Trenches and Volcanic Arcs
The subduction zone/plate boundary stands out in the ocean as a deep trench where one plate is bent downwards and drags down the leading edge of the opposite over-riding plate.
The Kermadec Trench is the southern section of a very active line of plate tectonic collision. It continues north all the way to Tonga (over 2500 km from New Zealand) where the rate of collision is up to 24 cm per year - the fastest anywhere on earth. Near to New Zealand this reduces to about 6 cm per year or less. This extended line includes other features such as a long ridge and volcanic arc with over 50 mostly submarine volcanoes, a back arc basin and a second ridge system further to the West. The Tonga Trench itself includes the second deepest point in the world’s oceans at -10,882 metres.
The high level of hydrothermal activity and wide variety of sea floor features makes for an extremely diverse and rich ocean ecosystem that is of great interest to scientists as you can see in this video.
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The back arc basin west of the volcanoes, is an area where the crust is actually being stretched apart and thinned into a long depression. This is mostly a result of the dragging force of the subducting plate. Here small amounts of magma are created to form the back-arc basalts. There is another ridge (the Lau-Colville ridge) on the far side of this depression. The Lau-Colville and Tonga-Kermadec ridges used to be a single ridge before the back-arc basin formed through rifting around 5 million years ago.
Most volcanoes along the Tonga-Kermadec arc show signs of hydrothermal activity, where hot seawater containing lots of dissolved elements is discharged into the sea water. If the temperature is high enough, black smokers can be built. The chimneys of black smokers can contain high concentrations of metals such as gold and copper.