West Mata is a submarine volcano at 1,100 meters depth 200 kilometres (120 mi) southwest of the Samoas. Its eruptions are currently the deepest observed. West Mata is just west of the north in Tonga Ridge where the Tonga Trench makes a sharp turn to the west.
It is in the NE Lau Basin about 35 km east of the Lau spreading center and about 70 km northeast of the currently erupting NE Lau spreading center.
The Research Vessel Thompson surveyed the area during November 2008 and May 2009. In November 2008, the researchers detected a plume of material over West Mata which indicated likely erupting lava flows and/or pyroclastic materials. The sulfur and iron rich plume contained mineral and/or glass shards.
The shard compositions indicated that the erupted material was boninite (an olivine-bronzite andesite with little or no feldspar) which had never been observed from an active volcano.
According to en.wikipedia