GLOSSARY – D
by: Charles O’Dale
RADIOMETRIC DATING Reports of the National Center for Science Education – Brent Dalrymple,
The parent isotopes and corresponding daughter products most commonly used to determine the ages of ancient rocks are listed below:
|Parent Isotope||Stable Daughter Product||Currently Accepted Half-Life Values|
|Hafnium-182||Tungsten-182||9 Million years|
|Uranium-235||Lead-207||704 million years|
|Potassium-40||Argon-40||1.25 billion years|
|Uranium-238||Lead-206||4.5 billion years|
|Thorium-232||Lead-208||14.0 billion years|
|Lutetium-176||Hafnium-176||35.9 billion years|
|Rhenium-187||Osmium-187||43 billion years|
|Rubidium-87||Strontium-87||48.8 billion years|
|Samarium-147||Neodymium-143||106 billion years|
Potassium-40 decays slowly into argon-40, so that the more argon-40 present, the older the sample is. However, measuring the ratio of potassium-40 to argon-40 has the disadvantage of the potassium and argon needing to be measured separately. A more reliable variant of this method is to convert the potassium into argon-39. The rock sample is heated to release both the argon-39 and argon-40, so that the two isotopes can be measured at the same time. The amount of argon-39 that it is released indicates how much potassium-40 was originally in the rock.
|Name of Method||Age range of Application||Material Dated||Methodology|
|Radiocarbon||1 – 70,000 years||Organic material such as bones, wood, charcoal, shells||Radioactive decay of 14C in organic matter after removal from bioshpere|
|K-Ar dating||1,000 – billion of years||Potassium-bearing minerals and glasses||Radioactive decay of 40K in rocks and minerals|
|Uranium-Lead||10,000 – billion of years||Uranium-bearing minerals||Radioactive decay of uranium to lead via two separate decay chains|
|Uranium series||1,000 – 500,000 years||Uranium-bearing minerals, corals, shells, teeth, CaCO3||Radioactive decay of 234U to 230Th|
|Fission track||1,000 – billion of years||Uranium-bearing minerals and glasses||Measurement of damage tracks in glass and minerals from the radioactive decay of 238U|
|Luminescence (optically or thermally stimulated)||1,000 – 1,000,000 years||Quartz, feldspar, stone tools, pottery||Burial or heating age based on the accumulation of radiation-induced damage to electron sitting in mineral lattices|
|Cosmogenic Nuclides||1,000 – 5,000,000 years||Typically quartz or olivine from volcanic or sedimentary rocks||Radioactive decay of cosmic-ray generated nuclides in surficial environments|
|Magnetostratigraphy||20,000 – billion of years||Sedimentary and volcanic rocks||Measurement of ancient polarity of the earth’s magnetic field recorded in a stratigraphic succession|
|Tephrochronology||100 – billions of years||Volcanic ejecta||Uses chemistry and age of volcanic deposits to establish links between distant stratigraphic successions|
DATING GLOSSARY(from “The Nature Education“)
absolute dating: Determining the number of years that have elapsed since an event occurred or the specific time when that event occurred
atomic mass: The mass of an isotope of an electron, based on the number of protons and neutrons
atomic nucleus: The assemblage of protons and neutrons at the core of an atom, containing almost all of the mass of the atom and its positive charge
daughter isotope: The isotope that forms as a result of radioactive decay
electrons: Negatively charged subatomic particles with very little mass; found outside the atomic nucleus
electron spin resonance: Method of measuring the change in the magnetic field, or spin, of atoms; the change in the spin of atoms is caused by the movement and accumulation of electrons from their normal position to positions in imperfections on the crystal structure of a mineral as a result of radiation.
elements: Chemical substances that cannot be split into a simpler substances
fault: A fracture in a rock along which movement occurs
geomagnetic polarity time scale: A record of the multiple episodes of reversals of the Earth’s magnetic polarity that can be used to help determine the age of rocks
half-life: The amount of time it takes for half of the parent isotopes to radioactively decay to daughter isotopes
index fossil: A fossil that can be used to determine the age of the strata in which it is found and to help correlate between rock units
isotopes: Varieties of the same element that have the same number of protons, but different numbers of neutrons
magnetic field: A region where lines of force move electrically charged particles, such as around a magnet, through a wire conducting an electric current, or the magnetic lines of force surrounding the earth
magnetism: The force causing materials, particularly those made of iron and other certain metals, to attract or repel each other; a property of materials that responds to the presence of a magnetic field
normal polarity: Interval of time when the earth’s magnetic field is oriented so that the magnetic north pole is approximately in the same position as the geographic north pole
neutrons: A subatomic particle found in the atomic nucleus with a neutral charge and a mass approximately equal to a proton
optical stimulating luminescence: Dating method that uses light to measure the amount of radioactivity accumulated by crystals in sand grains or bones since the time they were buried
paleomagnetism: Remanent magnetization in ancient rocks that records the orientation of the earth’s magnetic field and can be used to determine the location of the magnetic poles and the latitude of the rocks at the time the rocks were formed
parent isotope: The atomic nucleus that undergoes radioactive decay
polarity (magnetic polarity): The direction of the earth’s magnetic field, which can be normal polarity or reversed polarity
potassium-argon (K-Ar) method: Radiometric dating technique that uses the decay of 39K and 40Ar in potassium-bearing minerals to determine the absolute age
principle of cross-cutting relationships: Any geologic feature that cross-cuts across strata must have formed after the rocks they cut through were deposited.
principle of faunal succession: Fossil species succeed each other in a definitive, recognizable order and once a species goes extinct, it disappears and cannot reappear in younger rocks.
principle of original horizontality: Layers of strata are deposited horizontally, or nearly horizontally, and parallel or nearly parallel to the earth’s surface.
principle of superposition: In an undeformed sequence, the oldest rocks are at the bottom and the youngest rocks are at the top.
protons: Positively charged subatomic particles found in the nucleus of an atom
radioactivity (radioactive): An unstable isotope spontaneously emits radiation from its atomic nucleus
radioactive decay: The process by which unstable isotopes transform to stable isotopes of the same or different elements by a change in the number of protons and neutrons in the atomic nucleus
radiocarbon dating: Radiometric dating technique that uses the decay of 14C in organic material, such as wood or bones, to determine the absolute age of the material
radiometric dating: Determination of the absolute age of rocks and minerals using certain radioactive isotopes
relative dating: Rocks and structures are placed into chronological order, establishing the age of one thing as older or younger than another
reversals (magnetic reversals): Changes in the earth’s magnetic field from normal polarity to reversed polarity or vice versa
reversed polarity: Interval of time when the earth’s magnetic field is oriented so that magnetic north pole is approximately in the same positions as the geographic south pole
strata (singular: stratum): Distinct layers of sediment that accumulated at the ea surface.
stratigraphy: The study of strata and their relationships
thermoluminescence: Dating method that uses heat to measure the amount of radioactivity accumulated by a rock or stone tool since it was last heated.
Lattice gliding due to stress subjected to a crystal. Twinned crystals may show deformation lamellae. Twinning in crystals may be caused by shock deformation.
DENSITY vs ROCK TYPE
Grams / milliliter (mL)
|Basalt or lava rock||
The crystalline form of carbon (C) is the hardest naturally occurring material. May be formed in meteorite impact when carbon, e.g. in graphite-bearing rocks, is subjected to extraordinarily high temperatures and pressures. Is observed in meteorites and impactites (e.g., Younger Dryas event, Ries, Popigai impact structures).
Partly isotropic crystal originating from patchy shock damage of the crystal lattice.
Glass formed by shock damage of a mineral and not by melting. According to current knowledge, diaplectic glass cannot be formed in endogenetic processes.