Periodically, the magnetic field of the earth reverses leaving a magnetic signal in volcanic and sedimentary rocks.
The bodies of these annelids are externally marked by annuli that are arranged in series with each other.This and other work shows that radiocarbon and isotope time-series derived from spines can be used for demographic or palaeoclimate studies.Annulus (zoology) In zoology, an annulus is an external circular ring.Circa Circa (from Latin, meaning 'around, about, roughly, approximately') – frequently abbreviated ca., or ca and less frequently c.,circ. – signifies "approximately" in several European languages and as a loanword in English, usually in reference to a date.Circa is widely used in historical writing when the dates of events are not accurately known.These paleomagnetic signatures are fixed when ferromagnetic materials such as magnetite cool below the Curie point, freezing the magnetic moment of the material in the direction of the local magnetic field at that time.
The direction and magnitude of the magnetic field of the Earth at a particular location varies with time, and can be used to constrain the age of materials.
This sub-discipline of paleoclimatology and ecophysiology is relatively new.
Acanthochronology is closely related to dendrochronology, dendroclimatology and isotope geochemistry and borrows many of the methods and techniques from these sub-disciplines of the Earth Sciences.
Another technique used by archaeologists is to inspect the depth of penetration of water vapor into chipped obsidian (volcanic glass) artifacts.
The water vapor creates a "hydration rind" in the obsidian, and so this approach is known as "hydration dating" or "obsidian dating", and is useful for determining dates as far back as 200,000 years.
Trees usually add growth rings on a yearly basis, with the spacing of rings being wider in high growth years and narrower in low growth years.