dating of rocks

• Stratigraphy

  • Scientific discipline
  • Describes rock successions and their interpretation in terms of a general time scale

  
• Rock layers (strata)


  • Found throughout the Earth
  • Most are sedimentary rocks formed from weathered old rocks

  
• Sediment


  Examples are gravel, sand, mud that bury dead plants and animals
• How sedimentary rocks are made


  1. Particles accumulate and become rocks

  2. Organisms buried with them become fossils

  
• Relative age


  • Compares chronologic sequence of a rock, fossil or geologic feature without using a number
  • Used to determine age of rock relative to rock units around it
• Principles of relative dating

  • Geologists use these guides to determine the relative order that geological events occurred
• Uniformitarianism


  • Changes in Earth's crust result from continuous and uniform processes
  • Processes such as storm, earthquake, volcanism, weathering have occurred multiple times throughout time and their result can be observed in the rock record
• Ripple marks


  • Formed by flow of water over sand
  • Modern in Lake Michigan (l), fossilized (r)
• Original horizontality


  • Sediments are deposited in flat, horizontal layers parallel to Earth's surface
  • Not horizontal rock layers have been altered by tectonic forces such as folding or faulting

  
• Folded rock layers


  • Layers F, I, H, M were deposited in horizontal layers, then folded
• Superposition


  Younger rock layers lie on top of older rock layers
• Superposition


  • D was deposited, then C, B, A
• Lateral continuity


  • Rock layers extend horizontally in all directions
  • Breaks or separations are caused by barriers at time of deposition or erosion
  • Breaks happen after layering and then sediments are placed in the gap
• Lateral continuity


  • A, F, E, C, D, B
• Cross-cutting relationship


  • Involves features that cut across older rocks
  • Younger than the units it cuts through
• Cross-cutting relationship


  • A, E, D, C, B
• Inclusions


  • Old rock pieces inside younger rocks
  • Younger than rock it is surrounded with
• Inclusions


  • B, D
• Unconformity


  Surface of erosion or non-deposition that separates rock layers of different ages

  
• Unconformity


  • E, D, A, B, C
• Unconformity


  • J, I, H, A, C, B, G, F, E, D
• Absolute dating


  • Used to determine age of a rock in millions of years before present
  • Radiometric dating is a technique that uses radioactive isotopes of elements in the rock to estimate its age
• Isotopes


  Atoms of an element that differ in number of electrons and atomic weight
• Radioactive isotopes


  Unstable and decay, meaning they spontaneously change from an unstable to stable form
• Half-life


  Amount of time it takes for 50% of radioactive parent isotope to decay to daughter isotope
• Radioactive dating information


  • Radioactive isotope name
  • Disintegration (parent and daughter)
  • Half-life (amount of time it takes for half of original substance to decay)
• Radioactive decay


  • Natural breakdown of an unstable, radioactive element into a stable element
  • Happens at a predictable rate, which makes it useful for determining the age of an object containing a radioactive element
• Radioactive dating examples


  • Mammoth (bone containing carbon 14)
  • Mastodon
  • Rubidium 87
• Stratigraphy

  • Scientific discipline
  • Describes rock successions and their interpretation in terms of a general time scale
• Types of dating of rocks

  1. Relative dating
  2. Absolute dating