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In
geology Geology (from the Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek ...

geology
, a rift is a linear zone where the
lithosphere A lithosphere ( grc, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial planet, terrestrial-type planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion o ...
is being pulled apart and is an example of
extensional tectonics Extensional tectonics is concerned with the structures formed by, and the tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mo ...
. Typical rift features are a central linear downfaulted depression, called a
graben In geology Geology (from the γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is a branch of concerned with both the liquid and , the of which it is composed, and the processes by which they change over ti ...

graben
, or more commonly a
half-graben A half-graben is a geological structure bounded by a fault Fault commonly refers to: *Fault (geology), planar rock fractures showing evidence of relative movement *Fault (law), blameworthiness or responsibility Fault(s) may also refer to: Arts ...
with normal faulting and rift-flank uplifts mainly on one side. Where rifts remain above sea level they form a
rift valley A rift valley is a linear shaped lowland between several Highland, highlands or mountain ranges created by the action of a geologic rift. Rifts are formed as a result of the pulling apart of the lithosphere due to extensional tectonics. The ...
, which may be filled by water forming a
rift lake A rift lake is a lake A lake is an area filled with water, localized in a basin, surrounded by land, apart from any river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another ...
. The axis of the rift area may contain
volcanic rocks Volcanic rock (often shortened to volcanics in scientific contexts) is a rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by th ...

volcanic rocks
, and active
volcanism Volcanism (or volcanicity) is the phenomenon of eruption of molten rock (magma) onto the Earth#Surface, surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics and volcanic gases erupt through a break in the surface called ...
is a part of many, but not all, active rift systems. Major rifts occur along the central axis of most
mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics File:Earth cutaway schematic-en.svg, upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate ...
s, where new
oceanic crust The oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates This is a list of tectonic plates on Earth's surface. Tectonic plates are pieces of Earth's crust 350px, Plates in the crust of Earth Earth's crust i ...
and lithosphere is created along a
divergent boundary In plate tectonics File:Earth cutaway schematic-en.svg, upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate tectonics (from the la, label=Late Latin, tectonicus, from ...
between two
tectonic plate This is a list of tectonic plates on Earth's surface Earth is the third planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibri ...
s. ''Failed rifts'' are the result of continental rifting that failed to continue to the point of break-up. Typically the transition from rifting to spreading develops at a
triple junction Triple is used in several contexts to mean "threefold" or a " treble": Sports * Triple (baseball) In baseball Baseball is a bat-and-ball games, bat-and-ball game played between two opposing teams who take turns batting (baseball), ba ...
where three converging rifts meet over a hotspot. Two of these evolve to the point of seafloor spreading, while the third ultimately fails, becoming an
aulacogen An aulacogen is a failed arm of a triple junction. Aulacogens are a part of plate tectonics File:Earth cutaway schematic-en.svg, upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to sc ...
.


Geometry

left, Topographic profile of the Malawi Lake Most rifts consist of a series of separate segments that together form the linear zone characteristic of rifts. The individual rift segments have a dominantly half-graben geometry, controlled by a single basin-bounding fault. Segment lengths vary between rifts, depending on the elastic thickness of the lithosphere. Areas of thick colder lithosphere, such as the Baikal Rift have segment lengths in excess of 80 km, while in areas of warmer thin lithosphere, segment lengths may be less than 30 km. Along the axis of the rift the position, and in some cases the polarity (the dip direction), of the main rift bounding fault changes from segment to segment. Segment boundaries often have a more complex structure and generally cross the rift axis at a high angle. These segment boundary zones accommodate the differences in fault displacement between the segments and are therefore known as accommodation zones. Accommodation zones take various forms, from a simple relay ramp at the overlap between two major faults of the same polarity, to zones of high structural complexity, particularly where the segments have opposite polarity. Accommodation zones may be located where older crustal structures intersect the rift axis. In the Gulf of Suez rift, the Zaafarana accommodation zone is located where a
shear zone A shear zone is a very important structural discontinuity surface in the Earth's crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones. Intervening (crus ...

shear zone
in the
Arabian-Nubian Shield File:Pannotia.svg, 300px, The Arabian-Nubian Shield in the supercontinent Pannotia c. 570 million years ago, before the rift, opening of the Red Sea. The Arabian-Nubian Shield (ANS) is an exposure of Precambrian crystalline rocks on the flanks of th ...

Arabian-Nubian Shield
meets the rift. Rift flanks or shoulders are elevated areas around rifts. Rift shoulders are typically about 70 km wide. Contrary to what was previously thought, elevated passive continental margins (EPCM) such as the
Brazilian Highlands The Brazilian Highlands or Brazilian Plateau ( pt, Planalto Brasileiro) are an extensive geographical region, covering most of the eastern, southern and central portions of Brazil Brazil ( pt, Brasil; ), officially the Federative Republic ...
, the
Scandinavian Mountains #REDIRECT Scandinavian Mountains#REDIRECT Scandinavian Mountains The Scandinavian Mountains or the Scandes is a mountain range that runs through the Scandinavian Peninsula. The Scandinavian Mountains are often erroneously thought to be equivalen ...
and India's
Western Ghats The Western Ghats is a mountain range that covers an area of in a stretch of parallel to the western coast of the Indian peninsula, traversing the states State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', ...

Western Ghats
, are not rift shoulders.


Rift development


Rift initiation

The formation of rift basins and strain localization reflects rift maturity. At the onset of rifting, the upper part of the lithosphere starts to extend on a series of initially unconnected
normal fault In geology Geology (from the Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers ...

normal fault
s, leading to the development of isolated basins. In subaerial rifts, for example, drainage at the onset of rifting is generally internal, with no element of through drainage.


Mature rift stage

As the rift evolves, some of the individual fault segments grow, eventually becoming linked together to form the larger bounding faults. Subsequent extension becomes concentrated on these faults. The longer faults and wider fault spacing leads to more continuous areas of fault-related
subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope move ...
along the rift axis. Significant uplift of the rift shoulders develops at this stage, strongly influencing drainage and sedimentation in the rift basins. During the climax of lithospheric rifting, as the crust is thinned, the Earth's surface subsides and the Moho becomes correspondingly raised. At the same time, the mantle lithosphere becomes thinned, causing a rise of the top of the asthenosphere. This brings high heat flow from the upwelling asthenosphere into the thinning lithosphere, heating the orogenic lithosphere for dehydration melting, typically causing extreme metamorphism at high thermal gradients of greater than 30 °C. The metamorphic products are high to ultrahigh temperature granulites and their associated migmatite and granites in collisional orogens, with possible emplacement of metamorphic core complexes in continental rift zones but oceanic core complexes in spreading ridges. This leads to a kind of orogeneses in extensional settings, which is referred as to rifting orogeny.


Post-rift subsidence

Once rifting ceases, the mantle beneath the rift cools and this is accompanied by a broad area of post-rift subsidence. The amount of subsidence is directly related to the amount of thinning during the rifting phase calculated as the beta factor (initial crustal thickness divided by final crustal thickness), but is also affected by the degree to which the rift basin is filled at each stage, due to the greater density of sediments in contrast to water. The simple 'McKenzie model' of rifting, which considers the rifting stage to be instantaneous, provides a good first order estimate of the amount of crustal thinning from observations of the amount of post-rift subsidence. This has generally been replaced by the 'flexural cantilever model', which takes into account the geometry of the rift faults and the flexural
isostasy Isostasy (Greek ''ísos'' "equal", ''stásis'' "standstill") or isostatic equilibrium is the state of gravitational Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Prope ...

isostasy
of the upper part of the crust.


Multiphase rifting

Some rifts show a complex and prolonged history of rifting, with several distinct phases. The North Sea rift shows evidence of several separate rift phases from the
Permian The Permian ( ) is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth ...
through to the Earliest
Cretaceous The Cretaceous ( ) is a geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of division ...

Cretaceous
, a period of over 100 million years.


Rifting to break-up

Rifting may lead to continental breakup and formation of oceanic basins. Successful rifting leads to seafloor spreading along a mid-oceanic ridge and a set of conjugate margins separated by an oceanic basin. Rifting may be active, and controlled by
mantle convection Mantle convection is the very slow creeping motion of Earth's solid silicate mantle caused by convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heteroge ...
. It may also be passive, and driven by far-field tectonic forces that stretch the lithosphere. Margin architecture develops due to spatial and temporal relationships between extensional deformation phases. Margin segmentation eventually leads to the formation of rift domains with variations of the Moho topography, including proximal domain with fault-rotated crustal blocks, necking zone with thinning of crustal
basement A basement or cellar is one or more Storey, floors of a building that are completely or partly below the storey, ground floor. It generally is used as a utility space for a building, where such items as the boiler, water heating, water heate ...
, distal domain with deep sag basins, ocean-continent transition and oceanic domain. Deformation and magmatism interact during rift evolution. Magma-rich and magma-poor rifted margins may be formed. Magma-rich margins include major volcanic features. Globally, volcanic margins represent the majority of passive continental margins. Magma-starved rifted margins are affected by large-scale faulting and crustal hyperextension. As a consequence, upper mantle peridotites and gabbros are commonly exposed and serpentinized along extensional detachments at the seafloor.


Magmatism

Many rifts are the sites of at least minor magmatic activity, particularly in the early stages of rifting.
Alkali basalt In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo dur ...
s and
bimodal volcanism Bimodal volcanism is the eruption of both mafic and felsic lavas from a single volcanic centre with little or no lavas of intermediate composition. This type of volcanism is normally associated with areas of extensional tectonics, particularly rift ...
are common products of rift-related magmatism. Recent studies indicate that post-collisional granites in collisional orogens are the product of rifting magmatism at converged plate margins.


Economic importance

The sedimentary rocks associated with continental rifts host important deposits of both minerals and
hydrocarbon In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, prop ...
s.


Mineral deposits

SedEx mineral deposits are found mainly in continental rift settings. They form within post-rift sequences when hydrothermal fluids associated with magmatic activity are expelled at the seabed.


Oil and gas

Continental rifts are the sites of significant oil and gas accumulations, such as the Viking Graben and the
Gulf of Suez Rift The Gulf of Suez Rift is a continental rift zone that was active between the Late Oligocene ( 28 Megaannum, Ma) and the end of the Miocene (c. 5 Ma). It represented a continuation of the Red Sea Rift until break-up occurred in the middle Miocene, w ...
. Thirty percent of
giant oil and gas fields The world's 932 giant oil and gas fields are considered those with of ultimately recoverable oil or gas equivalent. Geoscientists believe these giants account for 40 percent of the world's petroleum reserves. They are clustered in 27 regions of t ...
are found within such a setting. In 1999 it was estimated that there were 200
billion A billion is a number with two distinct definitions: *1,000,000,000 1,000,000,000 (one billion A billion is a number with two distinct definitions: *1,000,000,000 1,000,000,000 (one billion, short scale; one thousand million or milliar ...
barrels A barrel or cask is a hollow cylindrical A cylinder (from ) has traditionally been a Solid geometry, three-dimensional solid, one of the most basic of curvilinear geometric shapes. Geometrically, it can be considered as a Prism (geometry) ...

barrels
of recoverable oil reserves hosted in rifts.
Source rock In petroleum geology Petroleum geology is the study of origin, occurrence, movement, accumulation, and exploration of hydrocarbon fuels. It refers to the specific set of geological disciplines that are applied to the search for (). Sedimentary b ...
s are often developed within the sediments filling the active rift (
syn-rift File:GulfofSuezRift.png, 200px, Gulf of Suez Rift showing main extensional faults In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central li ...
), forming either in a
lacustrine A lake is an area filled with water, localized in a basin, surrounded by land Land is the solid surface of the Earth that is not permanently covered by water. The vast majority of human activity throughout history has occurred in lan ...
environment or in a restricted marine environment, although not all rifts contain such sequences. Reservoir rocks may be developed in pre-rift, syn-rift and post-rift sequences. Effective regional seals may be present within the post-rift sequence if
mudstone Mudstone, a type of mudrock Mudrocks are a class of fine-grained Granularity (also called graininess), the condition of existing in granular material, granules or Grain, grains, refers to the extent to which a material or system is composed ...

mudstone
s or
evaporite An evaporite () is a water-solubility, soluble sedimentation, sedimentary mineral deposition (geology), deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposit ...
s are deposited. Just over half of estimated oil reserves are found associated with rifts containing marine syn-rift and post-rift sequences, just under a quarter in rifts with a non-marine syn-rift and post-rift, and an eighth in non-marine syn-rift with a marine post-rift.


Examples

* The Asunción Rift in Eastern Paraguay * The Canadian Arctic Rift System in northern
North America North America is a continent A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continen ...

North America
* The
East African Rift File:East Africa Rift System GPS and stresses.png, Main rift faults, plates, plate boundaries, GPS plate velocities between adjacent blocks and minimum horizontal stress directions The East African Rift (EAR) or East African Rift System (EARS) is ...
* The
West and Central African Rift System The West and Central African Rift System (WCARS) is a rift system composed of two coeval Cretaceous The Cretaceous (, ) is a geological period that lasted from about 145 to 66 million years ago (mya). It is the third and final period of the Meso ...
* The
Red Sea Rift The Red Sea Rift is a spreading center between two tectonic plates, the African Plate and the Arabian Plate. It extends from the Dead Sea Transform fault system, and ends at an intersection with the Aden Ridge and the East African Rift, forming the ...
* The
Gulf of California The Gulf of California ( es, Golfo de California), also known as the Sea of Cortés (''Mar de Cortés'') or Sea of Cortez, or less commonly as the Vermilion Sea (''Mar Bermejo''), is a marginal sea of the Pacific Ocean The Pacific Ocea ...

Gulf of California
* The
Baikal Rift Zone factsheetThe Baikal Rift Zone is a series of continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly regarded as conti ...
, the bottom of
Lake Baikal Lake Baikal (; russian: Oзеро Байкал, Ozero Baykal ; bua, Байгал далай, Baigal dalai; mn, Байгал нуур, Baigal nuur) is a rift lake A rift lake is a lake A lake is an area filled with water, localized i ...

Lake Baikal
is the deepest continental rift on the earth. * The
Gulf of Suez Rift The Gulf of Suez Rift is a continental rift zone that was active between the Late Oligocene ( 28 Megaannum, Ma) and the end of the Miocene (c. 5 Ma). It represented a continuation of the Red Sea Rift until break-up occurred in the middle Miocene, w ...
* Throughout the
Basin and Range Province The Basin and Range Province is a vast physiographic region Physiographic regions of the world are a means of defining Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Eart ...
in North America * The Rio Grande Rift in the southwestern US * The rift zone that contains the
Gulf of Corinth The Gulf of Corinth or the Corinthian Gulf ( el, Κορινθιακός Kόλπος, ''Korinthiakόs Kόlpos'', ) is a deep inlet An inlet is an indentation of a shoreline, usually long and narrow, such as a small bay or arm, that often lea ...

Gulf of Corinth
in
Greece Greece ( el, Ελλάδα, Elláda, ), officially the Hellenic Republic, is a country located in Southeastern Europe Southeast Europe or Southeastern Europe () is a geographical subregion A subregion is a part of a larger region In geogr ...

Greece
* The Reelfoot Rift, an ancient buried failed rift underlying the
New Madrid Seismic Zone The New Madrid Seismic Zone (), sometimes called the New Madrid Fault Line, is a major seismic zone and a prolific source of intraplate earthquakes (earthquakes within a tectonic plate) in the Southern United States, Southern and Midwestern Unite ...
in the
Mississippi embayment The Mississippi Embayment is a physiographic feature in the south-central United States The United States of America (USA), commonly known as the United States (U.S. or US), or America, is a country Contiguous United States, primarily l ...
* The
Rhine RiftThe Upper Rhine Plain, Rhine Rift Valley or Upper Rhine Graben (German language, German: ''Oberrheinische Tiefebene'', ''Oberrheinisches Tiefland'' or ''Oberrheingraben'', French language, French: ''Vallée du Rhin'') is a major rift, about and on a ...

Rhine Rift
, in south western Germany, known as the
Upper Rhine valley The Upper Rhine Plain, Rhine Rift Valley or Upper Rhine Graben (German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of Germany, s ...
, part of the
European Cenozoic Rift System The European Cenozoic Rift System (ECRIS) is a long system of rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear Fault ...
* The
Taupo Volcanic Zone The Taupō Volcanic Zone (TVZ) is a volcano, volcanic area in the North Island of New Zealand that has been active for the past two million years and is still highly active. Mount Ruapehu marks its south-western end and the zone runs north-eas ...
in the north east
North Island The North Island, also officially named Te Ika-a-Māui, is one of the two main islands of New Zealand New Zealand ( mi, Aotearoa ''Aotearoa'' (; commonly pronounced by English English usually refers to: * English language ...

North Island
of
New Zealand New Zealand ( mi, Aotearoa ''Aotearoa'' (; commonly pronounced by English English usually refers to: * English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon Engl ...

New Zealand
* The
Oslo Graben #REDIRECT Oslo Graben#REDIRECT Oslo Graben The Oslo Graben or Oslo Rift is a graben formed during a geologic Rift (geology), rifting event in Permian time, the last phase of the Variscan orogeny. The main graben forming period began in the late Carb ...
in
Norway Norway, officially the Kingdom of Norway,Names in the official and recognised languages: Bokmål Bokmål (, ; literally "book tongue") is an official written standard for the Norwegian language Norwegian (Norwegian: ''norsk'') is a Nort ...

Norway
* The
Ottawa-Bonnechere Graben The Ottawa-Bonnechere Graben (also known as the Ottawa Graben) is a geological structure that coincides with a wide topographic depression extending from near Montréal Montreal ( ; officially Montréal, ) is the second-most populous city i ...
in Ontario and Quebec * The
Northern Cordilleran Volcanic Province The Northern Cordilleran Volcanic Province (NCVP), formerly known as the Stikine Volcanic Belt, is a geologic province defined by the occurrence of Miocene to Holocene volcano A volcano is a rupture in the crust of a planetary-mass object, ...
in
British Columbia ( en, Splendour without diminishment) , image_map = British Columbia in Canada 2.svg , Label_map = yes , coordinates = , official_lang = None , Slogan = Beautiful British C ...

British Columbia
,
Yukon Yukon ( ; ; formerly called Yukon Territory and sometimes referred to as The Yukon) is the smallest and westernmost of Canada's three territories. It also is the least populated province or territory in Canada, with a population of 35,874 peo ...

Yukon
and
Alaska Alaska (; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Central Alaskan Yup'ik language, Yup'ik: ''Alaskaq''; tli, Anáaski) is a U.S. state in the Western United States, on the northwest extremity of the country's West Coast of the United State ...

Alaska
* The
West Antarctic Rift System The West Antarctic Rift System is a series of rift valleys between East East is one of the four cardinal direction The four cardinal directions, or cardinal points, are the directions north North is one of the four compass points or car ...
in
Antarctica Antarctica ( or ) is Earth's southernmost continent. It contains the geographic South Pole and is situated in the Antarctic region of the Southern Hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Oc ...

Antarctica
* The Midcontinent Rift System, a late
Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale The geologi ...

Precambrian
rift in central North America * The Midland Valley in Scotland * The Fundy Basin, a
Triassic The Triassic ( ) is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth ...

Triassic
rift basin in southeastern Canada * The Cambay, Kachchh, and Narmada rifts in northwestern Deccan volcanic province of IndiaChouhan, A.K., Choudhury, P. & Pal, S.K. New evidence for a thin crust and magmatic underplating beneath the Cambay rift basin, Western India through modelling of EIGEN-6C4 gravity data. J Earth Syst Sci 129, 64 (2020). https://doi.org/10.1007/s12040-019-1335-y


See also

*
Rift zone upright=1.2, East Rift Zone on Kīlauea, Hawaii A rift zone is a feature of some volcanoes, especially shield volcanoes, in which a set of linear cracks (or rifts) develops in a volcanic edifice, typically forming into two or three well-defined re ...
* Wilson Cycle


References


Further reading

* * * {{Authority control + Structural geology Plate tectonics