Download 460:102 Notes Historical Geology Notes

460:102 Notes
Historical Geology Notes : Chp 1
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Introduction to Geology II: Historical
Objectives and Outline
Chapter 1 Earth as a System
Objectives:
° How our changing views of the planet are affecting the ways we treat it
° The historical development of the field of geology
° The development of the basic geologic principles employed by historical geologists.
° The evolution of the geologic time scale
Outline:
I. Exploring the Earth’s System
A. Physical
1. Plate Tectonics, mountains, the rock cycle, chemical reactions, climate
B. Biological
1. Bacteria ➔ the origin of plants and animals
C. Physical and biological components are intertwined
1. Must study system as a whole - not individual components
2. Physical environments influence life➔ creates niches for evolutions, hostile
environments
3. Biology can influence physical environment ➔ facilitate erosion, change the
atmosphere CO2 ,
II. The Principle of Uniformitarianism
A. Uniformitarianism-belief that there inviolable laws of nature that have not change in the
course of time. Extends to all sciences. Physics is physics.
B. Actualism: The Present is the Key to the past.
1. Differs from Uniformitarianism because it states that the processes observed at work
today can be applied to explain the rocks deposited in the past.
2. Exceptions:
a. Rocks formed under conditions that no longer exist ➔ Banded Iron Formations
b. Conditions exist but cannot be observed ➔ Depth, metamorphism - Lab results may
simulate these conditions.
c. Conditions exist at present, but require long time periods to form.
III. The Founders of Historical Geology (Most NOT in BOOK)- Forget Plate Tectonics for the
time being; What did we know and when did we know it? What led us to the understanding
of earth's history that we have at present? What did we overcome to reach our understanding
of change?
A. Greeks
1. 6 century BC a. Greek scholars began to record and analyze data within the context of the principles
of nature - assumed that the natural phenomena within their world could be
understood - search for natural laws concerning the causes and effects
b. Observations led to the conclusion of repeatability of processes. Impermanence of
details of the Earth's surface (Hutton's Cycles and Uniformitarianism?)
c. Examples - stratigraphical and geological - noted that shells not unlike clams and
snails that lived along the coasts were found on mountain tops - conclusion ocean
extended to the mountain tops
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2. Herodotus, a Greek historian, concluded in 400 BC that the Med. was at one time
more widespread and that the Nile delta was constructed from great volumes of
sediment that had been transported and deposited by the Nile River. (Modern
sedimentology?) But, he believed most features were the result of sudden, violent
processes.
3. Aristotle (384-322 BC) recognized river deposits and realized that fossil seashells
from rocks were similar to those found on the beach, indicating the fossils were once
living animals. He deduced that the positions of land and sea had changed and
thought these changes occurred over long periods of time.
4. Eratosthenes (250 BC) calculated the circumference of the Earth by measuring
noontime shadows at two localities of different latitude. He extrapolated the distance
from his reference points at Syene and Alexandria in Egypt and determined Earth's
circumference to be ~40,000 km, a remarkably accurate estimate and major scientific
accomplishment. (40,045km)
B. Romans - Grecian theories of scientific inquiry transcended time and culture and were
inherited by Roman scholars.
1. Pliny the Elder (23-79 AD) reached similar conclusions as those of the Greeks
concerning the distribution of land and sea; modern and fossil shell comparisons,
volcanic eruptions (documented Vesuvius), earthquakes and floods.
2. Augustine of Hippo (354-430 AD) stated that the crucifixion was a unique event from
which all events could be measured, thus initiating the BC/AD time scale. Note:
modern archaeologists sometimes use a bce/ce scale to denote "before common era"
and "common era".
C. --- The Dark Ages (~500-1100 AD) --Decline of the Roman Empire - Principle of uniformity of nature's processes, along with
scientific inquiry suffered a critical blow - The DARK AND MIDDLE AGES. For
1000 years, Religious dogma held sway and any departures from the structures of the
time was considered a serious offense.
D. --- Printing Press invented (1450) [Renaissance begins] --The Renaissance - advance in many scientific fields - development of scientific principles Copernicus, Kepler and Galileo put the Earth in a new cosmic context (Astronomy).
Geology - another story - religious climate still limited advances in geology - the science
of the Earth itself. Genesis portrayed an Earth that was 6000 yrs. old. Creationists
considered the Earth as stable since the Noachian flood catastrophe.
1. Leonardo da Vinci (1452-1519) recognized that material carried by rivers to the sea was
eventually compacted into sedimentary rock and later uplifted to form mountains. He
concurred with Aristotle's view that fossils were the remains of ancient life. As a byproduct of his engineering work on canals and rivers, he studied the deposits of the Po
River and concluded that they must be at least 200,000 years old. He surmised that the
whole of geologic time must be much longer.
2. James Ussher (1581-1665) was the first to estimate the age of the Earth using genealogies
of the Bible. Ussher stated in 1650 that the Earth was created on October 22, 4004 BC.
This date was later reproduced in many editions of the Bible and was incorporated into
the dogma of the Christian church. For nearly a century thereafter, it was considered
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heresy to assume that Earth and its puzzling geologic features were more than ~6000
years old. Thus, a very young Earth provided a basis for most chronologies until the 18th
century.
3. Nicholas Steno or Neils Stensen (Danish physician, 1638-1687)
a. Studies of Italian geology - settled in Florence Italy - Physician to the Grand Duke of
Tuscany - ample time to explore and follow interests
b. Developed basic geologic principles
1. Superposition - observed the layered strata in Tuscany as he tramped around the
countryside; witnessed the process of deposition of layers in an aqueous medium.
When Layers formed only the fluid was overlying. Therefore, no overlying layers
could have been present when the lower layers were formed. Superposition! The
oldest layer is at the bottom and successively higher layers are successively
younger in an sequence of undisturbed strata. Probably not the first to recognize
layering and may not have been the first to conclude Superposition. But he was
the first to articulate this and put it into perspective.
2. Original horizontality - settling particles under the influence of gravity - the
layering should form parallel to the surface on which it was accumulating;
important for understanding tilted strata - revealed an episode of crustal
disturbance
3. Original lateral continuity - as originally deposited, strata extend in all directions
until they terminate by thinning at the margin of the basin, end abruptly against
some barrier, or grade laterally into a different kind of sediment.
c. First to study stratigraphy in an organized way
E. First attempts at Subdivision: An Economic Incentive
1. During the Renaissance, interest in mining and mineralogy grew significantly,
Industrial Revolution. George Bauer, a.k.a. Agricola, became the Father of
Mineralogy, wrote extensively on mineral classification and mining activity. From
this, mining academies were started. Needed a scheme to understand the origin,
occurrence and relationship of ore minerals to help locate and exploit other regions
with minerals. Economic incentives vs. pure scientific endeavor?
Superposition to an extreme - stratified low lying rocks formed the foothills and igneous
and metamorphic rocks form the cores - All crystalline rocks formed at one time and
all sedimentary rocks formed later. Superposition is a good descriptive generalization
applicable to individual mountain belt but was stretched to a faulty unfounded
principle which was perpetuated through geology
2. Abraham Gottlob Werner (German, 1750-1817)
a. Professor Mineralogy, Freiberg Mining Academy
b. Internationally known - first great synthesizer of geologic knowledge - Carried the
idea that all crystalline rocks formed at one time to an extreme. Zealously nurtured
the idea that all rocks of the Earth’s crust, regardless of composition were precipitates
of the world encompassing primeval ocean. Hot and steamy ocean full of dissolved
minerals needed to form the rocks. As the ocean shrank (subsidence and cooling), it
left behind the precipitated layers arranged in superpositional order. Each layer was
considered to have its own composition and be of the same age everywhere.
"neptunism" because water was the forming agent.
3. Advocated "neptunism" theory and a "universal sea" to explain stratigraphy
4. Developed a four-fold classification of rock ages
a. Primitive Series - Granite at the bottom followed by gneisses, schists, and other
crystalline rocks
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b. Transition Series, slates, graywackes, quartzites, and limestones. Some were
fossiliferous and considered to be the early forms of life. Transition from
uninhabitable to habitable. Werner considered that the Primitive and Transition
series to have covered the entire surface at one time.
c. Secondary Series, fossiliferous succession of sandstone, limestone, slate, and coal
formed by chemical precipitation as the ocean shrank to its size. Some surface
reworking by running water.
d. Alluvial rocks topped the sequences, sand, gravel, clay, peat, ash
Neptunism was appealing because it was consistent with the biblical account of the
creation. Young Earth - 6000 yrs. old.
Problems - where did the water go? Same origin for basalt as for shales, limestones
etc. J.F. D’Aubisson de Voisins proved volcanic origin of basalts
3. James Hutton (Scottish, 1726-1797) Physician turned geologist. Not bound by the
arithmetic of the biblical chronology. Demonstrated that rock units and relationships
were not worldwide, but regional. Conflict - origin of basalt in a Primitive Series Neptunist - Ocean ppt. Arduino appreciated the igneous origin of basalts when he named
the volcanic subdivision of the Tertiary Series. Hutton demonstrated that basalt could
form as an intrusive body. Cross cutting granite dikes through stratified rocks led him to
igneous origin and that they were molten when they forced their way into fissures.
Plutonists.
a. Dynamic/cyclic concept of earth history - recognized that every rock formation no
matter how old, appeared to have been derived from other rocks, still older rocks.
Earth is always changing. “Succession of Worlds”. Destruction through erosion and
weathering.
b. Principle of Uniformitarianism (Actualism) - Hutton was convinced that the geologic
processes in the past were no different from the present. “No powers that are not
natural to the globe, no action to be admitted of except those of which we know the
principle, and no extraordinary events to be alleged in order to explain a common
appearance.”
Hutton never used Uniformitarianism (William Whewell). “The present is the key to
the past” Sir Archibald Geike.
c. First great geology text: Theory of the Earth (1785) - important book but his writing
was cumbersome and filled with too many details which obscured the thrusts of his
arguments. John Playfair a friend and colleague of the Royal Society assumed the
task of clarifying Hutton’s idea. Published a brilliant summary in 1802 - Illustrations
of the Huttonian Theory of the Earth.
Playfair - “Amid all the revolutions of the globe the economy of Nature has been uniform
and her laws are the only thing that have resisted the general movement. The rivers
and the rocks, the seas and the continents have changed in all their parts; but the laws
that describe those changes, and the rules to which they are subject, have remained
invariably the same.
d. Concept of immense length of geologic time - Recognized the importance of
unconformities and cross-cutting features. “The result, therefore, of our present
inquiry is that we find no vestige of a beginning and no prospect of an end”.
Irreconcilable with 6000 yr. old earth. Hutton credited with idea of Deep Time.
Father of Modern Geology even though much of his book was theoretical and based too
little on field evidence. Through time Biblical account of age of Earth faded,
Neptunism and young Earth lost ground because it was contradicted by too many
data.
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4. Charles Lyell (English, 1797-1875)
1. Advocate and author exposing views of ancient earth history and Uniformitarianism collection, organized and presentation the data in support of Uniformitarianism.
Hutton’s view of the Earth and Playfair’s clarification were slow to take hold. Lyell’s
crusade:
a) establishment of uniformitarianism at he expense of catastrophism as the
acceptable philosophy for interpreting the history of the Earth
b) establishment of geology among the sciences as a discipline based on inductive
principles
2. Author of important text: Principles of Geology (1830)
3. Synthesizer of stratigraphy who developed the first geologic time scale- Founded
Modern Historical Geology and reintroduced Unlimited time. Unlike Hutton, Lyell
was a historian who realized the primary task of geology was the unraveling of the
Earth events in time. Used Faunal succession. Successive cycles
4. Principles of Cross-cutting relationships and inclusions
5. Developed concept of relative age dating and sequence of events
IV. The Nature and Origin of Rocks
A. Igneous
1. Texture
a. Intrusive- coarse grained
b. Extrusive – fine grained
2. Composition
a. mafic - dark colored - Basalts
b. intermediate
c. sialic - Light colored - Granites
B. Sedimentary –
1. Clastic - grain size – Largely a function of energy of environment
a. sandstones
b. siltsone
c. claystone
2. skeletal remains - Limestone
3. Chemical - evaporites
C. Metamorphic - Alteration - metamorphosis - temperature and pressure
1. grain size - slate-phyllite-schists- gneiss
2. quartzite and marble
V. The Rock Cycle
A. All rocks are subject to change after they form
B. Sedimentary
1.Unconformity - a surface between sedimentary strata that representd an interval of time
during which erosion occurred
2.Angular unconformity - unconformity where the older layers were tilted
3. Disconformity - surface where little was disturbed, perhaps difficult to see
4. Nonconformity unconformity on top of eroded crystalline rocks.
VI. Global Dating of the Rock Record
A. Relative ages
1. Fossils and Markers
2. Cross-cutting and Inclusions
B. Absolute ages
C. Geologic Time
D. Development of the Time Scale
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VII. Imaging the Earth – Largely developed by Studying Earthquakes
A. The Density Gradient
1. Crust - Outer Skin
2. Mantle – Ultramfic rocks
3. Core – Inner sphere of Fe-Ni
4. Mohorovicic Discontinuity
B. Lithosphere vs. Asthenosphere
1. Plates
VIII. Plate Tectonics
A. Plate Movements
1. Ridges, Subduction Zones and Transform Faults
B. Volcanoes at Plate Boundaries
C. Mountain Building
IX. The engine of Plate Tectonics
A. Heat
1. Convection, Conduction and Radiation
B. Plate Tectonics and the Rock Cycle
X. The water Cycle
A. Reservoirs
1. Ocean = 97.25%
2. Glaciers = 2.05%
3. Groundwater = 0.68%
4. Lakes = 0.01%
5. Soils = 0.005%
6. Atmosphere = 0.001%
7. Rivers = 0.0001%
8. Land Plants = 0.00004%
B. Interactions with the Rock Cycle
1. Weathering, Erosion, Deposition
2. Evaporation
3.
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