The Science of Historical Geology

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Chapter 1

• The Science of Historical Geology

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Introduction

• The Earth formed about 4.6 billion years ago.
• Homo sapiens appeared on Earth between about
  300,000 and 150,000 years ago.

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• Humans ask questions about their surroundings.
• How did the Earth form?
• Why do earthquakes occur?
• What lies beneath the land and below the ocean
  floor?
• Curiosity leads to exploration.

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Why Study Earth History?

• The Earth has changed through time.
• Understanding past geologic events will help us
  predict future geologic events.

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• Past geologic events include
• Earthquakes.
• Volcanic eruptions.
• Continents flooded by inland seas.
• Drifting and colliding continents.
• Glaciers have covered large parts of continents.
• Meteorite and asteroid impacts.
• Changes in chemistry of oceans and atmosphere.
• Changes to life on Earth through time - sometimes
  slow, sometimes swift and deadly.

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Geology

• Geology is the study of the Earth.
• Two major branches of geology
• Physical Geology - deals with Earth materials and
  processes
• Historical Geology - deals with origin and
  changes of Earth and life through time and space.
  

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What do Geologists Do?

• Study the structure of mountain ranges
• Attempt to predict geologic hazards like
  earthquakes and volcanic eruptions.
• Identify minerals in meteorites to learn how
  Earth formed.
• Study rivers, floods, glaciers, and underground
  water.
• Look at results of past events and work backward
  in time to discover causes of those events.
• Search for fossil fuels and mineral resources.

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Scientific Method in Geology

• Science operates through the use of the
  Scientific Method.
• The scientific method is a method for finding
  answers to questions and solutions to problems.
  Scientists work like detectives to gather data,
  to try to figure out what happened. The data may
  be obtained through observations and/or
  experiments, which can be repeated and verified
  by others.

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Summary of Scientific Method

• A question is formulated.
• Observations (collect data)
• Develop multiple working hypotheses (ideas to
  explain the observations)
• Test the hypotheses by experimenting and either
  accept, reject, or modify the hypothesis.
• The simplest explanation is best.
• When a hypothesis has considerable experimental
  or observational support, it is accepted and
  others are rejected, and it may become a theory.
• A theory ultimately may become a scientific law.

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What is a Theory?

• A hypothesis that survives repeated challenges,
  and is supported by a large body of evidence, may
  be elevated to the status of a theory.
• A theory is not just an wild idea or a guess.
• Theories have survived close examination, and can
  be accepted with confidence.
• A theory has a very high probability of being
  correct.
• Examples of theories include the theory of
  relativity, plate tectonics theory, evolutionary
  theory, and atomic theory.

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Major Themes in Earth History

• Deep time
• Plate tectonics
• Evolution of life

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Deep Time

• Recognition of immensity of geologic time is
  geology's most important contribution to human
  knowledge.
• The science that deals with determining the ages
  of rocks is called geochronology.

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Methods of Dating Rocks

• Absolute age - The actual age. Quantifying the
  age of the rock or mineral in years.
• Relative age - Determining which rocks are older
  and which are younger.

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Absolute Age

• The discovery of radioactivity in 1896 gave us
  the tools to find the absolute age of a rock.
• Radiometric dating involves analysis of the
  breakdown of unstable radioactive elements in
  rocks.
• Radioactive elements decay by releasing subatomic
  particles from their nuclei. Through this
  process, the unstable radioactive element is
  converted to a stable "daughter" element.
• Example Uranium-235 decays to form lead-207.

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Radioactive Decay

• Many radioactive elements can be used as geologic
  clocks. Each radioactive element decays at its
  own nearly constant rate. The rate of decay can
  be measured.
• Once this rate is known, geologists can determine
  the length of time over which decay has been
  occurring by measuring the amount of radioactive
  parent element and the amount of stable daughter
  elements.

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Half Life

• Each radioactive element has its own unique
  half-life.
• A half-life is the time it takes for half of the
  parent radioactive element to decay to a daughter
  product.
• Example Uranium-235 has a half-life of about 704
  million years.

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• Uranium-235 decays to form lead-207. Uranium-235
  has a half-life of about 704 million years.
• After 704 million years, only half (50) of the
  uranium atoms in the mineral remain. (The rest
  have decayed to lead-207.)
• After another 704 million years, only half of
  that amount (or 25) of the uranium atoms remain.
  
• So, a rock with 25 uranium-235 and 75 lead-207
  must be 1,408 million years old (or 1.408 billion
  years old).

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• Using radiometric dating, some rocks found in
  Canada's Northwest Territories have been dated at
  4.04 billion years old.

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Relative age

• Determining which rocks are older and which are
  younger. Rock unit A is older than rock unit B".
• The geologic time scale was developed through
  relative dating.
• Relative age determinations provide a framework
  or geologic time scale in which to place events
  of the geologic past.
• Using radiometric dating, actual dates in years
  have been determined for the geologic time scale.

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Major Themes in Earth History

• Deep time
• Plate tectonics
• Evolution of life

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Plate Tectonics

• The theory of plate tectonics has revolutionized
  the understanding of geology. Plate tectonics
  explains many large scale patterns in the Earth's
  geological record.
• It is a "great unifying theory" in geology.

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Plate Tectonics

• The Earth's surface or lithosphere is divided
  into plates (about 7 large plates and 20 smaller
  ones).

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Plate Tectonics

• The lithosphere is about 100 km thick and
  consists of the rigid, brittle crust and
  uppermost mantle.
• Rigid lithospheric plates rest (or "float") on
  the asthenosphere, the easily deformed, or
  partially molten part of mantle below the
  lithosphere.
• The plates are moving, but their rates and
  directions of movement vary.

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Plate Movements

• Plate movement is due to convectional flow
  (circular movement of the asthenosphere due to
  hot material rising and cooler material sinking).
  
• The plates only move a few millimeters per year,
  about the rate at which your fingernails grow.

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Types of plate boundaries

• Divergent - where plates move apart from one
  another.
• Convergent - where plates move toward one
  another.
• Transform - where two plates slide past one
  another

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Major Themes in Earth History

• Deep time
• Plate tectonics
• Evolution of life

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Evolution of Life

• In biology, evolution is the
• "great unifying theory" for understanding
• the history of life.

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Evolution of Life

• As a result of evolution, plants and animals
  living today are different from their ancestors.
  They differ in appearance, genetic
  characteristics, body chemistry, and in the way
  they function.
• These differences appear to be a response to
  changes in the environment and competition for
  food.
• Fossils record the changes in organisms over
  time.

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Natural Selection

• Charles Darwin and Alfred Wallace were the first
  scientists to assemble a large body of convincing
  observational evidence in support of evolution.
• They proposed a mechanism for evolution which
  Darwin called natural selection.

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• Natural selection is based on the following
  observations
• Any given species produces more offspring than
  can survive to maturity.
• Variations exist among the offspring.
• Offspring must compete with one another for food
  and habitat.
• Offspring with the most favorable characteristics
  are more likely to survive to reproduce.
• Beneficial traits are passed on to the next
  generation.

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Lines of evidence for evolution cited by Darwin

• Fossils provide direct evidence for changes in
  life in rocks of different ages.
• Certain organs or structures are present in a
  variety of species, but they are modified to
  function differently (homologous structures).
• Modern organisms contain vestigial organs that
  appear to have little or no use. These structures
  had a useful function in ancestral species.
• Animals that are very different, had
  similar-looking embryos.

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Other lines of evidence for evolution come from
the fields of

• Genetics (DNA molecule)
• Biochemistry (Biochemistry of closely-related
  organism is similar, but very different from more
  distantly related organisms).
• Molecular biology (sequences of amino acids in
  proteins)

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Organic Evolution

• These discoveries indicate that plants and
  animals of each geologic era arose from earlier
  species by the process we call "organic
  evolution".
• Organic evolution refers to changes that have
  occurred in organisms with the passage of time.