Absolute Dating Methods
Determining the age of materials by means of their radioactive content is called Radiometric age dating or radiometric dating.
Earlier dating of rocks, minerals and other Earth materials resorted to deductive methods. Rock strata were compared in different regions and only by growing knowledge of the physical processes involved in their forming, could Scientists estimate the time needed for the specific formative process to have taken place.
Near the end of the 19th century a more precise tool, for dating past Earth-events, were gained. Radiometric age-dating is based on the fact that every radioactive element decays. The original or "parent" element emits radiation and particles until the loss thereof transforms it into a stable "daughter" element. A series of transformations into other radio-active elements marks its unparalleled pattern and rate of decay until it reaches stability.
The Law of Radioactivity describes the decay of radioactive elements and their resulting formation. A simple mathematical formula incorporates the decay rate of the parent element and calculates the ratio of the daughter element to the parent element after a given period of time. The concentration of the chemical elements must be measured as well as their isotopic composition.
K-Ar dating is based on naturally occurring radioactive potassium-40, decaying to argon-40. This decay sequence has been used to construct the geological time scale.
By emission of a beta particle, radioactive rubidium-87, decays to stable stronsium-87. Rubidium-bearing minerals, such as feldspars and micas, can be dated in this way, thus resulting that this procedure has been used to date the oldest known rocks on Earth as well as rocks from the moon.
U or Th-Pb Methods
All isotopes of uranium and thorium are radioactive and by emission of alpha and beta particles both decays to stable isotopes of lead. This method has been used to date granite gneisses of the Precambrian age that occur on all of earths continents.
In the upper levels of the atmosphere, carbon-14 is produced by cosmic rays. The carbon-14, which is incorporated in carbon dioxide molecules, is taken up by green plants in the course of photosynthesis. The level of carbon-14 in the tissue of the green plants, or the animals that feed on them, stays constant as long as they are alive, because of compensation by addition of carbon-14 from the atmosphere. When the organism dies, the radioactivity of the carbon-14 decreases at a well-established rate. This dating method is primarily to be used in materials such as wood and bones.
By measuring the thickness of the hydration layer left on the obsidian. These rinds are produced by the absorption of water from its surroundings that slowly diffuse into the artefact. This method can be used on types of glass that are 200 to 200 000 years old.
Taking into account the number, width and density of the annual growth rings on trees, events can be dated and conditions are determined. Dendrochronologists can accurately date events and conditions of 3000 to 4000 years ago.
By comparing the remnant magnetism caused by heat from a fireplace or furnace with the changing direction and strength of the earths magnetic field, ancient pottery can be dated.