Tonga Plate
My Blogs (olelog) are mainly based on my daily reading of earth science news. Here on whatonearth.olehnielsen.dk I try to weave some of the pieces together to a greater whole with added background info.

The Tonga Plate is a small southwest Pacific tectonic plate or microplate. It is centered at approximately 19° S. latitude and 173° E. longitude. The plate is an elongated plate oriented NNE - SSW and is a northward continuation of the Kermadec linear zone north of New Zealand. The plate is bounded on the east and north by the Pacific Plate, on the northwest by the Niuafo’ou microplate, on the west by the Australian plate and to the south by the Kermadec plate. The Tonga plate is subducting the Pacific plate along the Tonga Trench. This subduction turns into a transform fault boundary north of Tonga. An active rift or spreading center separates the Tonga from the Australian plate and the Niuafo’ou microplate to the west. The Tonga plate is seismically very active and is rotating clockwise.

At the Tonga Trench the Pacific Plate has been subducted beneath the Tonga Plate for the past 40 million years. At about 100 km down the Tonga Slab water is squeezed out of the sinking slab. When water is added to hot mantle rock, one of the main effects is to lower the melting temperature of the rocks. The resulting molten rock (magma) rise up through the mantle wedge and feed volcanoes at the surface. Due to seaward migration of the trench the Lau Basin is rifted apart with back-arc spreading.

A study finds that the structure of the mantle wedge may be far more complex than anyone had imagined. The new findings were published in the Online Early Edition of the journal Science (Sciencexpress of 12 April 2007) with the title “Pervasive Seismic Wave Reflectivity and Metasomatism of the Tonga Mantle Wedge". The Tonga mantle wedge is layered with zones of different composition.

The authors were seeing structure much deeper down than the 100 km mentioned above, at depths as much as 450 kilometres. They think the fluids don't come out all at once, but are released progressively as the pressure increases with depth and then have to percolate up through the overlying wedge.

Whereas the added water causes mantle rock to melt in the upper layers (blue dot A on my drawing), the researchers said different effects are likely to occur at greater depths (blue dot B on my drawing). Under the intense pressures found at depth, added fluids would cause changes in the composition of the mantle rock, and structures composed of these altered minerals within the mantle wedge would be detectable by their altered seismic reflectivity. The detection of layered structures was attributed to the mantle wedge being progressively flushed with fluids expelled from the sinking slab, producing not only melts that rise in volcanoes but also mineralogical structures with seismic reflectivity.

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