While uranium is water-soluble, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sedimentsfrom which their ratios are measured. Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event. The ions then travel through a magnetic field, which diverts them into different sampling sensors, known as " Faraday cups ", depending on their mass and level of ionization. This scheme has application over a wide range of geologic dates. Atoms of the same element that have different numbers of neutrons are called isotopes. It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration.
This plot shows the level of carbon in the atmosphere as measured in New Zealand red and Austria greenrepresenting the Southern and Northern Hemispheres, respectively.
Dormant volcanoes can also emit aged carbon. While the lighter isotopes 12 C and 13 C are stable, the heaviest isotope 14 C radiocarbon is radioactive. Once produced, the 14 C quickly combines with the oxygen in the atmosphere to form first carbon monoxide CO and ultimately carbon dioxide CO 2. A particular isotope of a particular element is called a nuclide. To be able to distinguish the relative ages of rocks from such old material, and to get a better time resolution than that available from long-lived isotopes, short-lived isotopes that are no longer present in the rock can be used. Chronostratigraphy Geochronology Isotope geochemistry Law of superposition Luminescence dating Samarium—neodymium dating.
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