sentences:  According to McCaffrey, this is fast enough to have produced geologically significant effects, such as unroofing of high-grade metamorphic rocks, and provides one explanation for why the forearc in southern IBM is tectonically more active than that in northern IBM.The evolution of the IBM arc system is among the best known of any convergent margin.Because IBM has always been an arc system under strong extension, its components encompass a broad area, from the Palau-Kyushu Ridge to the IBM trench (see first-right figure).