Deep trenches at the bottom of the sea, often bordering continental margins or island arcs.

Compensating for the newly formed crust from the oceanic ridges, the same amount of crust has to be torn back into the internal of earth at the deep-sea trenches. While this movement is accompanied by severe earth quakes, light material from the molten crust dwells up to the earth surface in a certain distance from the trenches. Because the magma formed in this process is more or less viscous, the resulting volcanoes are of dangerous, explosive nature.

Deep-sea trenches formed by the oceanic parts of two continental plates result in the formation of volcanic island arcs (e.g. in south-east asia). A deep-sea trench bordering the margin of a continent leads to the formation of volcanic mountains, like the andes in south america. The collision of the continental parts of two continental plates, leads to the formation of very high mountains like the himalaya.