135-690: The Quaternary ( / k w ə ˈ t ɜːr n ə r i , ˈ k w ɒ t ər n ɛr i / kwə- TUR -nə-ree, KWOT -ər-nerr-ee ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the Phanerozoic eon. It follows the Neogene Period and spans from 2.58 million years ago to
270-439: A consequence, a powerful source generating plate motion is the excess density of the oceanic lithosphere sinking in subduction zones. When the new crust forms at mid-ocean ridges, this oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes denser with age as it conductively cools and thickens. The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into
405-450: A few tens of millions of years. Armed with the knowledge of a new heat source, scientists realized that Earth would be much older, and that its core was still sufficiently hot to be liquid. By 1915, after having published a first article in 1912, Alfred Wegener was making serious arguments for the idea of continental drift in the first edition of The Origin of Continents and Oceans . In that book (re-issued in four successive editions up to
540-577: A formal proposal to the ICS for the establishment of the Anthropocene Series/Epoch. Nevertheless, the definition of the Anthropocene as a geologic time period rather than a geologic event remains controversial and difficult. An international working group of the ICS on pre-Cryogenian chronostratigraphic subdivision have outlined a template to improve the pre-Cryogenian geologic time scale based on
675-461: A geochronologic unit can be changed (and is more often subject to change) when refined by geochronometry while the equivalent chronostratigraphic unit (the revision of which is less frequent) remains unchanged. For example, in early 2022, the boundary between the Ediacaran and Cambrian periods (geochronologic units) was revised from 541 Ma to 538.8 Ma but the rock definition of the boundary (GSSP) at
810-699: A known geological context. The geological history of Mars has been divided into two alternate time scales. The first time scale for Mars was developed by studying the impact crater densities on the Martian surface. Through this method four periods have been defined, the Pre-Noachian (~4,500–4,100 Ma), Noachian (~4,100–3,700 Ma), Hesperian (~3,700–3,000 Ma), and Amazonian (~3,000 Ma to present). Plate tectonics Plate tectonics (from Latin tectonicus , from Ancient Greek τεκτονικός ( tektonikós ) 'pertaining to building')
945-571: A layer of basalt (sial) underlies the continental rocks. However, based on abnormalities in plumb line deflection by the Andes in Peru, Pierre Bouguer had deduced that less-dense mountains must have a downward projection into the denser layer underneath. The concept that mountains had "roots" was confirmed by George B. Airy a hundred years later, during study of Himalayan gravitation, and seismic studies detected corresponding density variations. Therefore, by
1080-617: A machine-readable Resource Description Framework / Web Ontology Language representation of the time scale, which is available through the Commission for the Management and Application of Geoscience Information GeoSciML project as a service and at a SPARQL end-point. Some other planets and satellites in the Solar System have sufficiently rigid structures to have preserved records of their own histories, for example, Venus , Mars and
1215-400: A misnomer as there is no force "pushing" horizontally, indeed tensional features are dominant along ridges. It is more accurate to refer to this mechanism as "gravitational sliding", since the topography across the whole plate can vary considerably and spreading ridges are only the most prominent feature. Other mechanisms generating this gravitational secondary force include flexural bulging of
1350-471: A relative interval of geologic time. A chronostratigraphic unit is a body of rock, layered or unlayered, that is defined between specified stratigraphic horizons which represent specified intervals of geologic time. They include all rocks representative of a specific interval of geologic time, and only this time span. Eonothem, erathem, system, series, subseries, stage, and substage are the hierarchical chronostratigraphic units. A geochronologic unit
1485-551: A secondary phenomenon of this basically vertically oriented mechanism. It finds its roots in the Undation Model of van Bemmelen . This can act on various scales, from the small scale of one island arc up to the larger scale of an entire ocean basin. Alfred Wegener , being a meteorologist , had proposed tidal forces and centrifugal forces as the main driving mechanisms behind continental drift ; however, these forces were considered far too small to cause continental motion as
SECTION 10
#17328377824821620-407: A solid crust and mantle and a liquid core, but there seemed to be no way that portions of the crust could move around. Many distinguished scientists of the time, such as Harold Jeffreys and Charles Schuchert , were outspoken critics of continental drift. Despite much opposition, the view of continental drift gained support and a lively debate started between "drifters" or "mobilists" (proponents of
1755-429: A specific and reliable order. This allows for a correlation of strata even when the horizon between them is not continuous. The geologic time scale is divided into chronostratigraphic units and their corresponding geochronologic units. The subdivisions Early and Late are used as the geochronologic equivalents of the chronostratigraphic Lower and Upper , e.g., Early Triassic Period (geochronologic unit)
1890-478: A static Earth without moving continents up until the major breakthroughs of the early sixties. Two- and three-dimensional imaging of Earth's interior ( seismic tomography ) shows a varying lateral density distribution throughout the mantle. Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). The manifestation of this varying lateral density
2025-532: A system/series (early/middle/late); however, the International Commission on Stratigraphy advocates for all new series and subseries to be named for a geographic feature in the vicinity of its stratotype or type locality . The name of stages should also be derived from a geographic feature in the locality of its stratotype or type locality. Informally, the time before the Cambrian is often referred to as
2160-458: A wider sense, correlating strata across national and continental boundaries based on their similarity to each other. Many of the names below erathem/era rank in use on the modern ICC/GTS were determined during the early to mid-19th century. During the 19th century, the debate regarding Earth's age was renewed, with geologists estimating ages based on denudation rates and sedimentary thicknesses or ocean chemistry, and physicists determining ages for
2295-438: Is mantle convection from buoyancy forces. How mantle convection directly and indirectly relates to plate motion is a matter of ongoing study and discussion in geodynamics. Somehow, this energy must be transferred to the lithosphere for tectonic plates to move. There are essentially two main types of mechanisms that are thought to exist related to the dynamics of the mantle that influence plate motion which are primary (through
2430-494: Is a numeric-only, chronologic reference point used to define the base of geochronologic units prior to the Cryogenian. These points are arbitrarily defined. They are used where GSSPs have not yet been established. Research is ongoing to define GSSPs for the base of all units that are currently defined by GSSAs. The standard international units of the geologic time scale are published by the International Commission on Stratigraphy on
2565-479: Is a subdivision of geologic time. It is a numeric representation of an intangible property (time). These units are arranged in a hierarchy: eon, era, period, epoch, subepoch, age, and subage. Geochronology is the scientific branch of geology that aims to determine the age of rocks, fossils, and sediments either through absolute (e.g., radiometric dating ) or relative means (e.g., stratigraphic position , paleomagnetism , stable isotope ratios ). Geochronometry
2700-475: Is a way of representing deep time based on events that have occurred throughout Earth's history , a time span of about 4.54 ± 0.05 Ga (4.54 billion years). It chronologically organises strata, and subsequently time, by observing fundamental changes in stratigraphy that correspond to major geological or paleontological events. For example, the Cretaceous–Paleogene extinction event , marks
2835-527: Is based on their modes of formation. Oceanic crust is formed at sea-floor spreading centers. Continental crust is formed through arc volcanism and accretion of terranes through plate tectonic processes. Oceanic crust is denser than continental crust because it has less silicon and more of the heavier elements than continental crust . As a result of this density difference, oceanic crust generally lies below sea level , while continental crust buoyantly projects above sea level. Average oceanic lithosphere
SECTION 20
#17328377824822970-456: Is called a plate boundary . Plate boundaries are where geological events occur, such as earthquakes and the creation of topographic features such as mountains , volcanoes , mid-ocean ridges , and oceanic trenches . The vast majority of the world's active volcanoes occur along plate boundaries, with the Pacific plate's Ring of Fire being the most active and widely known. Some volcanoes occur in
3105-533: Is called the geosynclinal theory . Generally, this was placed in the context of a contracting planet Earth due to heat loss in the course of a relatively short geological time. It was observed as early as 1596 that the opposite coasts of the Atlantic Ocean—or, more precisely, the edges of the continental shelves —have similar shapes and seem to have once fitted together. Since that time many theories were proposed to explain this apparent complementarity, but
3240-487: Is in motion, presents a problem. The same holds for the African, Eurasian , and Antarctic plates. Gravitational sliding away from mantle doming: According to older theories, one of the driving mechanisms of the plates is the existence of large scale asthenosphere/mantle domes which cause the gravitational sliding of lithosphere plates away from them (see the paragraph on Mantle Mechanisms). This gravitational sliding represents
3375-408: Is invoked as the major driving force, through slab pull along subduction zones. Gravitational sliding away from a spreading ridge is one of the proposed driving forces, it proposes plate motion is driven by the higher elevation of plates at ocean ridges. As oceanic lithosphere is formed at spreading ridges from hot mantle material, it gradually cools and thickens with age (and thus adds distance from
3510-415: Is still advocated to explain the break-up of supercontinents during specific geological epochs. It has followers amongst the scientists involved in the theory of Earth expansion . Another theory is that the mantle flows neither in cells nor large plumes but rather as a series of channels just below Earth's crust, which then provide basal friction to the lithosphere. This theory, called "surge tectonics",
3645-576: Is the scientific theory that Earth 's lithosphere comprises a number of large tectonic plates , which have been slowly moving since 3–4 billion years ago. The model builds on the concept of continental drift , an idea developed during the first decades of the 20th century. Plate tectonics came to be accepted by geoscientists after seafloor spreading was validated in the mid-to-late 1960s. The processes that result in plates and shape Earth's crust are called tectonics . Tectonic plates also occur in other planets and moons. Earth's lithosphere,
3780-401: Is the field of geochronology that numerically quantifies geologic time. A Global Boundary Stratotype Section and Point (GSSP) is an internationally agreed-upon reference point on a stratigraphic section that defines the lower boundaries of stages on the geologic time scale. (Recently this has been used to define the base of a system) A Global Standard Stratigraphic Age (GSSA)
3915-597: Is the responsibility of the International Commission on Stratigraphy (ICS), a constituent body of the International Union of Geological Sciences (IUGS), whose primary objective is to precisely define global chronostratigraphic units of the International Chronostratigraphic Chart (ICC) that are used to define divisions of geologic time. The chronostratigraphic divisions are in turn used to define geochronologic units. The geologic time scale
4050-488: Is to consider the relative rate at which each plate is moving as well as the evidence related to the significance of each process to the overall driving force on the plate. One of the most significant correlations discovered to date is that lithospheric plates attached to downgoing (subducting) plates move much faster than other types of plates. The Pacific plate, for instance, is essentially surrounded by zones of subduction (the so-called Ring of Fire) and moves much faster than
4185-407: Is typically 100 km (62 mi) thick. Its thickness is a function of its age. As time passes, it cools by conducting heat from below, and releasing it raditively into space. The adjacent mantle below is cooled by this process and added to its base. Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it
Quaternary - Misplaced Pages Continue
4320-573: Is used in place of Lower Triassic System (chronostratigraphic unit). Rocks representing a given chronostratigraphic unit are that chronostratigraphic unit, and the time they were laid down in is the geochronologic unit, e.g., the rocks that represent the Silurian System are the Silurian System and they were deposited during the Silurian Period. This definition means the numeric age of
4455-435: Is used. It asserts that super plumes rise from the deeper mantle and are the drivers or substitutes of the major convection cells. These ideas find their roots in the early 1930s in the works of Beloussov and van Bemmelen , which were initially opposed to plate tectonics and placed the mechanism in a fixed frame of vertical movements. Van Bemmelen later modified the concept in his "Undation Models" and used "Mantle Blisters" as
4590-646: The Anthropocene is a proposed epoch/series for the most recent time in Earth's history. While still informal, it is a widely used term to denote the present geologic time interval, in which many conditions and processes on Earth are profoundly altered by human impact. As of April 2022 the Anthropocene has not been ratified by the ICS; however, in May 2019 the Anthropocene Working Group voted in favour of submitting
4725-558: The Appalachian Mountains of North America are very similar in structure and lithology . However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for continental drift. Specifically, they did not see how continental rock could plow through the much denser rock that makes up oceanic crust. Wegener could not explain the force that drove continental drift, and his vindication did not come until after his death in 1930. As it
4860-529: The Brothers of Purity , who wrote on the processes of stratification over the passage of time in their treatises . Their work likely inspired that of the 11th-century Persian polymath Avicenna (Ibn Sînâ, 980–1037) who wrote in The Book of Healing (1027) on the concept of stratification and superposition, pre-dating Nicolas Steno by more than six centuries. Avicenna also recognised fossils as "petrifications of
4995-619: The Gelasian Stage, which was formerly considered part of the Neogene Period and Pliocene Epoch. This was later revised to 2.58 mya. The Anthropocene was proposed as a third epoch as a mark of the anthropogenic impact on the global environment starting with the Industrial Revolution , or about 200 years ago. The Anthropocene was rejected as a geological epoch in 2024 by the International Union of Geological Sciences (IUGS),
5130-537: The Milankovitch cycles and the associated climate and environmental changes that they caused. In 1759 Giovanni Arduino proposed that the geological strata of northern Italy could be divided into four successive formations or "orders" ( Italian : quattro ordini ). The term "quaternary" was introduced by Jules Desnoyers in 1829 for sediments of France 's Seine Basin that clearly seemed to be younger than Tertiary Period rocks . The Quaternary Period follows
5265-638: The Neogene Period and extends to the present. The Quaternary covers the time span of glaciations classified as the Pleistocene , and includes the present interglacial time-period, the Holocene . This places the start of the Quaternary at the onset of Northern Hemisphere glaciation approximately 2.6 million years ago ( mya ). Prior to 2009, the Pleistocene was defined to be from 1.805 million years ago to
5400-530: The Precambrian or pre-Cambrian (Supereon). While a modern geological time scale was not formulated until 1911 by Arthur Holmes , the broader concept that rocks and time are related can be traced back to (at least) the philosophers of Ancient Greece . Xenophanes of Colophon (c. 570–487 BCE ) observed rock beds with fossils of shells located above the sea-level, viewed them as once living organisms, and used this to imply an unstable relationship in which
5535-422: The chemical subdivision of these same layers into the mantle (comprising both the asthenosphere and the mantle portion of the lithosphere) and the crust: a given piece of mantle may be part of the lithosphere or the asthenosphere at different times depending on its temperature and pressure. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates , which ride on
Quaternary - Misplaced Pages Continue
5670-724: The fluid-like solid the asthenosphere . Plate motions range from 10 to 40 millimetres per year (0.4 to 1.6 in/year) at the Mid-Atlantic Ridge (about as fast as fingernails grow), to about 160 millimetres per year (6.3 in/year) for the Nazca plate (about as fast as hair grows). Tectonic lithosphere plates consist of lithospheric mantle overlain by one or two types of crustal material: oceanic crust (in older texts called sima from silicon and magnesium ) and continental crust ( sial from silicon and aluminium ). The distinction between oceanic crust and continental crust
5805-473: The lithosphere and asthenosphere . The division is based on differences in mechanical properties and in the method for the transfer of heat . The lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat by conduction , whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient. This division should not be confused with
5940-668: The Commission on Stratigraphy (applied in 1965) to become a member commission of IUGS led to the founding of the ICS. One of the primary objectives of the ICS is "the establishment, publication and revision of the ICS International Chronostratigraphic Chart which is the standard, reference global Geological Time Scale to include the ratified Commission decisions". Following on from Holmes, several A Geological Time Scale books were published in 1982, 1989, 2004, 2008, 2012, 2016, and 2020. However, since 2013,
6075-464: The Earth's Moon . Dominantly fluid planets, such as the giant planets , do not comparably preserve their history. Apart from the Late Heavy Bombardment , events on other planets probably had little direct influence on the Earth, and events on Earth had correspondingly little effect on those planets. Construction of a time scale that links the planets is, therefore, of only limited relevance to
6210-542: The Earth's rotation and the Moon as main driving forces for the plates. The vector of a plate's motion is a function of all the forces acting on the plate; however, therein lies the problem regarding the degree to which each process contributes to the overall motion of each tectonic plate. The diversity of geodynamic settings and the properties of each plate result from the impact of the various processes actively driving each individual plate. One method of dealing with this problem
6345-468: The Earth's time scale, except in a Solar System context. The existence, timing, and terrestrial effects of the Late Heavy Bombardment are still a matter of debate. The geologic history of Earth's Moon has been divided into a time scale based on geomorphological markers, namely impact cratering , volcanism , and erosion . This process of dividing the Moon's history in this manner means that
6480-447: The ICS has taken responsibility for producing and distributing the ICC citing the commercial nature, independent creation, and lack of oversight by the ICS on the prior published GTS versions (GTS books prior to 2013) although these versions were published in close association with the ICS. Subsequent Geologic Time Scale books (2016 and 2020 ) are commercial publications with no oversight from
6615-404: The ICS, and do not entirely conform to the chart produced by the ICS. The ICS produced GTS charts are versioned (year/month) beginning at v2013/01. At least one new version is published each year incorporating any changes ratified by the ICS since the prior version. The following five timelines show the geologic time scale to scale. The first shows the entire time from the formation of the Earth to
6750-415: The ICS. While some regional terms are still in use, the table of geologic time conforms to the nomenclature , ages, and colour codes set forth by the International Commission on Stratigraphy in the official International Chronostratigraphic Chart. The International Commission on Stratigraphy also provide an online interactive version of this chart. The interactive version is based on a service delivering
6885-665: The International Chronostratigraphic Chart; however, regional terms are still in use in some areas. The numeric values on the International Chronostratigrahpic Chart are represented by the unit Ma (megaannum, for 'million years '). For example, 201.4 ± 0.2 Ma, the lower boundary of the Jurassic Period, is defined as 201,400,000 years old with an uncertainty of 200,000 years. Other SI prefix units commonly used by geologists are Ga (gigaannum, billion years), and ka (kiloannum, thousand years), with
SECTION 50
#17328377824827020-445: The action of gravity. However, it is now known that not all sedimentary layers are deposited purely horizontally, but this principle is still a useful concept. The principle of lateral continuity that states layers of sediments extend laterally in all directions until either thinning out or being cut off by a different rock layer, i.e. they are laterally continuous. Layers do not extend indefinitely; their limits are controlled by
7155-533: The actual motions of the Pacific plate and other plates associated with the East Pacific Rise do not correlate mainly with either slab pull or slab push, but rather with a mantle convection upwelling whose horizontal spreading along the bases of the various plates drives them along via viscosity-related traction forces. The driving forces of plate motion continue to be active subjects of on-going research within geophysics and tectonophysics . The development of
7290-505: The age of rocks). It is used primarily by Earth scientists (including geologists , paleontologists , geophysicists , geochemists , and paleoclimatologists ) to describe the timing and relationships of events in geologic history. The time scale has been developed through the study of rock layers and the observation of their relationships and identifying features such as lithologies , paleomagnetic properties, and fossils . The definition of standardised international units of geologic time
7425-409: The amount and type of sediment in a sedimentary basin , and the geometry of that basin. The principle of cross-cutting relationships that states a rock that cuts across another rock must be younger than the rock it cuts across. The law of included fragments that states small fragments of one type of rock that are embedded in a second type of rock must have formed first, and were included when
7560-478: The assumption of a solid Earth made these various proposals difficult to accept. The discovery of radioactivity and its associated heating properties in 1895 prompted a re-examination of the apparent age of Earth . This had previously been estimated by its cooling rate under the assumption that Earth's surface radiated like a black body . Those calculations had implied that, even if it started at red heat , Earth would have dropped to its present temperature in
7695-399: The asthenosphere. This theory was launched by Arthur Holmes and some forerunners in the 1930s and was immediately recognized as the solution for the acceptance of the theory as originally discussed in the papers of Alfred Wegener in the early years of the 20th century. However, despite its acceptance, it was long debated in the scientific community because the leading theory still envisaged
7830-508: The base of the Cambrian, and thus the boundary between the Ediacaran and Cambrian systems (chronostratigraphic units) has not been changed; rather, the absolute age has merely been refined. Chronostratigraphy is the element of stratigraphy that deals with the relation between rock bodies and the relative measurement of geological time. It is the process where distinct strata between defined stratigraphic horizons are assigned to represent
7965-469: The base of the lithosphere. Slab pull is therefore most widely thought to be the greatest force acting on the plates. In this understanding, plate motion is mostly driven by the weight of cold, dense plates sinking into the mantle at trenches. Recent models indicate that trench suction plays an important role as well. However, the fact that the North American plate is nowhere being subducted, although it
8100-495: The bathymetry of the deep ocean floors and the nature of the oceanic crust such as magnetic properties and, more generally, with the development of marine geology which gave evidence for the association of seafloor spreading along the mid-oceanic ridges and magnetic field reversals , published between 1959 and 1963 by Heezen, Dietz, Hess, Mason, Vine & Matthews, and Morley. Simultaneous advances in early seismic imaging techniques in and around Wadati–Benioff zones along
8235-518: The bodies of plants and animals", with the 13th-century Dominican bishop Albertus Magnus (c. 1200–1280) extending this into a theory of a petrifying fluid. These works appeared to have little influence on scholars in Medieval Europe who looked to the Bible to explain the origins of fossils and sea-level changes, often attributing these to the ' Deluge ', including Ristoro d'Arezzo in 1282. It
SECTION 60
#17328377824828370-413: The concept was of continents plowing through oceanic crust. Therefore, Wegener later changed his position and asserted that convection currents are the main driving force of plate tectonics in the last edition of his book in 1929. However, in the plate tectonics context (accepted since the seafloor spreading proposals of Heezen, Hess, Dietz, Morley, Vine, and Matthews (see below) during the early 1960s),
8505-566: The cooling of the Earth or the Sun using basic thermodynamics or orbital physics. These estimations varied from 15,000 million years to 0.075 million years depending on method and author, but the estimations of Lord Kelvin and Clarence King were held in high regard at the time due to their pre-eminence in physics and geology. All of these early geochronometric determinations would later prove to be incorrect. The discovery of radioactive decay by Henri Becquerel , Marie Curie , and Pierre Curie laid
8640-415: The deep mantle at subduction zones, providing most of the driving force for plate movement. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone. For much of the first quarter of the 20th century, the leading theory of the driving force behind tectonic plate motions envisaged large scale convection currents in the upper mantle, which can be transmitted through
8775-453: The developments in mass spectrometry pioneered by Francis William Aston , Arthur Jeffrey Dempster , and Alfred O. C. Nier during the early to mid- 20th century would finally allow for the accurate determination of radiometric ages, with Holmes publishing several revisions to his geological time-scale with his final version in 1960. The establishment of the IUGS in 1961 and acceptance of
8910-404: The different layers of stone unless they had been upon the shore and had been covered over by earth newly thrown up by the sea which then became petrified? And if the above-mentioned Deluge had carried them to these places from the sea, you would find the shells at the edge of one layer of rock only, not at the edge of many where may be counted the winters of the years during which the sea multiplied
9045-530: The discussions treated in this section) or proposed as minor modulations within the overall plate tectonics model. In 1973, George W. Moore of the USGS and R. C. Bostrom presented evidence for a general westward drift of Earth's lithosphere with respect to the mantle, based on the steepness of the subduction zones (shallow dipping towards the east, steeply dipping towards the west). They concluded that tidal forces (the tidal lag or "friction") caused by Earth's rotation and
9180-466: The driving force for horizontal movements, invoking gravitational forces away from the regional crustal doming. The theories find resonance in the modern theories which envisage hot spots or mantle plumes which remain fixed and are overridden by oceanic and continental lithosphere plates over time and leave their traces in the geological record (though these phenomena are not invoked as real driving mechanisms, but rather as modulators). The mechanism
9315-619: The emergence of the straits of Bosphorus and Skagerrak during glacial epochs, which respectively turned the Black Sea and Baltic Sea into fresh water lakes, followed by their flooding (and return to salt water) by rising sea level; the periodic filling of the English Channel , forming a land bridge between Britain and the European mainland; the periodic closing of the Bering Strait , forming
9450-715: The establishment of the Glacial Theory. In time, thanks to the refinement of geology, it has been demonstrated that there were several periods of glacial advance and retreat and that past temperatures on Earth were very different from today. In particular, the Milankovitch cycles of Milutin Milankovitch are based on the premise that variations in incoming solar radiation are a fundamental factor controlling Earth's climate. During this time, substantial glaciers advanced and retreated over much of North America and Europe, parts of South America and Asia, and all of Antarctica. There
9585-473: The final one in 1936), he noted how the east coast of South America and the west coast of Africa looked as if they were once attached. Wegener was not the first to note this ( Abraham Ortelius , Antonio Snider-Pellegrini , Eduard Suess , Roberto Mantovani and Frank Bursley Taylor preceded him just to mention a few), but he was the first to marshal significant fossil and paleo-topographical and climatological evidence to support this simple observation (and
9720-691: The forces acting upon it by the Moon are a driving force for plate tectonics. As Earth spins eastward beneath the Moon, the Moon's gravity ever so slightly pulls Earth's surface layer back westward, just as proposed by Alfred Wegener (see above). Since 1990 this theory is mainly advocated by Doglioni and co-workers ( Doglioni 1990 ), such as in a more recent 2006 study, where scientists reviewed and advocated these ideas. It has been suggested in Lovett (2006) that this observation may also explain why Venus and Mars have no plate tectonics, as Venus has no moon and Mars' moons are too small to have significant tidal effects on
9855-414: The foundational principles of determining the correlation of strata relative to geologic time. Over the course of the 18th-century geologists realised that: The apparent, earliest formal division of the geologic record with respect to time was introduced during the era of Biblical models by Thomas Burnet who applied a two-fold terminology to mountains by identifying " montes primarii " for rock formed at
9990-588: The geographical latitudinal and longitudinal grid of Earth itself. These systematic relations studies in the second half of the nineteenth century and the first half of the twentieth century underline exactly the opposite: that the plates had not moved in time, that the deformation grid was fixed with respect to Earth's equator and axis, and that gravitational driving forces were generally acting vertically and caused only local horizontal movements (the so-called pre-plate tectonic, "fixist theories"). Later studies (discussed below on this page), therefore, invoked many of
10125-458: The geologic time scale of Earth. This table is arranged with the most recent geologic periods at the top, and the oldest at the bottom. The height of each table entry does not correspond to the duration of each subdivision of time. As such, this table is not to scale and does not accurately represent the relative time-spans of each geochronologic unit. While the Phanerozoic Eon looks longer than
10260-598: The glacial period ended about 11,700 years ago. Modern humans evolved about 315,000 years ago. During the Quaternary Period, mammals, flowering plants, and insects dominated the land. Period (geology) The geologic time scale or geological time scale ( GTS ) is a representation of time based on the rock record of Earth . It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochronology (a scientific branch of geology that aims to determine
10395-448: The governing body of the ICS. The 2.58 million years of the Quaternary represents the time during which recognisable humans existed. Over this geologically short time period there has been relatively little change in the distribution of the continents due to plate tectonics . The Quaternary geological record is preserved in greater detail than that for earlier periods. The major geographical changes during this time period included
10530-492: The ground work for radiometric dating, but the knowledge and tools required for accurate determination of radiometric ages would not be in place until the mid-1950s. Early attempts at determining ages of uranium minerals and rocks by Ernest Rutherford , Bertram Boltwood , Robert Strutt , and Arthur Holmes, would culminate in what are considered the first international geological time scales by Holmes in 1911 and 1913. The discovery of isotopes in 1913 by Frederick Soddy , and
10665-708: The interiors of plates, and these have been variously attributed to internal plate deformation and to mantle plumes. Tectonic plates may include continental crust or oceanic crust, or both. For example, the African plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. Some pieces of oceanic crust, known as ophiolites , failed to be subducted under continental crust at destructive plate boundaries; instead these oceanic crustal fragments were pushed upward and were preserved within continental crust. Three types of plate boundaries exist, characterized by
10800-573: The land bridge between Asia and North America ; and the periodic flash flooding of Scablands of the American Northwest by glacial water. The current extent of Hudson Bay , the Great Lakes and other major lakes of North America are a consequence of the Canadian Shield 's readjustment since the last ice age; different shorelines have existed over the course of Quaternary time. The climate
10935-412: The large scale convection cells) or secondary. The secondary mechanisms view plate motion driven by friction between the convection currents in the asthenosphere and the more rigid overlying lithosphere. This is due to the inflow of mantle material related to the downward pull on plates in subduction zones at ocean trenches. Slab pull may occur in a geodynamic setting where basal tractions continue to act on
11070-913: The latter often represented in calibrated units ( before present ). The names of geologic time units are defined for chronostratigraphic units with the corresponding geochronologic unit sharing the same name with a change to the suffix (e.g. Phanerozoic Eonothem becomes the Phanerozoic Eon). Names of erathems in the Phanerozoic were chosen to reflect major changes in the history of life on Earth: Paleozoic (old life), Mesozoic (middle life), and Cenozoic (new life). Names of systems are diverse in origin, with some indicating chronologic position (e.g., Paleogene), while others are named for lithology (e.g., Cretaceous), geography (e.g., Permian ), or are tribal (e.g., Ordovician ) in origin. Most currently recognised series and subseries are named for their position within
11205-561: The layers of sand and mud brought down by the neighboring rivers and spread them over its shores. And if you wish to say that there must have been many deluges in order to produce these layers and the shells among them it would then become necessary for you to affirm that such a deluge took place every year. These views of da Vinci remained unpublished, and thus lacked influence at the time; however, questions of fossils and their significance were pursued and, while views against Genesis were not readily accepted and dissent from religious doctrine
11340-537: The litho- and biostratigraphic differences around the world in time equivalent rocks. The ICS has long worked to reconcile conflicting terminology by standardising globally significant and identifiable stratigraphic horizons that can be used to define the lower boundaries of chronostratigraphic units. Defining chronostratigraphic units in such a manner allows for the use of global, standardised nomenclature. The International Chronostratigraphic Chart represents this ongoing effort. Several key principles are used to determine
11475-421: The lithosphere before it dives underneath an adjacent plate, producing a clear topographical feature that can offset, or at least affect, the influence of topographical ocean ridges. Mantle plumes and hot spots are also postulated to impinge on the underside of tectonic plates. Slab pull : Scientific opinion is that the asthenosphere is insufficiently competent or rigid to directly cause motion by friction along
11610-607: The lower boundary of the Paleogene System/Period and thus the boundary between the Cretaceous and Paleogene systems/periods. For divisions prior to the Cryogenian , arbitrary numeric boundary definitions ( Global Standard Stratigraphic Ages , GSSAs) are used to divide geologic time. Proposals have been made to better reconcile these divisions with the rock record. Historically, regional geologic time scales were used due to
11745-403: The lower mantle, there is a slight westward component in the motions of all the plates. They demonstrated though that the westward drift, seen only for the past 30 Ma, is attributed to the increased dominance of the steadily growing and accelerating Pacific plate. The debate is still open, and a recent paper by Hofmeister et al. (2022) revived the idea advocating again the interaction between
11880-405: The many geographical, geological, and biological continuities between continents. In 1912, the meteorologist Alfred Wegener described what he called continental drift, an idea that culminated fifty years later in the modern theory of plate tectonics. Wegener expanded his theory in his 1915 book The Origin of Continents and Oceans . Starting from the idea (also expressed by his forerunners) that
12015-429: The matching of the rock formations along these edges. Confirmation of their previous contiguous nature also came from the fossil plants Glossopteris and Gangamopteris , and the therapsid or mammal-like reptile Lystrosaurus , all widely distributed over South America, Africa, Antarctica, India, and Australia. The evidence for such an erstwhile joining of these continents was patent to field geologists working in
12150-563: The motion picture of the Atlantic region", processes that anticipated seafloor spreading and subduction . One of the first pieces of geophysical evidence that was used to support the movement of lithospheric plates came from paleomagnetism . This is based on the fact that rocks of different ages show a variable magnetic field direction, evidenced by studies since the mid–nineteenth century. The magnetic north and south poles reverse through time, and, especially important in paleotectonic studies,
12285-438: The motion. At a subduction zone the relatively cold, dense oceanic crust sinks down into the mantle, forming the downward convecting limb of a mantle cell , which is the strongest driver of plate motion. The relative importance and interaction of other proposed factors such as active convection, upwelling inside the mantle, and tidal drag of the Moon is still the subject of debate. The outer layers of Earth are divided into
12420-466: The north pole, and each continent, in fact, shows its own "polar wander path". During the late 1950s, it was successfully shown on two occasions that these data could show the validity of continental drift: by Keith Runcorn in a paper in 1956, and by Warren Carey in a symposium held in March 1956. The second piece of evidence in support of continental drift came during the late 1950s and early 60s from data on
12555-407: The oceanic crust is suggested to be in motion with the continents which caused the proposals related to Earth rotation to be reconsidered. In more recent literature, these driving forces are: Forces that are small and generally negligible are: For these mechanisms to be overall valid, systematic relationships should exist all over the globe between the orientation and kinematics of deformation and
12690-437: The oceanic lithosphere and the thicker continental lithosphere, each topped by its own kind of crust. Along convergent plate boundaries , the process of subduction carries the edge of one plate down under the other plate and into the mantle . This process reduces the total surface area (crust) of the Earth. The lost surface is balanced by the formation of new oceanic crust along divergent margins by seafloor spreading, keeping
12825-408: The pertinent time span. As of April 2022 these proposed changes have not been accepted by the ICS. The proposed changes (changes from the current scale [v2023/09]) are italicised: Proposed pre-Cambrian timeline (GTS2012), shown to scale: Current ICC pre-Cambrian timeline (v2023/09), shown to scale: The following table summarises the major events and characteristics of the divisions making up
12960-463: The planet. In a paper by it was suggested that, on the other hand, it can easily be observed that many plates are moving north and eastward, and that the dominantly westward motion of the Pacific Ocean basins derives simply from the eastward bias of the Pacific spreading center (which is not a predicted manifestation of such lunar forces). In the same paper the authors admit, however, that relative to
13095-399: The plate as it dives into the mantle (although perhaps to a greater extent acting on both the under and upper side of the slab). Furthermore, slabs that are broken off and sink into the mantle can cause viscous mantle forces driving plates through slab suction. In the theory of plume tectonics followed by numerous researchers during the 1990s, a modified concept of mantle convection currents
13230-426: The plates of the Atlantic basin, which are attached (perhaps one could say 'welded') to adjacent continents instead of subducting plates. It is thus thought that forces associated with the downgoing plate (slab pull and slab suction) are the driving forces which determine the motion of plates, except for those plates which are not being subducted. This view however has been contradicted by a recent study which found that
13365-477: The presence of traces of the passage of a glacier at a considerable distance from the Alps. This idea was initially disputed by another Swiss scientist, Louis Agassiz , but when he undertook to disprove it, he ended up affirming his colleague's hypothesis. A year later, Agassiz raised the hypothesis of a great glacial period that would have had long-reaching general effects. This idea gained him international fame and led to
13500-408: The present continents once formed a single land mass (later called Pangaea ), Wegener suggested that these separated and drifted apart, likening them to "icebergs" of low density sial floating on a sea of denser sima . Supporting evidence for the idea came from the dove-tailing outlines of South America's east coast and Africa's west coast Antonio Snider-Pellegrini had drawn on his maps, and from
13635-452: The present, but this gives little space for the most recent eon. The second timeline shows an expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, the most recent period is expanded in the fourth timeline, and the most recent epoch is expanded in the fifth timeline. Horizontal scale is Millions of years (above timelines) / Thousands of years (below timeline) First suggested in 2000,
13770-529: The present, so the current definition of the Pleistocene includes a portion of what was, prior to 2009, defined as the Pliocene . Quaternary stratigraphers usually worked with regional subdivisions. From the 1970s, the International Commission on Stratigraphy (ICS) tried to make a single geologic time scale based on GSSP 's, which could be used internationally. The Quaternary subdivisions were defined based on biostratigraphy instead of paleoclimate . This led to
13905-488: The present. The Quaternary Period is divided into two epochs: the Pleistocene (2.58 million years ago to 11.7 thousand years ago) and the Holocene (11.7 thousand years ago to today); a proposed third epoch, the Anthropocene , was rejected in 2024 by IUGS , the governing body of the ICS. The Quaternary is typically defined by the Quaternary glaciation , the cyclic growth and decay of continental ice sheets related to
14040-485: The principles of superposition, original horizontality, lateral continuity, and cross-cutting relationships. From this Steno reasoned that strata were laid down in succession and inferred relative time (in Steno's belief, time from Creation ). While Steno's principles were simple and attracted much attention, applying them proved challenging. These basic principles, albeit with improved and more nuanced interpretations, still form
14175-517: The problem that the proposed base of the Pleistocene was at 1.805 million years ago, long after the start of the major glaciations of the northern hemisphere. The ICS then proposed to abolish use of the name Quaternary altogether, which appeared unacceptable to the International Union for Quaternary Research (INQUA). In 2009, it was decided to make the Quaternary the youngest period of the Cenozoic Era with its base at 2.588 mya and including
14310-459: The relationships recognized during this pre-plate tectonics period to support their theories (see reviews of these various mechanisms related to Earth rotation the work of van Dijk and collaborators). Of the many forces discussed above, tidal force is still highly debated and defended as a possible principal driving force of plate tectonics. The other forces are only used in global geodynamic models not using plate tectonics concepts (therefore beyond
14445-428: The relative position of the magnetic north pole varies through time. Initially, during the first half of the twentieth century, the latter phenomenon was explained by introducing what was called "polar wander" (see apparent polar wander ) (i.e., it was assumed that the north pole location had been shifting through time). An alternative explanation, though, was that the continents had moved (shifted and rotated) relative to
14580-521: The relative relationships of rocks and thus their chronostratigraphic position. The law of superposition that states that in undeformed stratigraphic sequences the oldest strata will lie at the bottom of the sequence, while newer material stacks upon the surface. In practice, this means a younger rock will lie on top of an older rock unless there is evidence to suggest otherwise. The principle of original horizontality that states layers of sediments will originally be deposited horizontally under
14715-467: The rest, it merely spans ~539 million years (~12% of Earth's history), whilst the previous three eons collectively span ~3,461 million years (~76% of Earth's history). This bias toward the most recent eon is in part due to the relative lack of information about events that occurred during the first three eons compared to the current eon (the Phanerozoic). The use of subseries/subepochs has been ratified by
14850-399: The ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load. The result is a slight lateral incline with increased distance from the ridge axis. This force is regarded as a secondary force and is often referred to as " ridge push ". This is
14985-614: The rigid outer shell of the planet including the crust and upper mantle , is fractured into seven or eight major plates (depending on how they are defined) and many minor plates or "platelets". Where the plates meet, their relative motion determines the type of plate boundary (or fault ): convergent , divergent , or transform . The relative movement of the plates typically ranges from zero to 10 cm annually. Faults tend to be geologically active, experiencing earthquakes , volcanic activity , mountain-building , and oceanic trench formation. Tectonic plates are composed of
15120-630: The rock record to bring it in line with the post-Tonian geologic time scale. This work assessed the geologic history of the currently defined eons and eras of the pre-Cambrian, and the proposals in the "Geological Time Scale" books 2004, 2012, and 2020. Their recommend revisions of the pre-Cryogenian geologic time scale were (changes from the current scale [v2023/09] are italicised): Proposed pre-Cambrian timeline (Shield et al. 2021, ICS working group on pre-Cryogenian chronostratigraphy), shown to scale: Current ICC pre-Cambrian timeline (v2023/09), shown to scale: The book, Geologic Time Scale 2012,
15255-474: The sea had at times transgressed over the land and at other times had regressed . This view was shared by a few of Xenophanes's contemporaries and those that followed, including Aristotle (384–322 BCE) who (with additional observations) reasoned that the positions of land and sea had changed over long periods of time. The concept of deep time was also recognised by Chinese naturalist Shen Kuo (1031–1095) and Islamic scientist -philosophers, notably
15390-544: The second rock was forming. The relationships of unconformities which are geologic features representing a gap in the geologic record. Unconformities are formed during periods of erosion or non-deposition, indicating non-continuous sediment deposition. Observing the type and relationships of unconformities in strata allows geologist to understand the relative timing the strata. The principle of faunal succession (where applicable) that states rock strata contain distinctive sets of fossils that succeed each other vertically in
15525-486: The southern hemisphere. The South African Alex du Toit put together a mass of such information in his 1937 publication Our Wandering Continents , and went further than Wegener in recognising the strong links between the Gondwana fragments. Wegener's work was initially not widely accepted, in part due to a lack of detailed evidence but mostly because of the lack of a reasonable physically supported mechanism. Earth might have
15660-470: The theory of plate tectonics was the scientific and cultural change which occurred during a period of 50 years of scientific debate. The event of the acceptance itself was a paradigm shift and can therefore be classified as a scientific revolution, now described as the Plate Tectonics Revolution . Around the start of the twentieth century, various theorists unsuccessfully attempted to explain
15795-502: The theory) and "fixists" (opponents). During the 1920s, 1930s and 1940s, the former reached important milestones proposing that convection currents might have driven the plate movements, and that spreading may have occurred below the sea within the oceanic crust. Concepts close to the elements of plate tectonics were proposed by geophysicists and geologists (both fixists and mobilists) like Vening-Meinesz, Holmes, and Umbgrove. In 1941, Otto Ampferer described, in his publication "Thoughts on
15930-484: The time during which the rocks were laid down, and the collection of rocks themselves (i.e., it was correct to say Tertiary rocks, and Tertiary Period). Only the Quaternary division is retained in the modern geologic time scale, while the Tertiary division was in use until the early 21st century. The Neptunism and Plutonism theories would compete into the early 19th century with a key driver for resolution of this debate being
16065-730: The time of the 'Deluge', and younger " monticulos secundarios" formed later from the debris of the " primarii" . Anton Moro (1687–1784) also used primary and secondary divisions for rock units but his mechanism was volcanic. In this early version of the Plutonism theory, the interior of Earth was seen as hot, and this drove the creation of primary igneous and metamorphic rocks and secondary rocks formed contorted and fossiliferous sediments. These primary and secondary divisions were expanded on by Giovanni Targioni Tozzetti (1712–1783) and Giovanni Arduino (1713–1795) to include tertiary and quaternary divisions. These divisions were used to describe both
16200-562: The time scale boundaries do not imply fundamental changes in geological processes, unlike Earth's geologic time scale. Five geologic systems/periods ( Pre-Nectarian , Nectarian , Imbrian , Eratosthenian , Copernican ), with the Imbrian divided into two series/epochs (Early and Late) were defined in the latest Lunar geologic time scale. The Moon is unique in the Solar System in that it is the only other body from which humans have rock samples with
16335-476: The total surface area constant in a tectonic "conveyor belt". Tectonic plates are relatively rigid and float across the ductile asthenosphere beneath. Lateral density variations in the mantle result in convection currents, the slow creeping motion of Earth's solid mantle. At a seafloor spreading ridge , plates move away from the ridge, which is a topographic high, and the newly formed crust cools as it moves away, increasing its density and contributing to
16470-429: The trenches bounding many continental margins, together with many other geophysical (e.g., gravimetric) and geological observations, showed how the oceanic crust could disappear into the mantle, providing the mechanism to balance the extension of the ocean basins with shortening along its margins. All this evidence, both from the ocean floor and from the continental margins, made it clear around 1965 that continental drift
16605-467: The way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are: Tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere . Dissipation of heat from the mantle is the original source of the energy required to drive plate tectonics through convection or large scale upwelling and doming. As
16740-526: The work of James Hutton (1726–1797), in particular his Theory of the Earth , first presented before the Royal Society of Edinburgh in 1785. Hutton's theory would later become known as uniformitarianism , popularised by John Playfair (1748–1819) and later Charles Lyell (1797–1875) in his Principles of Geology . Their theories strongly contested the 6,000 year age of the Earth as suggested determined by James Ussher via Biblical chronology that
16875-560: Was a major extinction of large mammals globally during the Late Pleistocene Epoch. Many forms such as sabre-toothed cats , mammoths , mastodons , glyptodonts , etc., became extinct worldwide. Others, including horses , camels and American cheetahs became extinct in North America . The Great Lakes formed and giant mammals thrived in parts of North America and Eurasia not covered in ice. These mammals became extinct when
17010-429: Was accepted at the time by western religion. Instead, using geological evidence, they contested Earth to be much older, cementing the concept of deep time. During the early 19th century William Smith , Georges Cuvier , Jean d'Omalius d'Halloy , and Alexandre Brongniart pioneered the systematic division of rocks by stratigraphy and fossil assemblages. These geologists began to use the local names given to rock units in
17145-531: Was feasible. The theory of plate tectonics was defined in a series of papers between 1965 and 1967. The theory revolutionized the Earth sciences, explaining a diverse range of geological phenomena and their implications in other studies such as paleogeography and paleobiology . In the late 19th and early 20th centuries, geologists assumed that Earth's major features were fixed, and that most geologic features such as basin development and mountain ranges could be explained by vertical crustal movement, described in what
17280-599: Was formed. For a typical distance that oceanic lithosphere must travel before being subducted, the thickness varies from about 6 km (4 mi) thick at mid-ocean ridges to greater than 100 km (62 mi) at subduction zones. For shorter or longer distances, the subduction zone, and therefore also the mean, thickness becomes smaller or larger, respectively. Continental lithosphere is typically about 200 km (120 mi) thick, though this varies considerably between basins, mountain ranges, and stable cratonic interiors of continents. The location where two plates meet
17415-415: Was in some places unwise, scholars such as Girolamo Fracastoro shared da Vinci's views, and found the attribution of fossils to the 'Deluge' absurd. Niels Stensen, more commonly known as Nicolas Steno (1638–1686), is credited with establishing four of the guiding principles of stratigraphy. In De solido intra solidum naturaliter contento dissertationis prodromus Steno states: Respectively, these are
17550-535: Was not until the Italian Renaissance when Leonardo da Vinci (1452–1519) would reinvigorate the relationships between stratification, relative sea-level change, and time, denouncing attribution of fossils to the 'Deluge': Of the stupidity and ignorance of those who imagine that these creatures were carried to such places distant from the sea by the Deluge...Why do we find so many fragments and whole shells between
17685-424: Was observed early that although granite existed on continents, seafloor seemed to be composed of denser basalt , the prevailing concept during the first half of the twentieth century was that there were two types of crust, named "sial" (continental type crust) and "sima" (oceanic type crust). Furthermore, it was supposed that a static shell of strata was present under the continents. It therefore looked apparent that
17820-457: Was one of periodic glaciations with continental glaciers moving as far from the poles as 40 degrees latitude . Glaciation took place repeatedly during the Quaternary Ice age – a term coined by Schimper in 1839 that began with the start of the Quaternary about 2.58 Mya and continues to the present day. In 1821, a Swiss engineer, Ignaz Venetz , presented an article in which he suggested
17955-443: Was popularized during the 1980s and 1990s. Recent research, based on three-dimensional computer modelling, suggests that plate geometry is governed by a feedback between mantle convection patterns and the strength of the lithosphere. Forces related to gravity are invoked as secondary phenomena within the framework of a more general driving mechanism such as the various forms of mantle dynamics described above. In modern views, gravity
18090-560: Was supported in this by researchers such as Alex du Toit ). Furthermore, when the rock strata of the margins of separate continents are very similar it suggests that these rocks were formed in the same way, implying that they were joined initially. For instance, parts of Scotland and Ireland contain rocks very similar to those found in Newfoundland and New Brunswick . Furthermore, the Caledonian Mountains of Europe and parts of
18225-476: Was the last commercial publication of an international chronostratigraphic chart that was closely associated with the ICS. It included a proposal to substantially revise the pre-Cryogenian time scale to reflect important events such as the formation of the Solar System and the Great Oxidation Event , among others, while at the same time maintaining most of the previous chronostratigraphic nomenclature for
#481518