Tonga Trench

The Tonga Trench is a massive oceanic canyon located in the southwestern Pacific Ocean. It forms the northern half of the Tonga-Kermadec subduction system, which stretches between New Zealand and Tonga. This trench is a vital part of Earth's geological recycling process. It collects abyssal oceanic sediments and fragments of oceanic crust. It also acts as a site where mantle-derived melts are transferred to island arc systems.

This trench is famous for its extreme depth. The deepest point is called the Horizon Deep. It is the deepest spot in the Southern Hemisphere. It is also the second deepest point on Earth, trailing only the Challenger Deep in the Mariana Trench. The name Horizon Deep comes from the research vessel *Horizon*. That crew discovered the deep in December 1952.

The trench works through a process called subduction. In this area, the Pacific plate is being subducted westward into the trench. This movement is the fastest plate-tectonic velocity on Earth. GPS measurements in the northern trench show a convergence rate of about 24 centimeters per year. This high speed makes the area the most active zone of mantle seismicity. The process is driven by the opening of the Lau Basin. As the Lau Basin expands, it produces new crust. This new crust is created in front of the Tonga-Kermadec trenches. Meanwhile, old crust is consumed behind the trench.

Geology in this region is quite complex. The area between the trench and the Lau back-arc basin is called the Tonga-Kermadec Ridge. This ridge moves independently from the Australian and Pacific plates. It is divided into several small plates, including the Tonga, Kermadec, and Niuafo'ou plates. The Tonga plate faces the trench directly. Subduction rates are not the same everywhere. They decrease as you move south. The rate drops from 24 centimeters per year in the north to 8 centimeters per year in the south. The movement also becomes more oblique as you go south.

Earthquakes are a major feature of this subduction zone. Most large earthquakes happen at the contact zone where the two plates meet. This friction between the plates causes the shaking. However, other earthquakes happen within the Pacific plate itself. This occurs because the plate is bending as it sinks. The Pacific crust here is very old, between 100 and 140 million years old. Because it is old and cold, it can store a lot of elastic energy. As the slab reaches depths of more than 600 kilometers, it hits barriers in the mantle. This contortion produces deep mantle earthquakes.

Life exists even in the extreme pressure of the Horizon Deep. A 2016 study focused on the community of roundworms living in the sediment. Scientists found that the abundance of these individuals is six times greater at the bottom than at the trench edge. The difference in biomass is even larger. However, species diversity is actually twice as big on the trench slope. This is likely because a few opportunistic species live on the slope. These patterns of life are similar to those found in the Mariana Trench. They are much higher than those in the Peru–Chile Trench.

Humans have also explored this deep environment. The Five Deeps Expedition aimed to map and visit the deepest points of all five oceans. On June 10, 2019, Victor Vescovo made the first crewed descent to the bottom of the Horizon Deep. He used a submersible called the Triton 36000/2. This mission confirmed the depth of the Horizon Deep using direct CTD pressure measurements.

The trench also interacts with other massive underwater features. At its southern end, it collides with the Louisville Seamount Chain. This collision zone moves southward at a rate of 1 centimeter per year. The subducting seamounts cause significant erosion on the southern Tonga fore-arc. This process likely makes the Tonga Trench much deeper than the Kermadec Trench. Another example of interaction is the Capricorn Seamount. This large guyot is sitting on the eastern wall of the northern trench. The bending of the Pacific plate is currently slicing the seamount like a loaf of bread. Scientists expect the trench to completely consume the seamount within 500,000 years.