Continental arc

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A continental arc is a type of volcanic arc occurring as an "arc-shape" topographic high region along a continental margin. The continental arc is formed at an active continental margin where two tectonic plates meet, and where one plate has continental crust and the other oceanic crust along the line of plate convergence, and a subduction zone develops. The magmatism and petrogenesis of continental crust are complicated: in essence, continental arcs reflect a mixture of oceanic crust materials, mantle wedge and continental crust materials.

Contents

Origin

Schematic diagram of the formation of a continental arc. Active Margin.svg
Schematic diagram of the formation of a continental arc.

When two tectonic plates collide, relatively denser oceanic crust will be subducted under relatively lighter continental crust. Because of the subduction process, the relatively cooler oceanic crust, along with water, is subducted to the asthenosphere, where pressures and temperatures are much higher than the surface of Earth. Under such conditions, the downgoing plate releases volatiles such as H2O and CO2, which cause partial melting of the above asthenosphere. [1] This process can create relatively buoyant magma, which subsequently forms a series of volcanoes at the surface along the subduction zone. There are some researchers who argue that refertilization of arc lithospheric mantle may also be an important process associated with arc magmatism. [2] [3] Because the subduction zone (which is also the plate boundary) is generally an arc-shape, geologists named those volcanoes volcanic arcs . A volcanic arc built on continental crust is called a continental arc; when built on oceanic crust the volcanoes form an island arc.

Petrogenesis and magmatism

Petrogenesis

The origin of igneous rock, or petrogenesis, in continental arcs is more complicated than that in oceanic arcs. The partial melting of the subducting oceanic slab generates primary magma, which would be contaminated by the continental crust materials when it travels through the crust. Because the continental crust is felsic or silica while the juvenile primary magma is typically mafic, the composition of magmas in continental arcs is the product of mixing between igneous differentiation of mafic magmas and felsic or silica crust meltings. [2] The mixing of existing continental crust, lower part of lithosphere or lithospheric mantle under the continental crust, the subducting oceanic crust and sediments, the mantle wedge and the underplates materials together is the main source of continental arc rocks. [4]

Magmatism

Cross-sectional diagram of magmatic processes in a continental arc Continental Arc Sketch.jpg
Cross-sectional diagram of magmatic processes in a continental arc

The dehydration of the downgoing slab and the partial melting of asthenosphere together generate the primary magma of continental arcs. Primary magma is composed of olivine tholeiitic basalt because of mixture of peridotites from the mantle wedge and large ion lithophile enriched (LIL-enriched) fluids from the dehydrating subducting plate. [4] Because the larger thickness and lower density, the continental crust is likely to prevent the upwards rising of primary magma. Ascending primary magma is likely to pond at the bottom of continental crust, forming a magma chamber. In this chamber an underplating process will take place, the assimilation and fractional crystallization of primary magma and lower crustal rocks forms underplate at the bottom of crust. [4] [5]

Through those procedure the olivine tholeiitic primary magma would change to calc-alkaline magmas and more evolved and enriched alkaline or siliceous magmas. [6] A further enriched source may be provided by the tectonic erosion process that causes scraping and dragging of lower continental lithosphere into the melting zone. Thus, a high concentrations of Rb, Cs, Ba, K, Th, and LREE (light rare-earth elements) and enriched isotopes can be found in the continental arc magmas. [7]

Intensity of arc magmatism

The geothermal structure in a subduction zone determines the melting rate of subduction slab and asthenosphere. The change in isotherm structure may have significant impact on the intensity of magmatism. Some factors may contribute to the change in geothermal structure: a) the change in convergence velocity of two plates in subduction zone; [4] b) the dipping angle of subduction slab; [4] c) the amounts of subducted low temperature materials (water and oceanic sediments); [1] d) the mantle/asthenosphere upwelling event (slab window [8] /slab breakoff [9] ).

Petrology

The petrogenesis of continental arcs is generally different from that of oceanic arcs, so more calc-alkaline and alkaline rocks can be found at a continental arc, with fewer tholeiites and low-K rocks. [4]

Calc-alkaline phenocryst-rich dacite, andesite and rhyolite rocks are abundant in continental arc. These rocks contain hydrous minerals biotite and hornblende partially resorbed in magmatic process. Strongly-zoned plagioclase with sieve texture also occurs in those rocks. Granodiorite, tonalite and diorite are most common intrusive rocks found in continental arcs. [10]

Erosion process

The erosion of continental arcs is a part of the main process of global lithosphere circulation. According to relative study, [11] the contribution of continental arc erosion in total continental crust loss is nearly 25%. A process called tectonic erosion happens when friction force during convergence scrapes off huge amount of rocks from the base of continental arcs. Also, precipitation on the continental-arc orogen itself is another erosion process. The debris from the continental arc would deposit in the subduction zone as turbidite. The undergoing subduction forces sediments to accretively add to the accretionary wedge or to subduct into the asthenosphere. Then part of sediments would be recycled through volcanic activities, and thus return to the continental crust, while another part would form new mantle material.

Distinctions between different arcs

The concepts "island arc", "volcanic arc", "oceanic arc" and "continental arc" may be confused:

In some cases, both a continental arc and oceanic arc can form along the strike of a single subduction zone (e.g. Aleutian Islands and Alaska Peninsula).

Table of continental arcs

Continental arcCountryTrenchOverriding Plate Subducting plate
Cascade Volcanic Arc United States and Canada Cascadia Subduction Zone; no physical oceanic trench can be identified [13] [14] North American Plate Juan de Fuca Plate, Explorer Plate, and Gorda Plate
Aleutian Arc United States Aleutian Trench North American Plate Pacific Plate
Kamchatka Russia Kuril–Kamchatka Trench Eurasian Plate Pacific Plate
Andean Volcanic Belt Colombia, Bolivia, Peru, Ecuador, Chile and Argentina Peru–Chile Trench South American Plate Nazca Plate and Antarctic Plate
Central America Volcanic Arc Guatemala, El Salvador, Honduras, Nicaragua, Costa Rica, Panama Middle America Trench Caribbean Plate Cocos Plate
Gangdese batholith Tibet, ChinaNo longer existent Lhasa terrane Neotethyan ocean

See also

Related Research Articles

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Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.

<span class="mw-page-title-main">Convergent boundary</span> Region of active deformation between colliding tectonic plates

A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–Benioff zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.

<span class="mw-page-title-main">Dacite</span> Volcanic rock intermediate in composition between andesite and rhyolite

Dacite is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. It is composed predominantly of plagioclase feldspar and quartz.

<span class="mw-page-title-main">Andesite</span> Type of volcanic rock

Andesite is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende.

<span class="mw-page-title-main">Island arc</span> Arc-shaped archipelago formed by intense seismic activity of long chains of active volcanoes

Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle along the subduction zone. They are the principal way by which continental growth is achieved.

<span class="mw-page-title-main">Forearc</span> The region between an oceanic trench and the associated volcanic arc

Forearc is a plate tectonic term referring to a region in a subduction zone between an oceanic trench and the associated volcanic arc. Forearc regions are present along convergent margins and eponymously form 'in front of' the volcanic arcs that are characteristic of convergent plate margins. A back-arc region is the companion region behind the volcanic arc.

<span class="mw-page-title-main">Volcanic arc</span> Chain of volcanoes formed above a subducting plate

A volcanic arc is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc located further from the subducting plate than the trench. The oceanic plate is saturated with water, mostly in the form of hydrous minerals such as micas, amphiboles, and serpentines. As the oceanic plate is subducted, it is subjected to increasing pressure and temperature with increasing depth. The heat and pressure break down the hydrous minerals in the plate, releasing water into the overlying mantle. Volatiles such as water drastically lower the melting point of the mantle, causing some of the mantle to melt and form magma at depth under the overriding plate. The magma ascends to form an arc of volcanoes parallel to the subduction zone.

<span class="mw-page-title-main">Trans-Mexican Volcanic Belt</span> Active volcanic belt that covers central-southern Mexico

The Trans-Mexican Volcanic Belt, also known as the Transvolcanic Belt and locally as the Sierra Nevada, is an active volcanic belt that covers central-southern Mexico. Several of its highest peaks have snow all year long, and during clear weather, they are visible to a large percentage of those who live on the many high plateaus from which these volcanoes rise.

<span class="mw-page-title-main">Rock cycle</span> Transitional concept of geologic time

The rock cycle is a basic concept in geology that describes transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. Each rock type is altered when it is forced out of its equilibrium conditions. For example, an igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and change as they encounter new environments. The rock cycle explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle.

<span class="mw-page-title-main">Northern Cordilleran Volcanic Province</span> Geologic province in the Pacific Northwest of North America

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 volcanoes in the Pacific Northwest of North America. This belt of volcanoes extends roughly north-northwest from northwestern British Columbia and the Alaska Panhandle through Yukon to the Southeast Fairbanks Census Area of far eastern Alaska, in a corridor hundreds of kilometres wide. It is the most recently defined volcanic province in the Western Cordillera. It has formed due to extensional cracking of the North American continent—similar to other on-land extensional volcanic zones, including the Basin and Range Province and the East African Rift. Although taking its name from the Western Cordillera, this term is a geologic grouping rather than a geographic one. The southmost part of the NCVP has more, and larger, volcanoes than does the rest of the NCVP; further north it is less clearly delineated, describing a large arch that sways westward through central Yukon.

<span class="mw-page-title-main">Magmatism</span> Emplacement of magma on the outer layers of a terrestrial planet, which solidifies as igneous rocks

Magmatism is the emplacement of magma within and at the surface of the outer layers of a terrestrial planet, which solidifies as igneous rocks. It does so through magmatic activity or igneous activity, the production, intrusion and extrusion of magma or lava. Volcanism is the surface expression of magmatism.

<span class="mw-page-title-main">Adakite</span> Volcanic rock type

Adakites are volcanic rocks of intermediate to felsic composition that have geochemical characteristics of magma originally thought to have formed by partial melting of altered basalt that is subducted below volcanic arcs. Most magmas derived in subduction zones come from the mantle above the subducting plate when hydrous fluids are released from minerals that break down in the metamorphosed basalt, rise into the mantle, and initiate partial melting. However, Defant and Drummond recognized that when young oceanic crust is subducted, adakites are typically produced in the arc. They postulated that when young oceanic crust is subducted it is "warmer" than crust that is typically subducted. The warmer crust enables melting of the metamorphosed subducted basalt rather than the mantle above. Experimental work by several researchers has verified the geochemical characteristics of "slab melts" and the contention that melts can form from young and therefore warmer crust in subduction zones.

Partial melting is the phenomenon that occurs when a rock is subjected to temperatures high enough to cause certain minerals to melt, but not all of them. Partial melting is an important part of the formation of all igneous rocks and some metamorphic rocks, as evidenced by a multitude of geochemical, geophysical and petrological studies.

<span class="mw-page-title-main">Opening of the North Atlantic Ocean</span> Breakup of Pangea

The opening of the North Atlantic Ocean is a geological event that has occurred over millions of years, during which the supercontinent Pangea broke up. As modern-day Europe and North America separated during the final breakup of Pangea in the early Cenozoic Era, they formed the North Atlantic Ocean. Geologists believe the breakup occurred either due to primary processes of the Iceland plume or secondary processes of lithospheric extension from plate tectonics.

<span class="mw-page-title-main">Subduction zone metamorphism</span> Changes of rock due to pressure and heat near a subduction zone

A subduction zone is a region of the earth's crust where one tectonic plate moves under another tectonic plate; oceanic crust gets recycled back into the mantle and continental crust gets created by the formation of arc magmas. Arc magmas account for more than 20% of terrestrially produced magmas and are produced by the dehydration of minerals within the subducting slab as it descends into the mantle and are accreted onto the base of the overriding continental plate. Subduction zones host a unique variety of rock types created by the high-pressure, low-temperature conditions a subducting slab encounters during its descent. The metamorphic conditions the slab passes through in this process creates and destroys water bearing (hydrous) mineral phases, releasing water into the mantle. This water lowers the melting point of mantle rock, initiating melting. Understanding the timing and conditions in which these dehydration reactions occur, is key to interpreting mantle melting, volcanic arc magmatism, and the formation of continental crust.

<span class="mw-page-title-main">Divergent double subduction</span> Special type of Tectonic process

Divergent double subduction, also called outward dipping double-sided subduction, is a special type of subduction process in which two parallel subduction zones with different directions are developed on the same oceanic plate. In conventional plate tectonics theory, an oceanic plate subducts under another plate and new oceanic crust is generated somewhere else, commonly along the other side of the same plates However, in divergent double subduction, the oceanic plate subducts on two sides. This results in the closure of ocean and arc–arc collision.

<span class="mw-page-title-main">Plate theory (volcanism)</span>

The plate theory is a model of volcanism that attributes all volcanic activity on Earth, even that which appears superficially to be anomalous, to the operation of plate tectonics. According to the plate theory, the principal cause of volcanism is extension of the lithosphere. Extension of the lithosphere is a function of the lithospheric stress field. The global distribution of volcanic activity at a given time reflects the contemporaneous lithospheric stress field, and changes in the spatial and temporal distribution of volcanoes reflect changes in the stress field. The main factors governing the evolution of the stress field are:

  1. Changes in the configuration of plate boundaries.
  2. Vertical motions.
  3. Thermal contraction.

Intraplate volcanism is volcanism that takes place away from the margins of tectonic plates. Most volcanic activity takes place on plate margins, and there is broad consensus among geologists that this activity is explained well by the theory of plate tectonics. However, the origins of volcanic activity within plates remains controversial.

<span class="mw-page-title-main">Dharwar Craton</span> Part of the Indian Shield in south India

The Dharwar Craton is an Archean continental crust craton formed between 3.6-2.5 billion years ago (Ga), which is located in southern India and considered as the oldest part of the Indian peninsula.

<span class="mw-page-title-main">Chile Ridge</span> Submarine oceanic ridge in the Pacific Ocean

The Chile Ridge, also known as the Chile Rise, is a submarine oceanic ridge formed by the divergent plate boundary between the Nazca Plate and the Antarctic Plate. It extends from the triple junction of the Nazca, Pacific, and Antarctic plates to the Southern coast of Chile. The Chile Ridge is easy to recognize on the map, as the ridge is divided into several segmented fracture zones which are perpendicular to the ridge segments, showing an orthogonal shape toward the spreading direction. The total length of the ridge segments is about 550–600 km.

References

  1. 1 2 Van Der Pluijm, B. A.; Marshak, S. (2004). Earth Structure (2 ed.). New York: Norton. p. 442. ISBN   978-0-393-92467-1.
  2. 1 2 Chin, Emily J.; Lee, Cin-Ty A.; Tollstrup, Darren L.; Liewen, Xie; Wimpenny, Josh B.; Yin, Qing-Zhu (2013). "On the origin of hot metasediment quartzites in the lower crust of continental arcs". Earth and Planetary Science Letters. 361: 120–133. Bibcode:2013E&PSL.361..120C. doi:10.1016/j.epsl.2012.11.031.
  3. ↑ Chin, Emily J.; Lee, Cin-Ty A.; Barnes, Jaime D. (2014). "Thickening, refertilization, and the deep lithosphere filter in continental arcs: Constraints from major and trace elements and oxygen isotopes". Earth and Planetary Science Letters. 397: 184–200. Bibcode:2014E&PSL.397..184C. doi:10.1016/j.epsl.2014.04.022.
  4. 1 2 3 4 5 6 Winter, John D. (2001). An Introduction to Igneous and Metamorphic Petrology. Upper Saddle River, New Jersey: Prentice-Hall Inc. ISBN   978-0321592576.
  5. ↑ Pitcher, W. S.; Atherton, M. P.; Cobbing, E. J.; Beckinsale, R. D. (1985). Magmatism at a Plate Edge: The Peruvian Andes (1 ed.). Springer. ISBN   978-1489958228.
  6. ↑ Harmon, R. S.; Barreiro, B. A. (1984). Andean Magmatism: Chemical and Isotopic Constraints (Shiva Geology) (1 ed.). Boston: Birkhäuser. ISBN   978-0906812617.
  7. ↑ Pearce, Julian A.; Parkinson, Ian J. (1993). "Trace element models for mantle melting: application to volcanic arc petrogenesis". Geological Society of London, Special Publications. 76 (1): 373–403. Bibcode:1993GSLSP..76..373P. doi:10.1144/GSL.SP.1993.076.01.19. S2CID   128765455.
  8. ↑ Zhang, Zeming; Zhao, Guochun; Santosh, M.; Wang, Jinli; Dong, Xin; Shen, Kun (2010). "Late Cretaceous charnockite with adakitic affinities from the Gangdese batholith, southeastern Tibet: Evidence for Neo-Tethyan mid-ocean ridge subduction?". Gondwana Research. 17 (4): 615–631. doi:10.1016/j.gr.2009.10.007.
  9. ↑ Ji, Weiqiang; Wu, Fuyuan; Li, Jinxiang; Liu, Chuanzhou; Liu, Chuan-Zhou (2009). "Zircon U–Pb geochronology and Hf isotopic constraints on petrogenesis of the Gangdese batholith, southern Tibet". Chemical Geology. 262 (3–4): 229–245. doi:10.1016/j.chemgeo.2009.01.020.
  10. ↑ Roden-Tice, Mary. "Earth Materials II – Petrology Course" (PDF). State University of New York at Plattsburgh. Archived from the original (PDF) on 23 November 2015. Retrieved 9 January 2015.
  11. ↑ Clift, Peter; Vannucchi, Paola (2004). "Controls on tectonic accretion versus erosion in subduction zones: Implications for the origin and recycling of the continental crust" (PDF). Reviews of Geophysics. 42 (2): RG2001. Bibcode:2004RvGeo..42.2001C. doi: 10.1029/2003RG000127 .
  12. ↑ Filedner; Moritz, M.; Klemperer, Simon L. (2000). "Crustal structure transition from oceanic arc to continental arc, eastern Aleutian Islands and Alaska Peninsula". Earth and Planetary Science Letters. 179 (3): 567–579. Bibcode:2000E&PSL.179..567F. doi:10.1016/S0012-821X(00)00142-4.
  13. ↑ Lillie, Robert J. (4 June 2001). "Subduction in the Pacific Northwest: Geology training manual for Olympic National Park" (PDF). Geoscientists-In-the-Parks document, 1999-OLYM. Denver, Colorado: National Park Service. p. 17. Retrieved 29 December 2014.
  14. ↑ "Pacific Mountain System – Cascade volcanoes", Earth Science Concepts, United States National Park Service, archived from the original on 2007-05-31
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