From Simple English Wikipedia, the free encyclopedia
Diagram of geological time scale as a spiral.
Geologic time scale uses the principles and techniques ofgeology to work out the geological history of theEarth.[1] It looks at the processes which change the Earth's surface androcks under the surface.
Geologists usestratigraphy andpaleontology to find out the sequence of the events, and show theplants andanimals which lived at different times in the past. They worked out the sequence of rock layers. Then the discovery ofradioactivity and the invention ofradiometric dating techniques gave a way to get the ages of the layers (strata).
Prospecting forenergy sources and valuableminerals depends on understanding the geological history of an area. Such knowledge can also help lessen the hazards ofearthquakes andvolcanoes.
The largest defined unit of time is the supereon composed ofEons. Eons are divided intoEras, which are in turn divided intoPeriods,Epochs andStages. At the same time paleontologists define a system offaunal stages, of varying lengths, based on the kinds of animal fossils found there. In many cases, such faunal stages have been adopted in building the geologicalnomenclature, though in general there are far more recognized faunal stages than defined geological time units.
Geologists tend to talk in terms of Upper/Late, Lower/Early and Middle parts of periods and other units, such as "UpperJurassic", and "MiddleCambrian".Upper,Middle, andLower are terms applied to the rocks themselves, as in "Upper Jurassicsandstone," whileLate,Middle, andEarly are applied to time, as in "Early Jurassicdeposition" or "fossils of Early Jurassic age." The adjectives are capitalized when the subdivision is formally recognized, and lower case when not. Thus in 2018 when three divisions (subepochs) of the Holocene were formally recognized it became correct to write "Early Holocene" instead of "early Holocene".[2][3]
Because geologic units occurring at the same time but from different parts of the world can often look different and contain different fossils, there are many examples where the same period was historically given different names in different locales. For example, inNorth America the LowerCambrian is called theWaucoban series that is then subdivided into zones based ontrilobites. The same timespan is split intoTommotian,Atdabanian andBotomian stages inEast Asia andSiberia. A key aspect of the work of the International Commission on Stratigraphy is to reconcile this conflictingterminology and define universal horizons (time division) that can be used around the world.
The following table summarizes the major events and characteristics of the periods of time making up the geologic time scale. As above, this time scale is based on the International Commission on Stratigraphy. The height of each table entry does not correspond to the duration of each subdivision of time.(not shown to scale)
In North America, the Carboniferous is subdivided intoMississippian andPennsylvanian sub-periods or epochs.
Discoveries in the past quarter century have substantially changed the view of geologic and paleontologic events just before the Cambrian. The termNeoproterozoic is used now, but older writers might have used 'Ediacaran', 'Vendian', 'Varangian', 'Precambrian', 'Protocambrian', 'Eocambrian', or might have extended the Cambrian further back in time.
Dates are slightly uncertain, and differences of a few percent between sources are common. This is because deposits suitable forradiometric dating seldom occur exactly at the places in the geologic column where we would most like to have them. Dates with an * are radiometrically determined based on internationally agreedGSSPs.
