Complex sentence: The Central American Volcanic belt is a mountain range consisting of volcanoes, formed along the subducting plate boundary--(the western boundary, Caribbean Plate), that extends through Guatemala, El Salvador, Honduras, Nicaragua, Costa Rica and into Panama.
Split into simpler sentences: The Central American Volcanic belt is a mountain range and volcanic arc, formed due to subduction along the western boundary of the Caribbean Plate. The volcanic belt extends through Guatemala, El Salvador, Honduras, Nicaragua, Costa Rica, and into Panama.