Flexi Says: A tectonic boundary where two plates are moving toward each other. If the two plates are of equal density, they usually push up against each other, forming a mountain chain. The compression forces will produce folding and reverse faults. If the plates are of unequal density, one plate usually sinks beneath the other in a subduction zone. A volcanic arc forms over the subduction zone, fed by rising magma plumes. There may also be some folding along the subduction zone due to compression forces.