sentences:  The southern boundary is found where the IBM Trench meets the Kyushu-Palau Ridge near Belau.Thus defined, the IBM arc system spans over 25° of latitude, from 11°N to 35°20’N
  
  The IBM arc system is part of the Philippine Sea Plate, at least to the first approximation.Although the IBM arc deforms internally – and in fact in the south a small plate known as the Mariana Plate is separated from the Philippine Sea Plate by a spreading ridge in the Mariana Trough – it is still useful to discuss approximate rates and directions of the Philippine Sea Plate with its lithospheric neighbors, because these define, to a first order, how rapidly and along what streamlines material is fed into the Subduction Factory.