Geologic Time

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Geologic Time
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• Overview Explain the techniques for determining
  the age and composition of the Earth and the
  universe.
• Objectives
• Compare age of earth using radiometric aging, and
  expanding universe measurements
• Identify techniques for evaluating the
  composition of objects in space
• Explain geologic time scale and the major events
  in each era.
• Examine radioactive dating, and carbon-14 dating
• Explain fossil succession
• Explain relationship between plate tectonics and
  continent formation

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Rocks and the History of Earth

• Rocks record geological events and changing life
  forms of the past.
• Rocks have shown the earth is older than
  originally thought.
• Same geological process that formed the earth are
  still happening today.
• Uniformitarianism forces and process we observe
  today have been at work for a very long time.
  (James Hutton 1700s)
• Changes can take hundreds, thousands, or even
  millions of years

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Geological Time Scale

• Revolutionized the way people thought about time
  and how our planet was perceived.
• Was developed to show sequence, or order, of
  events based on principals of relative dating
• Relative dating tells us the sequence in which
  events occurred, now how long ago they occurred.
• Nicholas Steno, a Danish anatomist ,geologist,
  and priest is credited with describing a set of
  geologic observations that are the basis of
  relative dating

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Geological Time Scale

• Principals governing the geologic time scale
• Law of superposition
• Principal of original horizontality
• Principal of cross-cutting relationships
• Inclusion (sometimes)

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Geological Time Scale

• Law of Superposition -
• In a underformed sequence of sedimentary rocks,
  each bed is older than the one above it and
  younger than the one below it

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Geological Time Scale

• Principal of Original Horizontality
• Layers of sediment are generally deposited in a
  horizontal position as in the deposition of
  sediments within lakes, rivers, and oceans.

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Geological Time Scale

• Processes such as tilting, folding, faulting, and
  intrusions of igneous rocks can distort the
  original strata.
• Tilted geologic layers were originally
  horizontal. Sediments will fill in uneven
  geologic layers. Geologists observe the tilted
  layer and try to visually reposition it
  horizontally.

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• The sedimentary layers in this large roadcut near
  Denver, CO. can be clearly recognized by the
  variation in color. These layers can be
  recognized as having been deformed because they
  have been tilted so they are dipping to the east
  (the left side of photo). This deformation was
  related to the uplift of the Rocky Mountains.

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Geological Time Scale

• Cross-Cutting Relationships
• Faults cut through layers or magma intrudes other
  rock and crystallizes, it is assumed the fault or
  intrusion is younger than the rocks affected.

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Geological Time Scale

• Inclusion
• Pieces of one rock unit that are contained within
  another.
• the rock unit from which the pieces came have to
  be older in order to provide the fragment.

Inclusion of metamorphic rock in lava
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Geological Time Scale

• Unconformities
• Breaks in rock record
• Long periods which deposition stops, and erosion
  removes previously formed rock, then deposition
  resumed.
• Uplift and erosion are followed by subsidence and
  renewed sedimentation
• Each represents significant geologic events in
  history

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Geological Time Scale

• 3 types of unconformities
• Angular
• easily recognized
• Disconformities
• More common
• Nonconformities
• Separates rock

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Geological Time Scale

• Correlation of rock layers
• Rocks of similar age located in different regions
  match up
• Allows a more complete view of the geological
  history of a region
• Use to trace rock formations over a relatively
  short distance

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Fossils

• Fossil remains or traces of prehistoric life.
• Important time indicators (help correlate rocks)
• Interpret and describe ancient environments
  including temperature
• Must have rapid burial and
• posses hard parts
• Types of fossils
• Unaltered remains
• Altered remains
• Trace remains ( indirect )

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Fossils

• Unaltered may not have been altered over time.
• Less common to find
• Teeth, bones, shells, entire animals, like
  insects or plant parts trapped in amber

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Fossils

• Altered remains that have likely been altered
• Petrified turned to stone mineral-rich water
  soaks into dead organism and replaces the cell
  walls or other solid material
• Molds/Casts
• Molds - organism is buried in sediment and then
  dissolved by underground water leaving an
  accurate representation of the organisms shape.
• No internal information
• Casts minerals fill hollow spaces in the molds

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Fossils
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Fossils

• Carbonization preserving leaves and delicate
  animal animals / buried under fine sediment
• Thin layer of Carbon residue is left after
  pressure squeezes out all the gas and liquid
  components.
• Very rare

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Fossils

• Trace / Indirect Evidence indirect evidence of
  prehistoric life
• Made in soft sediment
• Later filled with mineral water and preserved
• Some of the oldest known fossils
• Provide information regarding food habits
• Worm burrows, coprolites (dung and stomach
  contents)

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Fossils and Time Correlations

• William Smith (18th Century) fossils were not
  randomly distributed.
• Fossils show a connection between rock layers and
  times
• Fossils are distinct
• Principle of Fossil Succession
• Fossils succeed one another in a definite and
  determinable order. Therefore, any time period
  can be recognized by its fossil content.

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Fossils and Time Correlations

• Principle of Fossil Succession Order
• Age of Trilobites (early arthropods)
• Age of Fish
• Age of Coal Swamps
• Age of Reptiles
• Age of Mammals
• ages correspond to a particular time period

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Fossils and Time Correlations

• Obtaining numerical dates for geological past
• Radioactivity unstable nuclei spontaneously
  break apart or decay
• Decay continues until a stable or non-radioactive
  isotope is formed
• Half-Life necessary amount of time for half of
  the nuclei to reach a stable isotope

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Fossils and Time Correlations

• Radiometric Dating calculating the ages of
  rocks and minerals that contain certain
  radioactive isotopes it has provided 1000s of
  dates for events in Earths history
• Decay occurs at a constant rate
• Decay accumulation occurs at a constant rate
• Only accurate if mineral accumulation remains in
  a closed system (no loss of isotopes)
• Carbon-14 - dates more recent events
• All organisms contain a small amount of carbon-14
• When organisms die amount of carbon-14 decreases
• Carbon-12 is compared to Carbon-14 to determine
  age

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Fossils and Time Correlations

• Geological Time Scale Earths 4.5 billion year
  history divided into specific amounts of time
• Eons greatest expansion of time
• Eras division of eons
• Paleozoic ancient life
• Mesozoic middle life
• Cenozoic recent life
• Periods subdivision of eras
• Epochs smallest division of time

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Rocks

• Rock any solid mass of mineral or mineral-like
  matter that occurs naturally as part of our
  planet
• Three Types of Rock
• Igneous
• Sedimentary
• Metamorphic

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Rocks

• Igneous Rock formed from hardening lava
• Can be formed above or below the Earths surface
• Classified based on texture and composition
• Classifications Textures
• granitic course-grained
• basaltic fine-grained
• andesitic glassy
• ultramafic porphyritic

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Rocks

• Sedimentary Rock solids settle out of a fluid
  such as water or air and eventually become
  cemented
• Two classifications
• Clastic
• Chemical

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Rocks

• Metamorphic Rock when existing rocks are
  changed by heat and pressure
• Classifications
• Foliated Metamorphic
• Banded appearance
• Nonfoliated Metamorphic
• Not banded

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Rocks

• How rocks come to be The Rock Cycle
• - interactions between water, air, and land can
  cause rocks to change from one type to another.
• - driven by heat from Earths interior
• 1 - magma forms beneath the Earths surface
• 2 magma cools and solidifies to form igneous
  rock
• 3 surface rocks are broken down into sediments
• 4 sediments are compacted and cemented to form
  sedimentary rock
• 5 sedimentary rock changes to metamorphic rock
  under extreme pressure and temperature
  conditions.

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Minerals

• Must have the following to be a mineral
• Naturally Occurring
• Solid Substance
• Orderly Crystalline
• Definite Chemical Composition
• Inorganic

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Minerals

• How minerals form
• 4 major processes
• Crystallization from magma
• Minerals rich in iron, calcium and magnesium
• Precipitation
• Ex halite, calcite
• Pressure and Temperature
• Ex talc, muscovite
• Hydrothermal Solution
• Ex quartz, pyrite