An oceanic birth certificate
Deep beneath the surface of the Philippine Sea lies the Kyushu-Palau Ridge (KPR), a massive, 2,600-kilometer-long remnant of a formerly active volcanic island arc. This submarine mountain range records a deep planetary event: the birth of a subduction zone. Dredge samples—rocks scraped from the seafloor by research vessels—contain the physical evidence of the Izu-Bonin-Mariana (IBM) arc's formation, an event precisely dated to the Eocene epoch. Using 40Ar/39Ar dating methods on these dredged rocks, scientists have determined that this process began approximately 48 to 52 million years ago.
The KPR is what geologists call a "remnant arc." It was once the main site of volcanic activity as the Pacific Plate began its descent, or subduction, beneath the Philippine Sea Plate. Around 25 to 30 million years ago, a process called back-arc spreading began, effectively rifting the original arc in two. This created the Shikoku and Parece Vela Basins and pushed the active volcanic front eastward to its present location, leaving the now-extinct KPR behind. The rocks dredged from this quieted ridge are therefore a snapshot of the initial, violent moments of the arc's life, preserved for millions of years in the deep ocean.
A story told in lava
The dredged lavas from the KPR are not uniform; they tell a sequential story of arc development. The oldest rocks, dated to around 52 million years old, are called fore-arc basalts (FABs). Geochemically, these FABs resemble lavas found at mid-ocean ridges, suggesting they formed during an initial phase of seafloor spreading as the upper plate stretched and cracked just before subduction took hold.
Stratigraphically above these basalts lies a much rarer and more significant type of rock: boninite. Boninites are distinctive lavas with high magnesium and silica content but very low titanium. These chemical signatures indicate they were formed from a mantle that had already undergone a previous melting event, making it "depleted." To melt such refractory mantle requires immense heat and large amounts of water. This specific recipe only occurs during the initiation of subduction, when a hot, shallow mantle is rapidly fluxed with water released from the descending oceanic plate.
The clear sequence found in the dredge samples—fore-arc basalts followed by boninites—is a step-by-step model for how a new convergent plate margin is born. First, the crust rifts apart, producing MORB-like basalts, and then, as the slab begins to sink and release water, the unique boninite magmas are generated, heralding the start of true arc volcanism.