The Second Deepest Place On Earth

Today's selection -- from The Underworld by Susan Casey. The Tonga Trench:

“[If the island of] Tonga lacks in terrestrial heft, its surrounding waters are magisterial. To sail 180 miles south from the capital city of Nuku'alofa, on Tongatapu, is to find yourself floating atop thirty-five thousand feet of unquiet ocean above a seabed laceration known as the Horizon Deep. It's the deepest point in the 850-mile-long Tonga Trench, and the world's second-deepest spot, period—just a whisper shy of the Mariana Trench's Challenger Deep. You won't read about it in tourist brochures, but Tonga's ultradeep realm is one of the wonders of the underworld.

“In a contest of extremes, the Tonga Trench and the Mariana Trench are well matched. They're the inverted summits of Hades—a regal pair of forbidding destinations, as implacable as interstellar space. Like all hadal trenches they were created by subduction: as one tectonic plate dives beneath another, that collision bends the downgoing plate, forming a deep, V-shaped trench. There are approximately twenty-seven hadal trenches in the ocean, twenty-three of

which are located on the Ring of Fire, the belt of subduction zones around the Pacific margin. Only four of these trenches plunge below ten thousand meters (32,800 feet)—the Mariana, Tonga, Kermadec, and Philippine—and though they're well hidden from us, these titans are among the earth's most dramatic features.

The Tonga Trench constitutes the northern half of the Tonga-Kermadec subduction system, which extends 2,550 km (1,580 mi) between New Zealand and Tonga.

“The Mariana Trench has starred in undersea horror movies, but the Tonga Trench is scarier, and that's before you factor in the eight pounds of plutonium in its depths, jettisoned during the aborted Apollo 13 mission. It's steeper, more severe, more seismically volatile-busier. At the Tonga Trench's north end, the Pacific plate is subducting beneath the Australian plate at the startling rate of nine inches per year. Nowhere else is a tectonic plate being gobbled with such relish, seamounts and volcanoes ingested like dinner rolls. It's a buffet of geological havoc.

“Every so often the Tonga Trench gets indigestion and belches out an earthquake from way down in the mantle: a majority of the world's deepest quakes originate there, rumbling hundreds of miles beneath the seabed. In 2009, a slab of the Pacific plate cracked as it was being subducted, and the Tonga Trench roared. A magnitude 8.1 earthquake triggered two magnitude 7.8 earthquakes, and all three earthquakes shook simultaneously, generating tsunami waves that ravaged Tonga and Samoa.

“As an encore, one of Tonga's seafloor volcanoes burped up a new island, more than two miles long and a half mile wide, now known as Hunga Tonga-Hunga Ha'apai. (In January 2022, this same volcano would be historical in its fury, unleashing an eruption that blasted steam and ash thirty-six miles up into the mesosphere, created a two-hundred-and-ninety-foot tsunami at its epicenter, and sent shock waves around the globe.) In 2019, another Tongan island called Lateiki disappeared into the depths during a submarine eruption, only to pop up again in a slightly different location."

The Underworld: Journeys to the Depths of the Ocean
author: Susan Casey  
title: The Underworld: Journeys to the Depths of the Ocean  
publisher: Doubleday  
page(s): 117-119