The geology of continental arcs is characterized by four distinct entities: subduction zones, magmatic belts, forearc basins, and backarc basins. Subduction zones are regions where one tectonic plate slides beneath another, triggering the formation of volcanoes and earthquakes. Magmatic belts are chains of volcanoes that form above the subduction zone, while forearc basins are sedimentary basins located in the wedge between the subducting plate and the arc. Backarc basins are extensional basins located behind the magmatic arc, which can contain seafloor spreading ridges. These four entities collectively define the geological characteristics of continental arcs.
Structure of a Continental Arc
A continental arc is the result of the oblique subduction of an oceanic plate beneath a continental plate. This results in the formation of a series of volcanic arcs and magmatic belts parallel to the subduction zone. The structure of a continental arc is complex and varies depending on the age, size, and composition of the subducting plate, as well as the rate and angle of subduction. However, some general features are common to most continental arcs.
1. Forearc
The forearc is the region of the arc located between the subduction zone and the magmatic arc. It is composed of a series of sedimentary basins and accretionary wedges that are formed by the accumulation of sediments and oceanic crustal material that is scraped off the subducting plate.
2. Magmatic Arc
The magmatic arc is the region of the arc where volcanism occurs. It is composed of a series of volcanoes and plutonic rocks that are formed by the melting of the subducting plate. The composition of the magmas that form the magmatic arc varies depending on the composition of the subducting plate and the depth of melting.
3. Backarc
The backarc is the region of the arc located behind the magmatic arc. It is composed of a series of continental crustal blocks that have been accreted to the continental margin as a result of the subduction process. The backarc region is often the site of extensional tectonics and basin formation.
Table 1. Summary of the Structure of a Continental Arc
| Region | Description |
|---|---|
| Forearc | Region between the subduction zone and the magmatic arc; composed of sedimentary basins and accretionary wedges |
| Magmatic Arc | Region where volcanism occurs; composed of volcanoes and plutonic rocks |
| Backarc | Region behind the magmatic arc; composed of continental crustal blocks that have been accreted to the continental margin |
Question: What are the characteristics of a continental arc, and how do they form?
Answer: Continental arcs are curved, linear belts of volcanoes and associated plutonic rocks that form parallel to the margin of a continent and are typically found along the trailing edge of a subducting oceanic plate. They are formed by the subduction of oceanic crust beneath a continental margin, causing partial melting of the subducted oceanic crust and the generation of magma. The magma rises to the surface and erupts to form volcanoes, which can be either stratovolcanoes or shield volcanoes. The composition of the magma and thus the type of volcano produced depends on the nature of the subducted oceanic crust, the temperature and pressure conditions at depth, and the amount of water present.
Question: How does the geometry of a continental arc relate to the subduction zone?
Answer: The geometry of a continental arc is closely related to the geometry of the subduction zone. The arc is typically concave toward the subducting plate, and its curvature is greater than that of the subduction zone. This is because the subducted oceanic crust is forced to bend as it descends beneath the continental margin, and the magma generated by the partial melting of the oceanic crust rises along the bend. The depth of the subduction zone also influences the geometry of the continental arc. A shallower subduction zone produces a steeper arc, while a deeper subduction zone produces a flatter arc.
Question: What are the different types of crust found within a continental arc?
Answer: Continental arcs typically consist of three different types of crust: oceanic crust, continental crust, and arc crust. Oceanic crust is the oldest and densest of the three types of crust and is formed by the solidification of magma at mid-ocean ridges. Continental crust is the lightest and least dense of the three types of crust and is formed by the differentiation of magma in the lower crust. Arc crust is intermediate in age and density between oceanic and continental crust and is formed by the partial melting of the subducted oceanic crust and the assimilation of continental crust.
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