Lecture 6

• Uniformitarians


  Used Steno's laws and the law of succession to put rock strata in order from oldest to youngest
• Full geologic column does not exist at any one location
• Some layers are missing in some locations because of erosion, etc.
• Absolute ages were estimated later using radiometric dating
• In the 19th century, Geology became increasingly uniformitarian
• Realized Earth is ancient and changes gradually
• Challenged common assumption at that time that Earth is <10,000 years old
• Radiometric dating did not yet exist

  
• Time required for landscape formation was estimated from rates of sediment deposition, etc.
• The Atlantic Ocean is ~3700km wide and widens by ~2.5 cm per year
• New crust forms at the mid-oceanic ridge, pushing Americas & Europe/Africa farther apart
• Suggests Americas & Europe/Africa may have separated ~148 million years ago
• Uniformitarian rough estimate is consistent with modern methods like radiometric dating, palaeomagnetic data, etc., which together suggest a date of ~200 million years ago
• Lord Kelvin


  English physicist who studied heat flow
• Lord Kelvin claimed that the earth is <20 million years old
• Kelvin suggested the sun must be young or it would have used up its fuel
• Kelvin suggested warm rocks deep in mines prove Earth is young; if Earth were old, it would have lost all its heat from the core through its surface to space
• Darwin was troubled that 20 million years seems far too young for evolution to generate today's biodiversity through gradual natural selection
• 20th century physicists proved Darwin was correct and Kelvin was wrong
• Kelvin assumed the sun generates heat by burning fuel, but it generates heat by nuclear fusion [unknown in his time]
• Kelvin did not account for hot rock rising through the mantle and warming the crust, nor did he account for heat generated by radioactive decay of uranium (etc.) in the crust [unknown in his time]
• Atoms


  Comprise three types of particles: protons (positively charged) and neutrons (neutral) in nucleus, electrons (negatively charged) in orbit
• Element


  # of protons determines the element
• Isotope


  # of neutrons determines the isotope

  
• Charge


  # of electrons determines the charge
• 12C and 13C are stable; 14C is unstable (and rare)
• Unstable isotopes (=radioisotopes) undergo radioactive decay
• Nucleus emits energy by radiation and the element or isotope generally changes; e.g., 14C decays to 14N

  
• Radioactive decay


  Stochastic - decay of any individual radioisotope nucleus is unpredictable, but the rate of decay of many nuclei is predictable

  
• Half-life


  The time required for half of the nuclei in a sample to decay
• We don't need to know the original amount of rubidium in the rock to use rubidium–strontium dating, ratios are used instead

  
• When an igneous rock forms, the 87Sr:86Sr ratio can be presumed the same in each mineral, even though their 87Rb:86Sr ratios may vary

  
• As the rock ages, 87Rb decays into 87Sr at a predictable rate, 𝜆
• 87Rb is unstable, 87Sr is stable, 86Sr is stable and non-radiogenic
• Radiometric dating is typically used only on igneous rocks
• Sedimentary rocks are formed by accumulation of sediments that may be of different sources and ages
• Metamorphic rocks have complicated histories that can lead to uneven isotope distribution
• Radiometric dating cannot be used directly on most fossils (which form in sedimentary rock)
• Instead, igneous layers near the fossil are used to estimate its age

  
• Radiocarbon dating can be used on recent subfossils (e.g., Neanderthals) and other organic remains