The Tonga Trench and its forearc form an active convergent margin between two plates of the lithosphere, the Pacific plate being subducted into the earth's semi-molten aesthenosphere, and the northeastern corner of the Australian plate overriding it. The Tonga trench extends north-northeast from the North Island of New Zealand.
The convergence is taking place at breath-taking speed: a rate estimated at approximately 15 cm/yr (by Lonsdale, 1986); however, recent Global Positioning Satellite measurements indicate in places a convergence of 24 cm/yr across the northern Tonga Trench, which is the fastest plate velocity yet recorded on the planet (Bevis et al., 1995).
Such oceanic trenches are important sites for the formation of what will become continental crust and for recycling of material back into the mantle. Along the Tonga Trench mantle-derived melts are transferred to the island arc systems, and abyssal oceanic sediments and fragments of oceanic crust are collected.
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