Tonga. The most active seismic area in the world. (photo credit – Stefan Heinrich)

Tonga. It’s an archipelago for the seismic history books. Tonga is in the news again, this time the submarine volcano Hunga Tonga-Hunga Ha’apai was spotted by NASA satellites because the eruptions discoloured the Pacific waters amidst the island kingdom’s 176 islands. Locally, the area with the latest volcanics is known as “the place where islands jump back and forth”. The Tongans gave the region that name a long time ago. The jumping is seismic and that’s related to the volcanoes. The volcanoes are related to a deep subduction zone, a place where ocean crust is devoured in the Earth’s mantle.

Tonga – rather remote

Tongans are proud that they have never been colonized – they’ve never been anybody’s property. Considering the record of western domination, this is indeed something to take pride in – although part of the reason Tonga was never conquered may be that the thinly populated islands offered little more than subsistence farming and volcanic ash. There really wasn’t much reason for the French, Dutch, British, Portuguese, Spanish, or Americans to conquer and hold these dots of land. Nor would there have been much interest in a handful of islands that constantly shake.

Tonga and nearby Fiji are isolated but seismic geophysicists have been finding their way there since the 1950s. The area is a living laboratory of earthshattering activity. It was in this part of the Pacific that American scientists first got a clear glimpse of a subduction zone in action. Until then, the idea that the Earth consumes oceanic crust was an unproven scientific conjecture. But then Bryan Isacks and Jack Oliver showed up on the islands with their seismometers.

Serious worldwide seismic monitoring began with funding from the American defence department. In 1954, Jack Oliver, a young post-doc in New York City, accidentally monitored Nevada nuclear detonations on his east coast seismographs. It was immediately realized that such graphs could be used to keep an eye on Soviet nuclear tests. So the American government paid for installation of seismic recorders all over the world. Scientists jumped on the idea – perhaps they were saving the world from nuclear annihilation, but more importantly, they were gathering data about the Earth’s interior.

One use for all the new seismic data was mapping the Earth’s deep crust. In 1957, during the International Geophysical Year, the king of Fiji asked geophysicists to set up seismometers and to listen to the rumblings under his restless island. Recently, employing modern GPS equipment, scientists have learned that the spot Oliver and Isacks selected for their 1960s seismic study has the fastest plate movement on the planet – a rate of 24 centimetres per year, ten times faster than the global average of less than 3 centimetres. Movement of ocean crust at the Tonga Trench makes it the planet’s most energetic zone of seismicity. As Jack Oliver noted, in order to make an important discovery in science, be at the right place at the right time. He certainly picked the right place for seismic action.

For some great scientific breakthroughs, a measure of luck is involved. As Oliver writes in his book, Shocks and Rocks, plate tectonics was proven because “serendipity prevailed.” Serendipity, discovery by accident. Repeatedly, scientists have made accidental discoveries, often by gleaning data originally gathered for military use. Jack Oliver pointed out that the worldwide grid of seismic stations was not built for him to find slabs of ocean crust descending into the Tonga Trench, but scientists used the data for that purpose.

To investigate the deep-seated earthquakes revealed by military data, Bryan Isacks spent 15 months on various tropical islands, tinkering with seismometers and recording seismic stirrings within the deep crust and mantle. With his strategically located gadgets, Isacks found himself eavesdropping on the death groans of a gigantic slab of ocean crust sinking far below the geological structures that had created Tonga. The seismic noise was originating at least 600 kilometres below the Pacific’s calm surface. Scientists know that earthquakes result from violent shattering of brittle slabs of rock. Material six hundred kilometres below surface should be very hot and soft and pliable – bendable, not breakable, and not able to generate earthquakes. Enormous heat and pressure make such rock malleable. Nevertheless, they recorded deep earthquakes. The researchers concluded that a thick slab of cold ocean crust was being thrust deeply into the planet’s interior. But this was not a simple and direct conclusion. The idea contravened accepted wisdom – everything in the mantle should flow like cold molasses, not break like glass.

Seismic of an oceanic subduction zone – image modelled by Miksha

Although several geophysicists had speculated about subduction zones, it was not until Isacks, Oliver, and Lynn Sykes published “Seismology and the New Global Tectonics” in the Journal of Geophysical Research, that subduction zones were proven. Their paper was the first to document how the Earth recycles crust in ocean trenches. For such an important paper, it is an easy read and a good review of the development of plate tectonics up to late summer, 1968.

Their article acknowledges Alfred Wegener’s continental drift, Harry Hess’s seafloor spreading, and Tuzo Wilson’s transform faults. After that preamble is a nod to mid-oceanic ridges, the nursery of the Earth’s crust. They show that the spreading seafloor is revealed by seismicity, earthquake swarms, and the “young ages measured by radioactive and palaeontological dating and the general absence of sediment.” Thus the authors acknowledged the birthplace of the planet’s rocky crust. Logically, if the crust is spreading from mid-ocean rifts, and not surviving to an extremely old age, it follows there must be places where the (comparatively) youthful rock is being destroyed. Their seismic proof of the crust descending into deep trenches was the fundamental contribution of their paper. Their seismic observation of the digestion of ocean crust was as convincingly clear as an X-ray image of a python swallowing a gerbil.

The scientists concluded that island arcs are zones of destruction, rather like downward escalators carrying ocean crust into a deep dark recycling depot. As proof, Isacks had captured those seismic earthquake signals – noises not dulled by soft, pliable rock but instead the crisp sounds of rigid surface crust cracking within the abyssal trench. The low-frequency seismic rumblings recorded in the south Pacific were the cacophonous crushing of ocean crust within the Earth’s mantle. This discovery was one of the final proofs of the plate tectonics system.

Until the work of  Isacks, Oliver, and Sykes in the South Pacific, the theory of tectonics was floating on thin crustal plates. Underlying dungeons were not much considered. Oliver himself said, “As far as I know, no one before us had thought in terms of such a large-scale thrusting phenomenon that moved a 100-kilometer-thick slab of lithosphere from near the surface to depths of at least 720 kilometers, or had even brought the lithosphere-asthenosphere structure into the picture.”

Oliver’s “well-known picture” of a subduction plate

Oliver summed up the moment when he and Bryan Isacks first realized they had proof that trenches were swallowing ocean crust. “In retrospect, the final interpretation of the data seems obvious, but we pondered the data for months,” said Oliver. Then, one day, the team compared their records from Fiji to data from the Caribbean and “almost immediately, the well-known picture of the down-going slab beneath island arcs appeared on the blackboard.”

The current bit of Tongan volcanism and seismic shaking has wiped out wildlife on part of the island nearest the eruption, but the volcano has added a few hundred square metres of land to the kingdom. That new land, an upwelling of volcanic rock, is a by- product of the subduction zone gobbling up an incredibly huge swath of ocean seafloor. The bit of lava at Hunga Tonga-Hunga Ha’apai is but a small gift, an enlargement of the Tongan archipelago, given in exchange for seafloor crust – and given with much smoke and shaking to the little nation.