geologic

• Geologic Time Scale


  Standard way of dividing up the history of the Earth based on life-forms that have existed during specific times since the creation of the planet
• Divisions of the Geologic Time Scale

  • Eons
  • Eras
  • Periods
  • Epochs
• Eons


  • Longest subdivision, based on the abundance of certain fossils
• Eras


  • Next to longest subdivision, marked by major changes in the fossil record
• Periods


  • Based on types of life existing at the time
• Epochs


  • Shortest subdivision, marked by differences in life forms and can vary from continent to continent
• Early geologists had no way of knowing how the discoveries of the Earth were going to develop, so they put the time scale together piece by piece, naming units as they were discovered
• Cambrian


  • Named for exposures of strata found in a type-section in Wales by British geologist Adam Sedgwick
• Devonian


  • Named after significant outcrops first discovered near Devonshire, England
• Jurassic


  • Named for representative strata first seen in the Jura Mountains by German geologist Humboldt in 1795
• Cretaceous


  • From the Latin "creta" meaning chalk by a Belgian geologist
• The earliest time of the Earth is called the Hadean and refers to a period of time for which we have no rock record, and the Archean followed, which corresponds to the ages of the oldest known rocks on earth. These, with the Proterozoic Eon are called the Precambrian Eon. The remainder of geologic time, including present day, belongs to the Phanerozoic Eon.
• Geochronologic units


  The units making up the Geologic Time Scale
• Chronostratigraphic units

  The actual rock record of a period, called a system
• Geologic time


  The study of the age and history of the Earth
• Nicholas Steno


  • Danish physician (1638-1687)
  • Devised 3 main principles that underlie the interpretation of geologic time
• Steno's principles


  1. Principle of superposition

  2. Principle of horizontality

  3. Principle of original lateral continuity
• Steno's principles formed the framework for the geologic area of stratigraphy
• Opposing views on rock formation

  • Neptunists (Werner and followers)
  • Plutonists (Voisins and followers)
• James Hutton


  • Scottish physician and geologist (1726-1797)
  • Proposed the theory of uniformitarianism - "the present is the key to the past"
• William Smith


  • Surveyor who mapped England
  • First to understand that rock units could be identified by the fossils they contained
  • Gave rise to the principle of biologic succession
• Principle of biologic succession

  Each age in the Earth's history is unique such that fossil remains will be unique, allowing vertical and horizontal correlation of rock layers
• Rock outcrop correlation


  • Shark teeth fossils allow correlation of rock layers separated by 300 km
• Charles Lyell

  • English geologist who published "Principles of Geology"
  • Proposed the principle of cross-cutting relationships
  • Proposed the inclusion principle
• Charles Darwin


  • Unpaid naturalist who sailed on the HMS Beagle
  • Proposed the theory of natural selection
  • Wrote "Origin of Species" to convince the world that evolution had occurred and that natural selection was the mechanism
• Relative age


  The age of one object compared to the age of another, not the exact age
• Determining relative age


  1. Apply the 6 original stratigraphic principles

  2. Correlate strata by rock unit type or fossil type

  3. Oldest rock is on the bottom due to superposition
• Unconformities


  • Specific contacts between rock layers that help determine relative ages
  • Types: angular, disconformity, nonconformity
• Principle of cross-cutting relationships


  A rock feature that cuts across another feature must be younger than the rock that it cuts
• Index fossils


  • Biostratigraphically useful species that can be used to recognize chronostratigraphic units
  • Characteristics: widespread, short temporal durations, abundant, easily recognized
• Index fossil


  • Trilobites
• Absolute age


  The actual age of a rock in years, determined by radiometric dating
• Isotopes


  • Atoms of the same element with different numbers of neutrons
  • Some isotopes are unstable and undergo radioactive decay
• Radioactive decay


  The process where a parent isotope breaks down into a daughter isotope, characterized by beta decay and alpha decay
• Half-life


  The time it takes for 1/2 of the parent atoms in a radioactive isotope to decay
• Radiometric dating


  Using the proportion of parent isotope to daughter isotope to calculate the absolute age of a rock
• Radiocarbon dating


  A method used to date anything that was once alive, up to 70,000 years old, by measuring the remaining carbon-14
• Cambrian


  Latin name for Wales
• Devonian


  After significant outcrops
• Jurassic


  Representative strata