GEY 101 - Introductory Geology: Exploring Planet Earth
Geologic Time

Determining geological ages

• Relative age dates – placing rocks and events in their proper sequence of formation
• Numerical dates – specifying the actual number of years that have passed since an event occurred (known as absolute age dating)

Principles of relative dating

• Law of superposition
  • Developed by Nicolaus Steno in 1669
  • In an undeformed sequence of sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottom
  • Superposition is well illustrated by the strata in the Grand Canyon
• Principle of original horizontality
  • Layers of sediment are generally deposited in a horizontal position
  • Rock layers that are flat have not been disturbed
• Principle of cross-cutting relationships
  • Younger features cut across older feature
• Inclusions
  • An inclusion is a piece of rock that is enclosed within another rock
  • Rock containing the inclusion is younger
• Unconformity
  • An unconformity is a break in the rock record produced by erosion and/or nondeposition of rock units
  • Types of unconformities
    • Angular unconformity – tilted rocks are overlain by flat-lying rocks
    • Disconformity – strata on either side of the unconformity are parallel
    • Nonconformity – metamorphic or igneous rocks in contact with sedimentary strata

Correlation of rock layers

• Matching of rocks of similar ages in different regions is known as correlation
• Correlation often relies upon fossils
  • William Smith (late 1700s) noted that sedimentary strata in widely separated area could be identified and correlated by their distinctive fossil content
  • 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

Using radioactivity in dating

• Reviewing basic atomic structure
  • Nucleus
    • Protons – positively charged particles with mass
    • Neutrons – neutral particles with mass
    • Electrons – negatively charged particles that orbit the nucleus
  • Atomic number
    • An element’s identifying number
    • Equal to the number of protons in the atom’s nucleus
  • Mass number
    • Sum of the number of protons and neutrons in an atom’s nucleus
  • Isotope
    • Variant of the same parent atom
    • Differs in the number of neutrons
    • Results in a different mass number than the parent atom
  • Radioactivity -- Spontaneous changes (decay) in the structure of atomic nuclei
  • Types of radioactive decay
    • Alpha emission
      • Emission of 2 protons and 2 neutrons (an alpha particle)
      • Mass number is reduced by 4 and the atomic number is lowered by 2
    • Beta emission
      • An electron (beta particle) is ejected from the nucleus
      • Mass number remains unchanged and the atomic number increases by 1
    • Electron capture
      • An electron is captured by the nucleus
      • The electron combines with a proton to form a neutron
      • Mass number remains unchanged and the atomic number decreases by 1

Using radioactivity in dating

• Parent – an unstable radioactive isotope
• Daughter product – the isotopes resulting from the decay of a parent
• Half-life – the time required for one-half of the radioactive nuclei in a sample to decay

Radiometric dating

• Principle of radioactive dating
  • The percentage of radioactive toms that decay during one half-life is always the same (50 percent)
  • However, the actual number of atoms that decay continually decreases
  • Comparing the ratio of parent to daughter yields the age of the sample
• Useful radioactive isotopes for providing radiometric ages
  • Rubidium-87
  • Thorium-232
  • Two isotopes of uranium
  • Potassium-40
• Sources of error
  • A closed system is required
  • To avoid potential problems, only fresh, unweathered rock samples should be used

Dating with carbon-14 (radiocarbon dating)

• Half-life of only 5730 years
• Used to date very recent events
• Carbon-14 is produced in the upper atmosphere
• Useful tool for anthropologists, archeologists, and geologists who study very recent Earth history

Importance of radiometric dating

• Radiometric dating is a complex procedure that requires precise measurement
• Rocks from several localities have been dated at more than 3 billion years
• Confirms the idea that geologic time is immense

Geologic time scale

• The geologic time scale – a “calendar” of Earth history
  • Subdivides geologic history into units
  • Originally created using relative dates
• Structure of the geologic time scale
  • Eon – the greatest expanse of time
  • Names of the eons
    • Phanerozoic (“visible life”) – the most recent eon, began about 540 million years ago
    • Proterozoic
    • Archean
    • Hadean – the oldest eon
  • Era -- subdivison of an eon
  • Eras of the Phanerozoic eon
    • Cenozoic --> "recent life"
    • Mesozoic --> "middle life"
    • Paleozoic --> "ancient life"
  • Eras are subdivided into periods
  • Periods are subdivided into epochs

Precambrian time

• Nearly 4 billion years prior to the Cambrian period
• Not divided into smaller time units because the events of Precambrian history are not know in great enough detail
• First abundant fossil evidence does not appear until the beginning of the Cambrian

Difficulties in dating the geologic time scale

• Not all rocks can be dated by radiometric methods
  • Grains comprising detrital sedimentary rocks are not the same age as the rock in which they formed
  • The age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed
• Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth history and arrive at ages
  • Dating sedimentary strata using radiometric dating

Additional Study Guide

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