Island arcs

Island arcs are arc-shaped chains of volcanic islands formed by the collision of two oceanic plates. They are the sites of most of the world's explosive volcanic eruptions and large earthquakes. Tsunami and ash clouds generated by these events can affect people around the globe.

Features of Island Arcs

An island arc is a long, arcuate chain of volcanic islands with an ocean on the convex (or outer) side of the arc. Paralleling the arc lies a long, narrow trench with steeply sloping sides that descend far below the normal ocean floor. A map of the world shows that many island arcs occur in and around the Pacific Ocean, such as the Aleutians, Japan, Tonga, Indonesia, New Zealand, and the Marianas. The West Indies are an island arc bordering the Atlantic Ocean. The associated trenches contain the deepest places on the earth. The Mariana Trench near the island of Guam reaches a maximum depth of 10,924 meters (35,840 feet). This is farther below sea level than Mount Everest (at 8,848 meters, or 29,029 feet) is above sea level. These island arc features, though merely topographic ones, demonstrate that island arcs and deep-sea trenches are parts of the same earth structure.

There are six basic features common to island arc-trench systems: chains of volcanoes, deep ocean trenches, earthquake belts, a shallow sea behind the island arc, large negative gravity anomalies, and rock deformation in later geologic time. Some features are better displayed in one arc than another, but all are present.

All island arcs consist of an arc-shaped chain of volcanoes and volcanic islands. Many volcanoes are currently active or have been active in the recent geologic past. Some scientists group island arcs into two types: island arcs composed of volcanic islands located on oceanic crust (such as the Aleutians, Kurils, Marianas, and West Indies) and chains of volcanoes on small pieces of continental crust (such as Japan, Indonesia, New Zealand, and the Philippines). The main difference is that the continental-type arc is older, has had a more complex geologic history, and thus represents a later stage in the evolution of island arcs.

The volcanoes of both island arc types produce andesitic magma. Andesite is a light-colored, fine-grained igneous rock composed primarily of sodium-and calcium-rich feldspar. In terms of its composition, andesite lies midway between quartz-rich granites and iron-and magnesium-rich basalts. The andesitic magma also contains large amounts of gases that cause extremely explosive and destructive eruptions. Examples of island arc volcanic eruptions are Krakatau, Indonesia, in 1883; Mount Pelée, Martinique, in 1902; and La Soufrière, St. Vincent, in 1902.

Deep-Sea Trenches

Deep-sea trenches lie on the ocean side of all island arcs—long, narrow features that parallel the island chains. They have steep, sloping sides extending to great depths. Minor differences occur. Some trenches, such as the Mariana and the Kuril, have V-shaped cross sections and are rock-floored to their bottom. Others, such as the Puerto Rico and southwest Japan trenches, have a flat bottom. Detailed studies have shown that these flat-bottomed trenches are sediment-filled and that the underlying rock floor is also V-shaped.

Seismic Activity

Island arcs are active seismic regions and the sites of many of the world's largest and deepest earthquakes. The region in which an earthquake begins is called its focus. The foci of earthquakes in arc regions lie along a narrow, well-defined zone that dips from near the trench below the island arc. The number of earthquakes generally decreases with depth, with some foci reaching 600 to 700 kilometers (373 to 435 miles) below sea level. This dipping seismic zone is called the Benioff zone, after the seismologist Hugo Benioff, who first defined it.

Shallow Seas

Behind the island arc lies a shallow marginal sea; examples are the Sea of Japan, the Philippine Sea, and the Caribbean Sea. Below some marginal seas, the crust is partly continental but becomes oceanic toward the arc. Below other seas, the crust is entirely oceanic. The composition of the oceanic crust beneath the marginal seas is more like andesite than the basalt of the normal ocean floor.

Negative Gravity Anomalies

Large negative gravity anomalies are also common to arc-trench regions. Geophysicists have found that the value of gravity over the earth's surface varies by slight amounts. Most of the variations can be accounted for and result from irregularities in altitude and topography. However, after observed gravity readings are corrected, variations called gravity anomalies still remain. These are caused by differences in rock density from place to place below the earth's surface. A negative anomaly shows that a greater volume of lighter (less dense) rocks is present in one area than in surrounding ones. Large negative anomalies are associated with the deep-sea trench and imply the presence of a great volume of low-density rocks at depth.

Rock Deformations

Deformation of rocks in the recent geologic history of an island arc is common. Some rocks have been folded, others metamorphosed. Areas of local uplift and subsidence may also be related to shallow earthquakes and faulting. These features are more easily seen and studied on the island arcs located on continental crust, such as Japan or New Zealand. Deformation, however, is present in all island arcs.

Origin of Island Arcs

Earth scientists have long sought to explain the origin of island arcs. The remains of ancient marine volcanic islands and volcano-derived sediments are found in the core of the present-day Appalachian Mountains. These rocks and large amounts of continental sediments were compressed, faulted, and folded to form the ancient Appalachians. Yet the relation of the volcanic islands to the mountain-building process was unclear. The development of the concept of plate tectonics has provided an explanation.

The outer portion of the earth is composed of a number of rigid lithospheric plates. Driven by forces in the mantle and by the sinking of cold, dense crust, the plates are pulled over the face of the earth. A lithospheric plate are may contain continental crust, oceanic crust, or (more commonly) both. Island arcs form when the ocean portion of one of two colliding plates is forced under the other. The continuing collision of the plates may eventually result in the creation of a new mountain range, such as the ancient Appalachians. The process in which oceanic lithosphere is pushed under another plate is called subduction. Subduction of the plate causes the geological and geophysical features observed in the island arc system. (It should be noted that oceanic plates can also be subducted beneath continental plate margins. The resulting features are similar to those found in island arcs except that the andesite volcanoes form along the edge of the overriding continental margin. The Andes and Cascade Mountains are the island arc equivalent of subduction beneath a continent. Mount St. Helens is an andesite volcano.)

As the two plates converge, one bends and is pushed under the other. The line of initial subduction is marked by a deep ocean trench. Subduction is not a smooth process. Friction between the subducting plate and the overriding plate and between the downgoing plate and the mantle tries to prevent movement. When frictional forces are overcome, an earthquake occurs. The locations of earthquake foci outline the subducting lithospheric plate. As subduction continues, earthquakes occur at greater depths. The lack of earthquakes below 700 kilometers suggests that this is the maximum depth that the plate can reach before it becomes part of the mantle. The downgoing oceanic plate drags along any deep-sea sediments that have been deposited on it or in the trench area. Both the plate and the sediments are heated, primarily by friction and by the surrounding hotter mantle. At about a depth of 100 kilometers, partial melting occurs, giving a magma rich in sodium, calcium, and silica. This magma mixes with the iron-and magnesium-rich mantle, creating a magma that is less dense than the surrounding mantle. Forcing its way upward through zones of weakness in the overlying plate, the magma generates andesite volcanoes. The gravity anomaly associated with the trench is caused by the light crustal rocks of the subducting plate being held (or pushed) down by the overriding plate. The increased volume of less dense rocks produces a large negative anomaly.

Recently deformed rocks are also common to island arc systems. The overriding plate does not slip smoothly over the subducting plate. Instead, rocks in the leading edge of the plate are compressed (pushed together), causing faulting, folding, and uplift. Pieces of the subducting plate can be broken off and folded into the island arc. Heat from the mantle causes metamorphism in the overlying rocks. Behind the island arc, shallow faulting caused by tension (pulling apart) creates earthquakes.

The marginal sea between the island arc and the continent is called the back-arc basin. The presence of these basins is not totally understood. Some, such as the Aleutian and the Philippine basins, were formed from pieces of preexisting ocean. Others have features suggesting that they were once continental crust that has been turned into oceanic crust. Still, other basins appear to have been created by interarc spreading, like that seen along mid-ocean ridges. The origin of back-arc basins is a topic of active geologic research.

Study of Island Arcs

In studying island arcs, scientists use a wide range of geological and geophysical techniques. The geological methods generally study the accessible portions of island arcs and include mapping and sample collection and analysis. The geophysical methods study the deep features of island arcs using earthquake and explosion seismology, gravity surveys, and heat-flow measurements. Computers aid in the analysis of data and in the generation of island arc models. Earth scientists studying certain aspects or features of island arcs select a combination of tools most appropriate to their region of interest.

Geologic mapping requires direct access to the rocks forming island arcs. The geologist surveys a region, recording the type of rock found and its extent and the orientation of observed faults and folds. Rock samples are collected for later study. These may be supplemented by drilling to sample rocks below the surface. The field data are transferred to a topographic map and, with the aid of aerial photographs, a geologic map is drawn. Aerial and satellite photographs have become increasingly helpful in the mapping of regions covered by vegetation. The traces of faults and the effects of changing bedrock can be reflected in surface features visible from high altitudes.

The rock samples collected are subjected to chemical and mineralogical analyses. The presence of trace elements or certain minerals can provide clues to the source region of a rock's components or to the thermal history of the rock since it was formed. Minerals containing radioactive elements, such as potassium-40 and rubidium-87, can be used to obtain the age of the rock units. Microscopic analysis of the rocks yields information on their thermal and deformational history. Direct collection of rock samples is limited to the exposed portions of island arcs. Dredging is used for sample collection in shallow ocean regions, such as the back-arc basins. The use of submersibles, such as the Alvin, operated by Woods Hole Oceanographic Institution, has allowed scientists to photograph and collect rocks and other data from the ocean floor far below sea level, enabling them to extend the study of island arcs. However, these methods, do not reach the regions far below the earth's surface.

Indirect methods are used by geophysicists to study the deeper island arc regions. The two most widely used methods are earthquake and explosion seismology. Earthquake seismology studies the seismic waves generated by earthquakes and provides an average velocity structure of the crust and mantle. The distribution of earthquakes in arc regions, particularly Japan, led to the discovery of the dipping seismic zone beneath the arc. Scientists also study plate movements to learn the mechanism causing earthquakes.

Explosion seismology uses seismic waves generated by controlled explosions to study the detailed crustal and lithospheric structure of island arcs. Within the ocean regions, this technique has revealed the steep topography of the deep-sea trenches and the deformed rock layers near the base of the overriding plate.

Measurements of the earth's gravity field can be obtained on land and at sea. The data are corrected for irregularities in altitude and terrain. Any remaining differences relate to density variations deep in the crust. The negative anomalies in the trench areas reflect the great thickness of crustal rocks at the plate boundary. In other areas, such as Japan, gravity data suggest a thicker crust or a less dense mantle below the back-arc basin (Sea of Japan) than below the Pacific Ocean.

Heat-flow measurements also reflect regional features. On average, heat flow is the same over both continental and oceanic regions as a result of a deep, common source. In Japan, one of the most thoroughly studied island arc areas, a region of high heat flow coincides with the distribution of volcanoes and hot springs. A second high heat flow below the Sea of Japan suggests that the mantle is hotter than average, perhaps as a result of an interarc spreading center. A zone of low heat flow occurs on the Pacific side of the arc.

Earth scientists seek to unravel the history of island arcs to understand their formation. Computers are used to form models of island arcs so that theories of arc formation can be tested. By modifying the model to fit the observed geological and geophysical data, scientists can increase their overall understanding of the island arc system.

Principal Terms

andesite: a light-colored volcanic rock rich in sodium and calcium feldspar, with some darker minerals

basalt: a dark-colored igneous rock containing minerals such as feldspar and pyroxene, high in iron and magnesium

Benioff zone: the dipping zone of earthquake foci found below island arcs, named after Hugo Benioff, the seismologist who first defined it

earthquake focus: the region in the earth that marks the starting site of an earthquake

granite: a light-colored igneous rock containing feldspar, quartz, and small amounts of darker minerals

gravity anomalies: differences between observed gravity readings and expected values after accounting for known irregularities

lithosphere: the rigid outer shell of the earth, composed of a number of plates

subduction: the process by which a lithospheric plate containing oceanic crust is pushed under another plate

tsunami: a seismic sea wave generated by vertical movement of the ocean floor, caused by an earthquake or volcanic eruption

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“What Is an Island Arc?” World Atlas, 7 Dec. 2017, www.worldatlas.com/articles/what-is-an-island-arc.html. Accessed 20 Aug. 2024.