WGeochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is provided by tools such as palaeomagnetism and stable isotope ratios. By combining multiple geochronological indicators the precision of the recovered age can be improved.
WThe age of Earth is estimated to be 4.54 ± 0.05 billion years (4.54 × 109 years ± 1%). This age may represent the age of the Earth's accretion, or core formation, or of the material from which the Earth formed. This dating is based on evidence from radiometric age-dating of meteorite material and is consistent with the radiometric ages of the oldest-known terrestrial and lunar samples.
WThe geologic time scale (GTS) is a system of chronological dating that classifies geological strata (stratigraphy) in time. It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events in geologic history. The time scale was developed through the study of physical rock layers and relationships as well as the times when different organisms appeared, evolved and became extinct through the study of fossilized remains and imprints. The table of geologic time spans, presented here, agrees with the nomenclature, dates and standard color codes set forth by the International Commission on Stratigraphy (ICS).
WIn stratigraphy and geology, an eonothem is the totality of rock strata laid down in the stratigraphic record deposited during a certain eon of the continuous geologic timescale. The eonothem is not to be confused with the eon itself, which is a corresponding division of geologic time spanning a specific amount of years, during which rocks were formed that are classified within the eonothem. Eonothems have the same names as their corresponding eons, which means during the history of the Earth only four eonothems were formed. Oldest to newest these are the Hadean, Archean, Proterozoic, and Phanerozoic. A rock stratum, fossil or feature present in the "upper Phanerozoic" eonothem would therefore have originated within the "later Phanerozoic" eon. In practice, the rock column is discontinuous:Technically, a complete geologic record doesn't occur anywhere. For such a record to develop would require the area to have been receiving sedimentary deposits continually ever since the origin of the earth. Nowhere is such a situation known to exist. If it did exist, we could not effectively look at the strata because they would still be buried, and modern strata would continue to be deposited on top of them. The earth's surface has been far too dynamic to allow that to occur anywhere. No area has been in such a static condition throughout the earth's long history. Areas that have had sediment deposited on them at one time are later uplifted and eroded. In some places this has occurred many times. There is ample evidence to prove such a sequence of events.
WThe evolutionary history of life on Earth traces the processes by which living and fossil organisms evolved, from the earliest emergence of life to the present. Earth formed about 4.5 billion years ago and evidence suggests life emerged prior to 3.7 Ga. ago The similarities among all known present-day species indicate that they have diverged through the process of evolution from a common ancestor. Approximately 1 trillion species currently live on Earth of which only 1.75–1.8 million have been named and 1.6 million documented in a central database. These currently living species represent less than one percent of all species that have ever lived on earth.
WThe geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata. That is, deposits laid down by volcanism or by deposition of sediment derived from weathering detritus. This includes all its fossil content and the information it yields about the history of the Earth: its past climate, geography, geology and the evolution of life on its surface. According to the law of superposition, sedimentary and volcanic rock layers are deposited on top of each other. They harden over time to become a solidified (competent) rock column, that may be intruded by igneous rocks and disrupted by tectonic events.
WThe geologic time scale (GTS) is a system of chronological dating that classifies geological strata (stratigraphy) in time. It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events in geologic history. The time scale was developed through the study of physical rock layers and relationships as well as the times when different organisms appeared, evolved and became extinct through the study of fossilized remains and imprints. The table of geologic time spans, presented here, agrees with the nomenclature, dates and standard color codes set forth by the International Commission on Stratigraphy (ICS).
WThe geological history of Earth follows the major events in Earth's past based on the geological time scale, a system of chronological measurement based on the study of the planet's rock layers (stratigraphy). Earth formed about 4.54 billion years ago by accretion from the solar nebula, a disk-shaped mass of dust and gas left over from the formation of the Sun, which also created the rest of the Solar System.
WThe history of Earth concerns the development of planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to understanding of the main events of Earth's past, characterized by constant geological change and biological evolution.
WMarker horizons, chronohorizons, key beds or marker beds are stratigraphic units of the same age and of such distinctive composition and appearance, that, despite their presence in separate geographic locations, there is no doubt about their being of equivalent age (isochronous) and of common origin. Such clear markers facilitate the correlation of strata, and used in conjunction with fossil floral and faunal assemblages and paleomagnetism, permit the mapping of land masses and bodies of water throughout the history of the earth. They usually consist of a relatively thin layer of sedimentary rock that is readily recognized on the basis of either its distinct physical characteristics or fossil content and can be mapped over a very large geographic area. As a result, a key bed is useful for correlating sequences of sedimentary rocks over a large area. Typically, key beds were created as the result of either instantaneous events or very short episodes of the widespread deposition of a specific types of sediment. As the result, key beds often can be used for both mapping and correlating sedimentary rocks and dating them. Volcanic ash beds and impact spherule beds, and specific megaturbidites are types of key beds created by instantaneous events. The widespread accumulation of distinctive sediments over a geologically short period of time have created key beds in the form of peat beds, coal beds, shell beds, marine bands, black shales in cyclothems, and oil shales. A well-known example of a key bed is the global layer of iridium-rich impact ejecta that marks the Cretaceous–Paleogene boundary.
WRelative dating is the science of determining the relative order of past events, without necessarily determining their absolute age. In geology, rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another. Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating, archaeologists and geologists used relative dating to determine ages of materials. Though relative dating can only determine the sequential order in which a series of events occurred, not when they occurred, it remains a useful technique. Relative dating by biostratigraphy is the preferred method in paleontology and is, in some respects, more accurate. The Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of 'relative dating' as observed in geology from the 17th century to the early 20th century.
WTephrochronology is a geochronological technique that uses discrete layers of tephra—volcanic ash from a single eruption—to create a chronological framework in which paleoenvironmental or archaeological records can be placed. Such an established event provides a "tephra horizon". The premise of the technique is that each volcanic event produces ash with a unique chemical "fingerprint" that allows the deposit to be identified across the area affected by fallout. Thus, once the volcanic event has been independently dated, the tephra horizon will act as time marker.