East of Fiji, between Tonga and Samoa, is a feisty, earthquake-prone fault zone called the Tonga Trench that is the second-deepest submarine canyon in the world. Researchers have found that dozens of giant, flat-topped old undersea volcanoes quickly march toward the trench, ultimately taking the final plunge into the abyss.
Earthquakes and resulting tsunamis are a concern at the Tonga Trench, 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 marks the boundary where a westward-moving chunk of the earth's outer crust, the Pacific plate, is forced downward beneath the Indo-Australian plate next door.
Geologists long assumed that the destruction of giant volcanoes along these so-called subduction zones might add to the risk of earthquakes there. 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:
The 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 Earth's 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 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.
VIDEO: Courtesy Tony Watts, Oxford and Durham University (UK); NERC funded project Collaborative with IFM-GEOMAR (Germany) and NIWA and GNS Science (New Zealand).
IMAGE: Map of the Tonga Trench showing the seismic gap where the Louisville Seamounts meet their final demise. Shallow earthquakes (0-70 km) dating back to 1975 and greater than magnitude 7.5 are marked in red while those between magnitude 5.0 and 7.5 are marked in black. Active volcanoes, including the Monowai Cone, are marked in purple. (Courtesy T. Watts)