Chapter 1 An Introduction to Geology

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Chapter 1 An Introduction to Geology
GEOL 101 Introductory Geology
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The science of Geology

• Why Geology?
• Related to our daily life, such as
• Water and soil
• Provide guide to use of natural resources
• Provide knowledge of avoiding natural hazards
• Helping protecting the environment
• Physical geology - examines Earth materials,
  understand surface and subsurface processes
• Geology incorporates principles from physics,
  chemistry, biology, engineering

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The science of Geology

• Some historical notes about geology
• Catastrophism mid 1600s, James Ussher
• Sudden and violent change shape landscapes
• Religious belief of Earth created in 4004 B.C.
• Uniformitarianism late 1700s, James Hutton
• processes that operate today also operated in
  past
• geologic processes occur over extremely long time
• birth of modern geology

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Earth as a system

• Earth is a dynamic planet with many interacting
  parts or spheres
• Parts of the Earth system are linked
• Characterized by processes that
• Vary on spatial scales from fractions of
  millimeter to thousands of kilometers
• Have time scales that range from milliseconds to
  billions of years

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Earth as a system

• Powered by Sun - drives external processes
• Atmosphere
• Hydrosphere
• At Earths surface
• Powered by the Earths interior
• Heat remaining from Earths formation
• Heat generated continuously by radioactive decay
  - powers internal processes that produce
  volcanoes, earthquakes, mountains

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A view of Earth

• Earth is a planet that is small and
  self-contained, fragile, sensitive to change
• Environment is highly integrated with
  interactions between air, water, rock, life
• Earths four spheres
• Atmosphere- air
• Biosphere- life
• Hydrosphere - water
• Solid Earth- rock

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Atmosphere

• Gaseous environment that surrounds Earth
  provides life
• 90 of atmosphere within 10 miles of Earth
  surface (thin relative to radius of the solid
  Earth at 4000 miles)
• Provides air for breathing and protects from
  Suns heat and ultraviolet radiation
• Recent concern with atmosphere regarding Ozone
  holes and Global Warming

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Hydrosphere

• Water makes our Blue plant unique sustains
  life
• Water in motion Hydrologic cycle
• evaporation from oceans, precipitation from
  atmosphere, run off back to ocean
• 97 ocean water, 3 fresh water
• fresh water includes surface water (lakes,
  streams), ground water and glaciers
• Ocean covers 71 of Earth surface -average depth
  of 3800 m (12,500 feet)

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Earths Water Balance
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Biosphere

• Biosphere includes all life (including us!)
• Extends from ocean floor to several miles into
  atmosphere however, concentrated at Earth
  surface
• Life responds to countless interaction with the
  environment
• Successful adaptation to changing environment
  evolution of species
• Non-successful adaptation Extinction
• Survival of the fittest Darwins Legacy

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Solid Earth

• Solid Earth between atmosphere and oceans
  rocks and soil
• Surface and subsurface features reveal dynamic
  processes Plate Tectonics
• Face of the Earth
• Earths Interior
• Rock cycle - the loop that involves the processes
  by which one rock type changes to another

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The face of Earth

• Earths surface - two principal divisions
• Continents
• Ocean basins
• Significant difference between the continents and
  ocean basins is their relative elevations
• continents average 2750 feet above msl
• ocean floor average 12,500 feet below msl
• continents average 15,250 feet above ocean floor

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Continents and Ocean Basins
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Continents

• Mountain belts linear features, active
• Youngest Mountains
• Circum-Pacific Belt (surrounds Pacific Ocean)
• Island arcs active volcanic mountains,
• Older mountains
• Appalachian eastern United States
• Urals Russia
• Shields continental interiors, stable craton,
  oldest rocks (4 billion yrs)

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Ocean basins

• Continental shelf gently sloping platform
  adjacent to shore
• Continental slope steep dropoff from shelf to
  deep ocean floor
• Ocean ridge system most prominent topographic
  feature on Earth (43,000 mi)
• Deep-ocean trenches deep narrow depressions
  (36,000 feet)

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Earths internal structure

• Layers defined by chemical composition
• Crust thin, rocky outer skin
• Mantle solid, rocky shell
• Core iron-nickel alloy center
• Layers defined by physical properties
• Lithosphere sphere of rock
• Asthenosphere weak sphere
• Mesosphere middle sphere
• Core outer (liquid), inner (solid)

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Earths Layered Structure
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Layers Defined by Composition

• Crust
• Oceanic 7 km thick, dark igneous rock basalt,
  young (lt180 my), dense (3.0 g/cm3)
• Continental 70 km thick, granitic rock, old (up
  to 4000 my), less dense (2.7 g/cm3)
• Mantle
• contains 82 Earths volume, depth 2900 km
• rock called peridotite, more dense (3.3 g/cm3)
• Core
• iron-nickel w/ minor oxygen, silicon, sulfur
• extreme pressure highly dense (11 g/cm3)

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Layers by Physical Properties

• Lithosphere
• cold, strong rock exhibits rigid behavior
• 5 to 250 km thick
• Asthenosphere (upper Mantle)
• soft weak layer, partial melting
• depth of 660 km
• Mesosphere (lower Mantle)
• between 660 and 2900 km depth
• very hot, gradual flow of convection currents
• Core (differing mechanical strengths)
• Outer liquid, generates Earths magnetic field
• Inner behaves as solid due to immense pressure

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Earths internal structure

• How do we know about the Earths interior?
• Study of seismic wave generated by earthquakes
• Chapter 17

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Dynamic Earth

• The theory of plate tectonics
• continental drift the idea that continents
  moved about the face of the planet
• not widely accepted for more than 50 years
  because driving mechanism unknown
• magnetic patterns on sea floor (found from
  chasing submarines during WWII) suggested sea
  floor spreading
• Plate Tectonics accepted in 1965 as driving
  mechanism

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Theory of plate tectonics

• Symmetric magnetic pattern around ocean spreading
  ridges
• Black north magnetic pole, white south magnetic
  pole
• New ocean crust records magnetic pole
• Crust moves away from spreading ridges

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Plate Tectonics

• Earths rigid outer shell composed of numerous
  slabs (plates)
• mobile, continually changing size and shape
• create, consume, transform lithosphere
• Plate boundaries
• Most major interactions among individual plates
  occurs along their boundaries
• Plate tectonics first comprehensive model of
  Earths surface and internal workings

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Plate Boundaries

• Divergent two plates move apart, resulting in
  upwelling of material from the mantle to create
  new seafloor
• Convergent two plates move together with
  subduction of oceanic plates or collision of two
  continental plates consuming old lithosphere
• Transform located where plates grind past each
  other without either generating new lithosphere
  or consuming old lithosphere

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Mosaic of Earths Outer Shell
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Plate Boundaries
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Divergent Boundaries

• Plate spreading occurs mainly at mid-ocean
  ridges (sea floor spreading), rate of 2 to 20
  cm/yr
• Molten rock rises from asthenosphere through
  fractures (cracks) in hard rock, encounters sea
  water, cools to new rock
• New ocean crust is continually created oldest
  ocean floor rock is 180 million years

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Convergent Boundaries

• Older ocean plates consumed by subduction zone
  and returned to mantle
• One plate descends beneath another high
  temperature, high pressure environment producing
  molten rock
• Pacific ring of fire volcanic chains result
  in explosive eruptions
• Mount Saint Helens, WA 1980
• Mount Pinatubo, Philippines 1991

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Transform Fault Boundaries

• Plate grind past each other without creating or
  consuming lithosphere
• San Andreas Fault Zone, CA Pacific plate moving
  northwest relative to North American plate
• Capable of producing strong earthquakes
• Loma Prieta, San Francisco, CA in 1989 7.4
• Northridge, Los Angeles, CA in 1994 6.7

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Plate Collisions formed the Himalayas
Continental plate Continental Plate Convergence
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Geologic time

• Geologists today - accurate dates to geologic
  events in Earth history
• Relative dating and geologic time scale
• Relative dating events dates in sequence or
  order without knowing their age in years
• Law of superposition oldest sedimentary rock
  layer on bottom
• Principle of fossil succession fossils appear
  in a definite order, represent time period
• Absolute (radiometric) dating Chapter 8

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Geologic time

• The magnitude of geologic time
• Age of Earth 4.5 billion yrs (4500 million)
• Humans arrived 0.01 million yrs
• Count one year per second 24/7 take 150 yrs (two
  lifetimes) to reach 4.5 billion yrs
• An appreciation for the magnitude of geologic
  time is important
• many geologic processes are very gradual
• we are just a fraction of Earths history

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Geologic Time Scale
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The nature of scientific inquiry

• Science assumes the natural world is consistent
  and predictable
• Goal of science discover patterns in nature, use
  knowledge to make predictions
• Scientists collect facts by observation and
  measurements

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The nature of scientific inquiry

• How or why things happen can be explained by
• Hypothesis a tentative (or untested)
  explanation
• Theory a well-tested and widely accepted view
  that the scientific community agrees best
  explains certain observable facts
• Paradigm extensively documented, high degree of
  acceptance, explain interrelated aspects of the
  natural world

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The nature of scientific inquiry

• Scientific method - gathering facts through
  observations and formulation of hypotheses and
  theories
• No fixed path for scientists to follow that leads
  to scientific knowledge
• Scientific investigation typically involve
• collect facts by observation/measurement
• develop working hypothesis to explain facts
• observations/experiments to test hypothesis
• accept, modify, reject hypothesis