A Back-arc Basin is a geological formation that occurs on the ocean floor. This marine formation is most commonly identified along subduction zones, which occur where one tectonic plate moves beneath another, and island arcs, which are a kind of archipelago made up primarily of volcanos. A back-arc basin typically runs over a long distance, measuring over 600 miles in length. Additionally, this geological feature is characterized as very narrow, a detail thought to be caused by the lack of mantle convection near the ridge. Back-arc basins emit basalt rocks that are unique to other mid-ocean ridge basalts in that they contain high levels of magmatic water. In fact, these formations are well known for their hydrothermal activity. Where deep-sea vents emerge through these basins, a large number of biodiverse species and marine communities can often be found.
How Is A Back-arc Basin Formed?
Most researchers believe that back-arc basins are formed by a geological movement known as trench rollback. During this process, the subduction zone moves in the opposite direction of the tectonic plate that typically moves underneath another. This backward movement causes the tectonic plate located on the top of the subduction zone to stretch, creating a trench formation.
As this trench is formed, the crust stretches and becomes thin. This stretching movement is referred to as being under extension. This extension characteristic is one of the principal features found in back-arc basins. The now-thinned crust allows magma to escape to the surface of the ocean floor. The exposed magma creates pressure along the tectonic plate that is located at the top of the subduction zone. As this pressure increases, it may cause volcanoes above to erupt. All of this activity results in the sea floor spreading apart, which affects the shape and appearance of the land formations located above.
In other words, a line of volcanoes that are exposed and protrude above the surface of the ocean slowly change as the back-arc basin forms. As the backward movement thins out the crust and causes the ocean floor to separate, the volcanoes begin to separate. Eventually, the volcano is completely split in half, forming two separate volcanoes.
Where Can Back-arc Basins Be Found?
Most of the back-arc basins that have been identified are located in the western region of the Pacific Ocean along convergent plate boundaries. Other back-arc basins have been found in the following areas: Manus, Sea of Japan, South Scotia, Marianas, Sea of Okhotsk, Tyrrhenian Sea, North Fiji, and Tonga-Kermadec. Additionally, the Black Sea is home to two individuals back-arc basins. Researchers have also identified a number of extinct or no longer forming back-arc basins. Some of these fossilized basins include the: Kurile Basin, East Sea of Korea, and Parece Vela Shikoku Basin.
Difference Between Back-arc Basins and Mid-Ocean Ridges
As previously mentioned, back-arc basins may emit water-heavy basalt rocks, whereas other mid-ocean ridges emit dryer basalt rocks. In addition to this difference, back-arc basins are unique in that the rate of spreading occurs at a different rate throughout the area of the basin. This uneven spreading is known as asymmetry. Other mid-ocean ridges, however, experience symmetrical spreading. No scientific consensus has been reached to explain this phenomenon. Several theories put forth a number of possible explanations, like mantle wedge effects, hydration gradients, and the change from rifting behavior to spreading behavior.
The Mariana Trough Back-arc Basin
The Mariana Trough is one of the most well-known back-arc basins in the world. This geologic formation is located in the western region of the Pacific Ocean, where it is surrounded by the inactive West Mariana Ridge and the still-active Mariana volcanic arc to the east. At its northern end, the West Mariana Ridge and the Mariana arc come together. At its southern end lies the Challenger Deep, which is the deepest spot on the ocean floor that is known to researchers. It extends between 35,755 and 35,814 feet deep. The entire Mariana Trough stretches over a distance of around 807.78 miles, roughly the same distance as from London in England to Rome in Italy. Its widest point is found at around the middle of this basin and measures just over 149 miles.
The asymmetric seafloor spreading experienced in the Mariana Trough can be seen by comparing its western edge to its central area. For example, in the central region of this formation, the speed of spreading is estimated at between 2 and 3 times greater than that found in the western edge. The varying development of this back-arc basin has lead to its classification of 3 distinct areas: the southern platform, the northern rifting apex, and the central spreading basin.
The southern platform of the Mariana Trough is not as deep as the other 2 regions. In fact, its most profound point is only around 1.86 miles deep. Some much deeper and narrower troughs separate this region from the area of active volcanoes. The northern rifting apex is a prime example of the earliest stages of rifting development due to its close proximity to active volcanoes. Finally, the central spreading basin represents this geological development at its peak maturity.
The Lau Basin, located along the Australian-Pacific plate boundary, is an example of a relatively young back-arc basin. Geologists estimate that its age is less than 5 million years old and developed as a result of the Pacific plate moving beneath the Australian plate during the Pliocene era. The Lau Basin is bordered to the west by the Tonga-Kermadec Ridge and to the east by the Lau-Colville Ridge. This back-arc basin is unique in that it takes on a V-like shape, rather than the linear form of many others. Because of this shape, both of its openings face the same direction, to the south. These openings are referred to as the East Lau Spreading Center and the Central Lau Spreading Center. As seen with most back-arc basins, the spreading rate throughout this formation is asymmetrical. The East Lau Spreading Center, for example, moves at a rate of around 100 millimeters per year, while the entire average rate for this basin is approximately 150 millimeters per year. Given this rate, the Lau Basin is considered a rapidly spreading back-arc basin.