Earth History, Dating Methods, and Geological Time

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Earth History, Dating Methods, and Geological Time

• Geological Background to Primate Evolution

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Geological Background Rocks

• Igneous rocks are formed by volcanic processes
• Often by cooling of molten rocks (at surface or
  within Earths interior)
• They are important for radiometric dating (see
  K-Ar dating)

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Geological Background Rocks

• Sedimentary rocks are formed from deposition of
  sediments into layers
• Deposited by wind, water, gravity into layers
• Sediments formed by erosion of other rocks
• Layers are gradually hardened over time
• Sedimentary rocks are important because fossils
  are found in these rocks

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Geological Background Rocks

• Metamorphic rocks have morphed into another
  kind of rock
• Formerly sedimentary or igneous
• Changed by heat and pressure
• Not particularly important in paleontology
• But pretty!

How Rocks are formed
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Dating Methods

• Absolute and Relative Dating Methods
• Absolute methods provide ages in years BP
• Relative methods provide only relative sequence
  of events
• Often used in combination
• Direct and Indirect Dating Methods
• Direct methods date the object itself
• Indirect methods date something associated with
  the object of interest

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Stratigraphy Superposition

• The key to geology is understanding the layers of
  rock in the field
• Law of Superposition
• More recent strata are laid down on top of older
  strata, unless disturbed
• Faulting and folding can complicate matters
• Biostratigraphy is a relative dating technique
  that relies on correlation and the principle of
  superposition with respect to fossils embedded
  within strata

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Radiometric Dating Techniques

• Rely on decay of radioactive isotopes
• Decay constant
• Probability per unit time of a molecule of a
  radioactive element decaying
• Half-Life
• Time required for half the mass of radioactive
  mineral to decay to byproduct
• Different isotopes of same element have different
  atomic mass but same atomic number
• Atomic mass number of protons neutrons
• Atomic number number of protons
• Different elements have different atomic number

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Radioactive Decay

• Radioactive decay occurs at exponential or
  geometric rate
• Equal proportions are lost per unit time
• What is radioactive decay actually?
• Click here to find out

y
.5y
.25y
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2
3
X axis time in half lives Y axis amount
radioactive material remaining
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Potassium-Argon Dating

• 40K decays to 40Ar (and 40Ca)
• Half life is 1.25 billion years
• Ratio of 40K to 40Ar is measured to yield date
• Potassium is one of most common elements in
  Earths crust
• 39K is stable and most common isotope
• Potasssium is common in igneous rocks
• K-Ar clock is set with volcanic eruption
• High temperature releases any 40Ar and begins
  decay of 40K to 40Ar
• Argon gas is captured in crystal lattice of
  hardened lava (basalt, obsidian, etc)

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Radiocarbon (C-14) Dating

• 14C decays to 14N
• Half life is 5740 years
• Works back to 50 or 75,000 years BP
• 14C is created in atmosphere when cosmic rays
  bombard atoms of 14N
• 14C is a small percentage of atmospheric carbon
• It is incorporated into living things thru
  photosynthesis then heads up the food chain
• When an organism dies, it no longer incorporates
  14C
• The clock starts ticking at death, ratio of 14C
  to 12C changes, until all the 14C is gone
• Radiocarbon dating only works for organic remains
• Bone, teeth, wood or charcoal, etc.

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Web Sites to Study

• Two excellent web sites on dating methods in
  anthropology paleontology
• many other informative websites exist on these
  topics use your favorite Search Engine to find
  others
• Dating Methods, from UCSB
• Record of Time, from Palomar College

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Geology and Dating Methods QUIZ

• Describe, compare and contrast the K-Ar and C-14
  dating methods in a concise, informative and
  well-organized paragraph of complete sentences
  and good grammar.

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Alfred Wegener and Continental Drift

• 1880-1930
• German astronomer, meteorologist, and polar
  explorer
• 1915 The Origin of Continents Oceans

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Wegeners Theory

• Continents and oceans have moved across the face
  of the Earth in the past
• Complimentary contours of continents and
  continental shelfs
• Similar geological formations separated by
  oceanic basins
• Geographic distribution of past fossil forms
  across oceanic gaps

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South America Africa
These continents share all three kinds of data
suggesting they were once joined.
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Why Was Wegeners Theory Soundly Rejected?

• Built on mostly circumstantial evidence, most of
  which could be explained by other hypotheses
• Land bridges and sunken continents
• Major weakness was that he had no mechanism that
  could explain how continents could move across or
  through the earth

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Plate Tectonics A Revolution in the Earth
Sciences

• Paleomagnetism
• Earths magnetic field has switched many times in
  past from Normal to Reversed
• Apparent polar wandering
• Igneous rocks retain remnant magnetism from
  Earths magnetic field and bear the record of
  magnetic reversals and polar wandering
• Deep Sea Oceanography
• Parallel stripes of magnetism along both sides of
  mid-oceanic ridges, or spreading centers
• K-Ar dating shows older ages further from ridge

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Plate Tectonics A Revolution in the Earth
Sciences

• Sea Floor Spreading
• New crust created at spreading centers
• New crust cools and spreads laterally
• Plates and plate boundaries
• Oceanic and continental plates in lithosphere
  ride upon plastic asthenosphere
• Convection currents in asthenosphere, powered by
  heat from radioactivity deep within Earths mantle

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Plate Tectonics A Revolution in the Earth
Sciences

• Plate boundaries and geologic features explained
  by plate tectonics
• Mid-oceanic ridges or spreading centers
• Mid-Atlantic ridge
• Subduction zones, where oceanic plates are pulled
  down below continental plates
• Island arcs and marine trenches
• Volcanoes
• Earthquakes along many plate boundaries
• Transform faults, where plates slide by each
  other
• San Andreas fault
• Mountain building, where continental plates
  collide
• India and Asia form Himalayas

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Plates and Plate Boundaries
Plate tectonics earthquakes
Paleomap Project
USGS on plate tectonics
UCMP on plate tectonics
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Earthquakes are Associated with Plate Boundaries