In oceanic-oceanic convergence, one of the edges is forced down into the upper mantle, forming a deep trench in the ocean floor. Oceanic crust is able to sink into the mantle because its main component, cold basalt, is about the same density as the mantle rocks. The edge that goes down is called the "descending slab." Although the descending slab is mostly basalt, it also contains layers of clay, limestone, and serpentine that formed on the ocean floor. The clay and serpentine contain water. Limestone is made of minerals combined with carbon dioxide (CO2). As the slab containing all these different kinds of rocks descends into the mantle, it heats up. The heat causes the rocks containing water and CO2 to decompose, releasing these gases. Dry basalt will not melt at mantle temperatures, but when it comes in contact with water and CO2 its melting point is lowered. Consequently, some of the basalt melts and absorbs the water and CO2. The liquid basalt is less dense than the surrounding solid rocks, so buoyancy forces cause the liquid rock to rise through the overlying mantle and oceanic crust to form a volcano on the ocean floor. As the lavas pile up, they form groups of volcanic islands that parallel the trench, like Japan and Indonesia.  These types of volcanoes are called "island-arc volcanoes."

In oceanic-continental convergence, the oceanic crust always forms a descending slab beneath the continent because the granite in the continental crust is too light to sink into the mantle. In the descending oceanic slab, the same conditions for melting occur as before; except this time, as the liquid rock ascends through the continental crust, some of the granite is melted and mixed with the liquid lava. Granite contains lots of silica, so the lava becomes very viscous. Consequently, when these lavas reach the surface, the resulting eruptions tend to be even more explosive than basaltic eruptions. The Cascade volcanoes in the American Northwest are the best studied examples of this type and include Mount Rainier and Mount St. Helens.

In continental-continental convergence,the rocks on both plates are too light to sink into the mantle, so the edges simply crumble and fold into giant mountain ranges. The best example of this type of collision is the Himalayan Mountains in central Asia, which are the result of India crashing into Asia. Very little rock in these collisions is forced to great depths, so little melting occurs, and few volcanoes form.