Volcanic arcs represent chains of mountains or islands formed above a sinking tectonic plate, a process known as subduction. This geological phenomenon occurs where two plates converge, and the denser oceanic lithosphere descends into the Earth’s mantle. Although both continental and island arcs arise from the same fundamental mechanism, the nature of the overriding tectonic plate determines their distinct characteristics. The geological differences in their formation lead to significant variations in magmatic composition, crustal structure, and physical appearance. This article will delineate the distinctions between these two arc types based on their underlying geology and resulting features.
Defining the Tectonic Settings
The most fundamental difference between a continental arc and an island arc lies in the type of crust that serves as the foundation for the volcanic activity. An island arc forms in an oceanic setting when one oceanic plate subducts beneath another oceanic plate. The overriding plate consists of relatively thin, dense oceanic crust, which offers little resistance to the ascending magma.
A continental arc, by contrast, forms where an oceanic plate subducts beneath a much thicker, less dense continental plate. The resulting volcanic chain is built directly upon the edge of a continent.
The convergence of two oceanic plates creates an intra-oceanic subduction zone, setting the stage for an island arc. Because the overriding plate is thin, the newly formed volcanic chain is typically isolated from any large landmass. When oceanic crust dives beneath continental crust, the process establishes an active continental margin, defining a continental arc. The vast, silica-rich volume of the continental plate directly influences the chemistry of the magma that feeds the volcanoes.
Magmatic Evolution and Crustal Development
The distinct tectonic settings directly control the chemical evolution of the magma. In an island arc, the magma originates primarily from the melting of the mantle wedge above the subducting slab. Fluids released from the descending oceanic plate flux the overlying mantle, lowering its melting point and generating melt.
This initial magma is generally mafic and low in silica. As this magma rises through the thin, mafic oceanic crust, it undergoes limited differentiation and assimilation. Consequently, the resulting volcanic rocks are predominantly basalt and andesite, creating a relatively uniform, dense, and thin new arc crust.
The magmatic process is more complex within a continental arc due to the presence of the thick continental crust. As the mantle-derived mafic magma ascends, it must pass through tens of kilometers of pre-existing, silica-rich continental rock. This ascent causes crustal assimilation, where the hot, rising magma melts and incorporates surrounding host rock, fundamentally changing its chemical signature.
The magma also undergoes extensive fractional crystallization, where denser, early-forming minerals settle out, enriching the remaining liquid in silica. This combination transforms the initial mafic magma into a more felsic, or silica-rich, composition. The resulting rocks are dominated by andesite, dacite, and rhyolite at the surface, and large bodies of intrusive granodiorite and granite at depth, leading to massive crustal thickening.
Observable Physical Differences
These geological and magmatic differences manifest in observable physical characteristics, particularly regarding the scale and composition of the resulting landforms. Continental arcs are characterized by extremely thick crust, often reaching depths of 40 to 70 kilometers, which is necessary to accommodate the massive volumes of assimilated and differentiated material. This thickened, buoyant crust is responsible for the formation of towering mountain ranges, such as those found along the western Americas.
Island arcs, formed on oceanic lithosphere, have much thinner crust, typically averaging around 20 to 30 kilometers in thickness. This thin crust supports a chain of volcanic islands that are lower in elevation and often separated by water. The geological stability of continental arcs also tends to be greater, making them persistent features over hundreds of millions of years, while island arcs are often narrower and can be incorporated into continental margins over vast timescales.
Continental arcs typically feature intrusive rocks like granite and diorite, which are characteristic of the felsic and intermediate compositions created by crustal assimilation. The volume of these silica-rich rocks contributes significantly to the overall mass and height of the mountain belt. The volcanic cones of continental arcs are frequently stratovolcanoes composed of calc-alkaline andesite and dacite, reflecting the evolved magma chemistry. In contrast, island arcs often display volcanic landforms built from less-evolved, darker, and denser basalt and basaltic andesite.
Global Examples of Arcs
The Andes Mountains along the western edge of South America stand as the most prominent example of a continental arc. This immense mountain chain is the direct result of the Nazca Plate subducting beneath the South American Plate, leading to massive crustal thickening and the formation of a continuous, high-elevation volcanic belt.
Another notable continental arc is the Cascade Range in western North America, including volcanoes like Mount St. Helens and Mount Rainier. Here, the Juan de Fuca Plate is subducting beneath the North American Plate, producing a chain of volcanoes built upon the continental margin.
In contrast, the Aleutian Islands, which stretch across the northern Pacific, provide a classic example of an island arc. This curved chain of volcanic islands formed where the Pacific Plate subducts beneath the North American Plate, with the entire system situated on oceanic crust. Similarly, the Mariana Islands and the Japanese Archipelago are well-known island arcs formed by oceanic-oceanic subduction. These island chains are marked by the presence of a deep-sea trench on the convex side of the arc, a feature parallel to the volcanic islands themselves.