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• Geologic time scale


  A tool that shows the sequence of events based on relative dating principles
• Relative dating


  • Placing rocks in their proper sequence of formation, first, second, third, and so on
  • Cannot tell how long ago something took place, only that it followed one event and preceded another
• Law of superposition


  In an undeformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below
• Principle of original horizontality


  Layers of sediment are generally deposited in a horizontal position
• Principle of cross-cutting relationships


  A fault or intrusion is younger than the rocks it cuts through
• Inclusions


  • Unmelted remnants of surrounding host rock incorporated into an igneous mass
  • Igneous rock inclusions in younger sedimentary layers
• Nonconformity


  Older intrusive igneous rocks overlain by younger sedimentary layers
• Early attempts at determining Earth's age using sediment accumulation rates were unreliable
• Prior to the discovery of radioactivity, geologists had no accurate and dependable method of numerical dating and had to rely solely on relative dating
• Inclusions


  Remnants of the surrounding host rock that were broken off and incorporated at the time the magma was intruded
• Exposure and weathering of intrusive igneous rock

  Inclusions of surrounding rock
• Intrusive igneous rock


  Must be older than the overlying sedimentary beds because the sedimentary beds contain inclusions of the igneous rock
• Nonconformity


  When older intrusive igneous rocks are overlain by younger sedimentary layers
• Dike B and its associated sill are older than dike A, because dike A cuts the sill
• The batholith was emplaced after movement occurred along fault B, but before dike B was formed, because the batholith cuts across fault B and dike B cuts across the batholith
• Inclusions


  Pieces of one rock unit that are contained within another
• The rock mass containing inclusions is the younger of the two
• Conformable


  Layers of rock that have been deposited essentially without interruption
• Unconformity


  A break in the rock record where deposition ceased, erosion removed previously formed rocks, and then deposition resumed
• Angular unconformity


  • Tilted or folded sedimentary rocks that are overlain by younger, more flat-lying strata
• Disconformity


  Strata on either side are essentially parallel, with little evidence of erosion
• Nonconformity


  Separates older metamorphic or intrusive igneous rocks from younger sedimentary strata
• Rapid burial and the possession of hard parts are necessary conditions for an organism to be preserved as a fossil
• Flesh usually decays so rapidly that preservation is exceedingly unlikely, so hard parts such as shells, bones, and teeth predominate in the fossil record
• Principle of fossil succession


  Fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content
• The succession of dominant organisms, never out of order, is found on every continent
• Index fossils


  Fossils that are widespread geographically and are limited to a short span of geologic time, so their presence provides an important method of matching rocks of the same age
• Groups of fossils can be used to establish the age of a rock bed more precisely than using a single fossil
• Fossils can provide information about past environments, such as the type of water (shallow or deep, warm or cold) and the position of ancient shorelines
• Chemical fossils, or organic compounds derived from organisms, can also be used to determine the kind of organisms from which they are derived
• Fossils, the remains or traces of prehistoric life, are important inclusions in sediment and sedimentary rocks
• Paleontology


  The scientific study of fossils, blending geology and biology to understand the succession of life over geologic time
• Knowing the nature of the life-forms that existed at a particular time helps researchers understand past environmental conditions
• Fossils are important time indicators and play a key role in correlating rocks of similar ages that are from different places
• Types of fossils


  • Unaltered remains (teeth, bones, shells)
  • Entire preserved organisms (mammoths, sloths)
  • Petrified fossils (mineral matter fills internal cavities)
  • Replacement fossils (cell walls and solid material replaced with mineral matter)
  • Molds and casts
  • Carbonized fossils (thin carbon residue)
• Trace fossils include tracks, burrows, coprolites, and gastroliths
• Conditions favoring preservation


  Only a tiny fraction of organisms are preserved as fossils, normally remains are destroyed
• Correlation is the task of matching up rocks of similar age in different regions to develop a geologic time scale applicable to the entire Earth
• Within a limited area, correlation can be done by walking along the outcropping edges, but this may not be possible when rocks are mostly concealed
• Correlation over short distances is often achieved by noting the position of a distinctive rock layer or identifying a layer composed of very distinctive or uncommon minerals