East of Fiji, between Tonga and Samoa is a feisty, earthquake-prone fault zone called the Tonga Trench. Earthquakes and resulting tsunamis are a concern here, just as they are along the Japan Trench and the even deeper Mariana Trench to the south near Guam.
A whopping 10.9 kilometers deep in some areas, the Tonga Trench is the second-deepest submarine canyon in the world. It marks the boundary where a westward-moving chunk of the earth’s outer crust, the Pacific plate, is forced downward, beneath the adjacent Indo-Australian plate, along a tectonic subduction zone.
Geologists long assumed that the destruction of giant volcanoes along subduction zones might add to the earthquake risk. But Tony Watts of the University of Oxford and his colleagues noticed recently that the opposite might be true: Right at the spot where giant volcanoes are sliding to their doom, the Tonga Trench is surprisingly, seismically quiet. This past summer the researchers scanned the seafloor with sonar to find out why.
Last week at the annual fall meeting of the American Geophysical Union, they shared with journalists a stunning visual analysis of volcanoes now on the verge of entering the trench. And here’s what they found:
Dozens of giant, flat-topped old volcanoes, called the Louisville Ridge Seamounts, ride westward atop the Pacific tectonic plate at up to six centimeters per year. When they reach the trench, they start getting dragged down into the chasm. According to the old assumption, the chunky volcanoes would add friction to the movement of the two plates, leading to a greater build-up of strain and, consequently, to more violent earthquakes in that spot.
But Watts and his team could see that the doomed seamounts are already highly fractured, offering a possible reason why, contrary to expectations, that spot along the trench is virtually earthquake free. The team’s new hypothesis is that by breaking up early on, the volcanoes probably provide a kind of buffer that eases the subduction process—and actually reduces the risk of large, tsunami-generating earthquakes. Whew! Good news for a change!
The ultimate fate of the volcanoes is still unclear. Watts and his colleagues still don’t know whether the fractured volcanoes will be scraped off onto the Australian plate or chewed up and carried down deep into the mantle.
Either way, that sinking slab is producing new volcanic eruptions up top. On May 14, Watts and his team scanned the volcanic Monowai cone just across the trench from where the Louisville seamounts are subducting when soon after it erupted. The video below shows the Tonga trench, the active Monowai volcanic cone on the Indo-Australia side of the trench, and the march of the flat-topped and crumbling Louisville Seamounts into the abyss.