Izu–Bonin–Mariana arc

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The Izu–Bonin–Mariana arc (IBM arc) is a long chain of volcanic islands and deep trenches in the western Pacific. It marks the boundary where the Pacific Plate dives beneath the Philippine Sea Plate. The arc stretches about 2,800 kilometers from near Tokyo to beyond Guam and includes the Izu Islands, the Bonin Islands, and the Mariana Islands. Much of the arc is underwater, and its deepest point is the Challenger Deep in the Mariana Trench.

How it formed: The IBM arc began as the western Pacific plate began to subduct (sink) beneath the Philippine Sea Plate. As the sinking plate goes deeper, water and gases trapped in the crust are released into the mantle. This lowers the melting point of mantle rocks, creating magma that rises to form volcanic islands along the arc. The ages of the subducted crust vary along the line, with younger crust in the north and older crust in the south.

How fast it moves: Subduction rates in the IBM region range from about 2 centimeters per year in the south to about 6 centimeters per year in the north. This movement shapes the arc, the trenches, and the back-arc basins behind the volcanic front.

Three parts of the arc: The IBM arc has three main segments. The Izu segment in the north contains many volcanic islands and inter-arc rifts. The Bonin segment to the south is mostly underwater, with a few islands such as Nishino-shima. The Mariana segment in the east hosts a large back-arc basin called the Mariana Trough and a chain of volcanic centers that are partly submerged.

Back-arc basins: Behind the volcanic front, back-arc basins form as the subducting slab pulls the plate apart. The Mariana region has an active back-arc spreading center (the Mariana Trough). The Bonin segment generally has little or no back-arc spreading, while the Izu segment shows cross-arc features and more complex structure.

Crust and sediments: The seafloor east of the IBM trench records a history of very old crust in the north and younger, more varied crust in the south. Magnetic patterns help scientists piece together the Pacific’s spreading history. Sediments range from pelagic clays and cherts to volcanic deposits, reflecting the arc’s long evolution.

Earthquakes and deep subduction: The IBM subduction zone produces deep earthquakes in a zone called the Wadati–Benioff zone. The depth and distribution of earthquakes vary along the arc, revealing differences in the subducting slab’s geometry and behavior.

Why the IBM arc matters: Studying this arc helps scientists understand how subduction zones work, how fluids move from sinking plates into the mantle, and how magma forms to build arcs. The system also hosts hydrothermal systems that contribute to mineral deposits and to our knowledge of how the Earth’s crust and mantle interact at plate boundaries.

Notable places: The arc region includes Guam, the Bonin Islands, Iwo Jima, and other well-known Pacific islands, each with its own history and geography tied to this dynamic boundary.