depends on the amount of energy operating in
the environment. It would take more energy to move a boulder than a
grain of sand. The larger the sediment size, the more energy is needed
to move it. High- energy environments would include white water
mountain streams that are capable of moving almost all sizes of
particles. Low-energy environments include lagoons, lakes, deltas,
swamps, etc., that are capable of moving only the smaller particles.
3. Deposition is the stage where the sediment is deposited in a
particular geographic environment, which constitutes the sedimentary
environment. As in transportation, the area of deposition is also
determined by terrain. For example, large rocks formed on a mountain
range would be carried down the steep gradient and deposited at the
base of the mountain if the energy of the stream carrying them
decreased when it reached the base of the mountain. Since the stream
no longer has the high energy from the gradient, the large rocks are
deposited in a manner indicative of a mountain stream environment.
Sedimentary rocks can be interpreted to find out the environment in
which they formed.
Sedimentary Environments can be divided into several categories:
 Shoreline and Coastal Environments
 “Fluvial” or Stream, River, and Delta Environments
 Alluvial Fans or deposits at the bases of
mountains
 “Aeolian” or “wind-borne” deposits
There are numerous other sedimentary environments that your
instructor will inform you of at the appropriate time
4. Compaction is the final stage in the formation of a sedimentary
rock. At this stage the sediments are compacted due to the weight of
the overburden (overlying sediments) and can be eventually “lithified”
(turned to stone) as the particles are cemented together with substances
such as Calcite (CaCO3), Silica (SiO2), or forms of Iron Oxide (i.e.
Fe2O3), among other compounds..
V. Properties of Clastic Sediments:
These include certain characteristics of the sedimentary rock that give
specific information about the environment of deposition. These include
particle size, degree of roundness, degree of sorting, and color.
1. Particle Size: Clastic sediments are found in various sizes ranging
from <1/256 mm to >256 mm. Refer to Figure 1. The Wentworth
Scale of Particle Sizes. The name of a particular sediment size is
based on its particle size rather than its chemical composition. For
example, "sand" refers to particles having a size range between
0.125mm – 0.5mm. There can be quartz sand such as that found along
the Gulf Coast or there may be feldspar sands, gypsum sands, etc.
Remember that sediment size indicates the amount of energy operating
in the depositional environment and is therefore a useful clue in
determining what the sedimentary environment was. Boulders represent
a high- energy environment such as a river channel while clays
represent a low energy environment such as a floodplain or swamp.
The Wentworth Scale of Particle Sizes that is a list of sediment
particle sizes and the names used to describe them:
The Wentworth Scale of Particle Sizes (modified)
Particle Name
Approximate Particle Diameter in millimeters
Boulders
greater than 256 mm
Cobbles
64 to 256 mm
Pebbles
2 to 64 mm
Sand
1/16 to 2 mm
Silt
1/256 to 1/16 mm
Clay
less than 1/256 mm
2. Roundness: This is simply how “round” (or smooth) the particles in
the rock are. Particles in rocks that are angular, irregular in shape, and
have sharp edges are called “poorly rounded”. Particles that are
smooth and have no edges are called “well rounded”. The degree of
roundness indicates either the amount of agitation the particles were
subjected to before deposition, or the length of time it took to transport
the particle. “Well rounded” particles indicate that the particles were
subjected to a high amount of saltation (bouncing along as they were
transported) or being transported for a very long distance such as from
the center of a continent to its shoreline. Both of these factors indicate
how much the rock particle was hit by other fragments or was saltated
along the route of transportation. “Poorly rounded” sediments indicate
either a low amount of agitation, or a short distance of transportation
from the time the particle weathered or broke away from their parent
rocks. A high-energy environment, which allows for a long period of
exposure to weathering, such as a beach or in a stream, is condusive to
the formation to the formation of “well-rounded” sediments. On the other
hand, a high-energy depositional environment that does not allow a long
period of exposure to agitation, such as an alluvial fan, prevents the
sediments from becoming “well-rounded”
3. Sorting: refers to rock fragments separated according to particle
size. “poorly sorted” sediment would contain particles of varying size.
This usually represents a rapid deposition as the result of a rapid
decrease in the energy of an environment. Poorly sorted sediments are
many times found in alluvial fans at the base of a mountain. This
results in a "dumping effect" of sediments at the base of the mountain
(high- energy to low- energy). “Well Sorted” sediment contains
material that is made up primarily of all the same sized particles. This
indicates that the rate of deposition is slow enough to allow the materials
to be separated. Of course, the energy of the environment must be
sufficient to accomplish this. Beaches, such as those along the Texas
coast, allow sorting to occur. The high energy from the waves combined
with a proper depositional rate provides excellent conditions for sorting
of the sediments. Sediment is said to be "Mature" if it is well rounded
and well sorted. Poorly sorted and poorly rounded sediment is said to
be "Immature".
4. Color: The color of sediment can provide useful information about a
sedimentary environment. In general, colors of sedimentary rocks can
be interpreted in the following manner:
a.) Red, yellow, brown - oxidation conditions, probably marine in
origin.
b.) Black, gray, greenish-gray - reducing conditions, probably
marine except for floodplains and swamps.
c.) Light gray or white - little iron present, either marine or nonmarine; other characteristics of the rock must be considered such
as the presence of fossils, the type of fossils, whether or not there
is cross-bedding, etc.
VI. Chemical Precipitates:
Chemically formed sediments are produced under various conditions,
but generally speaking, when seawater becomes saturated with
chemicals, they will precipitate out of solution. This is similar to when a
lot of sugar is added to hot tea and then it is allowed to cool. Some of
the sugar will "crystallize" or settle out of solution because the tea was
"saturated" with sugar and it could not stay dissolved. Precipitates
usually form only in low energy environments such as lagoons or
deep-sea environments. Chemical Precipitates would not be found in
high- energy environments.
Limestone and Dolostone – These “carbonate rocks result from the
concentration and precipitation of Ca+, Mg+, and CO3- ions in the sea.
Limestone - Ca CO3 (primarily calcite)- forms offshore from the
precipitation of calcium and carbonate ions that have been dissolved off
of the continents. Limestones may also be formed from the
accumulation of microscopic calcareous tests (shells) of planktonic (or
other aquatic level) micro-organisms.
Dolostone - Ca,Mg (CO3)2 (primarily dolomite)- forms in a similar
manner, but contains magnesium as well as calcium. Dolostone may
start off as limestone and later is subjected to groundwater replacing Ca+
with Mg+. Or, some dolostones indicate having formed the
calcium/magnesium carbonate all at once.
“Bioclastic sediments” are formed by living organisms. Many aquatic
marine organisms produce shells or other protective coverings by
secreting calcium carbonate (limestone) or calcium magnesium
carbonate (dolomite). When these organisms die, their shells
accumulate along the sea floor forming layers of broken shell fragments.
Such material is biochemically produced and is ultimately broken by
water action They are then referred to as "bioclastic sediments". The
sedimentary rock coquina is a good example of a bioclastic deposit.
The availability of nutrients decreases the further from the shore
therefore most marine organisms live in the coastal, shallow water
areas. As the distance from shore increases, generally the number of
marine organisms decreases. The facies of bioclastic sediments such
as coquina usually indicates a beachfront.
“Organic Rocks” form as the result of organics (such as vegetative
matter) accumulating in low energy, reducing, anaerobic
environments such as swamps. The material does not rot quickly and
the volatiles are driven off leaving behind the carbon. A good example
of an organic rock is coal. The first stage is called peat. As the peat
gets compressed over time, it becomes lignite coal. As lignite
becomes compressed, it becomes bituminous coal. As bituminous
coal becomes compressed, it forms the metamorphic rock anthracite,
the final stage of coal. Other types of organic rocks may form from
accumulations of dead organisms (such as fish) in low energy lagoons.
VII. Bedding or Layering of Sedimentary Materials:
Sedimentary rocks are deposited in layers known as "beds". The type
of bedding will vary depending on the environment of deposition. Under
normal conditions, beds are deposited in horizontal layers with the
bedding planes (the line of contact between the beds) parallel to one
another. "Cross-bedding" occurs when the surface of deposition is
inclined (i.e. a delta) or a current is present (i.e. a stream). This type of
bedding is called "cross-bedding" and is indicative of these
environments.
The types of currents that form cross-bedding strata are:
a. Aeolian - wind action
b. Fluvial - river and stream action
c. Marine in Origin - current action
Types of cross-bedding include planar - the bedding planes separating
the cross-bedded units are parallel, wedged - the bedding planes are at
an angle to one another and form a wedge; and trough - the bedding
planes separating the cross-bedded units are curved.
Thick planar or wedged cross-bedding always indicates an aeolian
(wind) deposit such as a sand dune in the desert. Thin planar or
wedged units may be aeolian, fluvial, or marine. Because of this,
other characteristics such as color must be used to determine the
environment of deposition.
Many times paleocurrents of water (and sometimes wind) can be
traced by the ripple marks left in some sedimentary rocks indicating
ancient river channels or beachfronts. Mud cracks can also be
preserved indicating ancient low energy mud flats.
Another type of bedding is known as graded bedding. This is where
there is a gradation in the size of particles within a unit of deposition.
Larger particles are found on bottom with successively smaller
sediments on top. This type of bedding is formed by "turbidity
currents", which are the sudden flows of material down the continental
slopes. This causes the finer particles to be suspended in the water
while the larger particles fall out and are deposited on the bottom with
smaller and finer sediment on top. This results in a "gradation" in
particle size. The facies of graded bedding is deep water marine.
VIII. The Marine Lithofacies:
This refers to the depositional sequence found in a cross section of a
shore to deep- water environment. The usual sequences of rock types
are:
1. Sandstone formed on beach areas
2. Siltstone formed near-shore
3. Claystone/Shale formed further out
4. Limestone formed even further out in deeper waters
A schematic of the typical marine lithofacies is as follows:
The Marine Lithofacies
Transgression: - the advancement of the sea onto the land because of
a worldwide increase in sea level or a subsidence of the landmass.
Regression: - the retreat of the sea from the land due to a worldwide
drop in sea level or the uplift of the land.
Transgressional and Regressional sequences of strata can be used to
interpret and retrace ancient coastlines.
Transgressional Sequence
Regressional Sequence