REVIEW OF GEOLOGIC TIME

Relative Dating

The process by which geologic events are put in their proper time sequence.

Principles of Relative Dating

• Principle of superposition: Younger sediments lie on to of older ones so long as the sediments have not been overturned by geologic forces.
• Principle of original horizontality: Sediments are originally deposited in (nearly) horizontal layers.
• Principle of lateral continuity: A layer of sediment will have been originally continuous within the border of its depositional area.
• Principle of cross-cutting relationships: Where one geological feature cuts across another, the feature which has been cut must be older.
• Principle of inclusions: An object, such as a fragment of rock, that is found in a rock must be older than the rock itself.
• Principle of fossil succession: There is a unique progression with time of life forms preserved in the fossil record.
• Important example: A sedimentary rock is assumed be older than an igneous rock with which it is in contact if
  • the sedimentary rock is cut by the igneous rock.
  • the sedimentary rock has been metamorphosed by the igneous rock.
  • inclusions of the sedimentary rock are found in the igneous rock.

Unconformities

Unconformities are gaps in the geologic record. They mark times when erosion was occurring. There are three types.

• angular unconformity - where the sedimentary beds underneath the unconformity are not parallel to those above; easily seen in the field and indicates a large amount of missing geological record and therefore a substantial time gap.
• disconformity - where the sedimentary underneath the unconformity are parallel to those above; often difficult to see except with careful study of the contact between the beds above and below; may indicate a relatively short gap in the geological record.
• nonconformity - where sedimentary beds overlie crystalline (igneous or metamorphic rock); like angular unconformities, indicates a substantial missing geological record.

Correlation

Correlation is the process geologists use to match rocks of the same age between different locations. The following criteria are used.

• rock types - rocks that match closely in properties such as grain size and composition, cementation, chemistry, sedimentary structures (such as cross-bedding) may be correlatable; not a completely reliable method so caution must be used.
• fossil assemblages - rocks that contain similar to identical fossil types may be correlatable; much more reliable than rock type
• assemblage range zone - a particular fossil assemblage that has been shown to mark a particular interval in geological time; extremely reliable
• guide (index) fossils - fossils which have been shown to mark a particular interval in geologic time; very reliable; the best guide fossils
  • are widely distributed.
  • existed over a limited period of time.
• key (marker) bed - a distinctive and unique bed, such as a volcanic ash layer or a coal bed; up to 100% reliable and often capable of absolute dating (if volcanic)

Absolute Dating

"Absolute" dating allows you to put a number on the age of a geologic unit or event. The most common method of absolute dating is radiometric dating.

Atomic and Nuclear Concepts

• atom: An atom is the smallest bit of chemically identifiable matter. It consists of a positive nucleus containing positively charged protons and uncharged neutrons surrounded by electrons in smeared-out orbits called orbitals. Nearly all of the atom's mass is in its nucleus.

• element: An atom belongs to a particular element according to the number of protons in its nucleus. Hydrogen has one proton, helium two, lithium three, and so forth.

• atomic number: This is the number of protons in the nucleus. The atomic number of hydrogen is one, helium two, lithium three, etc.

• atomic mass: This is the total number of nucleons (protons plus neutrons) in the nucleus.

• isotope: Atoms of a particular element must contain a certain number of protons in their nuclei but may have different numbers of neutrons. Atoms with the same number of protons but different numbers of neutrons are isotopes of that element.

• radioactivity: This is the process in which an unstable nucleus decays into a different nucleus and emits radiation. The unstable nucleus is called the parent and the resulting nucleus is called the daughter. The daughter may or may not be itself radioactive.

• radioactive isotope: If an element has one (or more) isotopes that decay by radioactivity, they are called radioactive isotopes.

Types of Radioactivity

There are three types of radioactivity important in radiometric dating.

• alpha decay - in which an alpha particle, which is a particle with two protons and two neutrons (same as a helium nucleus), is expelled from the decaying nucleus; the atomic number is decreased by two and the atomic mass by four.

• beta decay - in which either and electron or an anti-electron (positron) is expelled from the decaying nucleus; the former increases the atomic number by one; the latter decreases the atomic number by one; the atomic mass is unchanged.

• electron capture - in which an electron in the atom is captured by the nucleus; the atomic number is reduced by one; the atomic mass remains unchanged, since the number of neutrons is increased by one.

Radiometric Dating

Radiometric dating involves measuring the number of daughter isotopes compared to the number of parent isotopes. The older the sample the greater the daughter-to-parent ratio. The time it takes for half the parent isotopes to decay into daughter isotopes is called the half-life of the radioactive isotope. The age of a rock or mineral determined this way is called the radiometric age.

To be able to apply radiometric dating to a sample you must have

• a radioactive isotope present with a half-life appropriate for the age of the sample,

• either no daughter isotope originally present in the sample or a way to determine how much daughter isotope was originally present, and

• the daughter isotope trapped in the sample since it formed.

A radiometric age can yield a false result when

• parent or daughter isotopes have been lost by, for example, metamorphism, and

• daughter isotopes have been added to the sample (for example, by sedimentation).

The best type of rock to date by radiometric means is igneous rock, because there are no daughter isotopes in the minerals that contain the parent isotopes, since the parent and daughter have different sizes and chemistry.

Types of Radiometric Dating

• fission track: When uranium decays it emits alpha particles that cause structural damage to the crystal containing the uranium. The density of the tracks is a measure of the age of the sample.

• radiocarbon: Radiocarbon dating is based on the decay of carbon 14, which has a lifetime of 5730 years.

  • creation of carbon 14 - occurs in the upper atmosphere by collisions between cosmic ray neutrons and nitrogen 14 atoms, resulting in the nitrogen absorbing the neutron and emitting a proton; it gradually decays back into nitrogen 14.

  • how used in dating - taken up by living organisms during their life, which ceases at death; the amount of carbon 14 that has decayed indicates the age of the organism.

  • necessity of calibration - due to the variable production of carbon 14 in the upper atmosphere; calibrated by tree-ring data to 11 800 years ago, by cross-referencing overlapping tree-ring records, including dead trees preserved at high altitudes; calibration beyond 11 800 years is by marine data and is not as reliable.

  • range of radiocarbon dating - from the present to around 50 000 years ago

• potassium/argon: Potassium 40 decays to argon 40 by electron capture. The argon is not originally present in the crystal, being a gas that escapes as the magma cools, but is trapped in the solid crystal as the potassium decays.

• uranium/lead: Both uranium 235 and 238 decay into lead isotopes. This form of dating is especially important, because zircon crystals, which are highly resistant to chemical change, often contain uranium and can make excellent samples.

Geologic Time and Stratigraphy

Geologic time is broken up into eons, eras, periods and epochs, from the longest to the shortest in time. The divisions between these time units are based on changes in the fossil record. A series of sedimentary beds belonging to a certain geologic period is called a system. The study of the layering of sedimentary rocks is called stratigraphy. The various rock layers are called lithostratigraphic units. Geologists define formations, which are mappable geologic units, such as a sedimentary bed or a sequence of related sedimentary beds. A formation that consists of number of different layers of sedimentary rock is often further divided into members. A number of related formations, lying stratigraphically one top of one another is often called a group. Biostratigraphic units are also defined, based on fossil content. The biozone is the biostratigraphic equivalent of the formation. Note that biozone boundaries do not necessarily corresond to formation boundaries.