Volcanic arc

Not to be confused with Volcanic belt.

Volcanic arc formation along a subducting plate

A volcanic arc (also known as a magmatic arc) is a chain of volcanoes formed above a subducting plate, positioned in an arc shape as seen from above. Offshore volcanoes form islands, resulting in a volcanic island arc. Generally, volcanic arcs result from the subduction of an oceanic tectonic plate under another tectonic plate, and often parallel an oceanic trench. The oceanic plate is saturated with water, and volatiles such as water drastically lower the melting point of the mantle. As the oceanic plate is subducted, it is subjected to greater and greater pressures with increasing depth. This pressure squeezes water out of the plate and introduces it to the mantle. Here the mantle melts and forms magma at depth under the overriding plate. The magma ascends to form an arc of volcanoes parallel to the subduction zone.

These should not be confused with hotspot volcanic chains, where volcanoes often form one after another in the middle of a tectonic plate, as the plate moves over the hotspot, and so the volcanoes progress in age from one end of the chain to the other. The Hawaiian Islands form a typical hotspot chain; the older islands (tens of millions of years old) to the northwest are smaller and have more soil than the recently created (400,000 years ago) Hawaii island itself, which is more rocky. Hotspot volcanoes are also known as "intra-plate" volcanoes, and the islands they create are known as Volcanic Ocean Islands. Volcanic arcs do not generally exhibit such a simple age-pattern.

There are two types of volcanic arcs:

• oceanic arcs form when oceanic crust subducts beneath other oceanic crust on an adjacent plate, creating a volcanic island arc. (Not all island arcs are volcanic island arcs.)
• continental arcs form when oceanic crust subducts beneath continental crust on an adjacent plate, creating an arc-shaped mountain belt.

In some situations, a single subduction zone may show both aspects along its length, as part of a plate subducts beneath a continent and part beneath adjacent oceanic crust.

Volcanoes are present in almost any mountain belt, but this does not make it a volcanic arc. Often there are isolated, but impressively huge volcanoes in a mountain belt. For instance, Vesuvius and the Etna volcanoes in Italy are part of separate but different kinds of mountainous volcanic ensembles.

The active front of a volcanic arc is the belt where volcanism develops at a given time. Active fronts may move over time (millions of years), changing their distance from the oceanic trench as well as their width.

Tectonic setting

A volcanic arc is part of an arc-trench complex, which is the part of a subduction zone that is visible at the Earth's surface. A subduction zone is where a tectonic plate composed of relatively thin, dense oceanic lithosphere sinks into the Earth's mantle beneath a less dense overriding plate. The overriding plate may be either another oceanic plate or a continental plate. The subducting plate, or slab, sinks into the mantle at an angle, so that there is a wedge of mantle between the slab and the overriding plate.

The boundary between the subducting plate and the overriding plate coincides with a deep and narrow oceanic trench. This trench is created by the gravitational pull of the relatively dense subducting plate pulling the leading edge of the plate downward. Multiple earthquakes occur within the subducting slab with the seismic hypocenters located at increasing depth under the island arc: these quakes define the Wadati–Benioff zones. The volcanic arc forms on the overriding plate over the point where the subducting plate reaches a depth of roughly 120 kilometres (75 mi) and is a zone of volcanic activity between 50 and 200 kilometers (31 and 124 mi) in width.

Ocean basins that are being reduced by subduction are called 'remnant oceans' as they will slowly be shrunken out of existence and crushed in the subsequent orogenic collision. This process has happened over and over in the geologic history of the Earth.

In the rock record, volcanic arcs can be seen as the volcanic rocks themselves, but because volcanic rock is easily weathered and eroded, it is more typical that they are seen as plutonic rocks, the rocks that formed underneath the arc (e.g. the Sierra Nevada batholith), or in the sedimentary record as lithic sandstones.

Petrology

In a subduction zone, loss of water from the subducted slab induces partial melting of the overriding mantle and generates low-density, calc-alkaline magma that buoyantly rises to intrude and be extruded through the lithosphere of the overriding plate. Most of the water carried downwards by the slab is contained in hydrous (water-bearing) minerals, such as mica, amphibole, or serpentinite minerals. Water is lost from the subducted plate when the temperature and pressure become sufficient to break down these minerals and release their water content. The water rises into the wedge of mantle overlying the slab and lowers the melting point of mantle rock to the point where magma is generated.

However, while there is wide agreement on the general mechanism, research continues on the explanation for focused volcanism along a narrow arc some distance from the trench. The distance from the trench to the volcanic arc is greater for slabs subducting at a shallower angle, and this suggests that magma generation takes place when the slab reached a critical depth for the breakdown of an abundant hydrous mineral. This would produce an ascending "hydrous curtain" that accounts for focused volcanism along the volcanic arc. However, some models suggest that water is continuously released from the slab from shallow depths down to 70 to 300 kilometers (43 to 186 mi), and much of the water released at shallow depths produces serpentinization of the overlying mantle wedge. According to one model, only about 18 to 37 percent of the water content is released at sufficient depth to produce arc magmatism. The volcanic arc is then interpreted as the depth at which the degree of melting becomes great enough to allow the magma to separate from its source rock.

It is now known that the subducting slab may be located anywhere from 60 to 173 kilometers (37 to 107 mi) below the volcanic arc, rather than a single characteristic depth of around 120 kilometers (75 mi), which requires more elaborate models of arc magmatism. For example, water released from the slab at moderate depths might react with amphibole minerals in the lower part of the mantle wedge to produce water-rich chlorite. This chlorite-rich mantle rock is then dragged downwards by the subducting slab, and eventually breaks down to become the source of arc magmatism. The location of the arc depends on the angle and rate of subduction, which determine where hydrous minerals break down and where the released water lowers the melting point of the overlying mantle wedge enough for melting.

The location of the volcanic arc may be determined by the presence of a cool shallow corner at the tip of the mantle wedge, where the mantle rock is cooled by both the overlying plate and the slab. Not only does the cool shallow corner suppress melting, but its high stiffness hinders the ascent of any magma that is formed. Arc volcanism takes place where the slab descends out from under the cool shallow corner, allowing magma to be generated and rise through warmer, less stiff mantle rock.

Magma may be generated over a broad area but become focused into a narrow volcanic arc by a permeability barrier at the base of the overriding plate. Numerical simulations suggest that crystallization of rising magma creates this barrier, causing the remaining magma to pool in a narrow band at the apex of the barrier. This narrow band corresponds to the overlying volcanic arc.

Examples

Cascade Volcanic Arc, a continental volcanic arc

The Aleutian Arc, with both oceanic and continental parts.

Two classic examples of oceanic island arcs are the Mariana Islands in the western Pacific Ocean and the Lesser Antilles in the western Atlantic Ocean. The Cascade Volcanic Arc in western North America and the Andes along the western edge of South America are examples of continental volcanic arcs. The best examples of volcanic arcs with both sets of characteristics are in the North Pacific, with the Aleutian Arc consisting of the Aleutian Islands and their extension the Aleutian Range on the Alaska Peninsula, and the Kuril–Kamchatka Arc comprising the Kuril Islands and southern Kamchatka Peninsula.

Continental arcs

• Cascade Volcanic Arc
• Alaska Peninsula and Aleutian Range
• Kamchatka
• Andes
  • Northern Volcanic Zone
  • Central Volcanic Zone
  • Southern Volcanic Zone
  • Austral Volcanic Zone
• Central America Volcanic Arc

Island arcs

• Aleutian Islands
• Kuril Islands
• Northeastern Japan Arc
• Japanese Archipelago including the Ryukyu Islands
• Izu–Bonin–Mariana Arc:
  • Izu Islands
  • Bonin Islands
  • Mariana Islands
• Luzon Volcanic Arc
• Philippines
• Tonga and Kermadec Islands
• Andaman and Nicobar Islands
• Mentawai Islands
• Sunda Arc
  • Lesser Sunda Islands
• Tanimbar and Kai Islands
• Solomon Islands
• Aeolian Islands
• Aegean or Hellenic arc
  • South Aegean Volcanic Arc
• Lesser Antilles, including the Leeward Antilles
• Scotia Arc:
  • South Sandwich Islands
• Mascarene Islands

Ancient island arcs

• Insular Islands
• Intermontane Islands
• Sakhalin Island Arc

See also

• Back-arc basin
• High island
• Island arc
• Subduction zone metamorphism
• Volcanic field

Further reading

• Subduction Zone Volcanism Archived 2018-12-29 at the Wayback Machine
• Tectonics and Volcanoes of Japan Archived 2016-03-03 at the Wayback Machine
• USGS: Island-Arc, Oceanic, and Continental Volcanoes