The planet's first recycling program
Fifty-two million years ago, during the Eocene Epoch, the Pacific Plate began to spontaneously sink into the mantle beneath the Philippine Sea Plate. This event, known as subduction initiation, was a major change in Earth's geology. It created a new 2,800-kilometer-long convergent plate boundary, now known as the Izu-Bonin-Mariana (IBM) arc system. This system, which extends south from near Tokyo, is an example of how plate tectonics may have started on a global scale. The rocks exposed here provide a unique record of the magmatic and tectonic processes that occur when a new subduction zone forms.
The first lavas to erupt were a special type of mid-ocean ridge basalt-like rock called fore-arc basalts (FAB). These basalts, dated to between 51 and 52 million years old, represent the initial seafloor spreading that occurred as the Pacific Plate began to roll back and sink. This process stretched and thinned the overriding plate, allowing hot mantle material to rise and melt through decompression. The FAB rocks are chemically distinct, showing features of both mid-ocean ridge basalts and the arc-related volcanism that would follow, recording the transition.
Shortly after the FAB volcanism, a very different and much rarer type of magma erupted. These lavas, called boninites, are high in both magnesium and silica. They are considered a sign of subduction initiation. Boninites are thought to form from the melting of mantle rock that has already been depleted by a previous melting event (which formed the FABs). This depleted mantle is then re-melted at high temperatures after being infused with water released from the sinking Pacific Plate. The presence of boninites, which are named after the Bonin Islands where they were first identified, shows the rapid development of a subduction zone. The Ogasawara (Bonin) Islands are one of the few places on Earth where well-preserved boninites are exposed above sea level.