Geology

Introduction

Geology - the study of the earth: "geo" + "ology"
  • its materials (e.g., rocks) and . . .
  • its processes (e.g., plate tectonics, earthquakes, etc.)

  • . . . and the history of the planet and its life forms from its origin to the present.
    'It is the job of the geologist to unravel this history and write the biography of the earth.'
  • relies heavily on multiple working hypotheses;

  • there are few "right" answers, . . . . . although this doesn't apply to tests!
Why be concerned with geology? It's important in many ways:
  • materials-minerals (gold, uranium, coal); rocks for building materials.
  • processes-earthquakes (can we predict them?); LA's big one, safer buildings;

  • tsunami warnings
    geothermal energy
    volcanic eruptions (Pinatubo, Mt. St. Helen's; landslides).
  • more practical:

  • -where to drill a well for water;
    -where to site a landfill, nuclear power plant;
    -flood control of rivers.
    -can I build on this wetland (delineation); should I live on this barrier island? floodplain?
Will Durant, "Civilization exists by geological consent, subject to change without notice." 

There are two major subdivisions of geology:
1) Physical geology-materials and processes; what's the world made of? What makes mountains? Subdisciplines:
a) structural b) petrology c) geochemistry d) geophysics, etc.

2) Historical geology-origin of the earth; a chronological history of the record of life; fossils, dinosaurs, etc. One subdiscipline is paleontology.

A Brief History of Geology
1) Aristotle-the first 'great' geologist; a philosopher who inhibited later research through the middle ages.

  • 17th and 18th centuries-theory of Catastrophism prevailed; major proponent was Georges Cuvier
  • Catastrophism:

    • proclaimed that earth features (mountains, valleys, etc.) were produced by cataclysmic events which no longer occur.
    • earth was thought to be quite young; Anglican Bishop James Ussher of Ireland traced all the 'begats' in the Bible and proclaimed the creation of earth to have occurred at 9:00 am, October 26, 4004 BC. This makes the world about 6000 years old!
    • late 18th century-James Hutton, 'Father of Modern Geology'; a Scottish physician; proposed the  Doctrine of Uniformitarianism. . . " The present is the key to the past."

    • A corollary of this is that the 'laws of nature are unchanging' throughout time and space.
    • we don't see such cataclysms; erosion is (and was) a slow process; this says that the world must be old to have produced the features we see.

    • An example of this logic: calculate the uplift rate of some mountains by fossils found at 10,000 feet which were dated to 15 million years. This gave a minimum rate of 0.2 mm/year.
      Erosion is lowering the North American continent at an average rate of 3 mm/1000 years.
      Charles Lyell-early 1800's.
    Nature of Scientific Inquiry-Scientific method
    -discuss hypothesis vs. theory vs. law
    • A hypothesis is not a guess. 
    • A theory is a widely held view which is the best explanation of scientific facts; no major inconsistencies. 
    • A law is a generalization for which we have no contradictory data.
    • Geologic Time

      How old is the Earth?  4.6 billion years (4,600,000,000 years).  The source of this number is radiometric dating using the Uranium-Thorium system.  See Chapter 10 (Geologic Time).

      Early ideas of the age of the Earth:

      1.  1654 Archbishop Ussher (Ireland), traced genealogy in Bible, using the 'Begats'.  He concluded that Earth was created October 26, 4004 BC, 9:00 am.  This makes the Earth 6000 years old.  This led to the Doctrine of Catastrophism which held that the Earth was shaped by series of giant disasters, requiring the fitting of many processes into a short time scale.

      2.  1770's, 1780's "Revolution" 
           James Hutton, Father of Geology (Scotland) 1726-1797.  Published Theory of the Earth in 1785.  Noting that Hadrian's Wall, built as a defense by Romans in England in the 122 A.D. and named for Emperor Hadrian, showed no significant change after 1500 years, he suspected that Earth was much older than Ussher's estimate.  He suggested that slow processes shape earth. Mountains arise continuously as a balance against erosion and weathering.  He proposed the Doctrine of Uniformitarianism: "The present is key to the past".  The physical and chemical laws that govern nature are uniform.  Unconformity at Siccar Point, Scotland was a major factor in his logic.  He suggested,  "No vestige of a beginning, no prospect of an end."

      3.  How old is Earth? Quantitative scientific methods.
      a.  In 1897, Lord Kelvin assumed that the Earth was originally molten and calculated a date based on cooling through conduction and radiation.  Age of Earth was calculated to be about 24-40 million years.
       Problem: Earth has an internal heat source (radioactive decay)

      b.  In 1899 - 1901, John Joly (Irish) calculated the rate of delivery of salt to the ocean. River water has only a small concentration of salts (~0 ppt). Rivers flow to the sea. Evaporative concentration of salts.  Salinity of the ocean averages 35 ppt.
                Age of Ocean = Total salt in oceans (in grams) divided by rate of salt added (grams per year)
                Age of Earth was calculated to be 90-100 million years.
       Problems: no way to account for recycled salt, salt incorporated into clay minerals, salt deposits.

      c.  Thickness of total sedimentary record divided by average sedimentation rates (in mm/yr). In 1860, calculated to be about 3 million years old. In 1910, calculated to be about 1.6 billion years old.  Early measurements of maximum thickness of sediment ranged from 25,000 m to 112,000 m. With more recent mapping, thickness of fossiliferous rocks is at least 150,000 m.  Sedimentation rates average about 0.3 m/1000 years.  At this rate, the age of the first fossiliferous rocks is about 500 million  years.
      Problems: did not account for past erosion or differences in sedimentation rates; also ancient sedimentary rocks are metamorphosed or melted.

      d.  Charles Lyell 1800's compared amount of evolution shown by marine mollusks in the various series of the Tertiary System with the amount that had occurred since the beginning of the Pleistocene. Estimated 80 million years for the Cenozoic alone.
      e.  Discovery of radioactivity by Henri Becquerel in 1896. In 1905, Rutherford and Boltwood used radioactive decay to measure the age of rocks and minerals. Uranium decay produces He, leading to a date of 500 million years.   In 1907, Boltwood suspected that lead was the stable end product of the decay of uranium. He published the age of a sample of urananite based on Uranium-Lead dating. Date was 1.64 billion years.  So far, oldest dated Earth rocks are 3.96 billion years. Older rocks include meteorites and moon rocks with dates on the order of 4.6 billion years.  Moon rocks, highland ~ 4.5 by, mare basalt ~ 3.2 - 3.8 by.  Meteorites - older than 4.5 by 
                Mass spectrograph was used after WWI (1918). Led to the discovery of over 200 isotopes.  Many radioactive elements can be used as geologic clocks. Each element decays at its own nearly constant rate. Once this decay rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent and the amount of stable daughter elements.  Example: Potassium-Argon dating.

      Why is the Earth younger than the moon and meteorites?

      •  Earth is geologically active. 
      •  Has a hot, molten interior. 
      •  Rocks are remelted and their internal clocks are reset. 
      • Also, rocks on Earth's surface are acted on by erosion and weathering. Rocks on Earth surface are not as old as the Earth, they are "recycled" rock materials.  Rocks broken down into sediment (gravel, sand, silt, clay).  Sediment will turn into sedimentary rock over time. Older rocks are buried deeply under younger rocks.


      Where do we find the oldest rocks on Earth?
      Canadian Shield. (NW Territories near Great Slave Lake, 3.96 by). Gneiss.  Narrows the gap between origin of Earth and first rocks to 640 million years. (Geotimes 12/1989).  Before this, oldest rocks known were from Isukasia region of Greenland (3.8 by).
      Glaciers 2 miles thick scraped off young recycled rocks.
      Land rose 250 ft since ice was removed => more erosion.
      Isostasy

      Very old rocks are at the surface in the Canadian Shield area.  Up to about 3.8 or 3.96 billion years old.

      Multicellular life did not appear until about 1 billion years ago. Before this, 3 billion years ago single celled life only. Hard parts like shells don't appear until 600 million years ago. (Trilobites) 

      How do we know that these things are millions or even billions of years old?

        Absolute dating tells exactly (+ or -) how old something is.
        e.g., radiometric dating allows us to age the earth at 4.6 billion yrs (4.6 x 109); would take 31.7 years to count to 1 billion at one count/second; 146 years to get to 4.6 billion. One billion beer bottles would circle the earth > 17x.
      Consider this*:
      Compress, for example, the entire 4.6 billion years of geologic time into a single year.  On that scale, the oldest rocks we know date from about mid-March.  Living things first appeared in the sea in May.  Land plants and animals emerged in late November and the widespread swamps that formed the Pennsylvanian coal deposits flourished for about four days in early December.  Dinosaurs became dominant in mid-December, but disappeared on the 26th, at about the time the Rocky Mountains were first uplifted.  Manlike creatures appeared sometime during the evening of December 31st, and the most recent continental ice sheets began to recede from the Great Lakes area and from northern Europe about 1 minute and 15 seconds before midnight on the 31st. Rome ruled the Western world for 5 seconds from 11:59"45 to 11:59:50.  Columbus discovered America 3 seconds before midnight, and the science of geology was born with the writings of James Hutton just slightly more than one second before the end of our eventful year of years.
      *Don L. Eicher, Geologic Time, 2nd ed.  (Englewood Cliffs, NJ:  Prentice Hall, 1978), pp 18-19.
       
      Click here for a brief chronology of what's happened on earth in the last 4.6 billion years!

      Relative dating compares one object to another w/o knowing the absolute age.

        a) law of superposition -the stratum on top is younger than the one below it (if strata are undeformed); but . . .
        b) principle of faunal succession -extensive fossil studies allow us to date an object by the fossils they contain.
      See Geologic Time Scale in lab book and text book; create a mnemonic, a device to remember these; quiz next time.
      (a) Q uick (quaternary)
      T emper(tertiary)
      C aused
      J eff
      T o
      P unch
      P aul
      (in the)

      M outh
      D uring (a)
      S cuffle
      O ver
      C arol

      Structure of the Earth

      atmosphere 
      hydrosphere
      (wet orange) 

      Crust (5-40 km) 

      Mantle (2885 km)
      km = .61 mile
      Outer Core (2270 km) 

      Inner Core (1216 km)
       
       

      The Earth is like a jellybean, only smoother: relief from trench to mountain top is ~12 mi.

        lithosphere-solid; composed of crust and upper mantle.
        asthenosphere -plastic; lower part of upper mantle.
      Surface is continents and basins.
      • boundary is the about 1/2 way down the continental shelf due to changing sea levels.

      Crust

      Continental (40%) Oceanic (60%)
      Thickness 35 km 5 km
      Density (SG) 2.7 3
      Rock type Igneous Igneous
      Andesite Basalt
      High levels of K, Na, Si Fe, Mg, Ca

      Origin of the Earth-excellent film (KC#195) The Third Planet, of Planet Earth series.
      1st half-geological origin of earth by accretion.  2nd half-mass extinctions by meteorites.

        • meteor Toutatis passed at 2.2 million mi in Dec. 1992
        • smaller earth-crossing asteroid passed w/in 106,000 mi. in 1990. (in between

        • us and the moon!!)
        • apparently on 26 million year cycle.