Earths History

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Earths History
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Geologic Events

• The present is the key to the past.
• This statement summarizes one of the basic
  principles of geologic history that the
  geologic processes going on today, such as
  weathering, erosion, volcanism, and earthquakes,
  also went on in the past.
• This concept is known as the principle of
  uniformitarianism.

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Sequence of Geologic Events

• Relative Age versus Absolute Age
• The relative age of a rock or event is the age as
  compared to other rocks or events.
• The absolute age of a rock or event is the actual
  age.

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Chronology of Layers

• There are two major principles used when
  interpreting geologic history
• Principle of original horizontality
• Principle of superposition.

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Principle of Original Horizontality

• The principle of original horizontality states
  that sediments are deposited in horizontal layers
  that are parallel to the surface on which they
  were deposited.
• This implies that tilted or folded layers
  indicate that the crust has been deformed.

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Original Horizontality
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Principle of Superposition

• The principle of superposition states that, in a
  series of undisturbed layers, the oldest layer is
  on the bottom and each overlying layer is
  progressively younger with the youngest layer on
  the top.

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Superposition
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Evidence of Events

• Several things provide geologists with evidence
  of events
• Igneous intrusions and extrusions
• Faults, joints and folds
• Internal characteristics.

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Igneous Intrusions and Extrusions

• When magma forces its way into cracks or crevices
  in crustal rock and solidifies, it forms a mass
  of igneous rock called an intrusion.
• When lava solidifies at the surface it forms a
  mass of igneous rock called an extrusion.
• Since the rock that the magma moved through, or
  over, existed prior to the intrusion, (or
  extrusion), it must be older.

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Igneous Intrusions
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Faults, Joints, and Folds

• Features such as faults, joints, and folds must
  be younger than the rocks in which they are
  found.
• A joint is a crack in a rock formation, similar
  to a fault but without any displacement.

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Internal Characteristics

• Cracks, veins, and mineral cement are younger
  than the rocks in which they appear.
• A vein is a mineral deposit that has filled a
  crack, or permeable zone, in existing rock.

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Correlation Techniques

• Correlation is the process of matching rocks and
  geologic events in one location to the rocks and
  events in another location.
• Methods used for correlation include
• Continuity of rocks
• Fossil evidence in rocks
• Volcanic time markers in rocks

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Correlation
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Bedrock and Outcrops

• Bedrock is the solid, unbroken rock of the crust.
• An outcrop is bedrock that is exposed at the
  Earths surface.
• Outcrops provide opportunity for geologists to
  directly study the layers of the bedrock, tracing
  them from one location to another called
  walking an outcrop.

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Bedrock / Outcrop
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Fossil Evidence

• Index fossils are fossils or organisms that lived
  over an extensive area, preferably over the
  entire Earth, for relatively short periods of
  time.
• Index fossils are useful in correlating the
  sedimentary rocks in which they are found.

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Index Fossils
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Fossil Correlation
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Volcanic Time Markers

• Severe volcanic eruptions can deposit a thin
  layer of volcanic ash over the surface of the
  entire Earth.
• These layers within a rock sequence may remain
  distinguishable and provide a time marker.
  (Similar to index fossils)

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Anomalies

• Anomalies are differences from what is expected.

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Determining Geologic Ages

• Without the rock record there would be no
  geologic history.
• The older the rock the more difficult to
  determine absolute age.
• The geologic history of an area is determined
  primarily by fossil evidence, the age of the
  rocks, and the erosional record in the rocks.

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Geologic Time Scale

• Geologic time is subdivided divided into units
  based on fossil evidence.
• There are 4 major divisions
• Precambrian represents the first 85 of Earths
  history (mostly devoid of fossils).
• Paleozoic Era represents 8.5 of Earths
  history (invertebrates, fishes, amphibians,
  vertebrates and land plants first appear).
• Mesozoic Era rep. 3.5 of Earths history
  (dinosaurs, earliest birds, and mammals).
• Cenozoic Era rep. 1.4 of Earths history
  (humanoids show up late 0.04 of history).

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Erosional Record

• Buried erosional surfaces, called unconformities,
  indicate gaps or breaks in the geologic time
  record.
• 4 steps combine in sequence to form
  unconformities
• Uplift, erosion, submergence and deposition
• 3 most common types of unconformities are
  angular unconformities, parallel unconformities,
  and nonconformities.

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Radioactive Decay

• Radioactive decay occurs when the nuclei of
  unstable atoms break down, changing the original
  atoms into atoms of another element.
• The rate of radioactive decay is measured in
  terms of half-life.
• Half-life is the amount of time it takes for half
  the atoms of a substance to decay into another
  element.
• Different substances have different half-lifes
• Examples are Uranium 238 and Carbon 14.

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History and Evolution of the Earths Atmosphere

• Earth is about 4.5 billion years old.
• The primary source of gases for the earliest
  atmosphere are thought to be from outgassing by
  volcanoes.
• Water vapor in the outer atmosphere would have
  been broken down into hydrogen and oxygen.

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Fossil Record

• Fossils preserved in the rocks provide evidence
  that many kinds of animals and plants have lived
  on Earth in the past under a variety of different
  conditions.