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					Geologic Time
“Nothing hurries geology”
      - Mark Twain
            What is Time?

           • Time is how we sense and record
             changes in the environment.

• It is our sense of one thing happening after
  another.
• We need standards of time for the convenience
  of everyday life.
                  Units of time
• As humans we think of time related to personal
  experience.
  –   Seconds
  –   Minutes
                  However, you will see that when we talk
  –   Hours         about Earth history, even using a
  –   Month        millennium isn’t sufficient because
  –   Years                of Earth’s vast age.
  –   Decades
  –   Centuries
  –   Millenia
                       Deep Time



• When we study history 200,
  1,000 or even 2,000 years seems
  like a long time.
• Geology involves vastly greater
  amounts of time
   – This is referred to as deep time.
        Geologic Processes
• Most are slow but relentless
  – Reflecting the pace at which the heat engines work
  – It’s unlikely that a mountain will visibly change shape
    or height during a human lifetime
             Geologic Processes
• In a geologic time frame
   – Most mountains are probably eroding “rapidly”




Uluru is a great block of uptilted sandstone in the heart of Australia's Outback.
           This region may be the world's best-developed peneplain,
                     and Uluru its best-known monadnock.
         To a geologist . . .
• “Rapidly” may mean within a few million
  years.
• A “fast” process
  – Started and completed within a few million
    years.
  – For example . . .
         Rate of Plate Motion




• At a mid ocean ridge
   – 1 cm/yr movement
   – 100,000 years for the
     seafloor to move 1,000
     km (100,000 cm in one
     km)
Some geologic processes do occur quickly
               Mt. St. Helens
Landslides Occur Quickly
• Tully Valley landslide on April 27, 1993
• Occurred after heavy precipitation of
 190 mm (7.5 in) during April in conjunction
 with melting of a winter snowpack
Processes of Change
   Through time
      Spatial Scales of Observation

• Geologists study all of
  Earth’s materials from:
  – The spatial scale of
    atoms
  – To the scale of our planet
    (global scale)




                                 Figure 1-02
      Spatial Scales of Observation

• Geologists study all of
  Earth’s materials from:
  – The spatial scale of
    atoms




                            Figure 1-02
      Spatial Scales of Observation

• Geologists study all of
  Earth’s materials from:
  – The spatial scale of
    atoms




                            Figure 1-02
      Spatial Scales of Observation

• Geologists study all of
  Earth’s materials from:
  – The spatial scale of
    atoms
  – To the scale of our planet
    (global scale)




                          Figure 1-02t   Figure 1-02
               Global Perspective of Earth




Figure 1-18B
• Thickness on the
  model is calculated
  using a proportion.

      119 mm          X
     6371 km        16 km

(6371 km) X = (119 mm)(16 km)

   X = (119 mm)(16 km)

          6371 km
   X = 0.3 mm

On this scale:
• Mt. Everest (8.8 km or 5.5. mi.)
  would be 0.16 mm high.
• Deepest location in the ocean
  (Mariana Trench – 10.9 km or
   6.8 mi) would be 0.2 mm deep
    The Geologic Time Scale
• Earth history is subdivided into time units based
  on:
  – The fossil record
  – Extinctions
     Geologic Time Scale – Temporal Scales of
                   Observation
• Like in a book, events and
  relationships are grouped into
   – Paragraphs
      • Sections
           – Parts




                                   Phanerozoic
• These are the geologic time
  scale’s
   – Eons
       • Eras
           – Periods
               » Epochs

• The “calendar” into which
  geologic events are placed.
     Temporal Scales of Observation




• Grand Canyon, AZ
  – Exposed rocks at the bottom of the
    canyon are more than a billion years
    old
  – Sediments in the Colorado River
     • May have formed just seconds ago

                                           Figure 1-01
Scale Model of Earth History

				
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posted:2/17/2013
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